i
Development of a CNC
Cold Spray Laboratory
Facility by
Dylan Greene
A Thesis submitted to the University of Dublin in partial fulfilment of
the requirements for the degree of
Masters in Mechanical & Manufacturing
Engineering
Trinity College Dublin, April 2014
Supervisor
Dr. Rocco Lupoi
ii
i
Declaration:
I declare that I am the sole author of this dissertation and that the work present in it,
unless otherwise referenced, is entirely my own. I also declare that the work has not been
submitted, in whole or in part, to any other university as an exercise for a degree or any
other qualification.
I agree that the library of Trinity College Dublin may lend or copy this dissertation
upon request.
Signed: _______________________________
Date: _______________________________
ii
Abstract:
The general objective of this project was to develop a CNC Cold Spray Facility
capable of coating flat and pipe substrates. The specific purpose of the machine build was
to provide control of the substrate feed rates accurately and safely into the path of the static
Cold Spray nozzle and to pave way for novel surface coating research to follow. A three
axis CNC machine was built, calibrated and used to conduct two basic Cold Spray tests.
All motion parameters of both the XY table and spindle axis are set, controlled and
monitored by the CNC controller software (Mach3) in open loop control mode. The XY
table positional accuracy is within 0.01 and the spindle accuracy is within 1 RPM. All
three CNC axes have a high degree of precision combined with excellent operational
repeatability. Both Cold Spray tests were conducted with (15-38) copper powder &
aluminium substrates; a Static Pipe Substrate Test & a Dynamic (Rotational) Pipe
Substrate Test. An approximate coating width at the set process parameters was taken from
the successfully Cold Sprayed Static Pipe Substrate and used to set the axial pitch for the
Dynamic (Rotational) Pipe Substrate Test. The Dynamic (Rotational) Pipe Substrate was
also successfully Cold Sprayed. These successful coatings tests verified the CNC machine
functionality and competency to conduct flat and pipe substrate Cold Spray experiments.
This Cold Spray facility will make TCD the National leader in Cold Spray technology and
applications and on this basis the impact of this work is extremely high. PIV analysis must
be conducted to measure the coatings particle velocity under set process parameters, a
camera must be installed to record Cold Spray tests for post analysis and way cover
bellows must be mounted to protect the platform ball screw mechanisms.
iii
Acknowledgements:
There are people that deserve sincere acknowledgement for their help and advice.
Mr. Seán Doonan, Mr. Alex Kearns, Mr. Jj Ryan, Mr. Gabriel Nocholson, Mr.
Michael O’Reilly & Mr. Danny Boardman were immensely helpful in the TCD workshop
throughout the year and regularly went out of their way to sort me out with any problems I
was having. I learned a lot from them and I can’t thank them enough for the continuous
support and genuine interest they showed in my project; they are truly class acts.
My supervisor Dr. Rocco Lupoi made time to discuss the project and answer any
queries I had at the drop of a hat and often worked with me after hours to ensure project
completion and success. He was a good motivator and a model supervisor from beginning
to end.
Mr. Rory Stoney put up with a lot from me as I bombarded him with hundreds of e-
mails about motor specs and electronics related questions all hours of the day. He never
failed to reply to a single question and frequently kept tabs on me to see how I was getting
on. He did so on his own free time, 24 hours a day, and for that I am extremely grateful.
I would like to thank Mr. Paul Normoyle for sanity checking my wired control bay
and for his general circuitry advice.
Thanks a lot Ms. Claudia Robbe for the help with the experiment set-up and I wish
you all the best in your own Masters.
I would to acknowledge Mr. Gordon O’Brien for the 3D model of the CNC XY
Stage he gave to me to adapt and add to. The finished product looked great with the spindle
included looked great.
Finally I would like to thank my parents, Emer & David. They provided me with
the nutritional & emotional support I needed to pull through and keep going when things
weren’t going so great in the project.
iv
Table of Contents:
a) Declaration – Page (i)
b) Abstract – Page (ii)
c) Acknowledgements – Page (iii)
d) List of Tables – Page (vi)
e) List of Figures – Pages (vi) – (x)
1. Introduction – Pages (1-3)
2. Literature Review - Pages (4-26)
2.1 Cold Spray Process Overview – (4)
2.2 Cold Spray Testing Facility Set-up – (7)
2.3 DC Motors – (8)
2.3.1 Steppers – Page (11)
2.3.2 Servos – Page (11)
2.3.3 Servo – Stepper Hybrids – (13)
2.4 Lead screw vs. ball screw & linear slide mechanisms – (15)
2.5 CNC Motion Control – (19)
2.6 Literature Review Conclusion – (23)
3. Proposed System Design – Pages (24-26)
4. Machine Build – Pages (26-63)
4.1 Planning & Organisation – (26)
4.2 Machine Housing – (29)
4.3 CNC XY Stage Assembly – (33)
4.4 Electronics Control Bay Installation – (36)
4.5 Motor Electronics Description – (39)
4.6 Safety Electronics Description – (40)
4.7 Spindle Design (Mechanical) – (45)
4.8 Hardware – Software Calibration & System Settings – Pages (49-55)
4.8.1 Set the Native Units – (49)
4.8.2 Engine Configuration – (50)
4.8.3 Motor Tuning – (52)
4.8.4 Limit Switch & Homing Set-up – (54)
4.8.5 Soft Limits – (55)
4.8.6 Spindle Axis Set-up & Calibration (Closed-Loop & Open-
Loop Control) – (56-61)
4.8.6.1 Open Loop Control – (56)
v
4.8.6.2 Closed Loop Control – (59)
4.8.7 System Functionality & Optimisation – (61)
5. Pressurised Gas & Powder Feeder System – Pages (64-65)
6. Spindle Speed Synchronisation – Pages (66-68)
7. Cold Spray Testing – Pages (68-76)
7.1 Mutual Test Process Parameters – (68)
7.2 Static Pipe Substrate – Pages (69-71)
7.2.1 Process Parameters & G-code – (69)
7.2.2 Results & Discussion – Pages (71)
7.3 Rotating Pipe Substrate – Pages (72-76)
7.3.1 Process Parameters & G-code – (72)
7.3.2 Results & Discussion – (75)
8. Problems & Delays – Pages (76-79)
9. Impact of Work – Pages (79-80)
10. Conclusion – Pages (80-81)
11. Future Work – Page (82)
12. References – Pages (83-85)
13. Appendix – Pages (86-107)
13.1 Supplemental Figures – (86)
13.2 Part Manuals & Specifications – (108)
vi
i) List of Tables:
Table 6.1 – Kinematic parameter table. – Page (68)
Table 7.1 – Process parameters used in both Cold Spray Tests. – Page (69)
Table 7.2 – Static Pipe Substrate Test specific process parameters & G-Code. – Page (70)
Table 7.3 - Static (Rotating) Pipe Substrate Test specific process parameters & G-code. –
Page (73)
ii) List of Figures:
Figure 2.1 - The evolutions of shapes of a 20 mm-diameter aluminium feed-powder
particle and a copper substrate-crater for the incident particle velocity of 6(15-38) m/s at
the times: (a) 5 ns; (b) 20 ns; (c) 35 ns; and (d) (15-38) ns (Grujicic et al., 2003). – Page (4)
Figure 2.1 - The evolutions of shapes of a 20 mm-diameter copper feed-powder particle
and an aluminium substrate-crater for the incident particle velocity of 6(15-38) m/s at the
times: (a) 5 ns; (b) 20 ns; (c) 35 ns; and (d) (15-38) ns (Grujicic et al., 2003). – Page (5)
Figure 2.2 – Schematic of a typical Cold Spray system (Grujicic et al., 2003). – Page (7)
Figure 2.3 – torque capability of stepper vs. servo motors over a range of speeds (green &
red = servo motor, blue = stepper motor) (Carlberg, 2012). – Page (10)
Figure 2.4 – (a) & (b) Experimental results displaying the removal of all stepper motor
harmonic resonances using an open-loop damping algorithm (Tsui et al., 2009). – Page
(12)
Figure 2.5 – Velocity Error Profiles with and without closed-loop damping algorithms at
different velocities (Tsui et al., 2009). – Page (14)
Figure 2.6 – Ball Screw Linear Stage/platform. – Page – (17)
Figure 2.7 – Friction – velocity relationship as described by Stribeck friction (Armstrong-
Hélouvry et al., 1994). – Page (18)
vii
Figure 3.1 – (a) De Laval Nozzle to be used (b) Lead Screw CNC platform model, (c) ‘T-
slot’ XY table model, (d) Complete Spindle axis assembly model & (e) Complete CNC
XY testing platform model assembly & (f) Complete CNC XY and spindle testing
platform model assembly. – Pages (24-26)
Figure 4.1 – Predicted Work Schedule (Gantt chart). – Page (27)
Figure 4.2 – The exterior of the machine housing. – Page (30)
Figure 4.3 – The inside of the process operating region (view room the retracted front
window). – Page (31)
Figure 4.4 – Passive Interlock switch on the right hand side housing window. – Page (31)
Figure 4.5 – Control bay location post stripping. – Page (32)
Figure 4.6 – Foundation of CNC XY Stage assembly. – Page (34)
Figure 4.7 – Top platform cable management system installation. – Page (35)
Figure 4.8 – Proposed layout of control bay before fixating components into place. – Page
(37)
Figure 4.9 – Complete control bay. – Page (38)
Figure 4.10 – Motion Signal Flow Chart. – Page (40)
Figure 4.11 - ON/OFF button circuit. – Page (41)
Figure 4.12 - DPDT latch relay circuit. – Page (42)
Figure 4.13 – Latched switch Illustration. – Page (42)
Figure 4.14 – Magnetic Proximity / Limit Switches installed on the X & Y platforms. –
Page (44)
Figure 4.15 – (a) Rough concept hand sketch (b) & (c) Colour coded and labelled Spindle
Axis Model. – Pages (46-47)
Figure 4.16 – Complete CNC Machine Assembly with XY Table & Spindle Axis. – Page
(48)
Figure 4.17 – Mach3 Interface in the ‘Program Run’ window with the ‘Config’ tab options
highlighted. – Pages (49)
viii
Figure 4.18 – Mach3 Interface with the ‘Engine Configuration’ Port# & Kernel Speed
options highlighted. – Page (51)
Figure 4.19 – Mach3 Interface with the X,Y & Spindle axis Step & Direction settings
highlighted. – Page (52)
Figure 4.20 – Mach3 Interface with the ‘Axis Calibration’ button highlighted. – Page (53)
Figure 4.21 - Mach3 Interface in the ‘Motor Tuning’ window with the motor tuning
settings highlighted. – Page (58)
Figure 4.22 – Mach3 Interface with ‘Special Functions’ used to activate Mach3 Closed
Loop Control Modes. – Page (60)
Figure 4.23 – Spindle Feedback Set-up Schematic. – Page (63)
Figure 4.24 – Motor Tuning Settings Summary. – Page (63)
Figure 5.1 – Pressurised Gas System Schematic. – Page (64)
Figure 5.2 – Main Pressure Valve connecting the pressurised gas to the Powder Feeder and
Cold Spray nozzle. – Page (65)
Figure 6.1 – Excel interface relating CS process parameters with a set of data inputted for
illustration. – Page (67)
Figure 6.2 – Coating pitch illustration for helical pipe coating tests.
Figure 7.1 – Mach3 Interface with ‘DYNAMIC_PIPE_TEST’ G-code file loaded and XY
table path displayed at the top right hand corner relative to the Home position (purple
crosshairs). – Page (70)
Figure 7.2 – Static Aluminium Pipe Substrate Cold Sprayed with - copper
powder (60mm coating length, 2 coating passes). – Page (71)
Figure 7.3 – Mach3 Interface with ‘DYNAMIC_PIPE_TEST’ G-code file loaded and XY
table path displayed at the top right hand corner relative to the Home position (purple
crosshairs). – Page (73)
Figure 7.4 – Close-up of Rotating Pipe Substrate Cold Sprayed Cold Sprayed with
- copper powder (15mm coating length, 4 passes). – Page (74)
ix
Figure 7.5 – Perspective shot of Rotating Pipe Substrate Cold Sprayed Cold Sprayed with
- copper powder (15mm coating length, 4 passes). – Page (75)
Figure 8.1 – Close-up shot of copper-oil mixture air sprayed out of the spindle bearing
(evidence of critical bearing copper powder exposure). – Page (78)
Figure 13.1 - Control bay location as before stripping. – Page (86)
Figure 13.2 - Control bay to be stripped and converted. – Page (87)
Figure 13.3 - Control bay installation development with the limit switch circuitry now
included (1). – Page (88)
Figure 13.4 - Control bay installation development with the limit switch circuitry now
included (2). – Page (89)
Figure 13.5 – Complete control bay with ventilation cover. – Page (90)
Figure 13.6 – Motor drivers PSU circuit. – Page (91)
Figure 13.7 – A SPST relay limit switch circuit. – Page (91)
Figure 13.8 – Chuck backing plate. – Page (92)
Figure 13.9 – Far side shoulder. – Page (93)
Figure 13.10 – Chuck side shoulder. – Page (94)
Figure 13.11 – ‘L-bracket’ spindle axis mount. – Page (95)
Figure 13.12 – DC motor mounting plate. – Page (96)
Figure 13.13 - Spindle Shaft. – Page (97)
Figure 13.14 – Spindle Axis close up post 90 second Cold Spray Test (Rotating Pipe
Substrate). – Page (98)
Figure 13.15 – XY Table post 90 second Cold Spray Test (Rotating Pipe Substrate). – Page
(99)
Figure 13.16 – Close up of the two Cold Spray samples side-by-side. – Page (100)
Figure 13.17 – 220 bar Nitrogen Gas Tank, size W from BOC. – Page (101)
x
Figure 13.18 – Close up of the Powder Feeder (Angle 1). – Page (102)
Figure 13.19 – Close up of the Powder Feeder (Angle 2). – Page (103)
Figure 13.20 – Close up of the Cold Spray nozzle and its inlet connections. – Page (104)
Figure 13.21 – Component designed and made to (a) connect IGUS cable manager to the
X-axis & (b) to fixate and act as a guiderail for the IGUS cable manager. – Page (105)
Figure 13.22 – Pressure Gauge Mount Design Concept that was never built. – Page (106)
Figure 13.23 – Another Pressure Gauge Mount Design Concept that was never built. –
Page (107)
1
1. Introduction:
Cold spray is a process whereby metal powder particles are employed as a material
coating by means of mechanical impact upon an appropriate substrate (Papyrin, 2001, Van
Steenkiste et al., 2002, Stoltenhoff et al., 2002). The powder particle sizes vary from
and are injected into the pathway of a high velocity gas stream to accelerate
them. The high velocity gas is produced by the expansion of a pressurized preheated gas
through a converging-diverging nozzle. The gas is expanded to supersonic velocity at the
nozzle exit combined with an equivalent drop in pressure and temperature (Dykhuizen and
Smith, 1998, Kosarev et al., 2003, Grujicic et al., 2004). The particles are first carried by a
separate gas stream within a high pressure powder feeder device. The working gases are
primarily Helium and Nitrogen. Helium has been shown to produce higher nozzle outlet
velocities thus has superior particle deposition efficiency however it is relatively
expensive. The particles can be injected from the powder feeder into the high velocity gas
stream prior or downstream of the supersonic nozzle throat, dependent on the specific
machine set up and design. The design determines the powder feeder gas pressure and
whether a gas heater is necessary or not. Each particle must reach a critical velocity in
order to deposit itself on the substrate as a coating. Below the critical velocity the particles
impact on the substrate will have a corrosive effect and will not adhere to the component
surface (Gilmore et al., 1999, Wu et al., 2006). The accelerated particle impact deforms
the particles and forms a bond between them and the substrate (Dykhuizen et al., 1999,
Grujicic et al., 2003). The once spherical metal particles become flattened and elliptical in
shape. As the process continues the deposited material develops a uniform coating with
little porosity and strong bond cohesion. Desired coating thickness can be achieved by
continued processing. Applications of cold spray include corrosion resistance, enhance
components mechanical properties (rigidity and coefficient of friction for example) and
improved aesthetics. It is an effective surface coating technology that does not require
melting of the substrate or the coating material. This eliminates oxidation, thermal
distortion and heat-induced cracking. Cold Spray does not require the metal particles and
the substrate to be metallurgically compatible for coating-substrate consolidation either.
