Gina M. Blaze-Dugala

Glenn Research Center, Cleveland, Ohio

Stirling Convertor Control for a Concept Rover

at NASA Glenn Research Center

NASA/TM—2009-215513

March 2009
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Gina M. Blaze-Dugala

Glenn Research Center, Cleveland, Ohio

Stirling Convertor Control for a Concept Rover

at NASA Glenn Research Center

NASA/TM—2009-215513

March 2009

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Prepared for the

Sixth International Energy Conversion Engineering Conference (IECEC)

sponsored by the American Institute of Aeronautics and Astronautics

Cleveland, Ohio, July 28–30, 2008
Acknowledgments

The work described in this paper was performed for the Science Mission Directorate (SMD) and

the Radioisotope Power System (RPS) Program, which provided funding for these projects.

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Trade names and trademarks are used in this report for identifi cation 

only. Their usage does not constitute an offi cial endorsement, 

either expressed or implied, by the National Aeronautics and 

Space Administration.

Level of Review: This material has been technically reviewed by technical management. 
NASA/TM—2009-215513 1

Stirling Convertor Control for a Concept Rover 

at NASA Glenn Research Center 

Gina M. Blaze-Dugala 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Abstract 

The U.S. Department of Energy (DOE), Lockheed Martin Space Systems Company (LMSSC), 

Sunpower Inc., and NASA Glenn Research Center (GRC) have been developing an Advanced Stirling 

Radioisotope Generator (ASRG) for potential use as an electric power system for space science missions. 

This generator would make use of the free-piston Stirling cycle to achieve higher conversion efficiency 

than currently used alternatives. NASA GRC initiated an experiment with an ASRG simulator to 

demonstrate the functionality of a Stirling convertor on a mobile application, such as a rover. The ASRG 

simulator made use of two Advanced Stirling Convertors to convert thermal energy from a heat source to 

electricity. The ASRG simulator was designed to incorporate a minimum amount of support equipment, 

allowing integration onto a rover powered directly by the convertors. Support equipment to provide 

control was designed including a linear AC regulator controller, constant power controller, and Li-ion 

battery charger controller. The ASRG simulator is controlled by a linear AC regulator controller. The 

rover is powered by both a Stirling convertor and Li-ion batteries. A constant power controller enables the 

Stirling convertor to maintain a constant power output when additional power is supplied by the Li-ion 

batteries. A Li-ion battery charger controller limits the charging current and cut off current of the 

batteries. This paper discusses the design, fabrication, and implementation of these three controllers. 

Nomenclature 

LMSSC Lockheed Martin Space Systems Company 

DOE Department of Energy 

ASC Advanced Stirling Convertor 

ASRG Advanced Stirling Radioisotope Generator 

NASA National Aeronautics and Space Administration 

GRC Glenn Research Center 

FTB Frequency Test Bed 

TDC Technology Demonstration Convertor 

FET Field-Effect Transistor 

PWM Pulse Width Modulator 

μF Micro Farad 

RMS Root Mean Square 

AH Amp Hour 

Hz Hertz 

VDC Volt, Direct Current 

VAC Volt, Alternating Current 

PID Proportional-Integral-Derivative 

IC Integrated Circuit 
NASA/TM—2009-215513 2

Introduction 

An Advanced Stirling Radioisotope Generator (ASRG) simulator was designed for demonstrating 

Stirling power conversion outside the laboratory environment. The setup is shown in Figure 1. Two 

Frequency Test Bed (FTB) Stirling convertors were mounted in dual-opposed configuration. FTBs are an 

early version of the Advanced Stirling Convertor (ASC) for component development and testing. Fins 

were placed on the heat rejection zone to allow for air cooling. The outer panels of the enclosure allow 

observation of the convertors inside. Two fans located on the top of the enclosure draw air in from the 

environment and discharge into the enclosure through the top panel. The cooling fins and fans were sized 

for operation in ambient air temperatures up to 43 °C. 

Thermal energy is supplied to each hot end by an array of cartridge heaters inserted into a nickel heat 

collector. The hot end and regenerator sections were insulated using Kaowool (Thermal Ceramics, Inc.) 

ceramic blanket. The inner insulation directs the air flow exiting the cooling fins along the radial direction. 

