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M N D - P - 2 3 8 0

ENGINEERING REPORT 4055

SNAP I POWER CONVERSION SYSTEM CONTROL DEVELOPMENT

PREPARED BY

NEW DEVICES LABORATORIES, TAPCO GROUP THOMPSON RAMO WOOLDRIDGE INC.

AS AUTHORIZED BY

THE MARTIN C O . PURCHASE ORDER N O , OE 0101

FOR

THE UNITED STATES ATOMIC ENERGY COMMISSION PRIME CONTRACT AT(30-3)-217

1 FEBRUARY 1957 TO 30 JUNE 1959 PUBLISHED 20 JUNE 1960 PREPARED BY; W, E, DAUTERMAN M . W, MUELLER E, J , VITON

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LEGAL NOTICE

This report was prepared as an account of Government sponsored work. Neither the United States, nor the Commission, nor any person acting on behalf of the Commission:

A . Makes any warranty or representation, expressed or implied, with respect to the accuracy, completeness, or usefulness of the information contained in this report, or that the use of any information, apparatus, method, or process disclosed In this report may not infringe privately owned rights; or

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FOREWORD

SNAP I Is the first of a family of devices to convert nuclear energy to electrical for use In space. The SNAP Systems for Nuclear A u x i l i a r y Power - programs are sponsored by the Atomic Energy Commission; the SNAP I prime contractor is The Martin Company. SNAP 1 was designed to u t i l i z e a radio Isotope as the energy source.

The SNAP I Power Conversion System utilizes mercury as the working fluid for a Ranklne c y c l e . A radioisotope Is used as the energy source to vaporize mercury In a boiler; turbo-machinery extracts the useful energy from the vapor and converts it into electrical energy; the exhaust vapor Is condensed by rejecting the waste thermal energy to space in a condenser-radiator.

During the SNAP I Power Conversion System development, Thompson Ramo Wooidridge has been responsible for the development of the following Items:

Turbo-machinery

Mercury vapor turbine Alternator

Lubricant and condensate pump Mercury lubricated bearings Speed Control

Conde nse r-Rad I ator

A series of eight Engineering Reports have been prepared describing Thompson Ramo Wooldrldge's SNAP I Power Conversion System development program. These are as follows: ER-4050 Systems ER-4051 Turbine ER-4052 Alternator ER-4053 Pump ER-4054 Bearings ER-4055 Control ER~4056 Condenser-Radiator ER-4057 Materials

The material In this report deals specifically w i t h the developmental history of the control for the SNAP I Power Conversion System. This report Is submitted as part of the require™ ments of Purchase Order OE-0101 from The Martin Company, Issued under the Atomic Energy Commission prime contract AT(30-3)-217.

TAPCO GROUP _{^ Thompson Ramo Wooidridge inc.}

P

_{TABLE OF CONTENTS}

1.0 Summary 1 2 . 0 Introduction 2 3.0 System Studies 3 4 . 0 Design and Development of Control . . . 26

TAPCO GROUP ^ ^ Thompson Mams Wooidridge inc.

P

1.0 SUMMARY

The results and efforts of work on the controls for the Snap 1 Power Conversion System are presented In this report. The areas of a c t i v i t y discussed here Include preliminary analysis of the system and controls requirements, determination of system specifications, two complete computer simulations of the system with the second simulation Incorporating a l l available component data, a development and test program for all controls h a r d -ware, and Investigation of special system and hardware requirements.

The results of the analysis work and computer studies are presented briefly to substantiate the development program established. A description of test and prototype hardware and performance data Is presented. The satisfactory conclusion to the controls effort Is substantiated by the demonstrated a b i l i t y of all major components to function separately In test stands or c o l l e c t i v e l y in system tests for periods of several thousand hours.

The control package In Its final design consists of two parts, a pressure regulating t h r o t t l e , and a speed sense and amplifier. The pressure regulator Is utilized in the line feeding the boiler to eliminate an Inherent instability In the system due to a positive feedback condition in the pump loop. The speed sense and amplifier are used to detect errors in system operation and provide a corrective signal to reset the % operating point of the regulator. This combination of regulator and speed sensitive

feedback provides satisfactory steady state operation and corrects for minor system disturbances or changes In component characteristics.