This technological process is known as ‘Cold Spray’ due to the relatively low temperature
range at which the expanded particle-gas mixture exits the nozzle ( ).
2
To produce a homogeneous coating thickness or to coat a component locally, an
integrated control system must be put in place. A control must be placed on the nozzle
pressure which will determine the gas-particle acceleration and whether or not it reaches
the critical velocity. The substrate and or nozzle must be precisely manoeuvred such that
the chosen coating depth and density has been deposited on the substrate and at the exact
position where it was meant to be. To obtain high precision engineering control and run the
process in a relatively short timeframe with a degree of automation, Numerical Control
(NC) is introduced.
Numeric Control is a manufacturing technique that uses programmed instructions
to control a machine that mills, cuts, punches, grinds, bends or turns raw stock into a
finished part (Xu and He, 2004, Xu et al., 2005). The instructions are transferred to the
machine via a storage medium. Computer Numerical Control (CNC) utilizes Computer
Aided Design (CAD) and Computer Aided Manufacturing (CAM) software systems to
produce and deliver these instructions by means of an electronic file in a variety of
formats. The role of CAD is to generate the final component geometry and the role of
CAM is to plan, manage and control manufacturing operations through either direct or
indirect computer interface with the plant’s production resources so that the design can be
materialized (Xu and He, 2004, Xu et al., 2005). As a 3D model contains the necessary
information for NC cutter path programming, many turnkey CAD/CAM packages exist
which facilitate an interface for the neutral data exchange between CAD and CAM systems
(Xu and He, 2004). Mach3 is a software package available that turns a typical computer
(with the appropriate drivers) into a fully featured 6 axis CNC controller. Mach3 allows the
motion control of servo and stepper motors by processing Gcode. It is compatible
importing DXF, BMP, JPG, and HPGL CAD/CAM files through LazyCam. Mach3 can
also generate iso-Gcode via LazyCam or Wizards. Wizards are ‘mini-programs’ that allow
users to quickly perform convenient operations without the need of pre-writing G-code.
The intention of this project is to construct a fully operational 3 axis CNC Cold
Spray facility with which to conduct a series of basic flat plate (primary goal) and pipe
coating (secondary goal) experiments. An XY testing platform is necessary for flat plate
substrates however an additional spindle axis is necessary for cylindrical substrates. Both
the flat plate and pipe substrates are aluminium and the powder coating material is copper.
3
This project is a comprehensive machine build with the aim of establishing a fully
functional testing facility for future Cold Spray investigations. Strong cohesive copper
coatings upon the aluminium substrates complete with functional and reliable safety
features will deem the project a success. There is no currently no Cold Spray testing
facility in Ireland, making it the first of its kind Nationwide.
4
2. Literature Review:
2.1 Cold Spray Process Overview:
Cold Spray is a novel surface coating technique in which a gas-particle mixture is
propelled at a substrate at supersonic velocity whereby the particles mechanically bond to
the substrate surface. The particles and gas are fed into the high pressure-end of the nozzle,
contracted towards the throat and expanded in the supersonic nozzle section. The
mechanical impact of the particles on the substrate is known as the pancake effect (see fig.
2.1 & 2.2). The originally spherical particles flatten and elongate forming thin elliptical
shapes upon collision with the substrate and stack on top and around one another forming
layers of coating.
Figure 2.1 - The evolutions of shapes of a 20 mm-diameter aluminium feed-
powder particle and a copper substrate-crater for the incident particle velocity
of 6(15-38) m/s at the times: (a) 5 ns; (b) 20 ns; (c) 35 ns; and (d) (15-38) ns
(Grujicic et al., 2003).
5
Figure 2.2 - The evolutions of shapes of a 20 mm-diameter copper feed-powder
particle and an aluminium substrate-crater for the incident particle velocity of 6(15-
38) m/s at the times: (a) 5 ns; (b) 20 ns; (c) 35 ns; and (d) (15-38) ns (Grujicic et al.,
2003).
The particles are accelerated by a supersonic jet of gas before impacting the
metallic or dielectric substrate (Papyrin, 2001). In the Cold Spray process, powder particles
are accelerated by the supersonic gas jet at a temperature that is always lower than the
melting point of the material, resulting in coating formation from particles in the solid state
(Papyrin, 2001). As a result the damaging effects of high-temperature oxidation,
evaporation, melting, crystallization, residual stresses, de-bonding, gas release, and other
common problems for traditional thermal spray methods are minimized or eliminated
(Papyrin, 2001). In addition to this, Cold Spray facilitates the property retention of particle
material and prevents the formation of unwanted phases associated with melting. The
6
process produces a high density, high hardness, cold-worked microstructure inducing a
compressive stress on the substrate surface upon impact. Early Russian studies examined
the dependence of deposition efficiency on particle velocity at ambient stagnation
temperature of the jet, where it was shown that there was a critical velocity for each
material for particle deposition to occur (Papyrin, 2001). If the particle velocity was less
than the critical velocity, no particle deposition and a degree of substrate erosion would
occur (Papyrin, 2001). The deposition coating process starts as the particle velocity
approaches the critical velocity, around which point the deposition efficiency briskly rises
from (15-38)-70% (Papyrin, 2001). Particle deposition efficiencies of 0%, 53% and 95%
were achieved at the nozzle outlet at velocities of 495, 652, and 784 m/s respectively
(Gilmore et al., 1999). Copper powder, helium and a rectangular were the particle material,
working fluid and nozzle exit aperture respectively (Gilmore et al., 1999). Typical values
for numerous metals (Al, Cu, Ti) ranged from (15-38)0-700 (Papyrin, 2001). From
these results and studies conducted at the Institute of Theoretical and Applied Mechanics,
the basic requirements for the coating formation from particles in a solid-state were
formed: jet temperature must be lower than the heat softening and melting temperature of
the particles, particle size range must be from 1-(15-38) and particle velocity must be
in the range of 300-1300 (dependent on particle size) (Papyrin, 2001). Supersonic jet
gas pressure ranges from 1-3 MPa, the nozzle mach number ranges from 2-4, stagnation jet
temperature ranges from 0-700 K and Gas preheating can increase the gas discharge speed
and particle velocity (Papyrin, 2001). The high-pressure gas is heated electrically via large
copper inductance coil and typical working gases are air, helium and nitrogen (Papyrin,
2001). Helium is the working gas of choice because it provides the fastest jet velocities due
to its small molecular weight and higher specific heat ratio (Dykhuizen and Smith, 1998).
A de Laval nozzle with a circular cross-section is used and consists of a contracting inlet
zone, a restriction cross-section (throat, 1-5 mm in diameter) and an expanding supersonic
exit zone ((15-38)-200 mm in length) (Kosarev et al., 2003). The nozzle is typically made
of tool steel or tungsten carbide which are both very hard materials, to resist abrasive wear
from the accelerated particles rubbing against the internal walls at supersonic velocities.
Other advantages of Cold Spray high are deposition rates, you can collect and re-use non-
deposited particles, minimal preparation to the substrate is required and the Standoff
distance is relatively short (Papyrin, 2001). (Kosarev et al., 2003) investigated some gas
dynamics and thermal effects related to the supersonic gas jet coming from a nozzle with a
rectangular cross section and its interaction with the substrate in the cold spray process
(Kosarev et al., 2003). The advantage over a conventional circular cross-section nozzle is
7
that a wider beams with a short length can be used for in the short direction of the substrate
and similarly narrow beams with a short width can be used in the long direction of a
substrate (Kosarev et al., 2003). It was found that rectangular nozzles with a large length to
width ratio significantly affected the flow parameters, reducing the Mach number by 10-
20% (Kosarev et al., 2003). It was shown that the width to length ratio of the nozzle was
the dominant factor in determining the Mach number and that w/L 0.025 was necessary
for the boundary layers of the opposite walls to overlap (Kosarev et al., 2003). A typical
Cold Spray set-up can be seen in fig. 2.3 below.
Figure 2.3 – Schematic of a typical Cold Spray system (Grujicic et al., 2003).
2.2 Cold Spray Testing Facility Set-up:
The primary goal of this project is to coat aluminium flat plate substrates. The Cold
Spray facility will move the component into the path of the nozzle along the XY plane, as
opposed to moving the nozzle across the component. The basic XY table set-up consists of
two CNC platforms which are simply linear translation mechanisms. The position, speed
and movement direction of the platforms are controlled by DC motors. One platform (base
platform) is bolted to the machine housing and the other platform is bolted to the table of
the base platform at exactly 90 to its longitudinal and transverse orientation. A ‘T-Slot’
table is typically bolted on top of the top platform to easily fixate components in place with
8
‘T-Slot nuts’ for testing, whatever the CNC process may be. With this system in mind,
certain specific technological decisions have to be made.
2.3 DC Motors:
In regards to the DC motors that will be used to control the position, speed and
movement direction of the XY table, I have to opt for brushed or brushless motors and
whether the motors will be steppers or servos. It’s important to understand the underlying
theory and key differences between brushed and brushless motors and likewise the
difference between stepper and servo systems to determine the right choice for the
application. Brushed motors consist of a permanently magnetised stator that encapsulates a
rotor separated by a small air gap. The rotor has one or more coil windings known as the
armature that produce a magnetic field when energised. The stator coils are energised by
the commutator, which is a thin sleeve of copper that is fitted around the rotor shaft. The
copper sleeve is divided into segments with small gaps separating them, each segment
energising a different coil winding. Carbon based brushes with a voltage applied across
them come into contact with the commutator as the rotor rotates. These brushes energise
the coil windings through the commutator. The polarity of the coil windings is switched in
the transition between commutator segments, thus the current direction is mechanically
switched by commutator rotation. On the other hand, the armature is on the stator in
brushless DC motors and there is a different method of commutation. Brushless DC motors
have a permanent magnet rotor and a stator with coil windings. Current is interchanged
between coil windings such that opposite poles of the rotor and stator align, moving the
rotor shaft either clockwise (CW) or counter-clockwise (CCW) depending on the
synchronisation of the current feed to the armature. The wear phenomenon experienced at
the contact interface is influenced by mechanical, electrical and thermal properties, the rate
of which is dependent on application (Shin and Lee, 2010). In a wear experiment by (Shin
and Lee, 2010), the wear rate of the brushes was shown to be affected by contact load,
sliding speed and current flow. Sliding speed and contact load had a minimal affect whilst
current change predominately affected the wear rate (Shin and Lee, 2010). There was also
a difference in positive and negative brushes caused by current flow, the positively charged
brush wearing at a faster rate (Shin and Lee, 2010). Although brushed DC (BDC) motors
are relatively low in cost and easy to control, the mechanical collector ( copper sleeve
9
commutator) has a limited life span (Moseler and Isermann, 2000). In conjunction with
this the brush sparks can destroy the rotor coils, inhibit the motors electromagnetic
capability (EMC) and lessen the insulation resistance to an unacceptable limit (Moseler
and Isermann, 2000). On the other hand brushless DC (BLDC) motors use an electronic
inverter to perform the commutation and are consequently more reliable (Moseler and
Isermann, 2000). As the price of power electronics such as DC motor drivers is
continuously decreasing and the demand for cost-effective servo systems is increasing,
BLDC motors are gradually phasing out BDC motors for many applications (Moseler and
Isermann, 2000, Hameyer and Belmans, 1996).
Both servos and steppers are synchronous motors consisting of a permanently
magnetised rotor and a stator with coiled windings, where the rotation period is an integral
number of alternating current cycles (Carlberg, 2012). The electromagnetic torque is
obtained by controlling the current components and the applied current to the coiled
windings is directly proportional to the motor torque (Pacas and Weber, 2005). The rotor
and stator have a set number of protruding poles / teeth. Permanent polarity interchanges
between North (N) and South (S) from tooth to tooth on the rotor and similarly for the
stator, only current must flow through the coil windings that are wrapped around the stator
teeth for them to become magnetised. The most common DC motor has (15-38) rotor teeth
spaced 7.2 apart and is capable of a 1.8 step using the basic excitation method (full
stepping). Typically the rotor consists of permanent magnets that axially extend the length
of the rotor and stator and form (15-38) N-S pole pairs (Goluba, 2000). The rotor stack is
magnetised axially with a single permanent. In the basic excitation method current flowing
through the ‘A’ poles (A coil windings) is cut off and current is fed into the ‘B’ poles
(Goluba, 2000). At one end of the stator the ‘B+’ are S poles and at the opposite end of the
stator the ‘B-’ are N poles (Goluba, 2000). This induces a 1.8 rotation to re-align the N
rotor teeth with the now ‘B+’ teeth faces (Goluba, 2000). Current is cut off from the ‘B’
poles and fed into the ‘A’ poles in the opposite direction (Goluba, 2000). The ‘A-’ teeth
faces of the stator attract the S rotor teeth to align with them rotating another 1.8 in the
same direction (Goluba, 2000). Current is cut off from the ‘B’ poles and fed back into the
‘A’ poles in the direction opposite to the direction of during the previous excitation of the
‘B’ poles. This switches the polarity of ‘B+’ & ‘B-’ (Goluba, 2000).
10
Figure 2.4 – torque capability of stepper vs. servo motors over a range of speeds
(green & red = servo motor, blue = stepper motor) (Carlberg, 2012).
‘According to (Teschler and Meyer, 1998) the motion control approach to be taken
is dependent on 3 critical factors – time, torque and inertia. For example there can be time
constraints for certain processes which require a certain applied torque load, torque
acceleration and deceleration to hit a specified process rate. These torque parameters are
dependent on the inertial mass of the system hence all 3 factors interact and affect one
another. Key performance motor benchmarks to consider are the torque-to-inertia ratio, the
torque-to-volume and the torque-to-weight ratio (Teschler and Meyer, 1998). Torque-to-
inertia ratio is a good means of assessing a motors ability to accelerate and decelerate and
both load and inertial mass of the motor resist acceleration/deceleration. The torque-to-
volume ratio is important when space is a critical factor. Brushless servo motors currently
display the highest torque-to-volume ratio (Teschler and Meyer, 1998). Torque-to-weight
ratio is closely linked to the torque-to-volume ratio. Smaller volume motors tend to be
lighter and weight is a crucial parameter when it comes to the robotics industry, where the
weight of each motor becomes part of the load in the next supporting axis (Teschler and
Meyer, 1998).
