American Institute of Aeronautics and Astronautics

1

Control and Monitoring System for an Inductively

Coupled Plasma Torch System

Joseph R. Sargent* and Randolph D. Friend

†

Norwich University, Northfield, Vermont, 05663, USA

A 30 kW inductively coupled plasma torch (ICP) facility been has recently developed at

the University of Vermont to conduct research in the testing of new materials to be used for

the thermal protection systems for the next generation of space exploration vehicles. The

objective of this work was to develop a PC-based control and monitoring system that would

automate control system inputs and data collection from outputs of the ICP torch. The

control software selected was National Instrument’s Laboratory Virtual Instrumentation

Engineering Workbench, LabVIEW. A control and monitoring system was designed that

was able to meet the following requirements: (1) Adjust and record the flow rate of up to five

mass flow rate controllers during startup and operation of the torch. (2) Use a bypass valve

to control and maintain pressure between a specified range at the chamber exhaust. (3)

Measure and record the cooling water temperature (seven separate thermocouples) and

having an exhaust temperature warning system for the user when upper or lower limits are

reached (4) Incorporate shielding to protect all system components and wiring from

electromagnetic interference produced by the 30 kilowatt power supply. The control and

monitoring system improves the operation of the plasma torch facility, allowing a single

operator to control and monitor the system from a computer interface.

Nomenclature

CO2 = carbon dioxide

DAQ = data acquisition

EMI = electromagnetic interference

GUI = graphical user interface

ICP = inductively coupled plasma

K = Kelvin

kW = kilowatt

lpm = liters per minute

MHz = megahertz

NI = National Instruments

PID = proportional integral derivative controller

sccm = standard cubic centimeters per minute

TC = thermocouple

V = volts

I. Introduction

n important consideration for the design of next generation space exploration vehicles is the thermal protection

system to protect the vehicle during entry into planetary atmospheres and re-entry into earth’s atmosphere.

Inductively coupled plasma (ICP) torch facilities are important in the aerospace industry to conduct research in the

development of new materials to be used for new thermal protection systems. The ICP torch simulates the extreme

conditions that a vehicle will experience when going through the earth’s or another planet’s atmosphere. By using a

mixture of different gases, atmospheric conditions for specific planets can be simulated. In anticipation of future

Mars exploration missions, the ICP torch facility at the University of Vermont (UVM) was designed specifically to

be able to simulate atmospheric conditions for space vehicles entering the Mars atmosphere.1

* Student, Mechanical Engineering Department, 158 Harmon Dr. Northfield, VT 05663, Student.

† Associate Professor, Mechanical Engineering Department, 158 Harmon Dr. Northfield, VT 05663, AIAA member.

A

50th AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition09 - 12 January 2012, Nashville, Tennessee

AIAA 2012-0597

Copyright © 2012 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

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A major goal of the ICP torch facility at UVM is to provide a test bed for the fundamental study of

aerothermodynamic gas/surface interactions specific to ablative materials used on thermal protection systems. A

current research focus is the interaction of pyrolysis gases (gases released from ablation products) with the hot

plasma gases in the boundary layer over the material. Uhl et al. have used the facility to perform pyrolysis

simulation studies through the use of a porous graphite plug to simulate pyrolysis gases released from ablative

materials.2 The facility was designed to be able to use emission- and laser-spectroscopic techniques to investigate

pyrolysis gas chemistry. Meyers et al. have developed a diode laser absorption spectroscopy sensor to measure CO2

concentrations near the sample surface to investigate whether or not appreciable amounts of CO2 recombine at the

surface of Martian entry vehicles.3,4

Dougherty et al. have used the facility to investigate surface catalyzed reactions

in the Mars atmosphere.5 Preliminary results suggest that oxygen atom recombination reactions are occurring,

which could lead to improved models for designing thermal protection systems. Furthermore, Lutz et al. have used

the facility to measure the carbon nitridation reaction rate that occurs when atomic nitrogen extracts solid carbon

from the surface of the heat shield. Results from this investigation are leading to a better understanding of the

required thickness for ablative heat shields.

