Gina M. Dugala and Linda M. Taylor

Glenn Research Center, Cleveland, Ohio

Mark E. Bell and James L. Dolce

Vantage Partners, LLC, Brook Park, Ohio

Martin Fraeman and David P. Frankford

Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland

Advanced Stirling Convertor Dual Convertor

Controller Testing at NASA Glenn Research Center 

in the Radioisotope Power Systems System

Integration Laboratory

NASA/TM—2015-218880

December 2015

AIAA–2015–3807
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Gina M. Dugala and Linda M. Taylor

Glenn Research Center, Cleveland, Ohio

Mark E. Bell and James L. Dolce

Vantage Partners, LLC, Brook Park, Ohio

Martin Fraeman and David P. Frankford

Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland

Advanced Stirling Convertor Dual Convertor

Controller Testing at NASA Glenn Research Center 

in the Radioisotope Power Systems System

Integration Laboratory

NASA/TM—2015-218880

December 2015

AIAA–2015–3807

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Prepared for the

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sponsored by the American Institute of Aeronautics and Astronautics

Orlando, Florida, July 27–31, 2015
Acknowledgments

This work is funded through the NASA Science Mission Directorate. The authors wish to acknowledge the Radioisotope Power 

Systems System Integration Laboratory development team, John Hopkins University Applied Physics Laboratory personnel, and 

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NASA/TM—2015-218880 1 

Advanced Stirling Convertor Dual Convertor Controller Testing at 

NASA Glenn Research Center in the Radioisotope Power Systems 

System Integration Laboratory 

Gina M. Dugala and Linda M. Taylor 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Mark E. Bell and James L. Dolce 

Vantage Partners, LLC 

Brook Park, Ohio 44142 

Martin Fraeman and David P. Frankford 

Johns Hopkins University Applied Physics Laboratory 

Laurel, Maryland 20723 

Abstract 

NASA Glenn Research Center developed a nonnuclear representation of a Radioisotope Power 

System (RPS) consisting of a pair of Advanced Stirling Convertors (ASCs), Dual Convertor Controller 

(DCC) EMs (engineering models) 2 and 3, and associated support equipment, which were tested in the 

Radioisotope Power Systems System Integration Laboratory (RSIL). The DCC was designed by the Johns 

Hopkins University Applied Physics Laboratory (JHU/APL) to actively control a pair of ASCs. The first 

phase of testing included a Dual Advanced Stirling Convertor Simulator (DASCS), which was developed 

by JHU/APL and simulates the operation and electrical behavior of a pair of ASCs in real time via a 

combination of hardware and software. RSIL provides insight into the electrical interactions between a 

representative radioisotope power generator, its associated control schemes, and realistic electric system 

loads. The first phase of integration testing included the following spacecraft bus configurations: 

capacitive, battery, and supercapacitor. A load profile, created based on data from several missions, tested 

the RPS’s and RSIL’s ability to maintain operation during load demands above and below the power 

provided by the RPS. The integration testing also confirmed the DCC’s ability to disconnect from the 

spacecraft when the bus voltage dipped below 22 V or exceeded 36 V. Once operation was verified with 

the DASCS, the tests were repeated with actual operating ASCs. The goal of this integration testing was 

to verify operation of the DCC when connected to a spacecraft and to verify the functionality of the newly 

designed RSIL. The results of these tests are presented in this paper. 

Symbols 

ACU advanced Stirling convertor control unit 

A/D analog to digital 

ASC Advanced Stirling Convertor 

D/A digital to analog 

DAQ data acquisition 

DASCS Dual Advanced Stirling Convertor Simulator 

DCC Dual Convertor Controller 

DSP digital signal processor 

EM engineering model 
NASA/TM—2015-218880 2 

EMI electromagnetic interference 

FCDA facility control and data acquisition 

FET field effect transistor 

FPC failsafe protection circuit 

FPGA field programmable gate array 

GSE ground support equipment 

IEEE Institute of Electrical and Electronic Engineers 

IGBT insulated-gate bipolar transistor 

IV&V independent verification and validation 

JHU/APL Johns Hopkins University Applied Physics Laboratory 

PDCU power distribution control unit 

PID proportional integral derivative 

PMAD power management and distribution system 

PWM pulse width modulation 

rms root mean square 

RPS Radioisotope Power System 

RSIL Radioisotope Power Systems System Integration Laboratory 

SRU shunt regulator unit 

SSPC solid-state power controller 

Introduction 

The Radioisotope Power System (RPS) Program has worked to develop Advanced Stirling Convertor 

(ASC) technology for applications in future space missions. After extended operation of the ASCs at 

Glenn Research Center, it was determined a need existed for the ability to verify and validate RPS units 

that are unfueled. This is driven by the fact that contractors who build RPS units and spacecraft 

developers can change over time requiring the RPS Program to develop and sustain the capability to test 

generator to spacecraft electrical interfaces. This capability allows the RPS Program and Planetary 