11
2.3.1 Steppers:
Most steppers are open-loop systems with no feedback and on the other hand
servos are closed-loop systems with position and velocity feedback (Carlberg, 2012,
Teschler and Meyer, 1998, Trumper et al., 1996). A disadvantage of stepper motors is their
intrinsic harmonic instabilities (Carlberg, 2012, Teschler and Meyer, 1998, Tsui et al.,
2009). At low speeds typically between 30-90 RPM, excitation of the motor resonant
frequency can occur which can induce velocity ripple, a loss of steps, and a substantial
error in the system’s final position (Carlberg, 2012). A midrange instability occurs at
approximately (15-38)% of the maximum motor torque output, and may result in a stalled
motor and or the same problems associated with low range instability(see fig. 2.5)
(Carlberg, 2012). A stepper system cannot recognise a change in torque and will therefore
stall if torque demand exceeds that available at any given speed (Carlberg, 2012). As motor
speed increases, torque tends to decrease (see fig. 2.4) (Carlberg, 2012, Teschler and
Meyer, 1998). Stepper motors tend to have more steps and higher inductance coils relative
to servo motors. Consequently torque drops off at a faster rate as the speed is increased
thus the peak performance of the stepper motor is at relatively low speeds (Carlberg, 2012,
Teschler and Meyer, 1998).
2.3.2 Servos:
The servo-amplifier is substantially more complex than the stepper-amplifier
electronics. Servos have the ability to regulate the current they send to the coil windings
(Carlberg, 2012). A servo system is affectively a stepper with a rotary encoder. As current
is proportional to torque, the control loop in the servo-amplifier is often called the torque
loop. The servo-amplifier can handle variable loads during operation within the design
operating parameters (Carlberg, 2012). In doing so it acts as a velocity and position control
by varying the applied current if the applied loading is changing (see fig. 2.6). For example
if a servo motor had a set velocity and the torque load was to suddenly increase, the servo-
amplifier would apply more current to account for the increased load such that the velocity
would not change. Similarly if a servo motor had a set velocity and the torque load was to
suddenly decrease, the servo-amplifier would apply less current to account for the
decreased load such that the servo system velocity would not change. In layman's terms a
12
servo-amplifier will supply the coil windings with only enough current to provide enough
torque to produce the desired velocity. Servo systems are better for higher torque, higher
speed and variable load applications (Teschler and Meyer, 1998). However for very high
RPM applications, a servo system may require gearing down which substantially increases
motor complexity and consequently cost.
Figure 2.5 – (a) & (b) Experimental results displaying the removal of all stepper
motor harmonic resonances using an open-loop damping algorithm (Tsui et al., 2009).
13
2.3.3 Servo – Stepper Hybrids:
Servo steppers are hybrid motors that combine the benefits of steppers and servos
together. The key advantages of stepper motors is they don’t require a feedback control for
position and velocity control, positional error is non accumulative and does not require
tuning of a feedback control device which needs expertise and effort (Tsui et al., 2009).
However, stepper motors main disadvantage is the harmonic instabilities caused by the
resonance of the natural frequency of rotor oscillation about the equilibrium position (Tsui
et al., 2009). These mechanical resonance harmonics can be almost completely eliminated
using an open loop damping algorithm based on a simplified torque expression (Melkote
and Khorrami, 1999) and an identified motor-characteristic (Tsui et al., 2009). Furthermore
(Tsui et al., 2009) developed a another damping algorithm for the same motor in servo
mode, containing position proportional + integral + derivative (PID) control and harmonic
torque ripple compensation. The algorithms are efficient enough to be used in demanding
applications and run on commercial digital signal processor (DSP) based hardware
platform (Tsui et al., 2009). So in short, a servo stepper system is one that is primarily used
in open-loop control that through the use of intelligent signal processing algorithms can
eliminate the harmonic instabilities without the need of feedback control.
14
Figure 2.6 – Velocity Error Profiles with and without closed-loop damping algorithms
at different velocities (Tsui et al., 2009).
15
2.4 Lead screw vs. ball screw & linear slide mechanisms:
A standard CNC stage comprises of a platform, a lead/ball screw, two guide rails
and a motor. To convert the rotational displacement, velocity and acceleration of the
stepper motors into linear translation of the X-Y platforms, I had the choice between using
ball screw or lead screw mechanisms. Lead and ball screws both comprise of a threaded
shaft with a specified pitch. The ball/lead screw is placed on bearings at either end of the
stage. The motor is coupled to one end of the screw. Running parallel to the ball/lead screw
are guide rails. A platform consists of a flat surface with a nut underneath, through which
the screw is fitted through. The platform is tightly fitted to the threaded shaft and the two
guide rails. The guide rails are typically low friction shafts fixed around which there are
bearing systems. The way in which lead and ball screw mechanisms differ is how the
platform nut and screw interact with one another. A lead screw nut has the identical thread
as that of the shaft. If you were to simply have the nut fitted to the shaft and turn on the
motor, the nut would rotate with the shaft and there would be no linear motion of the nut
along the shaft axial direction. However the inclusion of the guide rails force the nut to
follow the thread path as the motor rotates the screw, transforming the rotational motion to
linear motion. Similarly a ball screw nut has tightly fitted ball bearings in between the nut
and screw threads and works in the same way. See Fig. 2.7 for a labelled ball screw
mechanism driven CNC platform.
Finite stiffness of the screw, friction and torsional displacement are the key
disturbances associated with position control of a CNC machining centre (Eun-Chan et al.,
2003). These mechanical problems can induce steady state errors and vibrations of an X-Y
table’s position (Lim et al., 2001). Screw rigidity at the screw-nut and screw-bearing
interfaces will determine the degree of elastic deformation the screw will undergo upon
loading. Elastic deformation of the screw causes a torsional displacement difference
between the screw and the motor shafts (also known as backlash) which can be
significantly large at high accelerations and during velocity reversal (Lim et al., 2001, Eun-
Chan et al., 2003). Table positioning error can be quantified using visual encoders and or a
laser interferometer (Ku et al., 1998). Backlash can be worsened by screw wear, decreasing
the transmission performance and lifetime (Wei et al., 2012). As ball screws are pre-
stressed and have superior tribological properties, the screw is stiffer and thus is less
16
susceptible to backlash. Although ball screws are more expensive, they have the capability
to deal with higher loads, achieve faster speeds and run continuous duty cycles (Lipsett,
2009). A lead screw cannot compete with a ball screw mechanism in efficiency (~90%)
due to difference in frictional energy dissipation at the screw-nut interface (Lipsett, 2009).
A ball screw uses re-circulating ball bearings to minimize friction and maximize efficiency
while a lead screw depends on low coefficients of friction between sliding surfaces
(Lipsett, 2009). Power transmission reliability is inherently less reliable for sliding friction
mechanisms as opposed to re-circulating ball technologies (Lipsett, 2009). Although lead
screws are cheaper, they are not suitable for high speed applications, they wear faster due
to higher friction and require greater torque (Keefer, 2013). Having said this lead screws
are self-locking, do not require lubrication to achieve their design life and are relatively
quiet (Keefer, 2013).
The linear slide mechanisms for the CNC stage guiderails are ball bearing
mechanisms like that of the ball screw nut and the ball screw bearing mounts at either end.
In order to minimize the friction linear at the linear slide – guiderail interface, it is essential
to understand the fundamental characteristics of friction of a linear slide mechanism in
operation.
17
Figure 2.7 – Ball Screw Linear Stage/platform.
Variance in frictional forces along the guiderails is highly undesirable as stepper
motors cannot account for variable loads. If the frictional load were to rapidly
increase/decrease during operation it would quicken/reduce the feed rate, cause a positional
tracking error (could potentially stall the motor in the frictional increase case). There are
two distinct friction regimes, the pre-sliding regime and the gross sliding regime. The pre-
sliding regime is where the adhesive forces at the asperity contacts are dominant such that
the friction force appears to be a function of displacement rather than velocity (Swevers et
al., 2000, Armstrong-Hélouvry et al., 1994). The asperities deform elasto-plastically acting
like non-linear springs, until the displacement reaches a stage where the asperities begin to
Linear Slide Guiderails
CNC Stage Table & Ball Nut Mechanism
Screw
Motor Coupling Shaft
18
break (“break-away displacement”) resulting in gross sliding (Swevers et al., 2000,
Armstrong-Hélouvry et al., 1994). The gross sliding regime is where all of the asperity
junctions have been broken apart and where friction is now more a function of velocity due
to the presence of lubricating films (Swevers et al., 2000). The transition between the two
friction regime is not considered a discontinuity builds up to the gross sliding regime
(Swevers et al., 2000). Three important dynamic linear slide characteristics to consider are
stick-slip, varying break-away force, and induced frictional lag (Swevers et al., 2000).
Stick – slip behaviour can occur when friction decreases locally or globally along the
sliding interface with an increase in velocity (Swevers et al., 2000). When the driving
torque is increased at a constant rate, the friction force opposing the drive torque increases
at the same rate as long as the system sticks (Swevers et al., 2000).
Figure 2.8 – Friction – velocity relationship as described by Stribeck friction
(Armstrong-Hélouvry et al., 1994).
When the system breaks away, the friction torque has reached a maximum and
decreases with velocity in the low velocity regime due to the Stribeck-effect (Swevers et
al., 2000). The Stribeck-effect describes the transition between pre-sliding and gross
sliding regimes (see fig. 2.8 (Armstrong-Hélouvry et al., 1994)). The Stribeck model is the
19
sum of the negative viscous, coulomb and viscous friction models (Armstrong-Hélouvry et
al., 1994). The maximum friction torque is larger for smaller rates and smaller for larger
rates (Swevers et al., 2000, Armstrong-Hélouvry et al., 1994). Thus the break-away torque
is the drive torque at which the system breaks away and where the friction torque reaches a
maximum and starts to decrease with an increase in velocity. The break-away point is the
transition point from pre-sliding to gross sliding regimes. In summation friction is a major
problem with regards to motion control along a linear slide mechanism. It is a non-linear
phenomenon consisting of two distinct regions. It is also dependent on surface smoothness,
hardness surface layer metallurgy and the presence of lubricating films (Armstrong-
Hélouvry et al., 1994, Swevers et al., 2000). Predictability and repeatability of frictional
forces and the rate of change of friction with displacement in the pre-sliding regime and
velocity in the gross sliding regime is crucial for accurate positional control. The guiderails
must have a homogenous surface and the bearings systems must fit tightly to provide such
constant frictional properties along the guiderails. The guiderails should also be greased /
lubricated appropriately such that the desired coefficient of friction is achieved.
2.5 CNC Motion Control:
In essence the motion control of a DC motor is dictated by the control systems
ability to regulate the current magnitude being applied to the armature, the rate at which
current is interchanged between coil windings and the direction in which coil windings are
energised. The two major types of control are open-loop control and closed-loop control.
Purely open-loop control involves the application of an input to a process with no means of
quantifying the output error or correcting it. Closed-loop control involves the application
of an input to a process that has the capability to track the system response, quantify its
error and vary the input to achieve the desired output. It does so through the use of a
feedback control loop. In the specific case of CNC of axes platforms, the crucial control
components are the CNC controller, the motion control card and the motor drivers. The
CNC controller interprets the G-code and sends the motion variables to the motion control
card. The motion control card decodes the motion variables with a DSP chip that runs on a
loop, generating pulse signals. The pulse signals are sent to the motor drivers and
amplified. The motor drivers send the amplified signals and current to the motor armature
along different connections. Described above was a standard open-loop control set-up. A
20
closed loop equivalent has feedback control which can send speed, distance and position
information back to the motion control card. Here the motion control card can vary the
pulse signal frequency and the drivers can amplify or attenuate the current to the motors.
The extent of change in pulse single frequency and applied power are dependent on the
degree of position or speed error of the system. In an open loop system the applied torque
to the ball screw shafts will remain constant however variation in shaft loading due to
friction, vibrations or other disturbances will result in a change in shaft rotational speed,
inducing a speed, distance and position error of the system that cannot be quantified or
rectified. Closed-loop control uses a rotary and or visual encoder to track the speed,
distance and position of the platform. This information is fed back to the motion controller
via feedback control loops and these values are compared to the desired values at a
summing junction in the motion control card. The dynamic or transient response of a
system is defined by its settling time, rise time, steady-state error (SSE) and overshoot. The
settling time is the time taken for the system to reach a steady-state value and the rise time
is the time taken for the system to change from 10-90% of its maximum value. SSE is the
amount by which the steady- state system response is offset from the desired system
response. Overshoot a percent measure of how much the system response exceeds the
desired response before settling to a steady-state value.
The simplest form of current control is Microstepping, which reduces the resonance
of stepper motors as the rotor moves in a sequence of very small steps (Yang and Kuo,
2003). Having said this, the un-damped nature of stepper motors does not change (Yang
and Kuo, 2003). Microstepping’ dampens low range instability and electronic damping
techniques can be used to minimize midrange instabilities (Carlberg, 2012). Microstepping
also reduces the motor induced vibrations, which are commonly referred to as ‘stepping
ripple’ (Tsui et al., 2009). Microstepping involves the introduction of pulsating motor
motion control (typically in the form of a pulse width modulation signal) at a specified
frequency into the stepper system and facilitates smaller step increments per motor
revolution. Consequently a smoother performance is achieved that dampens the inherent
instabilities to a certain extent and makes the system better equipped for handling variable
load (Carlberg, 2012, Teschler and Meyer, 1998). However an increase is step increments
per revolution brings with it a drop in torque output capability and the signal frequency is
limited by the DSP on the motion control card and motor drivers. Hence an optimised
degree of microstepping must be selected that provides the system with sufficient torque,
21
harmonic damping and smoothness. It is clearly evident from the literature that
microstepping is the base control algorithm used in hybrid stepper servo systems and
closed-loop control is added in the form of a compensator (Schweid et al., 1995, Yang and
Kuo, 2003, Tsui et al., 2009). A compensator is added to improve the performance of
hybrid stepper servo systems because microstepping typically produces highly under-
damped velocity profiles (Tsui et al., 2009, Yang and Kuo, 2003, Schweid et al., 1995).
Closed-loop compensators are typically PID controllers or lead-lag controllers. In
proportional control, the error signal is sent down a feedback loop to a summing junction.
The system output is then multiplied by a proportional gain value that is proportional to
that of the output error. This results in a steady-state system SSE. In proportional + integral
control, the error signal is multiplied by both a proportional gain value and an integral gain
value. The area between the desired output and actual output lines plotted against time are
computed. This area is added or taken away from the actual system response in an effort to
make the actual and desired output lines plotted against time coincide. This improves the
systems transient response and completely removes SSE. Proportional + integral +
derivative control involves the error signal being multiplied by a proportional gain value,
an integral gain value and a derivative gain value. Derivative control references the system
response slope to that of the desired signal slope (which is exactly 0 when plotted against
time) and corrects the system response slope when they do not coincide. This further
improves system response correlation to the ideal value. As mentioned closed-loop control
is introduced to hybrid stepper servo systems for position and velocity damping. Many
motion control applications require regulation of a constant velocity subject to torque
disturbances (Schweid et al., 1995). (Schweid et al., 1995) use nonlinear microstepping
terms to create an analogue positional control in which most of the position control can be
achieved without the need of feedback. (Schweid et al., 1995) implemented a compensator
with microstepping, velocity damping and integral damping (Schweid et al., 1995).