A helium management system made up of a pressure gauge, isolation valve and fill port allows for 

charge pressure adjustment. 

The ASRG simulator system was designed to require as little support equipment as possible, with the 

intended application being integration onto a rover. The ASRG simulator support equipment shown in 

Figure 2 includes a tuning capacitor bank, control electronics, and ground support equipment. The tuning 

capacitor bank provides power factor correction, and is connected in series with the alternator outputs. 

The control electronics include the linear AC controller, protection circuit, constant power circuit, and 

Li-ion battery charger. Power is supplied to the rover by one of the connectors on the front of the control 

electronics panel. The linear AC regulator, constant power circuit, and Li-ion battery charger will be the 

focus of this paper. The linear AC regulator controller parasitic loads are located on the sides of the 

container, and are capable of dissipating the full output power of the convertors while being air-cooled by 

natural convection. Any power not required by the rover is dissipated in the parasitic load. The protection 

circuit serves as a safety mechanism for the convertors by initiating shutdown during an over stroke of the 

piston. 

The ground support equipment is connected to the rover via a tether. It requires a connection to a 

120 VAC, 60 Hz source. It provides heater power and temperature control, cooling fan power, and 

centering and starting circuits. Proportional-Integral-Derivative (PID) controllers maintain the hot-end 

temperature set-points. If the upper temperature limit is exceeded on either heater, power is removed from 

the heaters by means of an alarm relay integrated into each PID controller. The centering and starting 

circuits are used to center the pistons in the FTB convertors before operation. 

Figure 1.—ASRG Simulator Setup. 
NASA/TM—2009-215513 3

Figure 2.—Ground Support Equipment. 

Li-Ion Battery

1 2

Linear AC 

Controller 

Constant Power 

 Circuit

Li-Ion Battery 

 Charger 20 VDC

 ASRG 

Simulator

User Load

Figure 3.—Rover Power System. 

Figure 3 shows a block diagram representation of the rover power system. The Li-ion batteries 

function to supply additional power to the rover when the rover power exceeds the power output of the 

Stirling convertors, 130 W. The change in power is detected by a diode in the system that becomes 

forward biased when the Li-ion battery voltage is greater than the constant power circuit voltage. The 

constant power circuit also functions to enable the Stirling convertors to continue to maintain full power 

while the Li-ion batteries supply additional power. 

The two FTBs in the ASRG simulator together supply 20 VDC, 6.5 A, and 130 W at a frequency of 

106 Hz. It should be noted the design and testing of the Stirling convertor controller was performed using 

a 24 VDC Stirling convertor output. Specifications received at a later time verified the output voltage to be 

20 VDC. 

Stable operation of a Stirling convertor is maintained by a controller. It regulates the alternating 

current produced by the linear alternator of the convertor, provides a specified output voltage, and 

maintains operation at a certain piston amplitude and hot-end temperature. The controller allows 

adjustment of the operating point; hot-end temperature or piston amplitude may be adjusted up or down. 

The piston amplitude is maintained by varying the load with the regulation stage of each controller. The 

load of the controller is sized to dissipate all the power being produced by the convertor. If the controller 

did not dissipate all the power, the excess would flow into the resonating piston motion, increasing 

amplitude, and ultimately resulting in damage to internal convertor components. Similarly, if the 

controller dissipated more power than that being produced, the balance would be extracted from the 

resonating piston motion, causing a stall of the piston. 

The goal of the Stirling convertor controller task was to design and build a small, efficient, and 

reliable controller for use on the ASRG simulator1. Six methods of control were considered; each method 

was analog and used tuning capacitors for passive power factor correction. No active power factor 
NASA/TM—2009-215513 4

correcting methods were investigated. The controller tuning capacitor provides stable convertor operation 

by placing the stator current in phase with piston velocity. Load current flows as a response to terminal 

voltage. When a tuning capacitor is used, terminal voltage is in phase with piston velocity. The phase 

discrepancy is caused by the inductance of the stator. It is balanced out by a capacitance to make a 

resistive circuit. In a resistive circuit, the voltage and current are in phase. One important consequence of 

this technique is the tuning capacitors ability to only cancel the effect of the alternator inductance at a 

single frequency. If the operating frequency deviates significantly from this value, the power factor 

correction will be reduced. 