Although the facility is being actively used to conduct research and experiments, most of the control and

monitoring of the facility in its first few years of operation has been by manual means. A need exists to introduce an

automated control and monitoring system during operation and testing. The objective of this work was to develop a

PC-based control and monitoring system that would automate control system inputs and data collection from outputs

of the ICP torch facility. A description of the ICP facility is presented followed by the details of the design of the

control system. The results from testing the hardware and software components of the control system are also

presented.

II. Facility Description

A schematic of the ICP facility at the University of Vermont is given in Fig. 1. The major components include

the test chamber, the coil/injector-block assembly, and the radio frequency power supply. The three subsystems that

control the operation of the ICP torch include the gas feed system, the cooling system and the exhaust system. In

order to meet the objectives of this work, additional components were added to the subsystems as shown the boxes

in Fig. 1. This paper addresses how these additional components and the addition of control software provide an

automated control system that did not exist prior to this work.

Figure 1. Overview of the Inductively Couple Plasma Torch System1

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Details of the facility design and operation can be found in Ref. 1, and a brief summary is presented as follows.

The power supply used at the facility is an induction-heating generator that delivers up to 30 kW of radio frequency

energy in a frequency range between 2.5 MHz to 5.0 MHz. The plasma is created in the induction region of the torch

and the components include the coil, the confinement tube, and the gas injection system. The plasma is generated

through an induced radio frequency magnetic field created by a load coil made of copper with five or six turns. The

confinement tube is made of quartz and is rated for a melting temperature of 2000 K. The injector block distributes

the gas, inserting it into the confinement tube. In order to avoid overheating, the injector block assembly is made

completely of brass.

The stainless steel test chamber includes four viewports to provide optical access for laser induced fluorescence

and emission detection while additional ports allow for instrumentation and sample access. A pyrometer is attached

through one of these viewports for measuring the sample surface temperature. The plasma enters the chamber

through a bottom base plate which houses the quartz tube adapter and also contains two angled viewports. Exhaust

gases exit the chamber through a stainless steel, conical shaped upper cap which constricts the flow through a heat

exchanger to promote cooling.

The torch is cooled with a closed loop water system for cooling the injector block, the quartz tube adapter, the

base plate, the calorimeters and both heat exchangers. Two pumps circulate the water through the loops. A rooftop

chiller is used to provide cooling for the power supply and water heat exchanger. Flow that exits the vacuum

chamber is ventilated to the roof by a vacuum pump. An attached secondary water cooling system is contained in the

room with a circulation holding tank. This secondary system cools the backside of the samples, prevents the holders

from melting, and cools the chamber exhaust gases.

The gas enters the vacuum chamber from secured tanks containing argon, nitrogen, carbon dioxide, and air. Each

tank has a regulator and a shut off valve. The feed lines contain mass flow meters/controllers to regulate gas

mixtures appropriately. Two gate valves control the gas flow rate.

III. Operational Requirements

When the ICP torch facility was initially built, most of the operation, controls, and system monitoring required

manual procedures. For example, startup and shutdown procedures involved monitoring and adjusting four manual

valves simultaneously by a three person team. Not only were the manual methods time consuming and labor

intensive, they often limited the ability to simultaneously control multiple parameters. Being able to simultaneously

control and monitor multiple mass flow rate controllers is an important requirement for obtaining specific ratios of

gas mixtures. When manually operated, the flow rate from only two different gases can be controlled

simultaneously. Furthermore, manual controls introduced errors that could adversely affect the operation of the

torch. This project focused on improving the overall efficiency and reliability of the system by replacing the existing

manual control and monitoring methods with a more precise and automated PC-based control system.

