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Page 5.1MFE3004 Mechatronics IC. Pace
MFE 3004Mechatronics I
Actuation SystemsDr Conrad Pace
Page 5.2MFE3004 Mechatronics IC. Pace
Actuators• Actuators are the elements of a mechatronic system that are responsible for transforming the output of a control system into a physical controlling action on a machine or device
• Examples• An electrical output from a controller that has to be transformed to a linear or rotational motion to move a load• An electrical output that has to be used to control the flow rate through a pipe, etc..
Controller
Actuation
Sensing
ControlledEnvironment
2
Page 5.3MFE3004 Mechatronics IC. Pace
Actuators
• Actuation systems can be classified according to the source of the driving power
Pneumatic (Compressed Air power)Hydraulic (Pressurised Hydraulic Fluid Power)Electrical (Electrical power)
• Each of the above actuation systems have particular characteristics that make them suitable for certain applications.
Page 5.4MFE3004 Mechatronics IC. Pace
Actuators
• Two components of an actuation system– Power Amplification and Modulation Stage
Power Amplification and Modulation Stage
Control Signal
(Low Power)
Power Required to Drive Actuator
(High Power)Un-modulated Power Source
• Provide high power switching in electrical drives
• Provide flow switching and fluid flow control in fluid power drives
3
Page 5.5MFE3004 Mechatronics IC. Pace
Actuators
• Two components of an actuation system– Energy Conversion Stage
Energy Converter
Modulated High Power Input
Output Work
(Physical output of Actuator)
• Examples :
• Electrical Drives
• Fluid Power Cylinder
Page 5.6MFE3004 Mechatronics IC. Pace
Actuators• Two components of an actuation system
– Power Amplification and Modulation Stage– Energy Conversion Stage
4
Page 5.7MFE3004 Mechatronics IC. Pace
Electrical ActuationElectrical Actuators
Solenoids Linear Motors
Linear Motion Rotary Motion
DC Motors AC Motors Stepper Motors
Brush Type
Brushless
Asynchronous
Synchronous
Variable Reluctance
Permanent Magnet
Page 5.8MFE3004 Mechatronics IC. Pace
Electrical ActuationDC Brush Type Motors
FieldPole
ArmatureFieldCoil
ArmatureConductors
Yoke
A four-pole DC motor
Current through armature coil
I = (V - kφω)R
V = voltage across coilR = coil resistancek = back e.m.f. constantφ = magnetic flux per poleω = rotational speed
• A magnetic field is generated by the stator winding / permanent magnet
• The flow of current through the rotor winding and the presence of a magnetic field causes a force to act on the winding which induces a rotating torque
• For continuous rotation the current must be reversed once for each pole pair, thus requiring a commutator and brushes.
Torque generated by Armature current flow
T = kTI kT = Torque constant
5
Page 5.9MFE3004 Mechatronics IC. Pace
Electrical Actuation
DC Motors with Field Coils
ArmatureCoil
Field Coil Series Wound
• Highest starting torque
• Highest no-load speed
Shunt Wound
• Lowest starting torque
• Lowest no-load speed
• Stable speed over torque range
ArmatureCoil
FieldCoil
ArmatureCoil
FieldCoil
Field Coil Compound Wound
• Has characteristics of both series and shunt wound Armature
Coil
FieldCoil
Separately Excited
• Ideal set-up for speed control
• Speed control - Armature voltage control or Field voltage control
Page 5.10MFE3004 Mechatronics IC. Pace
Electrical Actuation
AC motors - Asynchronous Motors• Rotor consists of a set of conductors connected to each other by end rings forming a squirrel cage
• The stator winding is driven by an A.C. voltage which induces a rotating magnetic field
• The magnetic field induces an e.m.f. in the rotor which causes a current flow.
