Historical review and update to the state of the art of automation for plasma coating processes

Thin Solid Films, 118 (1984) 445 456

PREPARATION AND CHARACTERIZATION 445

H I S T O R I C A L REVIEW A N D U P D A T E TO T H E STATE O F T H E ART OF A U T O M A T I O N FOR PLASMA C O A T I N G PROCESSES*

PHILIP MEYER AND STEPHAN MUEHLBERGER Electro-Plasma Inc., 16842 Milliken Avenue, lrvine, CA 92714 (U.S.A.)

(Received April 4, 1984; accepted May 9, 1984)

Automation is the technique by which man is removed from the process and the resulting system operates automatically. A properly designed automated system will best take advantage of the man-mach ine combination using the machine for tedious or repetitive tasks and will use the man for those functions for which it is too difficult or costly to teach the computer or controller. Automated plasma coating facilities have been used historically to remove the operator from direct involvement in the coating process for several purposes, i.e. safety, process consistency and increased productivity. There is some natural overlap in these areas and the decision to automate a process can be motivated by all three concerns and perhaps even others such as available space or the search for prestige.

Each of these areas is discussed with special interest given to increased productivity. Some specific systems are reviewed which illustrate the present state of the art of automation for the plasma spray process including both part handling and control systems.

1. INTRODUCTION

Automation is the technique by which man is removed from the process and the resulting system operates automatically. When man is removed from direct involvement in the process, his role is usually changed to an indirect or supervisory role. A properly designed automated system will best take advantage of the m a n - machine combination using the machine for tedious or repetitive tasks and will use the man for those functions for which it is too difficult or costly to teach the computer or controller. One example is the role of the crew on a space shuttle mission. Man can react to the unexpected or abnormal operation more readily than the machine. Automated plasma coating facilities have been used historically to remove the operator from direct involvement in the coating process for several purposes, i.e. safety, process consistency and increased productivity. There is some natural overlap in these areas and the decision to automate a process can be

* Paper presented at the International Conference on Metallurgical Coatings, San Diego, CA, U.S.A., April9 13, 1984.

0040-6090/84/$3.00 © Elsevier Sequoia/Printed in The Netherlands

446 P. MEYER, S. MUEHLBERGER

motivated by all three concerns and perhaps even others such as available space or the search for prestige.

In the past, as with similar processes such as arc welding, plasma operators typically held the plasma gun and moved it across the workpiece. Round parts such as tubes or shafts were turned in a lathe or similar device. The operators were responsible for establishing the gun-to-part distance, the surface speed and the proper plasma conditions. They often compensated for variations in the plasma jet and powder stream by varying some undefined parameter. The coating quality greatly depended on the operator's skill and diligence. Even the best operators, however, had difficulty in producing consistent-quality coatings on a repetitive basis. In addition, the operators' intimate involvement in the process necessarily exposed them to inherent hazards. Recent emphasis by the Occupational Safety and Health Association and related agencies has increased the awareness of this problem on the part of plant planners and management.

In general, the trend toward automation has been led by the larger original equipment manufacturers where volume of work was the prime factor. Some early examples can be seen in the automobile industry where a few applications have been automated when plasma coatings were considered for production. In 1973, General Motors decided to automate the coating process of end-plate seals on their rotary engine program. In t975 a completely automatic assembly line plasma coating facility was delivered to Russia for incorporation into a diesel piston production factory. However, more recent examples are emerging in the turbine engine industry as coatings are increasingly being specified on newer engines; some of these are later examined in greater detail. By considering each of these factors (safety, process consistency and increased productivity) which have motivated the trend toward automation we can trace the progression of automation from the early stages up to present-day computerized automated systems.

2. SAFETY

The safety of operators for an atmospheric spray system is improved by removing them from direct exposure to noise, dust or powder, UV radiation, fatigue and machinery. This was originally accomplished by mounting the spray gun on a mechanical motion device such as an x - y or x - y - z mechanism and then incorporating a sound-proof room and dust removal (wet or dry) system. A motion mechanism is required to simulate the motions previously supplied by the hand-held gun of the operator, i.e. gun-to-part distance, surface speed and angle of incidence. However, the operator is still responsible for mechanical set-up and selection of the plasma conditions.