•

- T A P C O GROUP ^ ^ Thompson Ramo Wooidridge inc.

2 . 0 INTRODUCTION

The Snap I Povver Conversion Sysii-jm represents a unique application of a well established electrical power generation method - the Rankins cycle power plant. However,

development of this system presented several new and Interesting problems due to its departure from the normal power plant concepts.

One of these areas is the mod.2 of control to be a p p l i e d . A control is required to stabilize the power delivered to the electrical load and also to control the speed of the rotating package. For the application for which Snap I was designed, the electrical power required by the load was to be held constant. The electrical load control was to have been furnished by the user, and was not part of the Thompson Ramo Wooidridge responsibility.

Thermal energy Input to the Ranklne cycle from the radioisotope energy was to have been controlled by means of a heat rejection and temperature control system. This device was to have been furnished by The Martin Company.

The power conversion system control development program was devoted to the develop-ment of the turbine power and speed c o n t r o l , This report describes the analysis of various control modes, the selection of the f i n a l control mode, and the development and performance of the hardware required in this control system.

As a result of analysis and analog simulation of various control systems, the final c o n -trol consisted of a pressure regulator to con-trol boiler Inlet pressure as a function of speed. The control system Incorporated a frequency discriminator to sense speed, a magnetic amplifier to provide the modulation signal, a torque motor to modulate the regulator, and the pressure regulator Itself. It was shown In the analytical and testing program that this simple control is capable of correcting for all disturbances encountered by a space power system.

TAPCO CROUP / \ Thompson Ramo Wooidridge inc.

3.0 SYSTEM STUDIES

At the beginning of the SNAP 1 program It was recognized that the controls area was one of major Importance In the successful development of the power conversion system. Early investigations of the system were made In the following areas to establish a controls program:

1 . System specifications 2. Load variations

3 . Steady state system analysis

4 . Dynamic systems analysis using Laplace and differential solutions 5 . Controls component evaluations

6 . Effects of Inventory variations

7. Summary of system component performance to be used In preparing an analog computer simulation

8 . System startup and shutdown concepts

The first step in the control study program was to define as clearly as possible the exact system requirements to which the control must adhere. Once this was established. It was then necessary to define the system characteristics with which the control must work. Finally, a description of the system was obtained In a form suitable for control analysis. To achieve a power conversion unit of maximum r e l i a b i l i t y and smallest size and weight, it is desirable to maintain a constant Input and output power. One of the objectives of the control study was to achieve such a system despite known variations In the power source and the load and disturbances to the system of a random nature which could not be predicted.

It was necessary that the control system be capable of performing properly when subjected to both the norma! and random system disturbances,

3.1 Computer Studies

Figure 3-1 illustrates the components In the o v e r - a l l power conversion system, Including the associated load banks and system load, heat rejection control and electrical cooling system. Each of the various portions of the power conversion system was examined separately to obtain the required information for that part of the of the mathematical model. To tie the complete system together, a general Information flow diagram Is given in Figure 3 - 2 . This information flow diagram w i t h computed parameter values was the basis of the computer program to simulate the complete system. The objects of this performance and control study were;

1 . To study the basic uncontrolled system on an electronic analog computer. This simulation was capable of simultaneously presenting many Intermediate system quantities such as turbine speed, turbine power, alternator power, pump pressure,

system pressure, boiler and condenser temperature, e t c . , which are all of c o n -siderable Importance In the final selection of the design for the power conversion system and control system components.

OVERALL SYSTEM SCHEMATIC ^

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2 . To study the effects on system performance of idealized control applied to the basic system when the syitem Is subjected to various perturbations or disturbances.

3 . To study the effects on system performance when a control having physically r e a l -izable characteristics is applied to the basic system.

4 . To determine the optimum system gain and speed of response of each of the control systems studied and to select the p'-eferred control system.

5 . To determine the Individual component gains and responses for the control system selected.

6 . To write the specifications for the Individual components to be fabricated and used in a test rig control system.