Originally, each control concept was designed for Technology Demonstration Convertor (TDC) 

operation (Ref. 2). The designs required modification to accommodate the lower alternator voltage and 

higher current and power output of the FTB convertors. Each controller was designed for both single 

convertor and dual-opposed pair operation. The circuit simulation software PSpice (Cadence Design 

Systems, Inc.) was used to simulate operation of the FTB convertor pair on each of the controllers by 

creating a model of each controller. The linear alternator was modeled as an AC voltage source using its 

nominal values for resistance, inductance, frequency, and output voltage. This paper discusses the 

controller that was selected from this process (Ref. 1). 

Linear AC Regulator Controller 

A schematic of the linear AC regulator controller can be viewed in Figure 4. The rectified alternator 

AC voltage is sensed by a voltage divider connected to an operational amplifier. The AC voltage is passed 

through a separate diode bridge with no energy storage capacitor that would convert it to DC. The op-amp 

generates a voltage proportional to the difference between the divided voltage and the set point. This 

output is used to drive Field Effect Transistors (FET) in their linear range, rather than discrete on-off 

states. The FETs connect load resistors onto the DC bus, but also dissipate power themselves. The user 

may change piston amplitude by adjusting the voltage divider that controls the input to the operational 

amplifier. The linear AC regulator was designed to have continuously variable loads, which avoid the 

ON/OFF behavior of a zener diode controller (Ref. 1), but retains multiple redundant load stages. 

The linear AC regulator was designed to exhibit voltage stability, require few components, and have 

high efficiency. A disadvantage of the linear AC regulator is its use of transistors for power dissipation. 

The reliability of these transistors is not a major concern because the power dissipation is below the derated value. 

10uF

8k

0

0

0

0

0

0

VCC

1 2

VRef

+ 3

- 2

V+ 8

V- 4

OUT 1

0

VCC

499

499

499

100

Out

15000uF

0 4 4 4 4

0

Tuning Capacitor

Tuning Capacitor

50K

MC7812C

IN 1 OUT 2 GND 3

1k

10k

1meg

1 2

0

499

1nF

0

100

0.1nF

1uF

0

Figure 4.—Linear AC Regulator Controller Schematic. 
NASA/TM—2009-215513 5

Linear AC Regulator Bench Testing 

The purpose of the bench test was to verify the assembly of the controller and the PSpice simulations 

used to design the controller. 

Procedure 

Figure 5 shows the bench test set up of the linear AC regulator controller. 

A resistor placed in series with a DC power supply produced a current through the controller. The 

controller voltage potentiometer was set to produce a 24 VDC output. DC voltage was used during the first 

test to avoid serious damage if a short was created in the circuit during assembly. The current meter on 

the DC power supply aided in this determination by indicating a spike in current as the input voltage 

increased. A significant increase in current indicated that a short was present in the circuit. Similarly, if 

the current increased slightly; the op-amp was regulating and assembly of the controller was successful. 

An AC voltage test followed this test. 

A resistor placed in series with an AC voltage source produced a current through the controller. This 

test more accurately represents the operation of the controller. The input voltage, current, and power and 

DC output voltage were monitored with meters while the regulation stages of the op-amp were monitored 

on an oscilloscope. 

Results 

Figure 6 displays the op-amp stage data collected during the AC test. The results of this test verify the 

ability of the op-amp to regulate at the desired 5 VDC. The limitations of the oscilloscope did not allow all 

four stages to be displayed simultaneously. All four stages were observed and each had the same 

characteristics as those displayed in Figure 6. The op-amp stages obtained from the PSpice simulation as 

shown in Figure 7 match the experimental results. 

Figure 8 displays an op-amp regulation stage and the controller DC output voltage. The results of this 

test verify the ability of the controller to maintain the output voltage at 24 VDC. The output voltage 

obtained from the PSpice simulation as shown in Figure 9 matches the experimental results. 

Conclusion 

The bench test verified the controller assembly and operation. In addition, it verified PSpice as an 

accurate tool for controller design.