• The rotor current flow in the magnetic field causes a torque on the rotor, causing rotary motion
• The motor is not self-starting
Synchronous speed of motor = 2f/p
f = frequency of stator signal
p = no. of stator poles
6
Page 5.11MFE3004 Mechatronics IC. Pace
Electrical Actuation
AC motors - Synchronous Motors• Stator similar to asynchronous motors, and carries an a.c. voltage
• Rotor consists of a permanent magnet
• Synchronous motors run at exactly the synchronous speed
• Speed control obtained through the variation of the signal frequency
~~
~~
Motor
Variablefrequency
A.C.Supply
VoltageInverter
RectifierVoltageregulator
AC motors - Speed Control
Page 5.12MFE3004 Mechatronics IC. Pace
Electrical ActuationStepper Motors
• Motors that provide discrete angular displacements in steps (ex. 5° angular steps)
• Motors driven through a sequence of pulses where each pulse corresponds to an angular step.
• Continuous rotation can be provided by giving a continuous train of pulses, the speed being determined by the frequency of the pulses.
Variable Reluctance Stepper Motor• The ferromagnetic rotor consists of a number of poles (less than the stator poles)
N
S
Rotor
Stator
This pair of poles are energised by thecurrent being switched to them and therotor rotates to the position show adjacent.
N
S
This pair of poles are energisedby the current being switched tothem to give the next step.
7
Page 5.13MFE3004 Mechatronics IC. Pace
Electrical Actuation
Permanent Magnet Stepper Motor• The rotor consists of a permanent magnet that aligns itself with the energised stator poles
• All stepper motors can have more than one stack so as to decrease the angular step
• The stacks are energised in sequence.
Page 5.14MFE3004 Mechatronics IC. Pace
Electrical Actuation
Hybrid Stepper Motor• The rotor consists of a permanent magnet
• The magnetic rotor is toothed so as to have a number of poles
8
Page 5.15MFE3004 Mechatronics IC. Pace
Electrical Actuation
Typical Control Set-up for a motor• Motor Speed Control
• Motor Angular Displacement Control
+ _ + _
Position Error
Position Input Demand
Velocity Error Amplifier Motor
Motor Power Input Gear
Box
Rotational Output
Load
Position Output
Velocity Measurement
Position Measurement
Position Feedback
Velocity Feedback
Page 5.16MFE3004 Mechatronics IC. Pace
Pneumatic Actuators - Cylinders
Bearing
Seal Piston rodExhaustInlet
Return SpringPiston
Bearing CapCylinder barrelPiston sealBase Cap
Single Acting Cylinders
Double Acting Cylinders
B A
Ao
AA
area on piston side area on piston rodside
Symbols
9
Page 5.17MFE3004 Mechatronics IC. Pace
Pneumatic Actuators
Rodless Cylinder
Limited Rotation Rotary Actuators
Symbol
Page 5.18MFE3004 Mechatronics IC. Pace
Vane Type
Pneumatic Rotary Actuators
Continuous Rotation - Air Motors
Radial Piston Type
Axial Piston Type Symbol
10
Page 5.19MFE3004 Mechatronics IC. Pace
Control of Pneumatic Actuators
PressureControl
DrivingMeans
Regulator
Dire
ctio
nal
Con
trol
cylinder pistonpiston rod
Lubricator
AirCompressor
Filter
Air StorageTank
Pneumatic System
• The normal operating pressure in the system is generally 6 bar
Page 5.20MFE3004 Mechatronics IC. Pace
Direction Control Elements
RP
A
P R
A
Operation of a Single Acting Cylinder via a 3 port / 2 position (3/2 way) valve
Operation of a Double Acting Cylinder via a 4/2 way valve
A
P R
B A
P R
B
11
Page 5.21MFE3004 Mechatronics IC. Pace
Pneumatic Actuation
Application Example : A pick-and-place Pneumatic System
Page 5.22MFE3004 Mechatronics IC. Pace
Hydraulic Actuation
Agriculturalmachinery
Tractors, Combines
Constructionmachines andpublic servicevehicles
Excavators, graders, loaders,cranes, winches, elevatingplatforms, lifting and conveyingdevices, hydro-transmissions
Plasticmanufacturingmachines
blow-moulding machines,injection moulding machines
Heavy industries Steel mills,continuous-casting plant
Machine toolsnon-cutting
Materials-testing presses,ceramics and plastics presses,folding and bending presses,drawing presses.