It was determined that the basic uncontrolled system was Inherently unstable, because of a positive feedback through the pump-boller-turblne loop. The positive feedback effect

Is due to the fact that If turbine speed decreases, the pump speed and hence. Its outlet pressure decrease also. Thus, boiler Inlet, and consequently, turbine inlet pressures d e -crease, causing a further decrease in turbine and pump speed. This effect continues to zero speed. An opposite effect occurs when the turbine speed Increases from the nominal value.

Elimination of this positive feedback through the pressure loop was required to Insure system stability. In order to study the effects of system disturbances on the basic system. It was necessary to open the pressure loop and assume a perfect pressure regulator which would Ideally hold system pressure constant, or to Incorporate a pressure regulator with a negative characteristic. ^The more negative the characteristic of the regulator, the greater the system stability within gain limits.

Due to the problems associated with developing a regulator to operate In the high temper-ature mercury vapor atmosphere af the boiler outlet, studies were conducted with the regulator on the boiler Inlet. Because of the small boiler inventory and the nearly constant boiler wall temperature. It was demonstrated that any pressure transient Induced In the liquid phase of the boiler was felt in a very short time as a change In the pressure of the vapor fed to the turbine.

3 . 1 . 1 Type of Control Systems

On the basis of the studies of the uncontrolled system It was determined that a double c o n -trol was in order to stabilize the power delivered to the electrical load, and to maintain the operating point of the turbine within certain predetermined limits despite the Influence of disturbing forces acting on the system. All the systems discussed In this section Incorpo-rate a primary control In the pressure loop to maintain turbine speed and power output, and a secondary load control designed to maintain the electrical load power at the design point.

TAPCO GROUP AA Thompson Ramo Wooidridge inc.

The various systems studied are listed and consist of the basic system with certain Idealized control mechanisms superimposed to provide a complete system for comparative analysis:

1. The basic system with a boiler pressure regulator to control turbine speed and power, and a saturable reactor current control to limit load power variations.

2 . The basic system with an electrical power sense to provide a feedback signal to modulate boiler pressure thereby maintaining the power developed by the turbine and ultimately the power delivered to the load. Turbine speed Is free to drift In this system.

3 . The basic system with a saturable reactor constant electrical power control and a speed modulated pressure regulator to maintain turbine operating speed.

4 . The basic system with a fixed pressure regulator and a frequency discriminator and amplifier controlling a parasitic load In parallel with the system load.

5 . The basic system with a saturable reactor current llmiter to control load power variation and a turbine back pressure regulator to control turbine speed.

6 . The basic system with an electrical control to match the power absorbed by the load with the power put out by the turbine, and a modulated pressure regulator to maintain turbine power. (Both control mechanisms receive a signal from a speed sensitive frequency discriminator.)

It was determined that all of the proposed systems are stable. However, systems 4 and 5 were excessively Inefficient and system 2 was subject to prohibitive speed excursions. Consequently, subsequent studies were confined to the more favorable systems (numbers

1, 3 and 6) with some consideration given to other problem areas. Systems 1, 3 and 6 were evaluated for a variety of gains and time delays and the most suitable values deter-mined. The limits of stability were found for each system for both Integral and proportlona controls.

A summary of the responses of the three systems studied was prepared and used to determine the most desirable gain and permissible lags in the control components. Figures 3-3 and 3-4 show the limits of stability, response times and effects of time lags on the over-all system; also, suggested operating points are Indicated, The plots are a composite of res-ponses obtained from all runs made and therefore are Intended as a general summary to apply to any system.

The addition of a true "dead time" to the system simulation, such as is approached In the boiler, is of major importance. The result of introducing "dead time" is shown in Figure 3-5 for a control of the proportional type possessing a 0,1 sec response lag characteristic. It may be seen that to prevent over correction and excessive phase shift within the loop.

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TAPCO GROUP ^ Thompson Ramo Wooidridge inc.

gain musf be reduced by a factor of 50. To insure stability It was suggested that a nominal gain of noi- more than 2 , 5 psi/rps be specified for the control regulator. This represented a compromise between the straight time delay Influence and a possible dead time influence within the sysiem. This nominal gain still insures close control of speed for moderate load disturbances.