Based upon input from the users of the facility, the specific requirements for controls and automation were

outlined. They were: (1) Adjust and record the flow rate of up to five mass flow rate controllers during operation of

the torch. (2) Use a bypass valve to control and maintain pressure between 80 and 120 torr (typically 100 +/- 1.0

torr) at the chamber exhaust. (3) Measure and record the cooling water temperature (seven separate thermocouples),

the sample surface temperature from the pyrometer, and the exhaust temperature warning the user when upper or

lower limits are reached. (4) Incorporate shielding to protect all system components and wiring from

electromagnetic interference produced by the 30 kW power supply.

IV. Design

The design of the embedded control system was divided into five major areas that include data acquisition, the

bypass system, mass flow control, temperature monitoring, and electromagnetic shielding. The data acquisition

system (DAQ) consists of a PC computer, sensors or actuators, and a data acquisition device to interface between

the sensors and computer. A schematic of the DAQ system is given in Fig. 2.

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Arrows pointing away from the DAQ device toward actuators require controlled inputs. Arrows pointing from

sensors toward the DAQ device represent required data to be measured and monitored (outputs from the system).

Based upon the requirements presented in section III, the data acquisition system needed to be capable of monitoring

18 data points and controlling six actuators. The following sections detail the design of each of the components of

the control system.

A. Data Acquisition

Selection of a data acquisition device was based on the inputs and outputs desired for the system. Eight

thermocouple signals were required to measure the temperature from different cooling locations in the system. Five

0-5V digital outputs turned on and off the mass flow rate controllers. A total of seven analog inputs were required

including one to measure the pyrometer converted voltage, one for the pressure transducer and five mass flow rate

signals. Finally, six analog outputs including five mass flow rate set points and one proportional control valve signal

were required to complete the input/output of the control and monitoring system.

Laboratory Virtual Instrumentation Engineering Workbench, also known as LabVIEW is a visual programming

language by National Instruments that allows engineers and scientists to control various systems and measurements.

LabVIEW allows the programmer to create a user-friendly interface for controlling, monitoring and recording data.

The graphical user interface (GUI) allows the user to operate the system without having to understand the

background program in great detail. Due to its ease of use and flexibility in designing automated data acquisition

and control systems, LabVIEW was selected as the application software for the system.

Figure 2. Schematic of the Data Acquisition System (DAQ)

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National Instruments’ 4-slot CompactDAQ USB Chassis was selected because it has more than 50 hot-

swappable I/O modules and is designed to interface with LabVIEW. The NI-9403 (Digital I/O) provides 32 digital

input and output channels and is used for the 5V on-off switches of the mass flow rate controllers. The NI-9213

provides 16 thermocouple channels with a spring terminal. The NI-9201 provides eight analog inputs for reading the

mass flow rates, pyrometer and pressure transducers, and the NI-9264 provides 16 analog outputs for controlling the

five mass flow rate controllers and the proportional control valve.

B. Bypass System The development of a new bypass pipe system was required to automate the pressure control inside the

chamber and increase in the steady-state stability and performance. The new bypass system did not replace the

existing system, and can be easily disabled to operate on the preexisting manual controls. The bypass pipe system

design is shown in Fig. 3. The design was customized to connect to the exhaust of the torch chamber. Two reducing

tees to go from 1-inch to ¾-inch piping were attached to the main exhaust manifold. A type E thermocouple was

inserted to measure the midstream exhaust temperature

with a ¼-inch OD SS304 pipe connecting the ¼ turn

manual shutoff valve to the proportional control valve. To

maintain uniformity, the piping components were

obtained from MDC Vacuum, and included the Kwik

Flange System®. The assembly of the system proved to

be quite simple. The only complicated task was the

insertion of the O-ring between the flanges and clasping

them together.

The inline proportional control valve was calibrated

to 125 liters per minute at 100 torr with a 0-5V input

amplifier board purchased from Kelly Pneumatics

Pressure & Flow Control Solutions. The 0-5V input to the

amplifier board from the DAQ was scaled up to 0-12V

and supplied the 0.5 amp maximum required current.

That output was then sent through a protected cable to the

proportional flow valve. A Proportional-Integral-

Derivative (PID) controller adjusted the valve position

automatically with fluctuating pressures from the pressure

transducer in the chamber. The valve opened with the

increase of pressure from an increase in gas flows or

temperature and vice versa to stabilize the internal

chamber pressure.