Science Division Missions to validate and verify current and future electrically heated RPS unit 

operations, providing for a better understanding and characterization of fueled units. In addition, 

spacecraft vendors will require data on the RPS units that only NASA, as an independent agency, can 

provide. This avoids duplication of work among contractors and potential conflicts of interests and 

ensures that information is properly shared across the community and archived for future application. The 

resulting testbed, the Radioisotope Power Systems System Integration Laboratory (RSIL), is capable of 

simulating an end-to-end power system in order to provide insight into the electrical interactions among a 

representative RPS and its associated control scheme and realistic electric system loads. The RSIL 

addresses five major goals: 

(1) Provide the ability to independent verification and validation (IV&V) RPS generators for missions 

(2) Provide future missions with the capabilities to verify their spacecraft power system designs 

(3) Provide electrical testbed for current missions in development and those already deployed 

(4) Support the RPS product line development by providing the capability to test new technologies 

related to RPSs 

(5) Provide future missions with the capabilities to verify their fault protection and flight software, 

relating to RPSs 

This paper presents the first RSIL test series that utilized two ASC convertors: a Dual Advanced 

Stirling Convertor Simulator (DASCS) and Dual Convertor Controller (DCC) engineering models (EMs) 

2 and 3. 
NASA/TM—2015-218880 3 

Testing Components 

Advanced Stirling Convertor 

The ASC–1 #3 and #4, second-generation prototypes (Fig. 1) were designed and fabricated by 

Sunpower, Inc. These convertors were designed to operate at the parameters shown in Table 1. 

ASC–1 #3 and #4 are oriented in the dual-opposed configuration with the heater heads facing outward 

and the pressure vessel sections rigidly attached to each other. This configuration permits operation at near 

zero net vibration because the piston motions are equal but opposite in direction to achieve dynamic balance. 

The ASC support hardware was designed to allow adjustment of operating conditions, including hotend and rejection temperatures, piston amplitude of oscillation, and mean charge pressure of the working 

gas. The ASCs were instrumented to measure these parameters and all output characteristics, such as 

alternator voltage, alternator current, operating frequency, and alternator power. The hot end of each ASC is 

heated with electric cartridge heaters and the rejection temperature is maintained with a water/glycol bath. 

Figure 1.—Advanced Stirling Convertor (ASC)–1 #3 and #4. 

TABLE 1.—ADVANCED STIRLING CONVERTOR (ASC)–1 #3 AND #4 OPERATING PARAMETERS 

Maximum operating 

temperature 

Alternator voltage Alternator current Alternator power Charge 

pressure 

Thot, 

°C 

Tcold,

°C 

850 90 12 Vrms 7 A 80 W 515 psig 
NASA/TM—2015-218880 4 

Dual Convertor Controller Engineering Models 2 and 3 

A block diagram of the DCC connected to a convertor and a typical redundant spacecraft is shown in 

Figure 2. The DCC consists of two identical controller boards packaged in separate chassis. A photograph 

of the DCC is shown in Figure 3. Each board is capable of controlling both ASC channels. One controller 

board is actively in control of both convertors at all times, while the other is a hot standby used to recover 

from a fault. Each controller board contains the power handling, data acquisition (DAQ), signal 

processing, and secondary voltage conditioning circuits needed to control two convertors and deliver 

direct current (dc) power to the spacecraft. A block diagram of the controller board is shown in Figure 4. 

Figure 2.—Dual Convertor Controller (DCC) spacecraft application. 

Figure 3.—Dual Convertor Controller (DCC). 
NASA/TM—2015-218880 5 

Return

External Controller

Power

cmd/tlm

Power

External Shunt

Resistor

Other

Controller

Slice

Alternator

AC Power

A

Temperature

Internal

H‐Bridge

I‐V

Sense

I‐V

Sense

Emergency

Shunt PWM

Transient

Clamp

Temp

Sense

A/D

A/D

FPGA

OSC

Power Ckts

Signal Proc Ckts

Mem

Housekeeping

Supply

EMI DC Ckt

AC Circuit Brkr

Breaker

Alternator

AC Power

B

AC Circuit H‐Bridge

Breaker

Transient

Clamp

Buck

Reg

Figure 4.—Dual Convertor Controller (DCC) board. 

Power flows from each convertor through an electronic switch for each channel that acts as an 

alternating current (ac) circuit breaker. The ac circuit breaker is opened in the event of a fault to isolate 

the convertor from the failure and allow the redundant board to initiate control. Transient suppression 

devices on each controller board clamp voltage spikes that may occur when an ac breaker is opened. A 

field effect transistor (FET) H-bridge is pulse width modulated (PWM) as determined by the control law 

implemented in the field programmable gate array (FPGA). The H-bridge output is a dc voltage but with a 

large ripple current. That node is connected through an output buck regulator stage designed to minimize 

ripple in spacecraft output load current rather than maintain a specific output voltage. The regulator also 

limits current while connecting to the spacecraft load. An additional FET switch in combination with the 

buck regulator will isolate the load and controller from faults in either. An electromagnetic interference 

(EMI) filter on the board limits switching noise propagating from the controller to the spacecraft load. 

While the output regulator/dc breaker FETs are open, convertor power flows to an external shunt resistor 

controlled using a PWM FET-based regulator to maintain a fixed H-bridge output voltage. 