It had been specified that the power requirements of the satellite that would use SNAP - I would be held nearly constant. This case was Identical to that proposed In system 3 . In view of this consideration and general acceptability of system 3 dynamic characteristics as shown by the computer studies, development work was carried out to produce a mercury pressure regulator to stabilize and limit transient disturbances within the system.

The basic regulator possessed a negative flow-pressure characteristic and was modulated by appropriate electrical components to provide the desired gain and control over system excursions from design point operation.

3.2 Control System Component Specification

Since the pressure regulator was the primary loop control device It had to function to control system pressure despite a failure in the electronic modulation circuit components. This requirement necessitated that the negative feedback be Incorporated In the mechanical design of the regulator, that Is, an increase In pump discharge pressure would result In a decreased boiler inlet pressure.

The frequency discriminator which served as the speed sensing element and the amplifying device were specified w i t h more latitude In design selection as long as the system gain and speed of response were met.

3 . 2 . 1 Pressure Regulator

The pressure regulator was required to operate with an Internal pressure of 0-280 psig and an external pressure of 0 - 1 4 . 7 psla. The operational temperature range was 80-350 F. Due to the nature of the a p p l i c a t i o n , size and weight were minimum and no external leak-age was permitted. The following performance was required:

1 . The regulator was to have a negative regulation characteristic.

2 . Nominal flow of .031 pounds/second was to be controlled within ± 5% 3 . Regulation response time was to be less than ,001 second

4 . Regulator moving parts were to have a 1,000,000 cycle life

5 . The modulation mechanism must operate continuously for 1500 hours

~- TAPCO GROUP ^ ^ Thompson Ramo Wooidridge inc.

6 , Modulation response time was to be approximately .010 seconds 7 . Failsafe modulation was required

3 . 2 . 2 Frequency Discriminator

The frequency discriminator was required to produce a signal proportional to frequency error. The dynamic response of the discriminator was quite fast so that Its transfer function was represented as a gain f a c t o r .

1» The required gain was 0 . 3 volts/cps

2 . Operational frequency range was 1900 - 2109 cps 3 . Operational temperature range was - 30 C to +125 C

3 . 2 . 3 Magnetic Amplifier

The magnetic amplifier was required to amplify the frequency discriminator output to a value which the regulator torque motor could use. The following performance was required;

1 . Response speed was to be .010 seconds.

2 . Power was to be supplied to the magnetic amplifier at 125 volts and 2000 cps. 3 . The maximum amplifier poiwer output was to be 7 watts.

4 . Required operational temperature range was ~ 30 C to +125 C . 3 . 2 . 4 Torque Motor

The regulator torque motor had to be capable of transforming a low level electrical signal Into a mechanical motion with sufficient force to operate the pressure regulator. S p e c i f i -cations for the torque motor were:

1 . The torque motor was required to provide 8 pounds mldposltlon force, with a stroke of ± . 0 0 8 Inches,

2 . The maximum Input power allowed was 3.2 watts. 3 . Maximum allowable hysteresis was 2 % .

4 . Resonant frequency was to be at least 600 aps.

o o 5 . Operational temperature range was required to be -35 C to 200 C .

TAPCO GROUP ^ ^ Thompson Ramo Wooidridge inc.

3.3 Second Simulation of SNAP 1 System

Based on the established specifications a hardware development program was established. Using the performance of the prototype hardware a more refined computer simulation was undertaken. This simulation was intended to reflect the development work In the various component areas. Wherever possible, actual test data was used in establishing the com-puter program. Although the work done on the previous simulation was used as the foundation for this program, the second simulation was more comprehensive and presented a wide

range representation of a'l pertinent components based on actual performance in the laboratory. The simulation is summarized in the form of a block diagram in Figure 3 - 6 .

The complete system was observed during simulations of normal operation, step load tranlents, ramp load variations, and sun to shade transitions. In general, the particular combinations of component characteristics resulted In a stable system w i t h adequate functional performance. The system demonstrated an a b i l i t y to sustain Itself when operating with large lags or d e

-lays or with low performance components, but these circumstances made operation marginal. Various performance factors and pertinent observations are discussed in the following sec-tions.