Although many previously proposed controls attempt to linearize the system dynamics for
positional and velocity estimation using the d-q transformation, (Schweid et al., 1995) take
advantage of the inherently nonlinear control dynamics (analogue position control)
allowing the use of inexpensive sensors such as the Kalman filter for back EMF
measurements (Schweid et al., 1995). Although velocity damping is effective in reducing
the highly oscillatory microstepping response, the analysis shows that the dynamic
characteristics are a function of the operating point and will change as the constant external
22
torque level changes (Schweid et al., 1995). Integral damping solves this problem as it
demonstrates a linear system can be maintained with changes in constant external torque
level and provides zero steady-state positional error (Schweid et al., 1995). In other words,
integral damping makes the closed-loop stepper motor respond as a time invariant linear
system during velocity regulation (Schweid et al., 1995). The elimination of steady-state
position error means that the motor is operating about the optimally stable position within
the electrical cycle, so the tendency to lose step is significantly decreased (Schweid et al.,
1995). (Yang and Kuo, 2003) proposed a similar damping control scheme for reducing the
resonance of a hybrid stepper motor whereby the motor position and velocity were
estimated by phase-lock-loop based observer that tracks the phase angle of the motor back
EMF voltage with a feedback controller that closes a loop on an integrator. Proportional
gain control is also applied to the position and velocity feedback loops to regulate the
errors between the reference and the estimated motor speed and position (Yang and Kuo,
2003). Based on the work of (Schweid et al., 1995, Yang and Kuo, 2003) the use of
microstepping and PI control can produce excellent position and velocity accuracy even
with changes in external torque. Both (Schweid et al., 1995, Yang and Kuo, 2003) exploit
the nonlinear system dynamics to use a coarse method of positional and velocity tracking,
through the use of inexpensive sensors such as high bandwidth Kalman filters to measure
the back EMF voltage. It is apparent that it is not necessary or worthwhile to introduce
derivative controllers to achieve high positional and velocity accuracy is stepper servo
systems. Based on the literature it is difficult to distinguish a hybrid stepper-servo system
from a servo system. I believe the key difference between the two is that stepper servo
hybrids have the ability to be run in open-loop or closed-loop control.
23
2.6 Literature Review Conclusion:
I will be using a circular cross-section de Laval contracting-diverging supersonic
nozzle (fig. 3.1 (a)) placed fixed on a static Z-axis at an operation pressure between 30-
(15-38) bar using Nitrogen or Helium as the working fluid. Although both brushed and
brushless motors can be used for many of the same applications, brushless motors are
typically more reliable and have a longer life time. I am using BLDC motors based on their
superior reliability, longer life time and to have the option of introducing a degree of
feedback control via hybrid servo stepper or servo systems. I am using ball screw
mechanisms (fig. 2.7 & 3.1 (b)) for my linear platform translations because they are more
reliable, have longer life cycles and higher accuracy. I don’t consider operational noise an
important factor. Fundamentally stepper motors are best suited to control the position,
velocity and acceleration of the Cold Spray CNC X-Y platforms (fig. 3.1 (e)). They are
relatively low in cost, the system inertial load is constant, relatively fast
acceleration/deceleration is not necessary and only relatively low operating speeds are
required. There is an abundance of space available and weight is not an issue therefore I
did not have to factor in the torque-to-volume or torque-to-weight ratios. I am using an
open-loop ‘microstepping’ control algorithm for the X & Y platforms as it is relatively
simple, inexpensive and affective at damping out the inherent motor harmonic resonances.
I have chosen to use a standard ‘T-Slot’ table (fig. 3.1 (c)) for the testing platform due to
the ease of attaching/detaching a workpiece and the variability of workpiece fixation it
facilitates. In conjunction with this is allows for simple integration of a detachable spindle
axis (fig. 3.1 (d)). A stepper servo hybrid motor is the clear choice the dictating the motion
of the spindle with a good balance between cost and performance. With regards to health
and safety requirements, an abundant amount of information can be found on machinery,
electromagnetic compatibility, electrical equipment designed for use within certain voltage
limits and control panels in the 2006/42/EC, 2004/108/EC, 2006/95/EEC andEN60204-1
Directives of the official Journal of the European Union (EC, 2010, EC, 2004, EC, 2006a,
EC, 2006b).
24
3. Proposed System Design:
De Laval Nozzle
DC Motor
Linear Slide Guiderails
Screw
CNC Stage Table & Ball Nut Mechanism
Powder Inlet
Pressurised Gas Inlet
Nozzle Inlet
Pressure Gauge
b)
a)
25
T-Slot Cross Section
XY Plane T-Slot Table
Machine Mount
Y- Platform
X - Platform
Chuck
DC Motor
Chuck Bearing &
Mount System
Motor Mount
c)
d)
e)
26
Figure 3.1 – (a) De Laval Nozzle to be used (b) Lead Screw CNC platform model, (c)
‘T-slot’ XY table model, (d) Complete Spindle axis assembly model & (e) Complete
CNC XY testing platform model assembly & (f) Complete CNC XY and spindle
testing platform model assembly.
4. Machine Build:
4.1 Planning & Organisation:
In the beginning of the project I broke up the machine build into a number of
appropriate sections and made a list of tasks per section to be completed within specified
time periods. The list of tasks was subject to change, as was the associated timeline. A
Gantt chart (see fig. 4.1) and a list of milestones were constructed using this information.
Regularly comparing predicted against actual build status allowed for the early detection of
project delays as well for the readjustment of the predicted timeline.
f)
27
Projected Schedule & Milestones:
Figure 4.1 – Predicted Work Schedule (Gantt chart).
Milestone Title – (Date and Timeframe Completed / to be Completed) – Milestone
Description.
Replace bottom platform – (Week 1, 25/09/13 – 02/10/13) – The bottom platform ball
screw is partially damaged and is very difficult to rotate which could place its stepper
motor under excessive stress. It must be replaced by another platform with a smoother
running ball screw. This platform must be drilled and fitted to the machine housing at a
later stage.
Strip Control Bay in current machine housing – (Week 2, 3 & 4, 02/10/13 – 23/10/13)
– The housing to be used for the proposed CNC machine cold spray facility must be
stripped of all unnecessary wiring and components. Everything removed from the housing
must be kept to ensure nothing potentially useful is thrown away.
E-Stops, ON button & Interlocks – (Week 5 & 6, 23/10/13 –06/11/13) – Feed and return
wiring must be made from the E-Stops, ON button & Interlocks to the future location of
the control bay. Continuity tests must be undertaken to verify correct connections. Label
each wire at either end.
28
Complete platform assembly – (Week 7, 8 & 9, 06/11/13 – 27/11/13) – The top platform
must be fixed to the bottom platform. Both platforms must have their ball screws coupled
to the stepper motors. The stepper motors and magnetic proximity sensors are to be wired
to the future location of the control bay. Label each wire at either end. Continuity tests
must be undertaken to ensure correct connections and a cable management system such as
an IGUS cable router must be employed.
Install Control bay – (Week 10, 27/11/13 – 04/12/13) – Source an appropriate container /
shelf to house the power electronics (24V PSU, 48V PSU, electromagnetic relay switch,
motors drivers, RCD, DPDT switch...) and install them in the selected control bay location
within the machine housing. Carefully connect all motor, ON button, E-stop and interlock
wiring to the control bay (add in series along the mains path to ground). Design &
implement a holder bracket for the pressure regulator to be bolted to the front plate of the
machine housing.
Link hardware & software – (Week 11 & 12, 04/11/13 – 18/12/13) – Link the platform
hardware with the Mach3 CNC controller. Learn how the hardware and software interact
with one another and calibrate the equipment for optimum performance. Test basic
predefined M and G code programs.
Design & introduce Pipe Spindle Attachment – (Week 13, 14, 15 & 16, 18/12/13 –
25/11/13, 13/01/14 – 27/01/14) – Design a device capable of holding pipes of a range of
diameters (80-120 mm) with the ability to accurately rotate a specified angle or a at a set
RPM. The device is a pipe holder to be attached to the top platform for cold spray testing
on pipes. The design will include a stepper motor / stepper servo / servo motor that must be
integrated into the rest of the CNC system. Associated drivers and controllers must be
specified, acquired, installed and calibrated.
Cold Spray Testing – (Week 17, 18 & 19, 10/02/14 – 24/02/14) – Assemble the cold
spray nozzle, powder feeder and compressed gas system. Begin basic copper deposition
tests over a range of operating parameters on a range of aluminium substrate geometries.
Once the process parameters have been optimised on basic flat surface substrates,
experiment with pipe coatings.
29
4.2 Machine Housing:
The machine housing was donated by an Industrial Partner (see fig. 4.2 & 4.3). It
consists of a process operating region enclosed by a base plate, housing roof, two side
windows a front window and a back window. The front and back windows have integrated
passive interlock switches. Similarly the side windows have both passive and assertive
interlock switches built-in (see fig. 4.4). There are emergency buttons on the front and
right hand side of the machine housing. The interlock switches and emergency buttons will
be discussed in great detail in the Safety section. Below the base plate there is space for
housing electronics and built-in wire guiderails. Below the base plate and right hand side
window, there is a square grid onto which square nuts can be clipped into fixed positions
(see fig. 4.5). The square grid is covered by a window and is an ideal location for installing
a control bay.
30
Figure 4.2 – The exterior of the machine housing.
Process operating region
Front window
Right hand side window
Control bay location
Control bay location
Emergency stop button
31
Figure 4.3 – The inside of the process operating region (view room the retracted
front window).
Figure 4.4 – Passive Interlock switch on the right hand side housing window.
Metallic prongs
Passive Interlock switch
Base plate Lead screw platforms
DC stepper motors
32
The machine housing was fitted with circuit boards, programmable logic gates and
wiring (see fig. 13.1 in appendix). The first step was to strip the housing of everything
unnecessary (see fig. 4.5). The interlock switches and the emergency stop buttons were left
attached to the machine housing and the associated wiring was fed to the control bay
location and labelled. To ensure correct wire identification and labelling, continuity tests
were conducted with a multimeter. Spare wires and other electronic components were kept
in storage so as not to throw away anything useful that could be used at a later stage in the
build.
Figure 4.5 – Control bay location post stripping.
Square mesh grid Square clip-on nuts
Interlock switch & emergency stop wires
routed to the control bay location
33
4.3 CNC XY Stage Assembly:
The X and Y platforms are lead screw mechanisms and were recycled from
previous machine builds in the TCD Manufacturing Research Laboratory. The condition of
the platforms was thoroughly inspected and one of the platforms’ screw bearing systems
had been damaged. It was evident the bearings had been exposed to particulate matter (dust
and grit) previously in operation such that its articulating surface was rough and contained
small metal pieces between the ball bearings and their housing. Consequently the screw
required a lot more torque to twist/rotate with ones’ hand or a motor. The damaged bearing
system also induced variable required torque at any set RPM, as the particulate matter
exposure added a degree of frictional inhomogeneity between the ball bearings and their
housing. The platform had to be replaced to ensure position and velocity accuracy as the
platforms were to be controlled by open loop stepper motors (thus could not account for
variable loading conditions). There was an additional platform recycled from a previous
machine build that was not in use produced by the same manufacturer as the originally
intended platform pair Upon inspection it had a fully functional bearing system with
approximately the same ‘smoothness’ as the other platform to be used.
Before assembling the platforms together to form the foundation of the CNC XY
stage, the stepper motors were coupled to the platforms as it would have been more
difficult to do so after. Small ring shaft couplers fixed tight with small embedded bolts
were used to do so. The platforms were easily fixed into place, the base platform (Y -
platform) was bolted to the base plate and the top platform (X - platform) was bolted on
top of the base platforms stage and at 90 to base platform (see fig. 4.6).
34
Figure 4.6 – Early development of CNC XY Stage assembly.
Recycled wiring from the machine housing was used to route the stepper motor
power and signal wires to the control bay location. The recycled wires were matched with
an appropriate colour of the motor wires where possible. The wire ends were stripped and
intertwined around one another for compactness and neatness. Continuity tests were
conducted to ensure the wire pairings were correctly assigned. The intertwined wire
junctions were soldered together and encapsulated in heat shrink after to avoid problems
associated with exposed wires. It was critical that the motor wires were never subject to
any mechanical stress (tension, pulling, tugging etc…) whilst the CNC XY stage was
moving. The base platform wiring would never be under mechanical stress as it does not
move relative to the machine housing in operation. However the top platform does move
relative to the machine housing in operation and thus needed a cable management system.
Successful cable management of the top platform eliminates the chance of disturbing its
motor wiring. An appropriately sized IGUS cable rail was sourced and installed to solve
this problem (see fig. 4.6 & 4.7). The IGUS rail bends and moves with the top platform as
it moves linearly such that the wiring at either side (inlet and outlet) of the IGUS rail
remains static.
Y – platform
X – platform
35
Figure 4.7 – Top platform cable management system installation.
IGUS cable router
IGUS cable router mount
components
36
4.4 Electronics Control Bay Installation:
All of the machine components specification sheets and manuals are plisted and
provided an a USB stick (browse USB stick for the location of particular component
information). First a suitable container or shelf was needed to house all of the control
systems electronics. An electronics shelf recycled from the machine housing was thought
to be the best option (see fig. 13.2 in appendix). There was enough space to fixate all of the
CNC machine hardware and it contained useful parts such as colour coordinated wire
junctions, DIN rails, an RCD, cable feeders, a mains supply kill switch and a cover. The
shelf had ventilation slots on its sides and cover to dissipate excess heat produced by the
electronics. Numerous components were sourced from ‘StoneyCNC’, a CNC solutions
company. The DC stepper motors & drivers, the DC stepper servo motor and driver, the
break-out-board (BOB) and the CNC desktop software (Mach3) were all sourced by and
purchased from ‘StoneyCNC’ (see fig. 4.8 & 4.9). The next step was to determine the
layout of the electronic components within the control bay (electronics shelf) and how to
fixate them in place. The major electronic components were laid out in the control bay in
several different orientations until there was satisfactory space between system
components. The motor drivers and BOB already contained fixation holes or slots.
Similarly the 24 V & 48 V DC power supplies (PSU’s) had threaded holes along the side
of their casings. It was decided that holes would be drilled in the control bay base and the
motor drivers and BOB would be bolted directly to its base. However the 24 V & 48 V DC
PSU’s were to have aluminium backing plates made and bolted to the PSU’s, and the
backing plates were to be bolted to the control bay base. The threaded holes in the DC
PSU’s were not symmetrically spaced in a rectangular pattern for example, justifying the
use of back plates. It was far easier to have the awkwardly distributed holes made in the
base plate, to bolt the PSU to the base plate and to bolt the base plate to the control bay in
which appropriately symmetric spaced holes had been drilled. The DIN rail was bolted to
the control bay via its existing slots. The RCD, DPDT relay switch, the two SPST relay
switches, the 24 V DC busbar and the 0V REF busbar were slid onto and fixed to the DIN
rail before bolting it to the control bay.