C. Mass Flow Rate Controllers

Mass flow controllers had previously been manually controlled with regulators and valves and monitored

through a digital display. Automation of the controllers allows for a precise mixture of the gases and can be

monitored through the LabVIEW GUI. The five pre-calibrated mass flow rate controllers (one 100 lpm, one 1000

sccm, two 50 lpm, and a 2000 sccm) were manufactured by MKS Instruments. They are controlled by a Type 246C

single channel and a Type 247D four channel controller, power supply, and readout. The controllers included a 9-pin

DSub and a 25-pin DSub interface connection to communicate with the DAQ. The connection allows access to

scaled outputs from the mass flow controllers, flow on and off switches, and a set point input for remote control. The

cables that connected the two devices were two 50 ft shielded, twisted and grounded cables. Inside the faraday box

included interface pins, internal outputs, on and off digital switches, and the set points wired to the DAQ.

The scaled mass flow rate controller signal was displayed in the GUI. PID controllers smoothed the adjustment

of the change from the current flow rate to the user input. This was critical in maintaining stability of the plasma as

it was being created.

D. Temperature Monitoring

Thermocouples were needed to measure the different secondary water coolant temperatures throughout the

system. Seven Low Noise Type E 0.125” dia. sheath ungrounded Omega water probe thermocouples and a four-inch

exhaust temperature probe were utilized for monitoring temperatures throughout the system. The ungrounded

junction reduces the response time of the temperature readings, but the sheath is grounded to reduce noise.

Figure 3. Final design of bypass system.

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Interference holes were drilled into the plastic water coolant tubing and the probes were inserted to read the

midstream temperature. Thermocouple extension wire cut into 50 ft lengths provided a grounded, rigid, shielded,

and twisted wire from the thermocouples to the DAQ. The output from the pyrometer was connected to a 246 Ω

resistor so that the DAQ could measure the voltage and the LabVIEW GUI displayed the scaled surface temperature.

All temperature signals were displayed in the GUI and an upper and lower temperature could be set for each

thermocouple to monitor the temperature. Warning lights were placed in the GUI to allow the user to clearly see

when the temperature was outside of the acceptable range.

E. Electromagnetic Shielding

Many of the components of the control system were electrical, and shielding of these components was very

important because of the electromagnetic interference from 30 kW power source. The critical components to be

shielded were the DAQ and proportional flow valve. Shielding was accomplished by enclosing these components in

1/8-inch steel faraday boxes (Figs. 4 and 5). A basic principle followed was that the more current/voltage you have

running through a given wire, the less interference it could potentially receive. In designing the faraday boxes, all

connections allowed for quick wire removal and were accessible for future modifications. All wires were grounded,

shielded, and twisted to maximize the reduction of noise. Internally, the DAQ included a differential amplifier to

condition data from the thermocouples.

F. Faraday Boxes

The DAQ faraday box included a 10 amp fuse with a power switch, two 120 millimeter computer fans, and

interface pins for the major components. Internally, the power and USB cables were attached to the box. The power

ran through the fuse to the power switch and connected to a double gang outlet unit. Two of the outlets were used to

power the fans, one was for the DAQ and the other outlet was for the amplification board for the proportional flow

valve. All outlets, except for the outlet for the DAQ have 120V to 12V adapters reducing the voltage. The DAQ was

attached with Velcro to the middle of the faraday box, and was plugged into the power outlet. It was also connected

to a USB extension. The thermocouple wires were fed through the side of the box and wired directly to the DAQ.

The pressure transducer and both mass flow rate controllers interfaced through the connection pins with twisted,

shielded and grounded wires, and could easily be removed while leaving the connections to the box intact. The

proportional flow valve’s signal originated from the DAQ to the amplification board. The amplified signal (0-12V)

was sent through a coax connection with twisted, shielded, and grounded wire. The signal from the pyrometer was

also sent through protected wire and a 246 Ω resistor. The voltage across it was measured by the DAQ. The

openings of the fans were shielded with fine mesh to reduce EMI while allowing airflow.