3 . 3 , 1 System Performance

A number of recordings were made of system operation and transient response to several types of disturbances such as step load variations, ramp load application, and sun to shade transients. The following charts are presented as being representative of simulated system performance under normal circumstances w i t h the Inclusion of moderate lags In boiler res-ponse (2 sec) and the design controller gain of 2 . 5 psl per rps (Figures 3-7 and 3 - 8 ) .

Other system variables were found to respond satisfactorily for these same disturbances. The various inventory and area parameters were observed to change In proportion to v a r i -ations in system pressures. Large thermal capacity In the system tended to make the entire

loop very temperature stable,

A questionable area In this system simulation existed in the presentation of boiler dynamics. The existence of lags and dead times or delays In the boiler pressure loop had a profound Influence on system stability and performance. Since there was no data available defining the dynamics of the boiler at this time a wide range of boiler response characteristics were simulated.

In general, the addition of lags and dead times in the system results in decreased per-formance and s t a b i l i t y . Control gain also is important when considered in systems which Include time lags or delays since, as gain Increases, so does the likelihood of Instability for a given delay or lag c o n d i t i o n .

For small or zero delay times, the time constant or lag in the boiler was very significant

13

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TAPCO GROUP /X, Thompson Ramo Wooidridge inc.

SIMULATED SYSTEM PERFORAAANCE FOR SUN-SHADE TRANSIENT

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2.7 psla 4.74 BTU SEC (Redrawn to Scale) FIGURE 3-7

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in determining the magnitude of load resulting in excessive speed variation of the system- Figure 3-9 shows the maximum step load increase for various lags which would be allowable without producing underspeeding to the point where the pump would no longer sustain the system.

The curves in Figure 3-10 illustrate several characteristics of the system. Curve A indicates the maximum speed transient resulting from a 50 watt step load with various lags and a control gain of 2 - 1 / 2 psi/rps. Curve C then indicates the steady-state error that persists after the transient has died out, for various control gains. Curve B presents the maximum transients that occur from a 50 watt load versus control gain (assuming 0 lag).

Figure 3-11 shows that the magnitude of maximum speed excursions due to a step load change increases very rapidly due to an increase in lag (or boiler time constant). I n -creasing the control gain reduces the maximum speed variation but may tend to cause the system to oscillate or hunt.

Increasing the control gain also enables the system to withstand larger load transients without suffering from speed excursions beyond the nominal control limit ( 2 - 1 / 2 % ) . Figure 3-12 illustrates this fact by indicating that, as gain is increased, large loads may be applied to a system with nominal lags of 2 to 10 seconds while at the same time retaining operating speed w i t h i n 2 - 1 / 2 % of design.

Figure 3-13 may be used to determine the allowable control gain in a system having both lags and delays. As delays are Introduced, the gain must be reduced to prevent or l i m i t oscillations in turbine speed. It was noted that the addition of almost any delay time caused the system to hunt. To limit the magnitude of hunting excursions to within 2 - 1 / 2 % of design speed, the gain must be reduced as shown. The importance of small time lags In boiler response may be seen from these curves. In some instances, the system would make excessive speed variations, as a result of these oscillations and would exceed speed and voltage specifications. The effect of lag and delay time on the magnitude of oscillatory speed variations is shown In Figure 3 - 1 4 .

3 . 4 Motor Startup System

The feasibility of using the alternator as a motor to start the turbo-machinery was investi-gated. Although o permanent magnet alternator has no inherent starting capability with a conventional power source, the alferrwtor could be used for starting purposes if it were

locked into synchronism w i t h a variable frequency power supply. By Increasing the frequency, the speed could be increased to a value ot which the system sustained itself, A continuously variable frequency power source was designed using a permanent magnet alternator and a motor whc^e speed was controllable from 0 to 26,000 rpm by means of a variable transformer. The power output of the alternator could thus be varied from

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LOAD FAILURE VS. BOILER LAGS

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G A I N VS. DELAY FOR SYSTEM STABILITY

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SYSTEM SPEED OSCILLATIONS VS. DELAY AND LAG

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0 to 1300 cps with the voltage being proportional to speed. Preliminary testing of the Snap I alternator with the variable frequency power supply indicated the feasibility of starting the system by using a mechanical programmer and the motor-alternator com-bination.