37
Figure 4.8 – Proposed layout of control bay before fixating components into place.
Cable feeders
Break-out-board
48 V DC PSU
RCD
24 V DC PSU
DIN rail
X – platform motor driver
(DC stepper motor)
Y – platform motor driver
(DC stepper motor)
38
Figure 4.9 – Complete control bay.
Spindle motor driver
(DC stepper servo hybrid motor)
DPDT relay switch
powered by 24 V
DC PSU
24 V DC busbar
0 V REF busbar
24 V DC PSU
48 V DC PSU
Y – platform motor driver
(DC stepper motor)
X – platform motor driver
(DC stepper motor) Mains Power
Inlet Kill Switch
SPST relay switch used to trigger
limit switches for X & Y axes
39
4.5 Motor Electronics Description:
Mach3 software turns a typical computer into a CNC controller with a potential of
running 6 axes and a spindle axis. The computer is connected to a ‘USB SmoothStepper’
motion control card mounted on a BOB. Process instructions in the form of iso-G-code are
loaded into and interpreted by Mach3. Mach 3 fires the motion variables down the USB
cable to the motion control card. The motion control card has a Digital Signal Processor
(DSP) chip that runs on a loop continuously decoding the Mach3 data. The DSP breaks the
motion variables into 5V pulses of a certain frequency. These 5V logic pulse signals are
sent to the motor drivers and amplified. There are four connections between each motor
driver and the USB SmoothStepper motion control card. One connection is either a 5V OR
0V signal used to dictate motor motion direction (i.e. 5V CW, 0V CCW). The second
connection is a 5V pulse signal at a certain frequency used to generate motion. The third
and fourth pins are ground / reference pins for the first and second connections (all signals
require grounding).
The stepper motor drivers (for X & Y platforms) are connected to a 48 V DC PSU
(see fig. 4.9) as well as the four connections each to the motion control card. The 5V pulses
sent from the DSP on the motion control card to the motor drivers are in Pulse Width
Moderation (PWM) format. The 5V pulse is amplified by the driver electronics which is
powered by the 48 V PSU (see fig. 4.10). The amplified pulses are applied across the
motor armature to produce motion. The frequency of the PWM signal will determine the
motor speed and torque. Within the stepper motor drivers are dip switches which allow the
operator to set the ‘steps per revolution’, as in how many incremental steps of movement
the rotor makes to complete a full revolution. The more steps per revolution the high the
positional accuracy and the smoother the motion becomes. However torque capability
decreases with an increase in ‘steps per revolution’. A trade-off is made between torque
output and motion quality by ‘microstepping’ the motor drivers at 1600 pulses per
revolution via dip switches within the drivers. The spindle motor-drive combination is
different to that of the X & Y axes. The spindle motor has a feedback control loop between
it and its driver. Although the 5V pulse signals still control the speed of the motor the
spindle axis can account for variable loading. For example if the spindle speed is set and it
is not running as fast as it should be, its rotary encoder will pick this up and demand more
current to be applied across the motor armature so as to produce enough extra torque to
40
bring it up to speed. The motors need to be calibrated within Mach3 which will be covered
in detail in the ‘Hardware – Software Calibration & System Settings’ section.
In summation, Mach3 fires down the motion variables to the motion control card.
There are four connections each between the motion control card and each motor driver.
The first connection determines the direction of applied current across the rotor armature;
the second connection determines the frequency at which current is applied to the rotor
armature. The third and fourth are ground references for connection one and two. The
PWM signals fired to the motor drivers are amplified by the driver electronics and applied
across the motor armature. This produces rotary motion of the motor shafts which is
converted into linear translation of the CNC stages via their ball screw mechanisms.
Figure 4.10 – Motion Signal Flow Chart.
4.6 Safety Electronics Description:
The power circuit is set-up such that when the operator presses the ON/OFF button
(see fig.4.11) on the machine housing to fire up the motor drivers, he/she is only
‘potentially exposed’ to 24V DC as opposed to 230V AC. Note that the term ‘potentially
exposed’ is used because the ON/OFF button is electrically isolated from the current
running through its circuit and the 24V PSU is safely grounded. The operator would only
be exposed to ON/OFF button circuit current if the circuit was partially exposed (should
never happen) and finding a different path to ground (i.e. conducting through the machine
housing and operator to ground). Even as unlikely as it is, the user would only be subjected
to a non-lethal (harmless) 24 V DC and still isolated from AC completely.
G-Code Mach3 CNC
Controller
Motion Control
Card (DSP)
Motor
Drivers
41
Figure 4.11 - ON/OFF button circuit.
When the machine is plugged in, the live and neutral wires are in series with a
Residual Current Device (RCD). At the RCD outlet there are two sets of live and neutral
wires, one used to power the 24V DC PSU and the other used to power the 48V DC PSU
(see fig. 4.9). The RCD will immediately break the circuit between both DC PSU’s and
mains supply if there is an imbalance in current flow between the neutral and live wires
(i.e. something has gone wrong, irregular current draw). The neutral and live wires are
directly connected to the 24V DC PSU so it is always live when the machine is plugged in.
The neutral wire for the 48V DC PSU is connected directly from the RCD outlet however
its live wire is placed in series with a DPDT relay switch. Pressing the ON button on the
machine housing closes the 24V circuit which is used to power a DPDT electromagnetic
relay switch. This in turn connects the live wire from the mains supply to the 48V DC
PSU, firing it up along with the motor drivers for all axes. Green LED lights on the BOB,
DC PSU’s and motor drivers indicate that the electronics system is working as it should be.
The DPDT relay switch is ‘latched’ (see fig. 4.12), meaning that even once the ON button
on the machine housing has been let go the electromagnetic relay switch remains closed
providing the 48V DC PSU with its constant live supply. The latch relay switch is achieved
by providing another path from the 24V DC PSU to the switches electromagnetic coil. An
alternative path to the 24V DC PSU is wired in parallel with the ON button path (see fig.
42
4.13). So that when the ON button on the machine housing is let go, current running
through the alternate path to the relay switch PSU keeps the electromagnetic switch closed,
thus keeping the 48 V DC PSU and the motor drivers fired up. Pressing the OFF button on
the machine housing disconnects the 24V DC PSU’s reference link to ground, the relay
switch opens and the 48V PSU switches off as it no longer has its live wire power supply
(all signal voltages require a ground / point of reference i.e. 0V).
Figure 4.12 - DPDT latch relay circuit.
Figure 4.13 – Latched switch Illustration.
24 V DC
ON button
Alternate path
43
The emergency stop buttons on the front and right hand side of the machine
housing as well as the passive interlock switches on the left and right hand side are wired
in series with the 24V DC PSU and if triggered, cuts the supply to everything that requires
24V DC (the three electromagnetic relay switches). The emergency stop buttons and the
interlock switches are wired in normally open (NO) and normally closed (NC)
respectively. The interlock switches consist of a metal ‘prong’ fixed on the side doors that
slot into the interlock’s housing and complete its circuit when the doors are closed (see fig.
4.4). The emergency stop button’s circuit breaks if the emergency stop buttons are pressed.
Connecting the interlock circuits and emergency stop circuits in series, the circuit could
only be complete if both side doors were closed and the emergency stop buttons were not
pressed in. So to reiterate, this safety circuit is wired in series with the +24V DC output
line from the 24 V DC PSU. Consequently the motor drivers cannot be powered up if the
side doors and or the emergency stop buttons have been pushed. Similarly if the motor
drivers are already powered up and a side door or an emergency stop button is hit, power
will be cut to the driver motors instantaneously.
There are two limit switches (one for the X-axis and the other for the Y-axis, see
fig.4.14) wired to pins on the motion control card. A Limit switch consists of a pair of
magnets, one of which has two wires coming out of it that are not physically connected to
each other and the other is wireless. When the magnets come within close proximity of one
another, the circuit between the two wires in closed due to the magnetic field generated
between the two magnets. When the limit switches are triggered (i.e. come within close
proximity of each other), Mach3 ceases to send motion signals to the motor drivers and
prevents the XY table from moving any further in the triggered direction. Each limit switch
is wired into its own SPST relay switch powered by a 24V DC PSU that closes when the
limit switches are set-off. This in turn completes a circuit for a triggered switch connecting
the 5V signal pin it is wired to and a ground reference on the motion control card. This
informs Mach3 a limit switch has been set-off and all motion stops immediately.
44
Figure 4.14 – Magnetic Proximity / Limit Switches installed on the X & Y platforms.
X-axis Limit Switch
Y-axis Limit Switch
45
4.7 Spindle Design (Mechanical):
Starting from scratch; the design, build and integration of the spindle axis with the
CNC software was by far the most challenging aspect of the project. The spindle axis
design was divided into two sections; (i) identifying the parts available to buy and to
source them and (ii) identifying which parts needed to be made in the workshop and design
them. The only parts that were bought were those that could not be made in the workshop.
A three-jaw chuck was sourced and purchased to fixate pipe substrates of different
diameters. A stepper-servo hybrid motor and driver were sourced and purchased to rotate
the substrate at a desired RPM. A flexi coupler was sourced and purchased to couple the
motor and spindle shafts together. The remaining parts of the spindle axis assembly had to
be designed and made in the TCD workshop. I needed to design a robust bearing system
that could manage the inertia of the chuck and the workpiece whilst allowing the chuck to
rotate freely with minimal resistance. A shoulder either side of the bearing system was
needed to prevent axial motion along the axis of rotation that could damage the motor. A
shaft connected to the chuck to be coupled to the motor shaft and a mount for the motor
were also needed. The critical design specifications were as follows:
i) The spindle shaft and the motor shaft centres had to be aligned.
ii) The chuck must not be able to move in the axial or radial directions, simply
allowed to rotate CW/CCW.
iii) The chuck must rotate freely with minimal applied torque necessary.
iv) The spindle had to be capable of operating from 0 - 1000 RPM.
v) The design had to be modular so as to have the ability to change localised
damaged components with relative ease at minimum cost.
vi) The spindle had to be robust enough to withstand operational vibrations.
vii) The spindle had to require minimal maintenance and repair.
46
3 jaw chuck
Spindle bearing mount
Chuck back plate
Shoulder 1
Shoulder 2
DC motor
Bearing system
Shaft coupler
a)
b)
47
Figure 4.15 – (a) Rough concept hand sketch (b) & (c) Colour coded and labelled
Spindle Axis Model.
The final concept can be seen in fig. 4.15 above. The design process consisted of
basic part and assembly sketches, choosing a design concept and finalising the part
dimensions. CAD drawings of each part and assembly models were constructed in
SolidWorks. These drawings were sent to the workshop for the parts to be built and
assembled together. It consists of a ‘double-L’ bracket mount, a chuck backing plate, a ball
bearing fixture with shoulders either side, a DC motor and a shaft coupler. The ball bearing
fixture is interference fitted in to the front plate of the ‘double-L’ bracket mount and the
chuck backing plate is bolted to the chuck. The far side shoulder shaft is interference fitted
in to the chuck side shoulder slot. The chuck backing plate is bolted to the far side shoulder
shaft such that the shoulders are a tight fit either side of the bearing fixture and the chuck
backing plate is tightly fitted against the chuck side shoulder slot face. The spindle shaft
attachment is bolted to the far side shoulder and the flexi beam coupler is fixed to the shaft.
The DC motor shaft is fixed to the other end of the flexi beam coupler and is bolted to the
second plate of the ‘double-L’ bracket mount. The drawings of all manufactured parts (see
figs. 13.8-13.13 in Appendix) as well as the specification sheets of all sourced parts
included in the spindle axis assembly are documented in the Appendix. All critical design
specifications were met. All parts manufactured in the workshop are made from aluminium
bar stock or aluminium sheeting. The final result can be seen in fig. 4.16.
Spindle shaft
c)
48
Figure 4.16 – Complete CNC Machine Assembly with XY Table & Spindle Axis.
Thin aluminium plate used to protect
the ‘T-slot’ table during testing
49
4.8 Hardware – Software Calibration & System Settings:
Mach3 v2.0 is the software package from ArtSoft used to convert the desktop
computer into a fully functional CNC controller. The program and all associated files are
installed under the address ‘C:\Mach3’ as suggested by the manufacturers for support
services. The complete software configuration / set-up is as follows:
4.8.1 Set the Native Units:
Open the ‘Mach3 Loader’, click on the ‘Config’ tab (see fig. 4.17) and select ‘Native
Units’. The native units you select are the settings used for motor tuning, which will be
covered later on. Select ‘MM’s’ and click OK. This is the one and only time you will have
to set the native units thus it is not necessary to go near these settings again.
Figure 4.17 – Mach3 Interface in the ‘Program Run’ window with the ‘Config’ tab
options highlighted.
50
4.8.2 Engine Configuration:
Click on the ‘Config’ tab and select ‘Ports & Pins’ (see fig. 4.18). Here you should see
Port #1 with the port address ‘0x378’ which is a standard printer port address and select
‘Port Enabled’ if it is not already turned on. Enabling Port #2 would give the user access to
additional input signal but in the scope of this CNC system a 2nd
port is unnecessary.
Kernel Speed should always be left on 2(15-38)00 kHz. The Kernel Speed setting is the
maximum pulse rate at which the Mach3 drivers can provide the motor drivers with the
motion signal pulses. To run CNC axes using stepper motors at their maximum rated
speed, a Kernel Speed of 2(15-38)00 kHz is sufficiently high. A Kernel Speed setting is
also the least demanding on the system and is recommended initially for all start-up users
regardless of maximum motor driver pulse signal processing capability. Do not select any
other boxes in this window and click apply. Within the same window, select the ‘Motor
Outputs’ tab. Here click enable for the X , Y & Spindle axes. Check the connections
between the motion control card and the axes motor drivers to see which signal pins on the
card are wired to the step port (PUL+) direction port (DIR+) of each axis motor. Assign the
‘Step Pin #’ and ‘Dir Pin #’ for each axis accordingly (see fig. 4.19). Ensure that Port #1 is
selected for both ‘Step Port’ and ‘Dir Port’ for all axes. Clicking on ‘Dir Low Active’ will
switch the direction of an axis movement, one of many convenient features of Mach3.
Click Apply once again to save settings.
51
Figure 4.18 – Mach3 Interface with the ‘Engine Configuration’ Port# & Kernel Speed
options highlighted.
52
Figure 4.19 – Mach3 Interface with the X,Y & Spindle axis Step & Direction settings
highlighted.
4.8.3 Motor Tuning:
Click on the ‘PlugIn Control’ tab and select ‘USB SmoothStepper v17fd Config’.
Within this window under the heading ‘Max Step Frequency’ you must the X-axis, Y-axis
and Spindle Axis are all set to 256 kHz. If a motor’s driver signal input frequency
requirement at a certain speed exceeds that of the Mach3 signal output frequency then the
motors will cease to run. Hence it is imperative that the motion control card output
frequency capability is equal to or higher than that of the motor drivers it is sending the
signals to. Next click on the ‘Config’ tab and select ‘Motor Tuning’ (see fig. 4.21). Select
the X axis to begin with and you will be faced with three input parameters; ‘Steps per’,
‘Velocity’ and ‘Acceleration’. The ‘Steps per’ input refers to how many incremental steps
the motor has to undergo to move its associated CNC stage by 1mm. In order to find this
value, close the window and select the ‘Settings’ tab. Here under the heading ‘Axis
53
Calibration’ click on the ‘Set Steps per Unit’ button (see fig.4.20). Select the X axis first as
the axis you wish to calibrate. Mach3 will request you to enter a distance you wish the X
axis to move in either its negative or positive direction. A digital vernier callipers is
generously expanded and placed in contact perpendicularly against the CNC X axis stage.