The control law inputs and key status analog voltages are digitized by a 12-bit analog-to-digital (A/D) 

converter and processed by logic in the FPGA. The control law is implemented digitally by a coprocessor in the FPGA. The co-processor contains an Institute of Electrical and Electronics Engineers 

(IEEE) single precision compliant data path controlled by a finite state machine sequencer programmed to 

implement the control law algorithm. A 16-bit microcontroller, also implemented in the FPGA, adjusts 

control law parameters, accepts user commands, and produces status telemetry. A combination of analog 

circuits, FPGA logic, and microcontroller software monitor DCC operation and detect and correct 

potential faults. A custom-designed dc/dc converter accepts input power from the spacecraft bus or 

external source and provides secondary voltages to board circuitry. 

The DCC not only provides power to the spacecraft but it also must regulate operation of both ASCs 

to avoid damage to their internal components and maintain safe thermal conditions after fueling. The two 

controller boards are fully redundant. If a board fails, the redundant controller board will continue to 

safely operate the ASCs. Commands to the active controller board can also electrically isolate the backup 
NASA/TM—2015-218880 6 

board so that it can be replaced without use of external equipment to restore fault-tolerant operation. 

Analysis and simulation of the methods used to detect and recover from faults without damage to the 

ASCs as well as the ASC control algorithm are described in Reference 1. 

Dual Advanced Stirling Convertor Simulator 

A DASCS was developed using custom-designed circuits, commercial equipment, and a linearized 

ASC model (Ref. 2) implemented in software on an off-the-shelf digital signal processor (DSP) 

development board. A block diagram including user interface capabilities of the DASCS is shown in 

Figure 5 while a photograph of the DASCS is shown in Figure 6. 

Alternator inductance and resistance are modeled with physical components with electrical properties 

similar to the actual alternator in series with a back emf voltage source. A commercial current sensor is 

filtered and then digitized by an A/D in the DSP development system. The DSP is programmed to use the 

input current to calculate and produce an output voltage through a digital-to-analog (D/A) converter, also 

part of the DSP, proportional to the ideal converter back emf voltage based on the ASC model equations. 

The back emf voltage passes through an electrical isolator and is then used to control the output of a 

400 W commercial power supply. The power supply output is the equivalent of the back emf output of the 

convertor alternator. The DASCS output is calculated from ASC model equations and includes convertor 

mechanism dynamics beyond the alternator electrical inductance and resistance. The DASCS can be 

reprogrammed to mimic other types of ASCs by changing the linear model implemented in the DSP and 

the physical alternator inductance and resistance components. 

Previous engine simulator designs used an ac source, resistor, and inductor and only modeled 

electrical performance with no consideration of convertor mechanical behavior. Designers frequently 

found that their controllers did not function the same with an ASC as with their engine simulator. The 

impact of controller problems like ac input current sampling noise or spacecraft bus voltage changes on 

mechanical parameters like piston amplitude could only be investigated using time-consuming testing 

with a real ASC. In addition, damage to the ASC might occur during such evaluations. 

Figure 5.—Dual Advanced Stirling Convertor Simulator (DASCS) with user interface. 
NASA/TM—2015-218880 7 

Kepco CH 2 

Piston 

Amplitude 

Connection 

Alternator 

Current 

Connection 

Power Out 

to DCC 

Kepco CH 1 

Figure 6.—Dual Advanced Stirling Convertor Simulator (DASCS). 

The DASCS was used to test the DCC prior to operating with an actual ASC. The DASCS helped 

solve numerous nonlinearity and noise-related problems in sensing the alternator current. Overcoming 

those issues reduced output power fluctuations at the load, resulting in smaller peak-to-peak variations in 

piston amplitude and improved operating efficiency. The effectiveness of the DASCS was shown when 

the ASC was successfully controlled, at full power, on the first attempt by the DCC controllers. If DCC 

testing was only performed with an ASC, the success of the controller design and its 2-year time to fullpower ASC demonstration would not have been possible. The same behavior observed while testing with 

the ASC was also seen while testing with the DASCS including piston amplitude noise, controller 

efficiency, and sensitivity to load changes. The DASCS also provides an easy-to-use environment for 

debugging and testing fault detection and recovery of the DCC without risk of damage to a real ASC. 

Support Hardware 

The test rack comprises the data system, hard-wired failsafe protection devices, hot-end temperature 

control systems, power meters, and DCC commanding and support hardware. The data system utilizes 

National Instruments’ LabVIEW-based DAQ hardware and software to acquire data and monitor the test. 

It displays and records data on a computer, collects and saves data in various time frames, calculates 

parameters with received data, and provides safety to the convertors. The DAQ software, which was 

developed to operate in unattended mode, can control the support systems without user intervention. The 

user may specify upper and lower bounds for any parameter monitored by the DAQ system. The DAQ 

software will safely shut down operation of the test article when an out-of-bounds condition is sensed. 