However, when the power supply was connected to the turbo-machinery package, startup was not achieved because the starting torque requirement was higher than the power supply could produce. A t this time, the motor startup program was cancelled because of budget limitations and no further work was done.

Although the motor startup procedure was attractive from the standpoint of auxiliary and ground support equipment, there was a question of r e l i a b i l i t y in starting the system in a dry c o n d i t i o n . Consequently, the concept of a turbine start with an auxiliary pump or accumulator supplying pressurized mercury appeared more reliable and is the pre-ferred starting scheme.

3 . 5 Special System Considerations

In the course of conducting a thorough study of the Snap I system for the purpose of developing suitable control hardware other areas were studied. In some Instances these areas of investigation necessarily preceded the system simulation and were useful In establishing specifications for the system and associated hardware components. In other cases these studies were conducted to Insure that no serious problem areas r e -mained unnoticed to create difficulties later in the program. Some of the areas of investigation are mentioned in the following paragraphs.

Inventory changes were studied to determine the distribution of mercury in liquid and vapor form throughout the system. If proper flow continuity Is maintained In the

condenser-subcooler region no serious d i f f i c u l t y arises because of Inventory displacement. The majority of the total inventory Is always held in the subcooler and so long as flow forces the condensed mercury to the subcooler, no problems arise from minor disturbing influences such as changes In system temperature due to sun-shade variations or boiler heat transfer area variations.

In accordance with these findings, only minor variations in heat transfer areas result during normal system operation. The system has a natural tendency to stabilize Itself by adjusting condensing temperatures, inventories and heat rejection areas as a result of load changes or environmental changes such as sun to shade variations.

Investigations of the possibility of extending the life span of an orbiting Snap I proved favorable. It was shown that by incorporating several minor control items and by operat-ing at reduced load as the radioisotope fuel decays. It is possible to operate Snap I for a period of 6 months or longer. A variety of arrangements were considered and simulated on the analog computer to determine performance beyond the design life of 60 days for

TAPCO GROUP /A Thompson Rams Wooldridge inc.

the radioisotope f u e l .

Brief investigations of heat transfer considerations showed that the system Is capable of operation on the launch site and during boost into orbit. There Is enough heat r e

-jection area to maintain the condenser system at or near the design point while on the ground and there Is sufficient heat capacity In the system to prevent unreasonable temperature rises or variations from occurring during launch.

TAPCO GROUP /lA Thompson Ramo Wooldridge inc.

4 . 0 DESIGN A N D DEVELOPMENT OF CONTROL

Analysis and simulation of different types of control schemes resulted in the selection of a speed modulated pressure regulator to control turbine speed. Turbine speed was sensed by a frequency discriminator w i t h a magnetic amplifier and torque motor to provide the regulator Input signal. The design and performance of each of the control system components Is described herein,

4 . 1 Pressure Regulating Throttle

In the course of conducting the computer simulation of a controlled SNAP I system, many designs of regulation devices were evaluated. The results of the first computer study showed that these regulating mechanisms In combination with various load controls, provided a stable system. Since SNAP I would be supplying power to a constant load, it remained to develop a pressure regulator and the necessary frequency sense and amplifier. A design of a test regulator was undertaken to produce a single device to be used in evaluating a variety of configurations and permit selection of the best design for a prototype control. This test

regulator was mode to permit variations In regulation pressure, flow characteristics, hydraulic damping, modulation, spring rates, g a i n , and In basic configuration. By means of a series of spacers, springs, diaphragms, bellows, torque motors, " O " rings, collars, and special tubing It was possible to assemble a large number of test devices using these basic parts. The test regulator Is shown in Figure 4 - 1 , This regulator assembly used hydraulic modulation w i t h feedback through a speed sense, an amplifier and a torque motor. Internal f e e d -back is obtained through the lower compound diaphragm assembly. A photograph of the test regulator w i t h some of the spacers and accessory ports Is shown i n Figure 4 - 2 , 4.1.1 Test Facility