The callipers is held firmly in place and zeroed. An arbitrary distance for the X axis to
move is inputted into Mach3 and the actual distance moved is measured by the callipers
with accuracy. Mach3 requests the user to input the distance the CNC stage actually
moved. Inputting this information, an algorithm within Mach3 calculates the steps per mm
for the X axis and asks whether or not you would like to save this setting in the X axis
Motor Tuning set-up. Click OK and the ‘Steps Per’ box of the X axis Motor Tuning will be
automatically set (~315).
Figure 4.20 – Mach3 Interface with the ‘Axis Calibration’ button highlighted.
The ‘steps per mm’ for the Y axis is calibrated in an identical manner (~315 also).
Even though the motor-driver combo for the X & Y axes are the same and their lead screw
54
mechanisms have the same thread, the ‘steps per’ calibrated settings are not necessarily
identical. Go back to the ‘Motor Tuning’ section to continue motor calibration. The
‘Velocity’ and ‘Acceleration’ settings are less critical. ‘Velocity’ refers to the linear
velocity of the linear platform in mm/min. ‘Acceleration’ refers to the linear acceleration
of the linear platform in mm/min.min. Optimal values for these inputs are found by
iteratively trying combinations of the two that result in smooth motion, minimal
operational noise and no jerking which could result in the loss of steps and thus positional
accuracy. There is no maximum speed specification for the stepper motors controlling the
XY table as the maximum operating velocity will depend highly on the inertia of the
specific CNC stage set-up. ‘Velocity’ & ‘Acceleration’ are set to 6000 & (15-38)0 for both
axes respectively. Once you are satisfied with you motor tuning parameters click ‘Save
Axes Settings’ and OK to finish.
4.8.4 Limit Switch & Homing Set-up:
To configure the limit switches, go to the ‘Config’ tab and select ‘Ports & Pins’.
Within ‘Ports & Pins’ select the ‘Input Signals’ tab. Enable the ‘X Home’ & ‘Y Home’
input signals. Ensure that both input signals have Port #1 selected. Check the pin number
on the motion control that the X & Y limit switches are wired to and insert their
corresponding pin numbers. Click on Apply followed by OK to save settings. Within the
‘Program Run’ tab, press the tab button on the keyboard and set the ‘Slow-Jog Rate’ to
10%. Press tab once more followed by the ‘REF ALL HOME’ button beside the coordinate
system Direct Read-out (DRO) in the ‘Program Run’ tab. The ‘Slow Jog Rate’ is the feed
rate at which the X & Y platforms move at whilst ‘jogging’. Moving the XY table
manually with the up, down, left and right buttons on the keyboard or selecting ‘REF ALL
HOME’ will do so in jogging mode. Once ‘REF ALL HOME’ has been clicked, the X axis
will jog in its negative direction until its limit switch has been triggered and retract
1mm.Similarly the instant the X axis has retracted the Y axis begins to jog in its negative
direction until its limit switch has been triggered followed by a 1mm platform retraction.
This process is known as ‘Homing’. The current position of your stage is the home position
and is designated (0,0) in Cartesian coordinates. To develop the homing set-up further, the
different coordinate systems in play must be understood. If the red light above the
‘Machine Coords’ button on the coordinate DRO of the ‘Program Run’ tab is on, the
55
coordinate DRO is displaying the CNC stage coordinates relative to Home. If you have not
moved the CNC stage since ‘Homing’ it and the ‘Machine Coords’ button is on, the DRO
coordinates will be (0,0). Even though a Z axis has not been set-up, the Z coordinate is set
to its default value 0.
4.8.5 Soft Limits:
As well as the hard wired limit switches, Mach3 facilitates another degree of safety
with the ability to define a boundary on the XY plane that the XY table is not allowed to
cross or leave. These software limits or ‘Soft Limits’ can be toggled on or off via the ‘Soft
Limits’ button on the ‘Program Run’ tab. To set accurate limit distances, appoint of
reference is needed naturally. Home the XY table to its (0,0) ‘Machine Coords’ position by
pressing ‘REF ALL HOME’. Jog the X axis manually at a relatively slow speed moving it
to its maximum range or the point at which you don’t want the axis to move past for
whatever reason (i.e. factor of safety). With the ‘Machine Coords’ toggled on, note the
distance the X axis has moved on the coordinates DRO on the ‘Program Run’ tab. With the
XY in its current position, repeat this process identically for the Y axis.
56
4.8.6 Spindle Axis Set-up & Calibration (Closed-Loop &
Open-Loop Control):
4.8.6.1 Open Loop Control:
In order to know whether or not the spindle is running at the set RPM, the actual
spindle RPM needs to be known. Therefore a means of directly measuring the actual
spindle to implement control is absolutely necessary. An ‘iMach Spindle Tach’ is used to
measure the exact spindle RPM output with a resolution of 180 . On the spindle DRO,
there are three readouts. The top readout, ‘RPM’, is fed information from the tachometer
and displays the actual RPM of the spindle at that time. The ‘Spindle Speed’ readout
(bottom) displays the set/desired RPM of the spindle and the ‘S-ov’ readout displays the
overridden applied RPM (shown as a fraction of the ‘Spindle Speed’ readout on the RHS
also). This tachometer has a USB input and is integrated into Mach3 allowing the actual
spindle RPM output to be shown in the ‘RPM’ display on the spindle DRO as well as the
use of auto-calibration and spindle speed override functions embedded within Mach3 (for
Closed Loop Control modes on Mach3). The circumference of the far side spindle axis
shoulder shaft (chosen point of measurement for sensor) is covered in black tape. Half of
the circumference length is covered in white tape (the length of which is measured with a
callipers before applying it on top of the black tape). The ‘iMach Tach’ is a reflective
optical sensor that measures the reflectivity of light from the surface of the rotating spindle
at a frequency that can be changed within its plugin configuration. The sensor can take
between 1-10 readings every 100 s. The tachometer averages the readings in the 100 s
thus I set the number of readings to 10 so as to minimize RPM reading fluctuations. The
sensor is fixed horizontally and perpendicular to the far spindle shoulder shaft on the
spindle axis. To configure the tachometer, plug it into the computer and download/install
its latest plugin in the following location: (C:\Mach3\Plugins). Open the Mach3
application, click on the ‘Config’ tab and select ‘Plugin Config’. Here is the list of the
plugins currently installed with a ‘green tick’ or a ‘red x’ beside each of them specifying
whether or not they are currently enabled. Also beside each of them is a yellow
‘CONFIGURATION’ button. Click on the ‘iMach-Spindle-Tach’ configuration button. In
this window select ‘Use RPM DRO’, insert a ‘Total Ratio’ of 1, insert an ‘Average’ of 10,
insert a ‘Lowest RPM to Display’ value of 1 and under the ‘Action’ drop down tab select
57
‘NONE’ for action to be taken is measured RPM is less than ‘10’. The ‘Use RPM DRO’
tells Mach3 to output the tachometer reading directly to the spindle DRO readout. ‘Total
Ratio’ refers to the ratio between the spindle RPM and the RPM of the rotating component
it is measuring. If the sensor is measuring the spindle RPM directly then the ratio is set 1:1.
The ‘Average’ figure input is how many readings you want to take every 100 s as
previously mentioned.
Having already enabled the spindle motor output and assigned its step and direction
pins in ‘Ports & Pins Motor Outputs’ earlier on, select the ‘Spindle Set up’ tab within
the ‘Ports & Pins’ window. There are two methods of spindle motor control provided by
Mach3; ‘PWM Control’ & ‘Step/Dir Motor’. I have gone with the ‘Step/Dir Motor’ option
because it is straightforward to set-up requiring less system settings whilst achieving the
exact same functionality. Disable ‘Relay Control’, ‘Flood Mist Control’ and select the
‘Step/Dir motor’ & ‘Use Spindle Motor Output’ options under the ‘Motor Control’
heading. Under the heading ‘Special Functions’ select ‘Spindle Speed Averaging’
(somewhat damps the RPM DRO fluctuations making the spindle easier to calibrate and
control). Click Apply followed by OK. Next go to the ‘Config’ tab and select ‘Spindle
Pulleys’. Here you select the minimum and maximum spindle RPM range as well as the
‘Pulley Ratio’ you desire for your CS operation which must satisfy the bounds of the motor
RPM specification also. Here you have the option of inputting the minimum and maximum
RPM values that the motor is specified to operate at (0-3000 RPM). The ‘Pulley Ratio’ is a
proportional figure that accounts for the difference between the driving motor RPM and
the spindle RPM. If the spindle is driven directly by the motor (as it is in this spindle
design), the ratio is 1:1.By driven directly I mean there is no gearing system between the
motor shaft and spindle such that an angular displacement undergone by the spindle is
identical to that of the motor shaft. Mach3 enables the user to save 4 sets of spindle pulley
configurations for geared servo systems. The DC stepper servo hybrid spindle motor only
has 1 pulley, thus setting ‘Pulley 1’ is sufficient. Click OK to save the pulley settings.
Click on the ‘Config’ tab and select ‘Motor Tuning’. The motor tuning parameters for
the spindle are in completely different units. Select the Spindle Axis and input 4000 for
‘Steps per’. ‘Steps per’ in the spindle specific case is the number of incremental steps the
motor undergoes to complete 1 revolution (360 of rotation). The hybrid stepper servo
motor’s encoder is specified at 4000 Cycle per Revolution (CPR). CPR is the number of
58
output pulses per complete revolution of the encoder disk. The encoder CPR must match
with the motor Pulses per Revolution (PPR) in order for the closed loop system to function.
Each individual pulse applied to the motor armature will induce a single motor step, hence
the PPR is equivalent to the Steps per Revolution setting in the spindle ‘Motor Tuning’
section in Mach3. With regards to the ‘Velocity’ setting, it is measured in terms of RPM
and the maximum specified motor RPM is inserted (3000 RPM). The optimum
‘Acceleration’ setting on the other hand is somewhat subjective, as in the case of the X &
Y axes. ‘Acceleration’ is measured in terms of RPM/sec.sec. A range of ‘Acceleration’
settings are tested (starting from lowest to highest) and a final value is selected (12.5)
based on operational smoothness. Click ‘Save Axis Settings’ and OK to save these motor
tuning parameters for the spindle. The spindle set-up is particularly sensitive to the
‘Acceleration’ setting.
Figure 4.21 - Mach3 Interface in the ‘Motor Tuning’ window with the motor tuning
settings highlighted.
59
On the ‘Program Run’ tab enter a value between the set minimum and maximum
values as set in ‘Pulley 1’ in the ‘Spindle Speed’ input of the spindle DRO and turn on the
spindle by clicking the ‘Spindle CW’ (toggle on/off) button. The button blinks yellow
indicating that the spindle is now on. Test the spindle over the full range of speeds and
comparing its actual to the desired RPM output. Open loop control spindle calibration is
now complete.
4.8.6.2 Closed Loop Control:
Go back to the ‘Spindle Set-up’ tab within the ‘Ports & Pins’ window. Under the
heading ‘Special Functions’ (see fig. 4.22) select ‘Use Spindle Feedback in Sync Modes’,
& ‘Spindle Speed Averaging’ (somewhat damps the RPM DRO fluctuations making the
spindle easier to calibrate and control). ‘Use Spindle Feedback in Sync Modes’ gives
Mach3 permission to use the tachometer RPM readout as reference for control algorithms
such as spindle override, ‘S-ov’. Selecting ‘Closed Loop Spindle Control’ enables the
‘Spindle Calibration’ & actual RPM PID control. Click on the ‘Function Configs’ tab and
select ‘Spindle Calibration’. Press the ‘Spindle on/off’ button in this window and you are
presented with a graph that plots the spindle RPM in real-time. In this window press
‘AutoCal’ with the spindle still on. The ‘Control’ display will be automatically set to the
pulley’s set minimum RPM and the ‘RPM’ display to its RHS shows the actual RPM
output measured by the tachometer. Below the ‘RPM’ display is the ‘Control Ratio’.
‘AutoCal’ is Mach3’s integrated spindle auto-calibration function that runs the spindle
from 0-100% of its maximum output in small increments and compares the desired output
to the actual output of the spindle at each increment. The ‘Control Ratio’ is the fraction
from 0-1 of the maximum spindle output that the control output is trying to apply at any
given time during the calibration process. During spindle calibration there is a different
graph displayed on the LHS of the window. This graph plots control ration against RPM.
Two data series are plotted on this graph; ‘Calculated Curve’ & ‘Current Curve’. The
‘Current Curve’ plots the approximate behaviour the RPM should follow as control ratio is
varied (~linear). ‘Calculated Curve’ plots the actual spindle RPM as the control ratio is
60
varied. Effectively this graph is a visual interpretation of the deviation of actual to desired
RPM over the enter spindle speed range. Once calibration is finished click OK, turn off the
spindle via the ‘Spindle on/off’ button in the same window or the toggle button on the
DRO. Close the ‘Spindle Calibration’ window and test your spindle over the full range of
speeds. The hardware-software calibration process for Closed Loop Control mode is now
complete.
Figure 4.22 – Mach3 Interface with ‘Special Functions’ used to activate Mach3
Closed Loop Control Modes.
61
4.8.7 System Functionality & Optimisation:
The Cold Spray facility is a fully operational 3 axis CNC machine (X axis, Y axis &
Spindle axis). The XY table is controlled with open loop steppers in open loop control. The
X & Y axes linear displacement is approximately accurate as measured by a
vernier callipers during the X & Y axes calibration (when setting ‘Steps per’). To minimize
the potential of missing steps and backlash as could be the case with a rapid reverse in
direction of an axis at a high velocity, the stepper motor drives are ‘mircrostepped’ to an
optimal value of 1600 steps/rev. This value is optimal because it provides smooth linear
motion with no jerking of the platforms as well as providing ample torque to the motors to
move the XY stage over the desired range of process feed rates required. Even with rapid
reverse in direction of an axis at a high speed the loss of steps is extremely unlikely
provided that the motor – lead screw coupling and the lead screw – table nut are
completely flush. Hence if there is a loss in positional accuracy and repeatability of your
XY table the probable cause will is damage/wear to the X & Y axes lead screw
mechanisms. To absolutely ensure positional accuracy of the XY table, recalibrate it on a
regular basis and you should get the same number of ‘Steps per’ value for each axis if it is
functioning as it should.
The Spindle axis is powered by a stepper-servo hybrid motor drive combination. It can
be run on Mach3 with open loop control or closed loop control and the user can toggle
between the two with minimal difficulty. There are two separate closed loops in the spindle
set-up, a feedback loop between the motor and its driver (rotary encoder), as well as a
feedback loop (optical sensor) that directly measures the spindle RPM output and is
integrated with the Mach3 software (see fig. 4.23). In open loop control mode on Mach3
the spindle output is at least within 1 RPM of its actual output as verified from the optical
sensor (tachometer). Furthermore in open loop control mode you can manually override
the spindle speed with the ‘S-ov’ function, with which you can increase or decrease the
RPM output as a fraction of the set desired output in increments of 10% in order to exactly
match up the actual and desired spindle output. However the ‘S-ov’ function is not needed
in our case as our open loop system control is incredibly accurate (imperfect nonetheless).