Parameters that may trigger a shutdown include hot-end and rejection temperatures, convertor mean 

charge pressure, piston amplitude of oscillation, and loss of building power. Data collection allows for 

detailed analysis of ASC operation. To achieve this, LabVIEW collects and displays several parameters, 

including 

 Heater voltage and current 

 Electric heat-source temperature 

 Hot-end, cold-end, ambient, and pressure vessel temperature 
NASA/TM—2015-218880 8 

 Alternator root mean square (rms) voltage, rms current, and power 

 Piston amplitude 

 ASC operating frequency 

 Power factor 

 DASCS voltage, current, and power 

A complete data record of these parameters is stored every hour. In addition, the DAQ system 

maintains a buffer of data recorded over the last 24 hr at a 2-sec scan rate, records an average of the data 

over the last 5 min of every hour, and allows these data to be stored manually as needed for detailed 

analysis. From the stored data, numerous parameters are calculated including 

 Heater resistance and power 

 Average hot-end temperature 

 Cold-end temperature 

 Piston amplitude 

 Net heat input 

Hard-wired protection devices were also installed in the operations rack that function independent of 

the software-based protection. The hot-end temperature of each convertor is monitored by a limit 

controller. If either hot-end temperature exceeds the user-defined limit, the limit controller removes heater 

power from both ASCs via a relay. A failsafe protection circuit (FPC) was implemented to prevent piston 

overstroke. When the user-defined piston amplitude is exceeded, the controller is disconnected and an 

emergency load is applied across both alternators in less than one-half of a piston cycle. 

Hot-end temperature control is accomplished by use of programmable dc power supplies driven by 

closed-loop proportional-integral-derivative (PID) controllers. Each hot-end temperature is controlled 

individually. Power input and output are measured using power meters and are recorded by the DAQ 

system. 

Support hardware for the DCC include a dc electronic load, shunt resistors, capacitors, ground 

support equipment (GSE) power supply, and terminal emulator. The DCC relies on a dc electronic load to 

sink power in an output voltage range between 22.0 and 36.0 Vdc and is determined by the end user. The 

DCC will track this voltage, providing power as a constant-power source, and will sense a bus 

overvoltage condition. The DCC will provide constant power as long as the end user is in the required 

voltage range, but it will internally shunt power and disconnect itself from the bus if the voltage range is 

exceeded. Shunt resistors sized for maximum output power of the convertors are mounted on the test 

stand for this purpose. The bus is connected to a capacitor bank for transient energy storage that allows a 

range of 10,000 to 100,000 F. During motoring of the ASCs, power is provided to the internal DCC 

circuits by means of a dc power supply in the test rack. When the ASCs are producing positive power, the 

power supply is no longer needed and is turned off. The terminal emulator provides an interface for 

commanding the DCC. These commands include 

 Changing the voltage applied to the convertor, Vm 

 Convertor operating frequency 

 Disconnect from the spacecraft 

 Connect to the spacecraft 

 Operation with or without the backup controller card 

 Open load or short circuit fault injection 
NASA/TM—2015-218880 9 

Radioisotope Power Systems System Integration Laboratory 

The RSIL comprises five major components designed to simulate a prototypical spacecraft, which is 

powered by a RPS. The major components (Fig. 7) are RPS power sources (the test article, here a pair of 

ASC convertors), a power management and distribution system (PMAD), an energy storage system (i.e., 

bus capacitor, battery, or supercapacitor), electrical load simulators, and a flight control computer 

simulation. These components are connected together to simulate a spacecraft’s main electrical bus 

feeding a set of electrical loads all powered by RPS generators. This integrated electrical system tests 

RPS generators as part of an end-to-end spacecraft system rather than as stand-alone power generators. 

RSIL’s spacecraft simulation capabilities are 

 Generators: one, two, or three 200 W RPS generators 

 Energy storage: 100,000 F bus capacitor, 165 F supercapacitor, or 24 A-hr Li+ battery 

 Electrical loads: 20 channels at 100 W each 

 Main bus: 

o Regulation: 0.05 V with power shunt engaged 

o Control range: 22 to 38 V (26 to 33 V with Li+ battery) 

 Battery charging: Adjustable rate limit 2 to 25 A 

Figure 7.—Radioisotope Power Systems System Integration Laboratory (RSIL) system architecture.
NASA/TM—2015-218880 10 

The power management and distribution subsystem consists of an interface unit, power distribution 

control unit (PDCU) utilizing solid-state power controllers (SSPCs) for load switching, and a shunt 

regulator unit (SRU) to shunt excess power not used by the loads and to regulate charging of the energy 

storage device. The electronic loads consist of 20 independent channels capable of simulating constant 

resistance, constant current, constant power, or constant voltage loads. An overall facility control and data 

acquisition (FCDA) system manages the data storage and display. The RSIL can be configured in three 

ways: (1) stand-alone bus capacitor, (2) bus capacitor with a battery unit, and (3) bus capacitor with a 

supercapacitor. Since the ASCs are constant power sources, the energy storage unit is used to provide 

power at times when the load demand exceeds the source capabilities. 

In addition to these components, the RSIL incorporates an electrical safety system in the event of 

system failures. The control panel, which monitors various system signals, and associated alarm panel are 

shown in Figure 8. The safety system includes two levels of safety: alarms and shutdowns, with the 

emergency shutdown logic shown in Figure 9. 

Electric power distribution system parameters, voltages, currents, and temperatures are continuously 

monitored for out-of-tolerance operation. The monitors provide audible and visual alarms. The RSIL 

operator is responsible for correcting the out-of-tolerance condition. 