To assist In the development of the regulator controls a test booth was designed and f a b r i -cated. This test f a c i l i t y was a self-contained system capable of circulating mercury under a wide range of pressures and temperatures w i t h provisions for static and dynamic measure-ment and recording of regulator performance. The test rig also included provisions for Incorporating electronic hardware to provide for testing of the complete control system w i t h the discriminator and amplifier, and to permit integration Into the analog computer for on operational check w i t h a simulated system . Figure 4-3 Is a schematic of the test f a c i l i t y showing the principal Instrumentation and c a p a b i l i t y ,

4 . 1 . 2 Prototype Regulator Model

The selection of a configuration for a prototype mercury regulator was determined from an extensive series of tests of the various assemblies made w i t h the test model regulator. The selection was based on relative stability, the a b i l i t y to meet the specifications established by the computer study, r e l i a b i l i t y , and simplicity. It was determined during the test

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TAPCO GROUP / \ Thompson Ramo Wooldridge Inc.

program that a suitable negative slope in the pressure characteristic could be obtained by proper selection of the pintal and diaphragm areas and spring rate. Also, because the electrical power consumption of the system load was to be relatively constant, the modula-tion range required in the regulator needed to be only +10% to provide satisfactory control. The latter circumstance permlted the use of the torque motor thrust directly to effect the reset In operating pressure. The required gain was obtained in the external amplifier a n d / or discriminator c i r c u i t .

The prototype regulating throttle as shown in Figure 4--4 and Figure 4-5 Is a modulated force balance regulator. The equilibrium of the system depends upon the spring force, the regulated pressure on the upper diaphragm area, the differential pressure acting on the pintal area, the spring force due to the deflection of both the upper and lower diaphragms, and f i n a l l y the torque motor modulating force. Hence, the operation of the regulator is dependent on the areas, spring rates, and flow rates selected.

The Interna! force generating areas were selected to produce the desired regulator characteristics and to minimize the physical size. The diaphragms proved strong enough to w i t h -stand up to 300 psi mercury pressures without signs of excessive stressing. The diaphragm and pintal area combination was selected to give a slight negative slope pressure charac-teristic about the design point (50 psi increase in inlet pressure = 2 psi decrease in outlet pressure). This renders the regulated flow almost Insensitive to variations In inlet c o n d i -tions,

4,1»3 Prototype Test Performance

Several regulators of the type shown In Figure 4-5 were fabricated and subjected to extensive testing. Data was obtained for static and dynamic characteristics. Typical performance Is Illustrated In the recording shown In Figure 4 - 6 . Other test recordings show typical dynamic response to modulation signals as shown in Figure 4 - 7 .

Other typical test series are listed as follows: 1. Static tests at various flow rates,

2. Dynamic response to changes In supply pressure.

3. Dynamic response to changes In downstream requirements. 4 . Limited shock and vibration testing.

5. Overpressure capability tests.

6 . Modulation cycle l i f e (10,000,000 cycles). 7 . Endurance runs (100 hours minimum),

8 . Examination for mercury corrosion, orifice fouling, and erosion.

No opportunity was presented to conduct environmental tests or complete shock and v i b r a -tion tests, but indica-tions are that the regulator should comply w i t h the specifica-tion in

TAPCO GROUP /A Thompson Ramo Wooldridge inc.

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these areas. Also, no life tests were conducted though prototype regulators were operated in system tests for several thousand hours without sign of failure. In most Instances the prototype regulator met the specifications established by the computer study. However, some changes were necessary to provide optimum performance. The negative pressure regulation characteristic, for example, was reduced to Increase the inherent regulator stability. Other minor changes, such as in the spring rate, were made at a later date to comply with other changes in the system requirements for flow aid operating pressures,

4 . 2 Electrical Control

The purpose of the electrical components of the control wai to bias the pressure regulator of the control system to the correct mode of operation for any given set of conditions. Thus the control system operated over a wide range of conditions and still took advantage of the desired characteristics of the pressure regulator at its design point. The control components Included a speed or frequency sensor, an amplifier and an electromechanical transducer. As shown in the system block diagram. Figure 4 - 8 , a frequency discriminator was designed to provide a speed error signal proportional to changes In alternator frequency. This error signal was then amplified and used as the input current to drive a torque motor, thus producing a longitudinal mechanical force proportional to the frequency error of the system. This force was then employed as the means of continually adjusting the pressure regulator.