Figure 5.—Linear AC Regulator Bench Testing Setup. 
NASA/TM—2009-215513 6

 Time

250ms 300ms 350ms 400ms 450ms 500ms

V(M1:g) V(M2:g) V(M3:g) V(M4:g)

-20V

-10V

0V

10V

20V

Figure 6.—Op-Amp Stages: Bench Testing. Figure 7.—Op-Amp Stages: PSpice Simulation.

 Time

250ms 300ms 350ms 400ms 450ms 500ms

V(U13A:OUT) V(OUT)

-5V

0V

5V

10V

15V

20V

25V

Figure 8.—DC Output Voltage: Bench Testing. Figure 9.—DC Output Voltage: PSpice Simulation.

Linear AC Regulator Controlling a Pair of Convertors 

A pair of EE-35 convertors (Ref. 3) was controlled by the linear AC regulator. An EE-35 convertor is 

lower in power than the FTB convertors but provides the same output voltage. The purpose of controlling 

the EE-35 convertors with the linear AC regulator was to verify the operation of the controller and verify 

PSpice as an accurate tool for evaluating the performance of the controller with a Stirling convertor. 

Procedure 

The controller was connected to the linear alternator of the convertors. To start, the output voltage 

potentiometer was set to control the output voltage at 22 VDC and the hot-end temperature was increased 

to 300 °C. The EE-35 convertors were started on the AC bus and transferred to the linear AC regulator 

controller. The potentiometer was adjusted from its original setting to produce an amplitude of 3.75 mm. 

The temperature was increased to 400 and 470 °C in intervals of 5 min to allow for the convertors to 

reach steady state at each temperature. As the piston stroke increased with temperature, the potentiometer 

was adjusted to lower the stroke of the piston. A Yokogawa Scopecorder was used to capture waveforms 

of the power output, current, and DC output voltage of the EE-35 convertors. 
NASA/TM—2009-215513 7

Voltage SBIR EE-35-2 at 470°C

-50

0

50

0.00E+00 2.00E+01 4.00E+01 6.00E+01 8.00E+01 1.00E+02 1.20E+02 1.40E+02 1.60E+02 1.80E+02 2.00E+02

Time, Seconds

Volts

Results 

Table I shows the alternator voltage, alternator current, power output, and DC output voltage of the 

convertors. The data was collected at three operating temperatures: 300, 400, and 470 °C. The parameters 

in Table I were used to create a model in PSpice. This model represented the operation of the controller at 

each temperature; PSpice does not have a temperature parameter. Waveforms produced in PSpice were 

compared with those captured by the Yokogawa. This paper discusses the operation of the convertors at 

470 °C. 

TABLE I.—EE-35 CONVERTOR CONTROLLED WITH THE LINEAR AC REGULATOR 

Temperature, 

°C 

VAC,

V 

I, 

A 

Pout, 

W 

VDC, 

V 

300 24.25 0.66 11.99 25.78 

400 24.56 1.16 22.27 24.12 

470 24.3 1.42 27.41 23.24 

Figures 10 and 11 represent the alternator voltage data recorded during the operation of the 

EE-35 convertors with the linear AC regulator. 

Figure 12 represents the PSpice simulation of the alternator voltage with convertor operation at 

470 °C. Parameters in PSpice were adjusted to meet the power output, current input, and DC output 

voltage obtained during convertor operation. These parameters include the potentiometer and open circuit 

voltage. 

Voltage SBIR EE-35-1 at 470°C

-50

0

50

0.00E+00 2.00E+01 4.00E+01 6.00E+01 8.00E+01 1.00E+02 1.20E+02 1.40E+02 1.60E+02 1.80E+02 2.00E+02

Time, Seconds

Volts

Figure 10.—EE-35-1 Alternator Voltage at 470 °C. Figure 11.—EE-35-2 Alternator Voltage at 470 °C.

 Time

250ms 300ms 350ms 400ms 450ms 500ms

V(Ctune:1,R4:2)

-50V

0V

50V

Figure 12.—PSpice Simulation of Alternator Voltage at 470 °C. 
NASA/TM—2009-215513 8

Test Summary 

Operation of the EE-35 convertors with the linear AC regulator verified the ability of the linear AC 

regulator to control a pair of convertors. The simulation and experimental results match verifying PSpice 

as an accurate tool for controller design. 