Electrical power parameters that are out of limit trigger an automatic emergency shutdown procedure. 

This shutdown procedure can also be initiated by the RSIL operator. Visual and audible alarms 

annunciate occurrences. The emergency shutdown procedure for RSIL 

(1) Removes all energy sources from the spacecraft simulator via mechanical contactor. 

(2) Disconnects all loads from the simulator via mechanical contactor. 

(3) Notifies ASRG test controls of the event. 

The goal of the initial set of tests with the nonfueled RPS equipment, described in the “Testing 

Components” section, was to validate the functionality of the RSIL platform prior to integration with an 

Advanced Stirling Convertor Controller Unit (ACU) and a pair of DASCSs. 

Figure 8.—Control and alarm panels. 

Alarm Panel 

Control Panel 
NASA/TM—2015-218880 11 

Figure 9.—Emergency shutdown. 
NASA/TM—2015-218880 12 

Single Input Channel Testing 

For the single input channel testing, the DCC EMs 2 and 3 were connected to the RSIL and operated 

with ASC–1 #3 and #4 for a flight-like hardware simulation of an RPS connected to a spacecraft. A 

photograph of the test setup is shown in Figure 10. The ASC–1 #3 and #4 operating points for this test are 

listed in Table 2. 

Figure 10.—Radioisotope Power Systems System Integration Laboratory (RSIL) and Radioisotope Power 

System (RPS) setup. 

TABLE 2.—ADVANCED STIRLING CONVERTOR (ASC) OPERATING PARAMETERS 

Convertor power 

level, 

W 

Hot end, 

°C 

Cold end, 

°C 

Piston 

amplitude, 

mm 

Alternator 

voltage, 

V 

Alternator 

current, 

A 

Charge pressure, 

psig 

120 650 25 3.5 12.5 6.7 515 

138 650 25 4.0 13.7 7.0 515 
NASA/TM—2015-218880 13 

TABLE 3.—SUMMARY OF SINGLE INPUT CHANNEL TESTING IN RADIOISOTOPE 

POWER SYSTEMS SYSTEM INTEGRATION LABORATORY (RSIL) 

Test configuration Energy storage Test 

DASCS 120 and 138 W ASC power output Capacitor Load step change 

 Short circuit 

 Supercapacitor Load step change 

 Load profile 

 Voltage limit 

 Short circuit 

 Battery Load profile 

 Short circuit 

DASCS 138 W ASC power output Load step change 

ASC–1 #3 and #4 120 W ASC power output Capacitor Load step change 

 Voltage limit 

 Supercapacitor Load step change 

 Load profile 

 Voltage limit 

 Short circuit 

 Battery Load step change 

 Short circuit 

ASC–1 #3 and #4 138 W ASC power output Capacitor Load step change 

 Voltage limit 

 Supercapacitor Load step change 

 Voltage limit 

 Short circuit 

 Battery Load step change 

 Load profile 

 Short circuit 

Prior to integration of the ASCs with RSIL, a suite of testing with a DASCS, listed in the first part of 

Table 3, was performed to verify both the functionality of the DCC and RSIL. The DASCS has proved to 

be a reliable testing resource, due to the design and testing, as described in the “Dual Advanced Stirling 

Convertor Simulator” section, to mitigate risk to ASC hardware. For all tests, the DASCS, DCC, and 

RSIL operated as predicted with the DCC maintaining control of DASCS, and RSIL managing the power 

bus via energy storage and dissipative loads. The results of this testing are not shown in this paper for 

brevity. 

Single input channel testing included operations using capacitive, battery, and supercapacitor bus 

configurations. The results presented in this paper include DCC operation with ASC–1 #3 and #4 at both 

120 and 138 W convertor power output. RSIL scope traces are only included in this paper at the 120 W 

convertor power output level. It should be noted that the power received by RSIL is approximlatey 25 W 

less than that provided by the ASC due to line loss and an approximately 88 percent efficient DCC. The 

RSIL power is listed in each testing section that follows. 
NASA/TM—2015-218880 14 

Four main tests were completed during RSIL testing, which included load step change, load profile, 

voltage limit, and short circuit. 

1) The load step change test verified the RSIL’s ability to respond to a change in the load demand. It 

also verified that the DCC and ASC were not impacted by this load change. Step changes in 

electrical load were executed using the Chroma (Chroma Systems Solutions, Inc.) programmable 

loads with the following settings: 

 Constant load current regulation 

 2-A/μsec current slewing rate 

2) A load profile, based on data from several representative missions, tested the RPS’s and RSIL’s 

ability to maintain operation during load demands above and below the power provided by the 

RPS. The load profile used during the ASC 120 W power tests is shown in Figure 11. When the 

RSIL required more power than the RPS could accommodate, the energy storage device 

(supercapacitor or battery) supplied the additional power. When the RSIL demanded less power 

than what the source provided, the SRU dissipated the excess power. 

3) The voltage limit tests verified the DCC’s ability to disconnect from the RSIL when out of range 

from 22 to 36 V bus voltage for which it was designed. 