Machine toolsmetal-cutting
Clamping devices,planers and slotting machines,lathes, drilling and turningm/c’s, grinding machines
100 200 300Pressure (Bar)
• Hydraulic actuation is used in a wide variety of applications with varying operating pressures
12
Page 5.23MFE3004 Mechatronics IC. Pace
Hydraulic Actuation
• The consideration of the flow characteristics is important due to the effect of viscosity
• High viscosity gives rise to high frictional forces in the pipes giving rise to substantial pressure drops
• Viscosity is greatly dependent on temperature, thus affecting Flow rate
Fundamental Principles of Hydraulics
• The hydraulic fluid has to fulfil following tasks:Pressure Transfer Lubrication CoolingDamping oscillations Corrosion protection Scuff removalSignal transmission Eliminate presence of air
Page 5.24MFE3004 Mechatronics IC. Pace
Differentialcylinder(standard)
Piston return strokefaster than advancestroke
Synchronouscylinder
Pressurised area ofequal size. Advance andreturn speeds identical
Cylinder withend positioncushioning
To moderate the speedin the case of largemasses and prevent ahard impact
wiper seal
piston rodbearing
barrel
piston rod
piston withseals
end cap
rod sealventingscrew
Hydraulic Actuators - Cylinders
13
Page 5.25MFE3004 Mechatronics IC. Pace
Telescopiccylinder
Longer Strokes
PressureIntensifier
Increases pressure
TandemCylinder
When large forces arerequired and only smallcylinder diameters arepossible
Hydraulic Actuators - Cylinders
Page 5.26MFE3004 Mechatronics IC. Pace
Hydraulic Actuators - Motors
Hydraulic Motors
Gear Motors Vane Motors Piston Type Motors
Axial Piston Motors Radial Piston Motors
Characteristics of Hydraulic MotorsApplication Motor Type
Low torque, low power Gear; Vane
Low torque, medium power Vane; Axial piston
Medium torque and/or power Axial piston; Radial piston (inward working)
High power, very high torque Axial piston; Radial piston (outward working)
14
Page 5.27MFE3004 Mechatronics IC. Pace
Hydraulic Actuators - Motors
Radial Piston Motors
Gear Motor Axial Piston Motor
Page 5.28MFE3004 Mechatronics IC. Pace
Hydraulic Actuation
Hydraulic Power System
Driving Means
Pump
Non-return Valve
Pressure relief valve
To system control and working elements
Return
Oil reservoir (sump)
Return Filter
Suction filter
Pressure Filter
Hydraulic Circuits
15
Page 5.29MFE3004 Mechatronics IC. Pace
Hydraulic Actuation
Example of an Electro-Hydraulic Actuation System
SERVO VALVE
AMPLFIIER
Control Signal
HYDRAULIC FLUID SUPPLY
Hydraulic Fluid Drain
HYDRAULIC CYLINDER
Position Feedback
POSITION MEASUREMENT
Error Signal
Position Demand Signal
Page 5.30MFE3004 Mechatronics IC. Pace
Hydraulic Pneumatics Electrical
Working Fluid Mineral Oil Air Voltage-Current
Working Pressure 500 Bar (maximum 6-7 Bar Up to 11kV
Available Force 100MN 10kN 100kN
Speed Low High Very High
Conversion Efficiency Over 70% Under 20% Over 80%
Capital Costs High Low Intermediate
Proportional Control Easy Difficult Easy
Hold Load Power-Off/Stability
Possible No (air is compressible) Possible
Precise Positioning Easy Difficult Easy
EnvironmentalInfluences
Sensitive in case oftemperature fluctuation,risk of fire in case ofleakage
Explosion proof,Insensitive totemperature
Risk of explosion incertain areas,insensitive totemperature
Energy storage Limited with the help ofcompressed gases
Easy Difficult, only in smallquantities usingbatteries
Linear Motion Simple using cylinders Simple using cylinders Difficult and expensive– with motion converter
Rotary Motion Simple Simple Simple
Actuator Power Systems
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Page 5.31MFE3004 Mechatronics IC. Pace
Actuators
1 10 100 1,000 10,000
100,000
10,000
1,000
100
10
Pneumaticbellows
Pneumaticcylinders(rodless)
Pneumaticcylinders(rodded)