Linear AC Regulator Efficiency Testing 

An experiment was conducted to measure controller efficiency. Power was applied to the input of the 

controller using an AC voltage source to simulate convertor power. The voltage and current delivered to 

the input were measured to calculate power input. The voltage and current delivered to the loads were 

also measured to calculate power output. The efficiency was calculated as the ratio between the output 

and input power. True RMS meters were used for all measurements. True RMS meters measure the 

heating potential of an applied voltage. Unlike an “average-responding” measurement, a true RMS 

measurement is used to determine the power dissipated in a resistor. RMS amplitude measurement is the 

best way to relate AC quantities to DC quantities, or other AC quantities of differing waveform shapes. 

The output current of the linear AC regulator is not a pure sine wave. The output current meter was 

capable of measuring only up to 3 A, which limited the maximum power input to approximately 44 W for 

this experiment. At this power level, the efficiency was measured at 98.64 percent. Table II lists the 

calculated and experimental efficiencies. It should be noted that instrument error was not considered 

during experimental testing. The calculated efficiency was performed through PSpice simulation. The 

calculated and experimental efficiencies are at the same point in the circuit. The output current was 

measured as the current through the four stages. The input current was measured before the diode bridge. 

The circuit model predicted a controller efficiency of 95.68 percent at low power. 

TABLE II.—LINEAR AC REGULATOR EFFICIENCY SUMMARY 

 Experimental Simulation, 

low power 

Simulation, 

full power 

Power input, W 44.33 40.00 129.41 

Power output, W 43.72 38.27 123.88 

Efficiency, Percent 98.64 95.68 95.49 

Linear AC Controller and ASRG Simulator Integration 

Following successful checkout on the EE-35 convertors, the linear AC controller was integrated into 

the ASRG simulator with the FTB convertors and electronics support. Stable operation of the FTB 

convertors was demonstrated up to their maximum combined power output of 130 W. 

Stability was qualitatively evaluated by observing the linear alternator voltage. While operating at full 

power, the alternator voltage did not deviate more than 1 mV. A photograph of the controller integrated 

into the support electronics can be seen in Figure 13. The controller occupies the left half of the container. 

The loads and their sinks are located on the outer surfaces of the container. 

Figure 13.—Linear AC regulator controller integrated into ASRG simulator support electronics. 

Loads and 

heat sinks (4) 

Linear AC 

regulator 

Protection 

Circuit 
NASA/TM—2009-215513 9

Figure 14.—Linear AC Regulator Controller. 

Conclusion 

PSpice was the main instrument in designing the controller. The completed controller can be viewed 

in Figure 14. The controller was housed in a box 10.2 by 14 by 11.1 cm minus the load modules. 

Several tests were performed on the linear AC regulator: bench, EE-35 convertor, efficiency, and 

ASRG simulator testing. The bench and EE-35 convertor tests verify PSpice as an accurate tool for 

controller design. The efficiency of the controller was measured to be 98.64 percent at low power. 

Following successful checkout on the EE-35 convertors, the linear AC controller was integrated into the 

ASRG simulator electronics support. Stable operation of the FTB convertors was demonstrated up to their 

maximum combined power output of 130 W. 

Constant Power Circuit 

The constant power circuit maintains the Stirling convertor output to its maximum, 130 W, while the 

Li-ion batteries provide additional power. 

DC-DC Converter 

The DC-DC converter monitors the voltage and current provided by the Stirling convertors at the 

output of the linear AC regulator. The TRM pin of the DC-DC converter adjusts the regulation of the DCDC converter between 24 and 28 VDC according to the current and voltage supplied by the Stirling 

convertors. When the rover current demands exceed the current available from the Stirling convertors, the 

DC-DC converter output regulates at 28 VDC. At this point, the Li-ion batteries supply additional power to 

the rover. 

Multiplier 

A multiplier is used to measure the power supplied to the load. The voltage across the load and 

current through the load is multiplied by means of an analog multiplier integrated circuit (IC). The 

multiplier provides a voltage output proportional to the power supplied to the load. The voltage input of 

the multiplier was scaled with a voltage divider. A current sensor detects the current supplied to the load 

and provides a voltage output signal proportional to the load current. This signal served as the second 

input to the multiplier. 