The faraday box for the proportional control valve can be disassembled into three parts. The proportional flow

valve was mounted to the bottom and spaced with washers for access to the inlet and outlet pipes. The top is hinged

and latched to the sides of the box for access to the internal wiring for the valve to the interface connection pins. The

top assembly, which contains slots for the pipe, is hinged to the bottom section and can be completely removed.

Figure 4. DAQ Faraday Box

Figure 5. Valve Faraday Box.

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V. Testing

Testing was conducted on site at the ICP torch facility at the University of Vermont (UVM). All of the new

systems were implemented and tested individually. The LabVIEW program was written and verified with the DAQ

before being implemented into the system. Once implemented, it was confirmed that it operated successfully. The

bypass valve system was constructed on site at UVM and then tested in place. The proportional flow valve was

checked prior to installation for proper calibration. The mass flow rate controls were part of the LabVIEW program,

and were simulated before integration. Once the program worked, it was connected to the controllers and tested. All

of the thermocouples were checked using ice water and a flame in order to confirm that they worked before placing

them in their final locations within the torch system for further verification. The faraday boxes were mounted in

their optimal locations and tested for shielding against the electromagnetic interference from the power supply.

The LabVIEW program was able to collect all of the required data, display it in the GUI, and record data to an

Excel spreadsheet. The Excel spreadsheet also included facility information, user notes and a timestamp of data

collected over time. PID was incorporated into the program, and was tested using a voltmeter to verify analog input

and output data. The analog output was set and changed by the user, and operated correctly when the voltage was

adjusted to the proper set point. It is assumed that the program users understand common PID issues, both at initial

and at steady state conditions.

The bypass valve system, which adjusted the chamber pressure, was tested at the facility and maintained steady

state pressure to +/- 0.1 torr, surpassing our goal of 100 torr +/- 1.0 torr. The proportional flow valve did not

oscillate rapidly enough causing increased wear and failure. The ¼-inch pipe in the bypass system could not sustain

the combined flow rates, requiring the main manual shutoff valve to be kept slightly open. The manual valve had to

be adjusted when a major difference in mass flow rates occurred. The system could be completely disabled instantly

by shutting off the ¼ turn valve or by hitting the stop button for the bypass system since the valve is normally

closed.

The mass flow rate interface system testing was a success. The pressure transducer results are given in Appendix

A. Throughout the test, the pressure output matched the digital display from the transducer. An anomaly in the

signal was observed as the torch was turned off. The MDC controllers were switched to remote control. The scales

of the outputs and inputs were modified from their respective ranges to 0 to 5 volts. A digital signal was sent to turn

on each individual mass flow controller, and then a different analog input set point signal was used to set the flow

rate. The flow rate was verified using the display settings on the MDC controllers and the output displayed in the

LabVIEW GUI. Each mass flow rate controller successfully output the flow rate and was able to be adjusted by the

PID controller based upon the user inputs. The flow rate controllers were able to be turned on and off remotely from

the LabVIEW GUI.

The pyrometer current output calibration curve is shown in Appendix B. The optical pyrometer measures the

visible light of a very hot object. These results were obtained using a butane flame for the heat source, aiming the

laser at the flame with an arbitrary background. The input emissivity of the object was not necessary to determine

the actual temperature, but the correlation between the displayed temperature and the current output from the

pyrometer is required. Data points were collected for both Fahrenheit and Celsius scales and plotted on the same

graph. It was assumed in our program that the output was the same for both scales. The pyrometer output was

connected to a resistor so that the DAQ could measure the corresponding voltage.

The range of the thermocouples was verified using ice water and a butane flame. These temperatures exceeded

the water coolant temperatures at the facility. All of the grounded thermocouples successfully output a scaled

voltage. The DAQ module 9213 included a differential amplifier to reduce noise.