4) The short circuit test verified the RSIL’s ability to respond to a short on one of the loads. For the 

short circuit test, a switch-operated insulated-gate bipolar transistor (IGBT) was connected across 

the inputs to a Chroma load with the IGBT’s emitter connected to the power return. When the 

IGBT’s gate was energized, the device shorted the high side of the Chroma’s input to the return 

power connection. Shorting events were induced manually by the RSIL operator. The IGBT used 

was a Fuji Electric Corp. 1MBI400–060 unit with ratings of 400 A and 600 V. 

Prior to connecting to RSIL, the ASCs are brought to full power operation with the DCC dissipating 

power in external shunt resistors (as described in the “Support Hardware” section). When steady-state 

operation of the ASCs is achieved, a command is sent to the DCC to connect to RSIL. 

Figure 11.—The 106 W maximum load profile. 
NASA/TM—2015-218880 15 

Capacitor Bus 

Two tests were performed in the RSIL bus capacitor configuration at both the 120 and 138 W 

convertor power output level: load step change and voltage limit testing. Table 4 describes the bus voltage 

and current at each power level. Figure 12 depicts the shunt tracking (dissipating) of the source current at 

the 120 W convertor power output. Figures 13 and 14 show the ASC–1 #3 alternator voltage, current, and 

power and piston amplitude response during the capacitor bus tests. 

The load test at the lower power level consisted of a positive 3 A load step change, followed by a 2 A 

step decrease, and then removal of the remaining 1 A load. Figure 15 shows the results of the 3 A 

increase. All systems operated as expected with no disturbances seen at the RPS. 

The load step change test at the higher ASC output power level consisted of applying 2 A on each of 

two load channels. All systems operated as expected with no disturbances seen at the RPS. 

For the voltage limit tests, the bus voltage was raised above 36 V and then lowered below 22 V to 

verify the DCC’s ability to disconnect from the bus when out of range from its designed bus voltage 

range. Table 5 shows the voltage at which the DCC disconnected at each convertor power output level. 

The DCC successfully disconnected when out of range of the 22 to 36 V bus voltage for which the unit 

was designed. 

This paper presents tests results for ASC–1 #3 only, but ASC–1 #4 responded in the same manner. 

The variation of 2.5 W in alternator power occurs when connecting/disconnecting from the bus. This 

same variation was observed while operating with the DASCS. The reason for this power change has not 

been identified as of yet. 

TABLE 4.—BUS VOLTAGE AND CURRENT FOR CAPACITOR BUS TESTING 

Power level, 

W 

Bus voltage, 

V 

Bus current, 

A 

RSIL power, 

W 

120 28.6 3.54 101.2 

138 N/A N/A 113 

Figure 12.—The 120 W: no added energy storage. 
NASA/TM—2015-218880 16 

Figure 13.—The 120 W operation: Advanced Stirling Convertor (ASC)–1 #3 operating with Dual 

Convertor Controller (DCC) engineering models (EMs) 2 and 3 for Radioisotope Power Systems 

System Integration Laboratory (RSIL) capacitor bus testing. 

Figure 14.—The 138 W operation: Advanced Stirling Convertor (ASC)–1 #3 operating with Dual 

Convertor Controller (DCC) engineering models (EMs) 2 and 3 for Radioisotope Power Systems 

System Integration Laboratory (RSIL) capacitor bus testing. 
NASA/TM—2015-218880 17 

Figure 15.—The 120 W: 3 A load step change with bus capacitor. 

TABLE 5.—BUS VOLTAGE AT DISCONNECT FOR CAPACITOR BUS TESTING 

Convertor power output, 

W 

Low voltage disconnect, 

V 

High voltage disconnect, 

V 

120 20.8 36.05 

138 20.52 36.8 

Supercapacitor Bus 

Four tests were performed in the supercapacitor bus configuration at the 120 W convertor power 

output level: load step changes, load profile, voltage limit, and short circuit tests. Three tests were 

performed in the supercapacitor bus configuration at the 138 W convertor power output level: load step 

change, voltage limit, and short circuit. Table 6 lists the bus voltage and current levels for this test. 

Figures 16 and 17 show the ASC–1 #3 alternator voltage, current, and power and piston amplitude 

response during the supercapacitor tests. 

For both power levels, a positive 2 A step load change (Fig. 18) was initiated and all systems operated 

as expected. The load was then removed (Fig. 19), with no disturbances observed at the source. 

A load profile (Fig. 11) test was performed at the 120 W convertor power output level. The RPS and 

RSIL operated in the manner expected with the DCC maintaining control of the ASCs and RSIL 

providing power via energy storage and dissipating power via the shunt regulator. 

As described in the “Capacitor Bus” section, the voltage limit tests were performed. The DCC 

successfully disconnected when out of range of the 22 to 36 V bus voltage for which the unit was 

designed. Table 7 lists the voltage at which the DCC disconnected from the bus. 

For the 120 W power level short circuit test, one load channel was set to 2 A and then shorted with a 

trip time of 85.2 s. The results of this test are shown in Figure 20. For the 138 W power level short 

circuit test, one load channel was set to 3 A and then shorted. For both power levels, no disturbances or 

abnormalities were seen at the source and all systems operated as expected. 