The current sensor and the voltage divider were both scaled based on the following transfer function 

provided by the multiplier datasheet. 

( )( )

10

1 2 1 2

output

X -X Y −Y =

X is the current sensor output and Y is the scaled voltage across the load. 
NASA/TM—2009-215513 10

DC-DC Convertor Current Sensor

From Controller

sensor

in+ in- out+ out- 0

Sensor

1k

0

0

-15Vdc

0

LM158

+ 3

- 2

V+ 8 V- 4

OUT 1

0

VCC

0

2N2222A/ZTX

AD584

V+ 8

VBG 6

STB 5

2.5V 3

5V 2

10 1 CAP 7

COM 4

VEE

0

1.1k

1k

0

0

0

0

1k

User Load

0

AD734/AD

Z2 10

ER 9

VN 8

X1 1

Z1 11

U2 5

X2 2

U0 3

U1 4 W 12 DD 13

Y2 7 Y1 6

VP VCC 14

1k

PAH300

IN+ 1

IN- 2

OUT+ 3

OUT- 4

TRIM 5

15Vdc

0.01uF 100k

1

Sensor

Vref

Vref

User Load

Figure 15.—Constant Power Circuit Schematic. 

The voltage divider values were selected to scale the load voltage to 9.6 VDC. The combined 

knowledge of the maximum Stirling convertor current output, 6.5 A, and the conversion factor of the 

current sensor, 2.5 A/V, gave a maximum current sensor output of 2.6 VDC. 

( )( ) 2.5 V

10 V

9.6 V 2.6 V

output = =

Design 

A schematic of the constant power circuit can be viewed in Figure 15. The constant power circuit 

measures the load power by means of two analog inputs on a multiplier IC. The first input is load voltage. 

The second input is load current detected by a current sensor. This power signal is compared to a set point 

using an op-amp, which generates a voltage proportional to the difference between the two inputs. The 

output of the op-amp drives a transistor which activates the TRM pin of the DC-DC converter. As the 

power demand of the load changes, the TRM pin adjusts the output voltage of the DC-DC converter to 

maintain an output voltage of 28 VDC up to a maximum power load of 130 W. When the power demands 

exceed 130 W, Li-ion batteries supply additional power to the rover. 

Battery Charger 

The battery charger charges Li-ion batteries through a constant voltage technique. The battery charger 

serves as a safety mechanism for the Li-ion batteries by means of a cut-off current function. The charger 

will use a constant voltage charge method, limit the charge current to 3.5 A, and turn off when the current 

falls below 1.1 A. 

Lithium Ion Battery 

Li-ion cells are a high energy Li-ion chemistry that utilize a mixed metal oxide cathode material and a 

graphitic intercalation anode. They have an organic solvent-based electrolyte with an inorganic lithium 

electrolytic salt. Li-ion chemistry provides a much higher energy density and specific energy than other 

systems. The tolerance of a Li-ion battery is ±0.05 V/cell. Li-ion batteries are designed to operate safely 
NASA/TM—2009-215513 11

within their normal operating voltage but become increasingly unstable if charged to higher voltages. 

Overcharging can also cause the cells to heat up. 

Charging Technique 

A taper charge, constant voltage charge, with C/50 limit was used to design the charging of the Li-ion 

batteries. The C refers to the capacity of the battery in amp hours (AH). The capacity of the battery is 

55 AH. Based on the Li-ion specification sheet, the charger has a voltage limit of 4.1 V/cell, a maximum 

charge current of C/5, a charge cutoff of C/50. The battery consists of 8 cells in series for a total charger 

voltage limit of 32.8 VDC. The charger current limit is 3.5 A, and charger cutoff current is 1.1 A. The 

current limit is based on the maximum convertor output. Full charge is reached after the voltage has 

reached the upper voltage threshold and the current has dropped to 1.1 A. As the battery voltage rises 

during the charge, the current tapers down, and the voltage rises to a float voltage level of 4.1 V/cell. 

A trickle charge method was eliminated due to the Li-ion batteries inability to absorb overcharge. 