Seeing as in open loop control mode the tachometer feedback is not used other than to
display the actual RPM, Mach3 is not constantly trying to check and correct its pulse signal
62
frequency to send to the spindle motor driver. Consequently the tachometer reading does
not fluctuate much and settles down rapidly with an applied increase or decrease in spindle
speed. In closed loop control the spindle works perfectly from 100 RPM+ (actual and
desired outputs match exactly), however the tachometer readings are far less stable relative
to open loop control mode. I speculate this is instability is caused by acute rapid fluctuation
in actual spindle RPM speed as the software PID control in Mach3 continuously changes
the signal frequency by small amounts that the motion control card is sending to the
spindle motor driver in order for the actual & desired spindle outputs to exactly match.
Either these slight and rapid changes in signal input frequency fed to the spindle motor
driver doesn’t allow enough time for the tachometer to settle to finite value consistently
over the full RPM range or there are less signal fluctuations at higher spindle speeds ((15-
38)0 RPM+). More work must be conducted into fine tuning both the PID parameters
within the spindle motor driver and Mach3 as well as investigation into enhancing
tachometer reliability and resolution (see ‘Future Work’ section for details). Regardless of
not achieving completely successful closed loop control mode on Mach3 there is still
closed loop control between the motor and its’ driver and the spindle output accuracy is
within RPM in open loop control mode, which is an excellent result. The magnetic rotary
encoder on the back of the spindle hybrid motor accounts for variable load ensuring that
enough torque is provided to the motor to ensure it runs within 1RPM of the desired RPM
at all times, thus there is no need to calibrate the system upon mounting of different pipe
substrates (this is done for you by the closed loop between the hybrid motor and its driver).
The feedback loop between the spindle motor and driver is not integrated with Mach3 and
requires no tuning. See fig. 4.24 for all final motor tuning settings.
`
The spindle is also capable of completing partial rotational pipe coatings, i.e.
coating the top half of the pipe only. This is achieved by setting up the spindle as an axis
(A, B, C or Z-axis) as opposed to a spindle in ‘Motor Outputs’. The ‘Motor Tuning’
parameters are left as they were in the spindle set-up. It is an ad-hoc method whereby the
pipe substrate is marked in two places; one point of reference and another point exactly
180 from the point of reference. Starting horizontally or vertically from one of the points
and ensuring the ‘spindle axis’ has been zeroed, the spindle is ‘jogged’ at a very slow rate
until the pipe has rotated exactly 180 to the other marked point. The linear displacement
63
moved by the ‘spindle axis’ (i.e. Z-axis) should be noted on the coordinates DRO.
Now you are provided with vital information, as in the linear displacement value
Mach3 thinks the spindle has undergone thinking it was driving a ball-screw platform as
well as the actual angular displacement that the spindle has undergone. This ‘imaginary’
linear displacement and actual spindle angular displacement are directly proportional to
one another. Next the ‘imaginary’ spindle axis Feed Rate has to be related to the actual
spindle RPM. First calculate the desired spindle RPM ( (
)
) using your
desired linear coating velocity and outer pipe radius. Next iteratively find what ‘imaginary’
axis Feed Rate correlates to your desired RPM. Now the CNC system is enabled to
complete partial pipe coatings.
Figure 4.23 – Spindle Feedback Set-up Schematic.
Axis Step Pin# Direction Pin# Steps per Unit
[
]
Velocity
[
]
Acceleration
[
]
X 2 3 315 600 (15-38)0
Y 4 5 315 600 (15-38)0
Spindle 6 7 4000 3000 12.5
Figure 4.24 – Motor Tuning Settings Summary.
Chuck
Flexi Shaft Coupler
DC Motor
Rotary Encoder
Motor
Driver
Optical Sensor
Mach3 CNC
Controller
Feedback Loop between
Optical Sensor and Mach3
Feedback Loop between
Rotary Encoder and Motor
Motion Control
Card
64
5 Pressurised Gas & Powder Feeder System:
A 220 bar pressurised Nitrogen gas bottle size W was used as the working fluid for the
Static & Dynamic Pipe Substrate Cold Spray tests. The pressurised gas system is
controlled by three manual valves. The Nitrogen bottle was connected directly to a large
pressure valve (capable of transferring up to 50 bars of pressurised gas across it, see fig.
5.2) that was split into two gas output lines. One output line is fed straight into the De
Laval nozzle inlet and the other is attached to the powder feeder gas inlet. The powder
feeder is controlled by a separate computer with which the powder feed can be toggled
on/off and its powder feed rate can be set and controlled also. There are three pressure
gauges. The first is at the large pressure valve inlet and it displays the gas pressure
remaining inside the bottle with the other two valves closed and the gas bottle open. The
second pressure gauge displays the pressure of the working fluid fed directly to the nozzle
inlet and the third displays the pressure of the gas fed into the powder feeder inlet. See
Flow Chart below (fig. 5.1) for better understanding of the Pressurised Gas & Powder
Feeder System. 30 bar is the pressure used for the Cu-Al Cold Spray experiments.
XY Table &
Figure 5.1 – Pressurised Gas System Schematic.
Pressurised
Gas Bottle
Gas Valve before
Powder Feeder Inlet
Main Gas Valve
Powder
Feeder
De Laval
Nozzle Inlet Pressure Valve
Gas Valve
before
Nozzle
Inlet
65
Figure 5.2 – Main Pressure Valve connecting the pressurised gas to the Powder
Feeder and Cold Spray nozzle.
Gas Valve &
Pressure Gauge
before Powder
Feeder Inlet
Gas Valve &
Pressure Gauge
before Cold
Spray nozzle
Inlet
Main Gas Valve
& Pressure
Gauge
66
6 Spindle Speed Synchronisation:
XY Table & Spindle Speed Synchronisation is paramount for CS pipe coatings.
The reason being that the linear feed rate of the substrate into the path of the nozzle has to
be the same whether the test is a static line test or a dynamic rotational test in order to
achieve the same coating properties. The key coating properties include material deposition
rate, coating porosity, coating thickness, coating consolidation and coating adhesion to the
substrate. For a dynamic pipe coating test the spindle must rotate and the XY stage must
move linearly into the path of the CS nozzle such that there is a slight overlap in coating
layers. In layman’s terms by the time the spindle has completed one full rotation, the
substrate must have moved an exact axial distance. The point behind this is the starting
point of the spindles second rotation is offset axially from the starting position if its 1st
rotation. This axial offset distance should be set such that there is an overlap between
coating layers to avoid helical voids on the coating layer. A basic program was constructed
to output the spindle RPM and the XY table feed rate for a desired linear coating velocity
and overlap using elementary kinematic relationships. The axial offset per spindle
revolution is defined as a pitch, the exact value of which is dependent on the coating width
on the substrate with the CS De Laval nozzle at a fixed stand-off distance and the degree of
overlap desired. The program was written in excel and requests the user to input a linear
coating velocity, a coating pitch (which approximately determines the coating overlap
percentage per substrate revolution, see fig. 6.2) and the pipe substrate diameter. The
program outputs the required spindle speed and linear feed rate of the XY table to achieve
these desirable process conditions. Additional useful information provided by the program
is the time taken per pipe substrate revolution as well as the time taken for the entire
experiment to take place. The program interface and the list of rudimentary mathematical
relationships it relies on can be seen in fig. 6.1 and below it respectively.
67
[
] (
)
;
Figure 6.1 – Excel interface relating CS process parameters with a set of data
inputted for illustration as well the kinematic formulae used to construct it.
Figure 6.2 – Coating pitch illustration for helical pipe coating tests.
Table 6.1 – Kinematic parameter table.
Pipe Circumference
Pipe Diameter
Pipe Angular velocity
Pipe Radius
Coating pitch,
p (from centre
to centre)
Pipe Substrate
Helical
Coating
68
Pipe Angular Velocity
Time taken per Pipe Revolution
Linear Coating Velocity on Pipe Substrate
Time taken for Entire Experiment
Number of Substrate Coating Passes
Linear Platform Velocity
Linear Platform Velocity
Axial Coating Length of Pipe Substrate
Coating Pitch
7 Cold Spray Testing:
There are two Cold Spray tests to conduct; one Cold Spray line test on the outer surface
of a static pipe substrate and a helical Cold Spray test on the outer surface of a rotating
pipe substrate. The mutual test process parameters are listed below and the test specific
process parameters are listed in their respective section also. The ball-screw mechanisms
of the X & Y platforms were protected with temporary plastic sheeting and thoroughly
cleaned post-testing. The spindle ball bearings were covered partially protected with tape
and the bearings were adequately lubricated with oil for smooth rotational motion.
Appropriate industrial way cover bellows have been sourced for permanent platform
protection however the supplier is based in IL, USA. Unfortunately there was insufficient
time to buy, ship and install the way cover bellows within the project timeframe.
7.1 Mutual Test Process Parameters:
Table 7.1 – Process parameters used in both Cold Spray Tests.
69
7.2 Static Pipe Substrate:
7.2.1 Process Parameters & G-Code:
Table 7.2 – Static Pipe Substrate Test specific process parameters & G-Code.
F480
G00X0Y0
G01X-60
G01X0
70
Figure 7.1 – Mach3 Interface with ‘DYNAMIC_PIPE_TEST’ G-code file loaded and
XY table path displayed at the top right hand corner relative to the Home position
(purple crosshairs).
G-code
Start/Pause/Stop/Edit
Options
Soft Limits Boundary Line
Toolpath (XY Table)
generated from G-code
Time Elapsed in Test when
‘Cycle Start’ is initiated
Loaded G-code
instructions list
71
7.2.2 Results & Discussion:
Figure 7.2 – Static Aluminium Pipe Substrate Cold Sprayed with copper
powder (60mm coating length, 2 coating passes).
The completion of this Cold Spray line coating test verified that the Cold Spray facility
is working, deeming the integrated CNC-Pressurised Gas System functional (see fig.7.2).
The scope of this project was to build the CNC machine and to verify the machine build
competency, not to analyse the Cold Sprayed samples in immaculate detail in relation to
coating adhesive strength, bonding mechanism, bonding interface, coating thickness,
coating porosity, coating hardness, coating microstructure etc… This analysis is to be
undergone in future research projects. Having said this, the experiment did provide an
approximate coating width at the specified Process Parameters (~ 6 ). This crucial piece
72
of information enabled the selection of an appropriate coating pitch value in order to
achieve a certain degree of helical coating overlap for the Rotating Pipe Substrate Test.
7.3 Static (Rotating) Pipe Substrate:
7.3.1 Process Parameters & G-Code:
Table 7.3 - Static (Rotating) Pipe Substrate Test specific process parameters & G-
code.
F40
S10
M3
G00X0Y0
G01X-15
G01X0
G01X-15
G01X0
M5
73
Figure 7.3 – Mach3 Interface with ‘DYNAMIC_PIPE_TEST’ G-code file loaded and
XY table path displayed at the top right hand corner relative to the Home position
(purple crosshairs).
Loaded G-code
instructions list Soft Limits Boundary Line Toolpath (XY Table)
generated from G-code
Time Elapsed in Test when
‘Cycle Start’ is initiated
G-code
Start/Pause/Stop/Edit
Options
74
7.3.2 Results & Discussion:
Figure 7.4 – Close-up of Rotating Pipe Substrate Cold Sprayed Cold Sprayed with
copper powder (15mm coating length, 4 passes).
75
Figure 7.5 – Perspective shot of Rotating Pipe Substrate Cold Sprayed Cold Sprayed
with copper powder (15mm coating length, 4 passes).
This Cold Spray test result proves that this CNC facility is fully capable of coating
both flat surface substrates & the outer surface of helical substrates with a very high level
76
of control over the CNC process parameters, although the exact degree of coating overlap
cannot be verified as multiple Cold Spray passes were made (see fig. 7.4 & 7.5). As
already stated, the goal of this project is not to analyse the Cold Spray sample properties
and quality. The goal is but to develop the Cold Spray testing platform with which Cold
Spray Research is to be conducted. In that respect I have successfully met my project goals
in doing so.
8 Problems & Delays:
Whilst connecting the 24V PSU to the mains supply, it shorted and ceased to work.
At the time it was thought to be an inherently faulty product. An external 24V PSU was
used temporarily so as to have still have the ability to work with the machine and
familiarize myself with Mach3 software. A 2nd
24V PSU from the same manufacturer and
was working fine until the PSU backing plate was bolted to its casing. This PSU shorted
also when connected to mains verifying that the definite cause of component shorting was
the backing plate. It was concluded that the bolts had gone so far into the PSU casing that
they pierced the insulation. In all honesty I did not notice the bolt penetration depth on the
PSU specification sheet, I never considered it could ever be a problem (assumed insulation
would be more robust). A 3rd
24V PSU was sourced an installed in the Control Bay having
concluded the cause of failure of its predecessors.
The magnetic proximity sensors used to home the machine coordinate system were not
working. The proximity sensors were purposely triggered and the pins that they were wired
to were checked in the ‘USB SmoothStepper Data Monitoring’ window. There was no pin
response from either the X or Y axis sensor. Each sensor had two wires, one was a ground
reference and the other was wired to a signal pin on the motion control card. If a sensor
was triggered, the circuit would be complete and a 5V signal would activate the pin on the
motion control card indicating the limit switch had been triggered. The limit switches for
both axes were routed together in a four wire encapsulation to the control bay. It was
thought that the 5V signals could have been interfering with one another, hence the sensors
77
were then wired separately to rule this out. Upon monitoring the sensor response, the pins
were firing off when triggered but not consistently. The next prognosis was that the 5V
signal was too weak hence its inconsistent performance. To combat this, the limit switch
sensors were wired into a SPST electromagnetic relay switch. If the limit switches were to
be triggered, the link between the 24V DC and the relay should be complete and trigger the
relay switch. In turn triggering this relay switch should close the link between the ground
reference and the pin on the opposite side of the relay switch, thus firing off the limit
switch and alerting Mach3. The limit switches now fired off 100% of the time when
triggered in this particular configuration.
78
Figure 8.1 – Close-up shot of copper-oil mixture air sprayed out of the spindle
bearing (evidence of critical bearing copper powder exposure).
After the first Cold Spray test the operating space was thoroughly cleaned and
examined. Upon inspection it was noticed that the spindle bearing was noticeably ‘stiffer’
(15-38) Cu powder –
lubrication oil ‘sludge’ mixture
removed from the spindle
bearing
Chuck
Flexi Shaft Coupler
79
and more difficult to rotate. On the face of the ball bearings was a homogenous layer of
copper powder. As much powder as possible was removed with a cloth and 10 bar
pressurised air was shot into the front bearing face. Consequently a ‘sludge’ like mixture of
copper powder and lubricating oil trickled out the opposite side of the bearing on top of
one of the bearing shoulders (see fig. 8.1). The ‘sludge’ was wiped away with a cloth and
this cleaning process was repeated until the benefit was negligible and by adding more
lubrication oil to flush out as much powder as possible. More lubrication oil was added to
the bearing system post-cleaning. Although the bearing articulating surface had not been
badly damaged, it had been critically exposed by the copper powder and was very difficult
to clean. With prolonged use, the bearing would have to be cleaned after every test and
each exposure would induce permanent residual damage. Seeing as it is extremely difficult
to clean and its condition will only continue to deteriorate, it must be replaced with a
sealed bearing soon. A bearing from the same manufacturer with identical internal and
external radii has already been sourced and priced at €11.66 with an approximated transit
time of 1-2 working days.