Vm is a command that is sent to the DCC to control the piston amplitude of the ASC. Vm had to be 

adjusted for both the 120 and 138 W operation in order to connect to the bus. The reason for this has not 

been identified as of yet. 
NASA/TM—2015-218880 18 

TABLE 6.—BUS VOLTAGE AND CURRENT FOR SUPERCAPACITOR TESTING 

Power level, 

W 

Bus voltage, 

V 

Bus current, 

A 

RSIL power, 

W 

120 28.06 3.53 99 

138 28.88 3.9 113 

Figure 16.—The 120 W: Advanced Stirling Convertor (ASC)–1 #3 operating with the Dual Convertor 

Controller (DCC) engineering models (EMs) 2 and 3 supercapacitor testing. 

Figure 17.—The 138 W: Advanced Stirling Convertor (ASC)–1 #3 operating with the Dual Convertor 

Controller (DCC) engineering models (EMs) 2 and 3 supercapacitor testing. 
NASA/TM—2015-218880 19 

Figure 18.—The 120 W: 2 A load step change with supercapacitor. 

Figure 19.—The 120 W: 2 A load removal with supercapacitor. 
NASA/TM—2015-218880 20 

TABLE 7.—BUS VOLTAGE AT DISCONNECT FOR SUPERCAPACITOR BUS TESTING 

Convertor power output, 

W 

Low voltage disconnect, 

V 

High voltage disconnect, 

V 

120 21 36.09 

138 21.6 36.62 

Figure 20.—The 120 W: shorted load test with supercapacitor. 
NASA/TM—2015-218880 21 

Battery Bus 

Two tests were performed in the battery bus configuration at the 120 W convertor power output level: 

load step change and short circuit. Three tests were performed in the battery bus configuration at the 

138 W convertor power output level: load step change, load profile, and short circuit. Figures 21 and 22 

show the ASC–1 #3 alternator voltage, current, and power and piston amplitude response during the 

supercapacitor tests. 

Figure 21.—The 120 W: Advanced Stirling Convertor (ASC)–1 #3 operating with Dual Convertor 

Controller (DCC) engineering models (EMs) 2 and 3 for Radioisotope Power Systems System 

Integration Laboratory (RSIL) battery bus testing. 

Figure 22.—The 138 W: Advanced Stirling Convertor (ASC)–1 #3 operating with Dual 

Convertor Controller (DCC) engineering models (EMs) 2 and 3 for Radioisotope Power 

Systems System Integration Laboratory (RSIL) battery bus testing. 
NASA/TM—2015-218880 22 

For the 120 W power level, a positive 2 A step load change (Fig. 23) was initiated. The load was then 

removed, as shown in Figure 24. For the 138 W power level, a step change of 4 A was initiated by 

providing 2 A loads on each of two load channels (Table 8). No disturbances observed at the source for 

either of the load tests. 

Figure 23.—The 120 W: 2 A load step change with battery bus. 

Figure 24.—The 120 W: 2 A load removal with battery bus. 
NASA/TM—2015-218880 23 

TABLE 8.—BUS VOLTAGE AND CURRENT FOR BATTERY BUS TESTING 

Power level, 

W 

Bus voltage, 

V 

Bus current, 

A 

RSIL power, 

W 

120 31.2 3.24 101.1 

138 31.24 3.61 113 

Figure 25.—Shorted load test with battery unit. 

Figure 26.—The 141 W maximum load profile. 

For the 120 W power level short circuit test, one load channel was set to 2 A and then shorted. The 

results of this test are shown in Figure 25. For the 138 W power level short circuit test, one load channel 

was set to 3 A and then shorted with a trip time of 85.4 s. For both power levels, no disturbances or 

abnormalities were seen at the source and all systems operated as expected. 

A load profile (Fig. 26) test was performed at the 138 W convertor power output level. The RPS and 

RSIL operated in the manner expected with the DCC maintaining control of the ASCs and RSIL 

providing power via energy storage and dissipating power via the shunt regulator. 
NASA/TM—2015-218880 24 

Dual Input Channel Testing 

As a follow-on to previous single input channel Stirling generator testing in RSIL, a test was 

performed using a flight-like hardware simulation of two RPSs connected to a spacecraft. In order to 

demonstrate two channel operation, the DCC was configured to utilize both of its two controller cards 

simultaneously. Figure 27 describes the RPS setup for this test. The primary objectives were to verify 

stable operation of each DCC card with its DASCSs and to verify two input channel functionality of the 

RSIL with parallel RPS inputs. 

Dual input channel testing included operations using capacitive, battery, and supercapacitor bus 

configurations. Table 9 summarizes the dual input channel DCC testing that was performed in RSIL. The 

results presented in this section are at a convertor power output level of 100 W per DASCS. Table 10 lists 

the DASCSs and RSIL power levels. This testing could not be performed at a higher DASCS convertor 

power output due to limitations of the SRU. Table 10 lists the power level on each RSIL input channel. 

As with the single input channel testing, four main tests were completed, which included load step 

change, load profile, voltage limit, and short circuit. 

Figure 27.—Dual input channel Radioisotope Power System (RPS) test setup.