Trickle charge could cause plating of metallic lithium, a condition that renders the cell unstable. 

Design 

A schematic of the battery charger can be viewed in Figure 16. The current is detected by a current 

sensor. This current is compared to a set point using an op-amp, which generates a voltage proportional to 

the difference between the two inputs. The output of the op-amp drives a transistor which activates the 

TRM pin of the DC-DC converter. As the load current changes, the TRM pin adjusts the output voltage of 

the DC-DC converter to maintain a load current of 3.5 A. As the current draw of the Li-ion battery 

decreases below 3.5 A, the transistor acts as an open circuit and drives the TRM pin by means of an 

adjustable resistor to a charge voltage of 32.8 VDC. 

The cut off circuit disconnects the Li-ion battery from system when the current decreases to 1.1 A. 

The current is detected by a current sensor. This current signal is compared to a set point using an opamp, which generates a voltage proportional to the difference between the two inputs. The output of the 

op-amp drives a transistor which controls the activation of a normally open relay. A push button must be 

pressed to energize the relay. When the relay is de-energized the Li-ion battery is disconnected from the 

circuit. 

Li-Ion Battery

12Vdc -12Vdc

VCC VEE

0 0

1k 2N2222A/ZTX

0

0

VLimit

1k 0

sensor

in+ in- out+ out- 0

Sensor

1k

0

LM158

+ 3

- 2

V+ 8 V - 4

OUT 1

VCC

2N2222A/ZTX

VEE

0

1.4Vdc

error

0

1k

0

1k

PAH300

IN+ 1

IN- 2

OUT+ 3

OUT- 4

TR IM 5

0.01uF 100k

VCC

Sensor

MY4-4PDT

12

4

8

11

3

7

10

2

6

9

1

5

14

13

500k

SET = .71

VLimit

LM158

+ 5

- 6

V+ 8 V - 4

OUT 7

VCC

VEE

1k

0

0.44Vdc

Current Sensor DC-DC Converter

1N4001

From Controller

Sensor

Figure 16.—Battery Charger Schematic. 
NASA/TM—2009-215513 12

System Integration Bench Test 

The purpose of the system integration bench test was to verify the operation of the constant power 

circuit and battery charger as a system. 

Procedure 

The integration bench test was set up according to Figure 3. The Li-ion battery was represented by a 

DC power supply. To start, the voltage reference on the constant power circuit was changed to meet 

different power demands. The voltage reference was set to 0.75 and 1.12 VDC, which allows maximum 

power at 15 and 10 Ω, respectively. The battery voltage must be set below the maximum constant power 

circuit output voltage. The DC electronic load was set for resistances from 7 to 20 Ω. Current drawn by 

the DC power supply indicated additional power being supplied by the Li-ion batteries. 

Results 

Tables III and IV contain data for a maximum power at 15 and 10 Ω loads respectively. Both tests 

verify the systems ability to recognize the maximum load and signal the battery to draw current. 

TABLE III.—INTEGRATION TEST—MAXIMUM POWER AT A 15 Ω LOAD 

Constant power 

current, 

A 

Battery 

current, 

A 

Constant power 

voltage, 

V 

Vref constant 

power, 

V 

Load, 

Ω

Battery 

voltage, 

V 

Vin, 

V 

1.3 0 27.98 0.75 20 25 19.67 

0.99 0.59 25.73 0.75 15 25 19.67 

1.86 0 25.27 0.75 15 20 19.67 

TABLE IV.—INTEGRATION TEST—MAXIMUM POWER AT A 10 Ω LOAD 

Constant power 

current, 

A 

Battery 

current, 

A 

Constant power 

voltage, 

V 

Vref constant 

power, 

V 

Load, 

Ω

Battery 

voltage, 

V 

Vin, 

V 

2.09 0 27.97 1.12 12 26.529 19.67 

0.91 1.5 26.57 1.12 10 26.529 19.67 

0.93 2.04 26.47 1.12 8 26.529 19.67 

0.98 2.37 26.44 1.12 7 26.529 19.67 

Current is drawn from the battery if two conditions are met; the battery voltage is greater than the 

constant power circuit voltage and the load is at maximum power. A data point in Table III verifies these 

two conditions along with the functionality of the diode to remain reverse biased until the battery voltage 

is greater than the constant power circuit voltage. 