9 Impact of Work:
The machine build is effectively a stepping stone for novel surface coating research
to follow. Building a CNC Cold Spray Machine will provide TCD with a unique surface
coating facility which will be the first of its kind in Ireland. This machine build not only
introduces a new surface coating technology to TCD, but to the Country. This implicates
that TCD will be sought after and approached by Irish companies looking to invest in our
ground-breaking Cold Spray research. There is huge potential for industrial manufacturing
research grants and college-industry partnerships as companies constantly strive to find
cheaper, faster and more effective surface coating / repair processes. This Cold Spray
facility will make TCD the National leader in Cold Spray technology and applications and
on this basis the impact of this work is extremely high. There have been regular visits to
the developing CNC Cold Spray Facility in TCD in the past few months. The European
Space Agency (ESA) has already expressed their interest and inquired about the deposition
efficiency, the mass deposition rate and the potential for external / internal pipe coatings.
80
Funding for a 2 month Enterprise Ireland Project to conduct Cold Spray Experiments on
behalf of Schuf (Specialist Valve Company, Germany) has already been secured.
10 Conclusions:
The general objective of this project was to develop a CNC Cold Spray Facility capable
of coating flat and pipe substrates. The XY Table consists of two ball screw CNC
platforms; one bolted to the machine housing (Y-axis) and the other on top of the first (X-
axis) at a 90 relative angle. A ‘T-slot’ table is bolted to the top CNC platform (X-
platform) allowing for variable positional fixation of workpieces. A thin aluminium sheet
is bolted on top of the ‘T-slot’ table at its four corners to prevent potential degradation of it
surface as a result of the Cold Spray process. BLDC stepper motors are used to control the
position and speed of the XY table via open loop control. The spindle axis consists of a
chuck and a BLDC stepper servo hybrid motor held by a ‘double L-bracket’ style mount &
bearing system that are coupled together via a flexi shaft coupler. The spindle motor has a
rotary encoder that has a feedback loop connected to the motor driver that does not need to
be tuned and prevents loss of step synchronisation. All of the CNC hardware is configured,
tuned and calibrated with the CNC software, Mach3, on a desktop computer. All motion
parameters of both the XY table and spindle axis are set, controlled and monitored by
Mach3 in open loop control mode. The XY table positional accuracy is within 0.01
and the spindle accuracy is within 1 RPM. All three CNC axes have a high degree of
precision combined with excellent operational repeatability.
The user is completely isolated from dangerous alternating current thanks to the
latched relay switch in the control bay that toggles the power supply to the motor drivers
on/off. There are two emergency stops and two interlock switches wired in series with the
24 V DC input to the latched relay. If either of the emergency stop buttons are pressed or if
either of the side windows are opened, the relay switch will open and cut power to the
motors ceasing all motion. The mains PSU, the 24 V DC PSU & the 48V DC PSU are all
properly grounded inside the control bay so as to minimize electrocution potential. An
81
RCD is placed in series with the live and neutral wires of the mains input supply to kill the
mains supply input in the event of an abnormal current draw or a difference in current
flowing through the lie and neutral wires.
The Cold Spray CNC machine build was deemed successful in accomplishing a Cold
Spray line coating of (15-38) copper powder on an aluminium static pipe substrate and
a Cold Spray helical coating of (15-38) copper powder on an aluminium rotating pipe
substrate. The coating properties (density, particle cohesion, hardness, coating-substrate
consolidation, bonding mechanism, etc…) were not investigated as they were outside the
scope of this project. The machine build is effectively a stepping stone for such novel
surface coating research to follow. This machine build not only introduces a new surface
coating technology to TCD, but to the Country. This implicates this machine will make
TCD the National leader in Cold Spray technology and applications and on this basis the
impact of this work is extremely high.
82
11 Future Work:
The way cover bellows needed to protect the lead-screw platforms have already been
sourced. They need to be bought, shipped and installed before any further Cold Spray
testing can be conducted (see Appendix, Page 121).
The spindle bearing must be replaced with a sealed equivalent, which has already been
sourced also (see Appendix, Page 121). The spindle axis needs to be dismantled and
reassembled in the workshop with the new bearing in place.
A High Definition (HD) camera a closed-circuit television (CCTV) camera must be
installed inside the operating space to record future Cold Spray tests for experimental
analysis.
A means of measuring particle velocity at various set process parameters (set pressure,
set particle size & weight and using a particular working fluid) must be introduced. Particle
Image Velocimetry (PIV) experimentation in the TCD Fluids Laboratory can provide this
crucial information. In addition to this a device should be sourced, bought and installed to
monitor machine vibrations during operation.
At some stage a laser could be introduced to the Cold Spray Facility that would enable
Cold Spray testing with a laser-heated substrate. This process is known as ‘SprayLaze’ and
would stretch the scope of testing parameters for Cold Spray Research in TCD, making it
even more attractive to invest in. Naturally integrating a laser would require enhancement
of the current safety measures put in place, i.e. the Perspex windows should be replaced
with aluminium sheeting to prevent laser exposure to the user in the case of
unpredicted/uncontrollable laser deflection. The inclusion of a gas pre-heater could also be
extremely beneficial as it too would broaden the research spectrum.
83
12 References:
ARMSTRONG-HÉLOUVRY, B., DUPONT, P. & DE WIT, C. C. 1994. A survey of
models, analysis tools and compensation methods for the control of machines with
friction. Automatica, 30, 1083-1138.
CARLBERG, S. 2012. How SERVOS and STEPPERS stack up. Machine Design, 84, 42-
48.
DYKHUIZEN, R. C. & SMITH, M. F. 1998. Gas dynamic principles of cold spray.
Journal of Thermal Spray Technology, 7, 205-212.
DYKHUIZEN, R. C., SMITH, M. F., GILMORE, D. L., NEISER, R. A., JIANG, X. &
SAMPATH, S. 1999. Impact of high velocity cold spray particles. Journal of
Thermal Spray Technology, 8, 559-564.
EC 2004. DIRECTIVE 2004/108/EC OF THE EUROPEAN PARLIAMENT AND OF
THE COUNCIL of 15 December 2004 on the approximation of the laws of the
Member States relating to electromagnetic compatibility
and repealing Directive 89/336/EEC.
EC 2006a. DIRECTIVE 2006/42/EC OF THE EUROPEAN PARLIAMENT AND OF
THE COUNCILof 17 May 2006 on machinery, and amending Directive 95/16/EC
(recast).
EC 2006b. DIRECTIVE 2006/95/EC OF THE EUROPEAN PARLIAMENT AND OF
THE COUNCIL of 12 December 2006 on the harmonisation of the laws of
Member States relating to electrical equipment designed for use within certain
voltage limits.
EC 2010. Appliance of reference designations on machinery in accordance with
ISO/IEC/EN 81346 September 2010
EUN-CHAN, P., HYUK, L. & CHONG-HO, C. 2003. Position control of X-Y table at
velocity reversal using presliding friction characteristics. Control Systems
Technology, IEEE Transactions on, 11, 24-31.
GILMORE, D. L., DYKHUIZEN, R. C., NEISER, R. A., SMITH, M. F. & ROEMER, T.
J. 1999. Particle velocity and deposition efficiency in the cold spray process.
Journal of Thermal Spray Technology, 8, 576-582.
GOLUBA, R. 2000. Closing the gap between steppers and servos. Instrumentation &
Control Systems, 73, 43-46.
GRUJICIC, M., SAYLOR, J. R., BEASLEY, D. E., DEROSSET, W. S. & HELFRITCH,
D. 2003. Computational analysis of the interfacial bonding between feed-powder
84
particles and the substrate in the cold-gas dynamic-spray process. Applied Surface
Science, 219, 211-227.
GRUJICIC, M., ZHAO, C. L., TONG, C., DEROSSET, W. S. & HELFRITCH, D. 2004.
Analysis of the impact velocity of powder particles in the cold-gas dynamic-spray
process. Materials Science and Engineering: A, 368, 222-230.
HAMEYER, K. & BELMANS, R. J. M. 1996. Permanent magnet excited brushed DC
motors. Ieee Transactions on Industrial Electronics, 43, 247-255.
KEEFER, J. 2013. Lead Screws vs Ball Screws: It's All about the Application [Online].
Available: http://blog.helixlinear.com/bid/224687/Lead-Screws-vs-Ball-Screws-It-
s-All-about-the-Application [Accessed 09/12/2013.
KOSAREV, V. F., KLINKOV, S. V., ALKHIMOV, A. P. & PAPYRIN, A. N. 2003. On
some aspects of gas dynamics of the cold spray process. Journal of Thermal Spray
Technology, 12, 265-281.
KU, S.-S., LARSEN, G. & CETINKUNT, S. 1998. Fast tool servo control for ultra-
precision machining at extremely low feed rates. Mechatronics, 8, 381-393.
LIM, H., SEO, J.-W. & CHOI, C.-H. 2001. Torsional displacement compensation in
position control for machining centers. Control Engineering Practice, 9, 79-87.
LIPSETT. 2009. Ball Screws and Lead Screws – The Real World Difference [Online].
Available:
http://www.thomsonlinear.com/downloads/articles/Ball_Screws_and_Lead_Screws
_The_Real_World_Difference_taen.pdf.
MELKOTE, H. & KHORRAMI, F. 1999. Robust nonlinear control and torque ripple
reduction for permanent magnet stepper motors. Iee Proceedings-Control Theory
and Applications, 146, 186-196.
MOSELER, O. & ISERMANN, R. 2000. Application of model-based fault detection to a
brushless DC motor. Ieee Transactions on Industrial Electronics, 47, 1015-1020.
PACAS, M. & WEBER, J. 2005. Predictive direct torque control for the PM synchronous
machine. Ieee Transactions on Industrial Electronics, 52, 13(15-38)-1356.
PAPYRIN, A. 2001. COLD SPRAY TECHNOLOGY. Advanced Materials & Processes,
159, 49.
SCHWEID, S. A., MCINROY, J. E. & LOFTHUS, R. M. 1995. CLOSED-LOOP LOW-
VELOCITY REGULATION OF HYBRID STEPPING MOTORS AMIDST
TORQUE DISTURBANCES. Ieee Transactions on Industrial Electronics, 42, 316-
324.
85
SHIN, W. G. & LEE, S. H. 2010. An analysis of the main factors on the wear of brushes
for automotive small brush-type DC motor. Journal of Mechanical Science and
Technology, 24, 37-41.
STOLTENHOFF, T., KREYE, H. & RICHTER, H. J. 2002. An analysis of the cold spray
process and its coatings. Journal of Thermal Spray Technology, 11, 542-5(15-38).
SWEVERS, J., AL-BENDER, F., GANSEMAN, C. G. & PRAJOGO, T. 2000. An
integrated friction model structure with improved presliding behavior for accurate
friction compensation. Ieee Transactions on Automatic Control, 45, 675-686.
TESCHLER, L. E. & MEYER, S. 1998. How steppers stack up against servos. Machine
Design, 70, 100.
TRUMPER, D. L., KIM, W. J. & WILLIAMS, M. E. 1996. Design and analysts
framework for linear permanent-magnet machines. Ieee Transactions on Industry
Applications, 32, 371-379.
TSUI, K. W. H., CHEUNG, N. C. & YUEN, K. C. W. 2009. Novel Modeling and
Damping Technique for Hybrid Stepper Motor. Ieee Transactions on Industrial
Electronics, 56, 202-211.
VAN STEENKISTE, T. H., SMITH, J. R. & TEETS, R. E. 2002. Aluminum coatings via
kinetic spray with relatively large powder particles. Surface and Coatings
Technology, 154, 237-252.
WEI, C.-C., LIOU, W.-L. & LAI, R.-S. 2012. Wear analysis of the offset type preloaded
ball–screw operating at high speed. Wear, 292–293, 111-123.
WU, J. W., FANG, H. Y., YOON, S., LEE, C. & KIM, H. 2006. Critical velocities for high
speed particle deposition in kinetic spraying. Materials Transactions, 47, 1723-
1727.
XU, X. W. & HE, Q. 2004. Striving for a total integration of CAD, CAPP, CAM and CNC.
Robotics and Computer-Integrated Manufacturing, 20, 101-109.
XU, X. W., WANG, H., MAO, J., NEWMAN, S. T., KRAMER, T. R., PROCTOR, F. M.
& MICHALOSKI, J. L. 2005. STEP-compliant NC research: the search for
intelligent CAD/CAPP/CAM/CNC integration. International Journal of Production
Research, 43, 3703-3743.
YANG, S. M. & KUO, E. 2003. Damping a hybrid stepping motor with estimated position
and velocity. Ieee Transactions on Power Electronics, 18, 880-887.
86
13 Appendix:
13.1 Supplemental Figures:
Figure 13.1 - Control bay location as before stripping.
87
Figure 13.2 - Control bay to be stripped and converted.
Mains Supply Kill Switch
Colour Coordinated Wire junctions
88
Figure 13.3 - Control bay installation development with the limit switch circuitry now
included (1).
DPDT relay switch
SPST relay switches
89
Figure 13.4 - Control bay installation development with the limit switch circuitry now
included (2).
90
Figure 13.5 – Complete control bay with ventilation cover.
91
Figure 13.6 – Motor drivers PSU circuit.
Figure 13.7 – A SPST relay limit switch circuit.
92
Figure 13.8 – Chuck backing plate.
93
Figure 13.9 – Far side shoulder.
94
Figure 13.10 – Chuck side shoulder.
95
Figure 13.11 – ‘L-bracket’ spindle axis mount.
96
Figure 13.12 – DC motor mounting plate.
97
Figure 13.13 - Spindle Shaft.
98
Figure 13.14 – Spindle Axis close up post 90 second Cold Spray Test (Rotating Pipe
Substrate).
99
Figure 13.15 – XY Table post 90 second Cold Spray Test (Rotating Pipe Substrate).
100
Figure 13.16 – Close up of the two Cold Spray samples side-by-side.
101
Figure 13.17 – 220 bar Nitrogen Gas Tank, size W from BOC.
102
Figure 13.18 – Close up of the Powder Feeder (Angle 1).
Powder Feeder
Controller
103
Figure 13.19 – Close up of the Powder Feeder (Angle 2).
Powder Feeder
Outlet Line
104
Figure 13.20 – Close up of the Cold Spray nozzle and its inlet connections.
Pressurised Gas Line
Powder
Feeder Line
105
Figure 13.21 – Component designed and made to (a) connect IGUS cable manager to
the X-axis & (b) to fixate and act as a guiderail for the IGUS cable manager.
106
Figure 13.22 – Pressure Gauge Mount Design Concept that was never built.
107
Figure 13.23 – Another Pressure Gauge Mount Design Concept that was never built.
108
13.2 Part Manuals & Specifications:
*The list of all specification sheets & manuals are logged in PDF format on the
provided disk.
Recommended