In a low pressure plasma system the operator is inherently removed from these hazards since the process is taking place inside a vacuum tank where noise and UV radiation are minimally transmitted and dust is filtered out in the vacuum manifold. One factor to consider here is that, when automating a process or system, man must still interface with the machine at some point and this point is where care should be taken to provide the operator with a safe working environment. For example, it was recently reported that in Japan a worker was killed by a robot. Safety will result from good design practices where emphasis is placed on providing a safe working

UPDATE ON AUTOMATION FOR PLASMA COATING PROCESSES 447

environment. One of the most difficult aspects of accomplishing this task is anticipating the unexpected. It might be said that 20% of designing a machine is for normal operation and 80% is for abnormal operation.

3. PROCESS CONSISTENCY

Process consistency has been a natural result of replacing the operator with a machine. However, the eye and skill of the operator which must be replaced by the machine require an understanding of the cause and effects of plasma physics and definition of the critical parameters, which must be monitored and controlled. One of the most important conditions is to obtain a stable plasma where the critical parameters do not fluctuate. To accomplish this, the plasma stream must be homogeneous and constant in the area where the powder is injected. This is best achieved by working with a neutral plasma. The discharge area of the charged plasma is well back in the arc chamber, allowing a recombination of the majority of ions and electrons before reaching the powder injection area (Fig. 1). To maintain this stable plasma condition, enthalpy must remain constant. Since enthalpy is a function of power and mass flow of gases, these parameters must be controlled.

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,ANODE N 0 Z'Z..kE Fig. 1. Plasma arc equipment.

Power in a d.c. arc is the product of voltage and current. The voltage is a resultant of arc chamber, gas type and mass flow. Arc current is controlled directly by commanding the d.c. power supply which can be increased or decreased to obtain a desired power level. One variable to contend with is a decrease in arc voltage that occurs with wear or erosion of the arc chamber. This decrease in voltage can be compensated for within some allowable tolerance by increasing arc current. Thus arc power and enthalpy are maintained as constant. The voltage can also be increased by increasing the flow rate or volume of the arc pr imary or secondary gases. However, this results in a change in the mass flow and the enthalpy of the plasma and is to be avoided if possible.

The arc gas mass flows are critical and must be maintained at consistent values. Various methods are used to control gas flows such as rotameters, flowmeters with electric servo valves and critical-orifice mass flow controllers. Hand-controlled


rotameters were used on early systems but have inherent calibration problems in that the indicated flow is valid only at a given back pressure. Since the arc chamber pressure of a plasma gun can vary for different plasma conditions, the rotameter cannot give an accurate measure of gas flow. The more elaborate mass flowmeters utilizing hot wires and servo valves can be calibrated for different gases and flow ranges; however, these devices are primarily for laboratory use and may not be properly suited to production usage.

A preferred method of gas flow control uses critical orifices and servo-driven pressure regulators. The principle is illustrated in Fig. 2. The mass flow of a gas varies directly with the absolute upstream pressure P1 and the area of a critical orifice and inversely with the square root of the absolute upstream temperature (see Appendix A). The advantages of this approach are the ease of calibration and the independence of gas flow from downstream (arc chamber) pressure.

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Another gas flow control method is shown in Fig. 3. This method also uses critical orifices but rather than vary the pressure Px it works with a constant pressure P1 and allows selection of any combination of four orifices, resulting in 15 discrete gas flow levels for both the primary and the secondary arc gases.

On the assumption of a stable plasma condition as discussed above, the final critical parameter is control of the powder delivery to the sprayhead. It is important for a powder delivery system to provide consistent powder flow under a variety of conditions, i.e. various cannister levels, different temperatures and humidity conditions, negative or positive powder injection pressures and fine or coarse powder conditions. A powder feeder should be of the closed-loop type to ensure an accurate feed rate and should be capable of being commanded remotely in accordance with preprogrammed conditions. Another desirable feature is an instant on-of f isolation attachment. This device allows a powder feeder to be pressurized to its operational pressure with inert gas before use, allowing instantaneous operational gas flow and ensuring that the powder is not exposed to atmosphere and humidity. While the plasma gun is running without powder, gas flows through a bypass circuit to the gun for cooling the powder tube. When powder is called for, the bypasss is switched, the powder pinch valve opens and gas flows through the powder cannister to carry the powder to the gun.

Some systems require not only that the critical parameters be controlled but


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also that many additional data items be monitored and recorded for process control and a quality control historical file. In addition, the plasma control system must be capable of communicating with other controllers such as the motion controller or the overall system computer. Typically, the plasma console should (i) control all critical parameters of plasma, (ii) store preprogrammed plasma conditions, (iii) monitor, store and print data as required, (iv) provide automatic and manual operations of the plasma head and (v) provide safety interlocks for plasma operation.