Table IV data indicates that if maximum load is reached and continues to increase, the constant power 

circuit current and voltage remain the same. This data validates the operation of the constant power circuit 

to maintain the Stirling convertor output while the Li-ion batteries supply additional power. 

Test Summary 

The integration of the constant power circuit and the battery charger was successful. The Li-ion 

batteries, represented by a DC power supply, supplied additional power when the linear AC regulator 

reached its maximum power output. It also demonstrated that the diodes in the circuit operated properly; 

the batteries did not supply power unless the battery voltage was greater than the constant power circuit 

output voltage. 
NASA/TM—2009-215513 13

Conclusion 

This paper discussed the design of support equipment for the ASRG simulator. The ASRG simulator 

and support equipment were designed for use on a concept rover. The linear AC regulator controlled the 

FTB convertors in the ASRG simulator. Bench testing, integration testing, and efficiency testing were 

performed on the controller. 

A constant power circuit was designed to maintain the power output of the Stirling convertors at 

130 W while Li-ion batteries supply additional power to the load. The constant power circuit was 

designed and simulated in PSpice and bench tested to verify the design. 

The battery charger charges Li-ion batteries through a constant voltage technique. The battery charger 

serves as a safety mechanism for the Li-ion batteries by means of a cut-off current function. The charger 

will use a constant voltage charge method, limit the charge current to 3.5 A, and turn off when the current 

falls below 1.1 A. The battery charger was designed and simulated in PSpice and bench tested to verify 

the design. 

The constant power circuit and battery charger were integrated as a system and tested. The test 

verified the systems capability to combine the functions of each circuit; the constant power circuits’ 

ability to maintain Stirling convertor power at 130 W while the Li-ion batteries supply additional power. 

References 

1. Salvatore Oriti, Gina Blaze, “Advanced Stirling Convertor Testing at NASA Glenn Research Center,” 

International Energy Conversion Engineering Conference, St. Louis, 2007. 

2. Jeffrey Schreiber, “Developmental Considerations on the Free-Piston Stirling Convertor for Use in 

Space,” International Energy Conversion Engineering Conference, San Diego, 2006. 

3. Gina Blaze, “Stirling Convertor Control for a Lunar Concept Rover,” Master’s Thesis at Cleveland 

State University, Cleveland, 2007. 
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Stirling Convertor Control for a Concept Rover at NASA Glenn Research Center 

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Blaze-Dugala, Gina, M. 

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WBS 138494.04.01.01 

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John H. Glenn Research Center at Lewis Field 

Cleveland, Ohio 44135-3191 

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13. SUPPLEMENTARY NOTES

14. ABSTRACT

The U.S. Department of Energy (DOE), Lockheed Martin Space Systems Company (LMSSC), Sunpower Inc., and NASA Glenn Research 

Center (GRC) have been developing an Advanced Stirling Radioisotope Generator (ASRG) for potential use as an electric power system for 

space science missions. This generator would make use of the free-piston Stirling cycle to achieve higher conversion efficiency than 

currently used alternatives. NASA GRC initiated an experiment with an ASRG simulator to demonstrate the functionality of a Stirling 

convertor on a mobile application, such as a rover. The ASRG simulator made use of two Advanced Stirling Convertors to convert thermal 

energy from a heat source to electricity. The ASRG simulator was designed to incorporate a minimum amount of support equipment,

allowing integration onto a rover powered directly by the convertors. Support equipment to provide control was designed including a linear 

AC regulator controller, constant power controller, and Li-ion battery charger controller. The ASRG simulator is controlled by a linear AC 

regulator controller. The rover is powered by both a Stirling convertor and Li-ion batteries. A constant power controller enables the Stirling 

convertor to maintain a constant power output when additional power is supplied by the Li-ion batteries. A Li-ion battery charger controller 

limits the charging current and cut off current of the batteries. This paper discusses the design, fabrication, and implementation of these three 

controllers. 

15. SUBJECT TERMS

Controllers; Stirling cycle; Radioisotope generator; Convertors 

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