TABLE 9.—SUMMARY OF DUAL INPUT CHANNEL TESTING 

Capacitor bus Voltage limit 

 Short circuit 

Supercapacitor bus Load step change 

 Voltage limit 

 Short circuit 

Battery bus Load profile 

TABLE 10.—SUMMARY OF DUAL ADVANCED STIRLING CONVERTOR SIMULATOR (DASCS) AND 

RADIOISOTOPE POWER SYSTEMS SYSTEM INTEGRATION LABORATORY (RSIL) POWER 

DCC card no. DASCS no. RSIL input channel DASCS power, 

W 

RSIL power, 

W 

2 1 1 101 78 

3 2 2 100 71 
NASA/TM—2015-218880 25 

Capacitor Bus 

Two tests were performed in the RSIL bus capacitor configuration: voltage limit and short circuit. 

Table 11 lists the bus voltage and current on each RSIL input channel during these tests. Since this was 

the first operation with two RSIL input channels, this test was completed at a lower power level than the 

superacapactior and battery tests. The initial dual input test utilized the bus capacitor to determine 

undervoltage trips for each DCC card. 

For the voltage limit tests, the bus voltage on each RSIL input channel was raised above 36 V and 

then lowered below 22 V to verify each DCC’s ability to disconnect from the bus when out of range from 

its designed bus voltage range. Table 12 shows the voltage at which each DCC disconnected. 

For the short circuit test, one load channel was set to 2 A and then shorted. The trip was realized in 

85.6 s and the short did not impact RPS or RSIL operation. 

Supercapacitor Bus 

Three tests were performed in the RSIL supercapacitor configuration: load step change, voltage limit, 

and short circuit. Table 13 lists the bus voltage and current on each RSIL channel for these tests. Three 

load channels were each programmed for 2 A. The loads were connected concurrently, thus realizing a 

step from no load 0 to 6 A. No disturbances were seen in RSIL or RPS. Table 14 shows the results of the 

voltage limit test. 

For the short circuit test, two of the load channels was set to 2 A for a total load of 4 A. A short was 

imitated on channel 1 and the trip was realized in 83 s. This did not impact RPS or RSIL operation. 

TABLE 11.—RADIOISOTOPE POWER SYSTEMS SYSTEM 

INTEGRATION LABORATORY (RSIL) BUS VOLTAGE 

AND CURRENT FOR DUAL INPUT CHANNEL 

CAPACITOR BUS TESTING 

Input channel Voltage, 

V 

Current, 

A 

RSIL power, 

W 

1 28.53 1.53 43.7 

2 28.53 1.46 41.7 

TABLE 12.—VOLTAGE AT DUAL CONVERTOR CONTROLLER (DCC) 

DISCONNECT FOR DUAL INPUT CHANNEL 

CAPACITOR BUS TESTING 

Test Input channel 1, 

V 

Input channel 2, 

V 

Low voltage 20.77 21.15 

High voltage 36.37 33.8 

TABLE 13.—RADIOISOTOPE POWER SYSTEMS SYSTEM 

INTEGRATION LABORATORY (RSIL) BUS VOLTAGE 

AND CURRENT FOR DUAL INPUT CHANNEL 

SUPERCAPACITOR BUS TESTING 

Input channel Voltage, 

V 

Current, 

A 

RSIL power, 

W 

1 28.6 2.87 82.1 

2 28.6 2.71 77.5 

TABLE 14.—VOLTAGE AT DUAL CONVERTOR CONTROLLER (DCC) 

DISCONNECT FOR SUPERCAPACITOR BUS TESTING 

Test Input channel 1, 

V 

Input channel 2, 

V 

Low voltage 20.76 21.1 

High voltage 36.26 34.2 
NASA/TM—2015-218880 26 

Figure 28.—Dual input channel load profile (212 W max.). 

Battery Bus 

The battery unit was connected as the energy storage for the system with a bus commanded to 

30.57 V. The load profile shown in Figure 28 was run to determine whether there were any negative 

interactions between the two RPSs during load changes. All systems operated within specifications with 

no disturbances seen by RSIL or the RPSs. 

Conclusion 

The results of the tests showed not only the viability of the Radioisotope Power System (RPS) 

convertors to act as a source for future planetary exploration missions, but also demonstrate the ability of 

the Radioisotope Power Systems System Integration Laboratory (RSIL) to provide overall electrical 

power system testing and analysis. The short circuit, overvoltage, undervoltage, and load step transients 

provided by RSIL were effectively managed by the Advanced Stirling Convertors (ASCs) and associated 

Dual Convertor Controller (DCC) both during single and dual input channel testing. This testing paves 

the way for future RSIL validation and verification of the next generation of RPS technology. 

References 

1. Duven, Dennis J., et al.: Performance Analysis of a Fault Tolerant Controller for a Single Stirling 

Convertor. Presented at the 11th International Energy Conversion Engineering Conference, 

San Jose, CA, 2013.

2. Regan, T.F.; and Lewandowski, E.J.: Development of a Linear Stirling System Model With Varying 

Heat Inputs. Presented at the Fifth International Energy Conversion Engineering Conference, 

St. Louis, MO, 2007.