We have discussed briefly the automating of the plasma function. However, the various motions and in some instances peripheral functions must also be controlled. Early efforts to automate motions used standard d.c. motors and relay logic systems to sequence events and to turn on and offvarious valves, motors and other devices. These systems were hard wired and lacked the flexibility to allow variations in plasma and motion control functions.


Microprocessors were incorporated into the system which allow sequences to be preprogrammed and changes to be made without wiring changes. Most processors have internal clocks so that timed sequences are possible and coordinated activities can be accomplished. Motion control, however, was still limited to fixed velocity and somewhat cumbersome fixed limits. Feedback loops need to be provided to supply position information of an axis via potentiometers or other devices which can allow velocity changes as a function of position.

Continuous change requires an analog output to control the motor speed with a vai'iable rate. Some modern processors have both discrete and analog capability. However, only recently have programmable logic controllers (PLCs) added axis control capability to their other functions. Computer numerical control (CNC) machines, in contrast, have excellent motion control capability, but since they are designed for machining operations they do not have the flexibility of more general types of controllers. One major advantage of a CNC machine is that little or no initial software is required. However, they are more limited in their ability to communicate with other computers and to perform data collection and supervision functions. A general purpose computer, in contrast, has the ability to do all these things but requires a great deal of initial software. The application best determines which approach is to be preferred. Since most of these automated systems are unique or custom, each requires individual consideration of the objectives to be met, i.e. the number and kinds of equipment to be interfaced with, the number of axes to be controlled, what kind of motion and how many analog and digital inputs and outputs are to be dealt with.

4. INCREASED PRODUCTIVITY

Modern production systems use automation to increase productivity. This results in reduced set-up times, improved part handling and optimization of system operation. Robotics is the term generally given to a device that simulates a man's motions. The use of a robot to hold parts or the plasma gun accomplishes the safety and process consistency aspects of automation as well as providing a tireless worker that does not drink coffee or play cards. The robot can be used to configure a flexible system capable of spraying a variety of geometries with a minimum of set-up time. Different part programs can be stored and recalled as required. When coupled with different part motion mechanisms, as many as nine axes of coordinated motions are possible.

This approach has been used to automate an atmospheric plasma spray system for spraying a variety of geometric shapes. A single plasma gun can be coupled with multiple powder feeders so that a specific powder can be selected along with the appropriate plasma condition for a desired powder and coating requirement. Selection of the powder and plasma conditions are made from the part program without operator intervention. The part motions are controlled by the robot controller for either a two-axis turntable or a three-axis sting, depending on what is to be sprayed.

The same basic concept can be dedicated to a specific family of parts such as turbine blade airfoils and platforms as shown in Fig. 4. This facility uses a robot to provide the gun motions coupled with a part motion device with two axes of motion.

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The coordinated variable rotation and flip motions are specially designed for turbine blades and vanes. An automatic device loads and unloads parts outside the spray chamber while the part inside is being sprayed. As such parts can be loaded and unloaded onto the carousel at random time intervals, the operator is allowed more freedom to accomplish additional tasks.

A recently designed fully automated low pressure plasma system was built to coat turbine blades and vanes. Mechanically, the machine has been constructed of a stainless steel spray chamber with four transfer and preheat chambers where parts are automatically loaded and heated in a vacuum before entering the main spray chamber (Fig. 5). The part motion sting provides the translation and rotation of parts while each gun has four axes of motion. Parts can be processed in a variety of sequences depending on their relative preheat and coating requirements.

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Fig. 5. Automated low pressure plasma system.

One example of a sequence for a turbine blade is shown in Fig. 6. It can be seen that many events are being accomplished simultaneously with the result that four parts are completed every 288 s for a throughput of 50 parts per hour. This system uses three different controllers for different functions. The overall system host or supervisor computer is a Digital Equipment Corporat ion (DEC) 1123 plus with dual hard disc drives, a VT 100 operator terminal and a remote touch-sensitive Vuepoint operator interface terminal for manual operation. The 16 axes of motion are controlled by two General Electric (GE) series six PLCs with axis modules added. Two plasma guns and the automatic loaders are controlled by two Fluke model 2400 A minicomputers. The numers of input and output signals are as shown in Table I.

Many of the inputs are processed simply to allow coordination of the tasks to be performed; however, a great number of these inputs such as gas pressures, various


TIME SECONDS 50 IOO 150 200 250 288

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LOADING CYCLE

TRANSLATION '1N'

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CLEAN ~ COAT

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Fig. 6. Sequence chart for a turbine blade.