All of the pre-calibrated thermocouples produced data which will be analyzed in further calculations of the

overall torch efficiency. Data for different water coolant temperatures are shown in Appendix C. The torch was

initially turned on and a steady increase in the flow of argon was applied. Thermocouple (TC) 5 was located inside

of the brass chamber where the plasma is produced and the temperature continually increased throughout the test.

TCs 1 and 6 were located in the chamber and in the exhaust manifold, respectively. When probes 1 and 2 were

individually entered into the flame, the temperature of TCs 4 and 7 increased while temperatures of TCs 1 and 6

decreased. The exhaust temperature (Exhaust) remained reasonably constant while the water coolant tank

temperature (TC 2) increased slightly over time. This test successfully showed that all thermocouples located in

various locations responded properly with the changing conditions.

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VI. Conclusions

The control system was integrated effectively into the existing ICP facility at UVM. Each component was

installed and tested individually, and then the entire system was tested and operated. The design of the embedded

control system was a success, accurately collecting data and exporting it to an Excel worksheet for analysis.

The bypass system helped automate the pressure inside the chamber leading to an increase in steady-state

stability and performance. The program was successful in controlling and monitoring the mass flow controllers. The

temperature monitoring system, composed of the thermocouples and pyrometer were verified to produce and record

correct outputs. The electromagnetic shielding canceled out the interference created by the power supply.

The additions to the torch assembly at UVM were all successful improvements. The automation and control

system is a major step in the development of the ICP torch facility providing an increase in the amount of data that

can be collected and monitored.

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Appendix

Ap

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dix

A. P

ress

ure

Tra

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uce

r R

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Ap

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B.

Py

rom

eter

Res

ult

s

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Ap

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C.

Th

erm

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le R

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Acknowledgments I. Caird and G. Ma as senior project partners.

W. Owens, J. Meyers, and D. Fletcher for collaboration

between Norwich University and the University of Vermont.

Lee Hill from Silent Solutions for EMI shielding design.

This work has been supported by NASA Grant NNX07AT56A.

References 1Owens, W. P., Uhl, J., Dougherty, M., Lutz, A., Meyers, J., and Fletcher, D. G., "Design of a 30kW Inductively Couple

Plasma Torch for Aerospace Material Testing", AIAA 2010-4322, 10th AIAA/ASME Joint Thermophysics and Heat Transfer

Conference, Chicago, IL, June 2010. 2 Uhl, J., Owens, W. P., Dougherty, M. J., Lutz, A. J., Meyers, J .M., and Fletcher, D. G., “Pyrolysis Simulation in an ICP

Torch Facility”, AIAA-2011-3618, 42nd AIAA Thermophysics Conference, Honolulu, Hawaii, June 2011.

3 Meyers, J. M., Owens, W. P., and Fletcher, D. G., “Near Surface CO2 Detection in an Inductively Coupled Plasma

Facility Using Diode Laser Absorption”, AIAA-2011-3788, 42nd AIAA Thermophysics Conference, Honolulu, Hawaii, June

2011. 4 Meyers, J. M., Owens, W. P., Dougherty, M., Lutz, A., Uhl, J., and Fletcher, D. G., "Laser Spectroscopic Investigation of

Surface-Catalyzed Reactions for Mars Exploration Vehicles", AIAA-2010-4915, 27th AIAA Aerodynamic Measurement

Technology and Ground Testing Conference, Chicago, IL, June 2010.

5 Dougherty, M., Owens, W., Meyers, J., and Fletcher, D., "Investigations of Surface-Catalyzed Recombination Reactions

in the Mars Atmosphere", AIAA-2011-1326, 49th AIAA Aerospace Sciences Meeting, Orlando, Florida, January 2011. 6 Lutz, A., Owens, W., Meyers, J., and Fletcher, D., “Investigation of CN Production from Carbon Materials in Nitrogen

Plasmas”, AIAA-2011-901, 49th AIAA Aerospace Sciences Meeting, Orlando, Florida, January 2011.

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