Solenoids
Pneumaticdiaphragms
HydraulicCylinders
LinearElectric
Motor andscrew
Piezoelectricdevices
Distance(mm)
Force (N)
Page 5.32MFE3004 Mechatronics IC. Pace
1 10 100 1,000 10,000
100,000
10,000
1,000
100
10
1
NOTEServo & steppermotors may operateat low speeds,including incrementalmotion
Hydraulic DrivesElectrical DrivesPneumatic Drives
Pneumatic(limited rotation)
Hydraulic(limited rotation)
Hydraulicradial piston
Hydraulicaxial piston
Hydraulicvane
Hydraulic gear
PneumaticDrives
PermanentMagnet stepper
High FrequencyInverter Drives
Induction motorhigh power
inverter drives
Output Speed (rev/min)
Output Torque (Nm)
Actuators
17
Page 5.33MFE3004 Mechatronics IC. Pace
Electricalslipcouplings
Conepulley
Vee beltand chains
Torques to250,000Nm parallelshaft (marine gears)
Unit hydraulicdrives
Harmonicdrives
1 10 100 1,000 10,000
100,000
10,000
1,000
100
10
1Friction wire
wrap
ToothedBelt
Ball ordiscfrictiondrive
Parallel shaftWormFluid couplingsand torqueconverters
Epicyclicgears
Output Speed (rev/min)
Output Torque (Nm)
Actuators
Page 5.34MFE3004 Mechatronics IC. Pace
Actuator Performance Characteristics
• Motion Requirements• Type of motion required (linear or rotary)• Travel required (e.g. stroke lengths or angles or rotation)
• Velocity• Defining Acceleration and Deceleration Characteristics
• Operational Power• Power sufficient for application (effort x flow)• Power due to varying loads
18
Page 5.35MFE3004 Mechatronics IC. Pace
Actuator Performance Characteristics
• Resolution, Accuracy and Repeatability
• Resolution : the smallest controllable change in output possible by the actuator
• Accuracy : how close to the intended target an actuator is expected to locate (error between demanded and actual output)
• Dependent on internal actuator factors (looseness of mechanical couplings, stiction, etc..)
• Dependent on external actuator factors (sliding friction of someexternal slide)
• Repeatability : the ability of the actuator to return repeatedly to the same point.
• Influenced by component rigidity (under changing loads)• Effected by actuator hysteresis and backlash.
Page 5.36MFE3004 Mechatronics IC. Pace
Actuator Performance Characteristics
• Responsiveness/ Dynamic Behaviour• Dynamic response to a demand signal• Dependent on
• Actuator’s velocity and acceleration characteristics• Effectiveness of feedback loop and control
• Generally analysed based on the • Step response (Time Domain analysis)• Frequency response (Frequency Domain analysis)
• Compliance• The movement of a component in reaction to a force exerted on it. • Stiffness of the system (low compliance) or sponginess/softness (high
compliance)• Compliance requirements depend on the type of application
19
Page 5.37MFE3004 Mechatronics IC. Pace
Actuator Performance Characteristics
• Non-Linearities• General Assumption – Actuation System behaves linearly (follows the
principle of superposition)• Most actuation systems exhibit deviations from the linear behaviour (non-
linearities)• Static/ Coulomb Friction• Eccentricity• Backlash• Saturation• Deadband
Page 5.38MFE3004 Mechatronics IC. Pace
Actuator Performance Characteristics
• Non-Linearities
Static/ Coulomb Friction Eccentricity
20
Page 5.39MFE3004 Mechatronics IC. Pace
Actuator Performance Characteristics
• Non-Linearities
Backlash Saturation
Page 5.40MFE3004 Mechatronics IC. Pace
Actuator Performance Characteristics
• Non-Linearities
Deadband
21
Page 5.41MFE3004 Mechatronics IC. Pace
Actuator Performance Characteristics
• Other Actuation System Selection Parameters• Control Parameters
• parameters that influence actuator control• Power Source Requirements
• Type of power source available • Method of Actuator performance measurement• System Integration• Costs• Safety (as part of the system)