TABLE I INPUT AND OUTPUT SIGNALS FOR A TURBINE BLADE SEQUENCE

Type Number

Analog input 44 Discrete input 123 Analog output 12 Discrete output 108

temperatures and flows are categorized as alarms or warnings and trigger specific actions if they exceed set limits. This information can also be used to aid the operator in trouble shooting a problem.

The software package includes a diagnostic feature for both maintenance and trouble shooting. The machine was also designed to allow one half to operate while the other half is down for maintenance, thus maximizing the machine up-time. Although the communication between the controllers was a formidable task, the system was configured to allow each its area of expertise. The GE device was selected to provide independent axis control, the Fluke microprocessor is primarily a process controller and data monitor while the DEC 1123 provides overall system supervision sequencing and mass storage for programs and historical data. The motion control uses single resolvers for position feedback and tachometers for velocity feedback (Fig. 7).

The demand for accuracy in a plasma spray operation is generally less than in a machining operation but, when parts are being mechanically transferred, a reasonable degree of accuracy is still required. For example, the sting motion is driven by a lead screw with 0.5 in pitch. The drive motor is directly coupled with the lead screw and is coupled to the resolver with a 2:1 gear ratio, so the motor turns two


revolutions for one revolution of the resolver and 1 in travel of the lead screw. The resolver has 1000 counts per revolution so the resolution of this axis is 0.001 in. The relationship between resolution accuracy and repeatability for the GE series six is described below 1.

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The resolution is the smallest unit of distance detected by the position control system. The resolution is the distance represented by one count.

The accuracy is the tolerance to which the controlled machine element can be positioned. The accuracy is affected by (i) the stability of the electronic circuits in the axis positioning module and servo amplifier, (ii) the blacklash and errors in the machine lead screw or gearing system and (iii) the deflections of machine elements under load. The accuracy can never be better than the system resolution. In positioning systems, the accuracy is usually several times worse than the system resolution.

The repeatability is the tolerance to which the controlled machine element can be repeatedly positioned to the same point in its travel. The repeatability is generally worse than the system resolution but somewhat better than the system accuracy.

As can be seen, removing the operator from the direct involvement in spraying a part has resulted in many improvements in the process, i.e. improved safety, more process consistency (and thus better quality) and increased productivity, resulting in lower coating costs per unit. However, it has demanded a great deal more in the way of equipment costs and improved technology. Newer systems will demand in some instances greater sophistication and, at the same time, simplification. Reliability of equipment is becoming increasingly important as equipment costs rise. Control systems will require standardization. More large companies are demanding that control systems are compatible with their existing "mother computers". The urge to standardize will consistently be challenged by the introduction of innovative new controllers.

REFERENCE

1 Axis positioning module manual for the series six programmable controller, Manual GEK-25368, General Electric Co., February 1983, pp. 3 14.


APPENDIX A

Gas flow through a nozzle or orifice can be described in fluid dynamics as isentropic flow. This is considered to be a frictionless adiabatic flow "with no heat transfer since the short distances travelled and changes that a particle undergo are slow enough to keep velocity and temperature gradients small ''A1.

The general expression for mass flow through an orifice for subsonic flow is

where A is the area of the orifice, Po is the absolute upstream pressure and K = %/cv is the specific heat ratio. Let Pt denote the throat pressure. When

Po = \ K + 1j

or flow through the orifice is critical or sonic, then eqn. (A1) reduces to

mmax - - To 1/2 (R-\K~I J (A2)

where T O is the absolute upstream temperature and R is the gas constant. In the case where K = 1.40 (for air), eqn. (A2) reduces to

Jh = 0.686 AP° ( R To) 1/2

This means that the "mass varies linearly with A and inversely as the square root of absolute temperature ''A2. Thus it can be seen that the condition for critical flow which results in eqn. (A2) is

P1 \ K + 1 ]

where P2 ---- Pt is the downstream pressure and P1 = Po is the upstream pressure. The following cases have been calculated to determine the critical pressure

ratios for the most common plasma arc gases. For helium and argon,

K = 1.665

P2 = 0.487 P1

o r

P1 - - = 2.052

For hydrogen,

K = 1.405

For nitrogen,

K = 1.399


or

Pz 0.527 P1

P ll = 1.89 P2

Referencesjor Appendix A A1 H.W. Liepmann and A. Roshko, Elements of Gas Dynamics, Wiley, New York, 1957, p. 51. A2 V. Streeter and E. B. Wylie, FluidMeehanics, McGraw-Hill, New York, 1957, p. 274.

Documents

Historical review and update to the state of the art of automation for plasma coating processes