4 . 2 . 1 Frequency Sensor

The frequency sensor, as shown in Figure 4 - 9 , was a discriminator circuit consisting of a pair of LC resonant circuits with one tuned to resonate above the operating frequency and one tuned to resonate below this frequency. The output of each resonant circuit was

rectified by a full-wave diode bridge and the two outputs, as shown In Figure 4-10, were then connected to give a differential output characteristic curve shown by the dashed curve. This differential output voltage was developed across R2 and R-, with C3 and C^ acting as a filter. Capacitors C] and C2 were glass dielectric type to give a minimum change with respect to temperature variations.

Since the discriminator was designed to have its null at the desired operating frequency of the system, the d . c . output was polarized with respect to the null. If the alternator frequency were above this null the error signal to the amplifier had one polarity and If the speed were low the error signal had the opposite polarity. As shown by the dashed curve the output was linear over approximately 25 cps on either side of the 2000 cps operating frequency. A thermistor was Incorporated In the discriminator output circuit to compensate for temperature variations.

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TAPCO GROUP AA Thompson Ramo Wooldridge inc.

4 . 2 . 2 Amplifier

Initially it was decided to develop a transistor amplifier capable of supplying the required current to drive a torque motor. Two configurations of a two stage push-pull amplifier were considered. One woi a common collector direct coupled to a common collector which proved to have inadequate g a i n . The second was a common emitter to common collector type. By using germanium transistors, this circuit proved to be a feasible approach.

A later attempt to convert this circuit to silicon transistors, in order to meet higher tempera-ture requirements, presented some problems In obtaining similar characteristics to the

germanium t y p e . This conversion also entailed changing some of the circuit constants to obtain satisfactory performance. This circuit is shown in Figure 4 - 1 1 ,

During the development of the silicon transistor amplifier a parallel program In the design and development of a magnetic amplifier was taking place. Several magnetic amplifier configurations were considered, including two stage schemes. To satisfy all requirements a minimum of four cores were i n d i c a t e d . The configuration using the least number of components was the one employed.

During the design the effective bias resistance was kept as large as possible and the diodes were selected to have low reverse current to maintain a high g a i n . To obtain minimum size and to prevent torque motor overload, an attempt was made to vary the saturation angle over the f u l l range (35° to 180°), The excess gain obtained was reduced through negative current feedback which tended to linearize the gain, to stabilize the gain for load resistance changes, to reduce the effect of other changes, and in general to Improve the time constant. The zero signal torque motor current was reduced where possible in order to reduce power consumption and motor heating. Figure 4-12 is a circuit diagram of the final version which was tested and proved satisfactory.

The magnetic amplifier was chosen in preference to the transistor amplifier and develop-ment of the transistor amplifier was stopped. Some of the reasons that favored use of the magnetic amplifier were as followss

U Reliability

2 . No need for 28 v d . c . 3 . Lower cost,

4 . Ease of procurement.

5. Less of an aging factor,

4 . 2 . 3 Torque A l i t o r

The torque motor was designed to convert a small differential current Into a proportional mechanical force. It was made of two C magnets arranged In opposition, a flat-armature supported by torsion springs, and two coils around the armature wound In opposite directions.

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When a differential current was applied to the coils, a net magnetic f i e l d or control flux was produced In the armature which produced a torque on the armature due to the unbalanced flux distribution. The armature thus rotated until the back torque developed by the torsion springs became equal to the torque produced by the unbalance In f l u x . Figure 4-13 shows the displacement vs differential current relationship. The full stroke of the torque motor was ±,008 Inches,

4 . 2 , 4 System

The final control system w i t h the exception of the torque motor and pressure regulator Is shown in Figure 4 - 1 4 , It was tested and temperature compensated to give less than 1/2% total drift in null between the temperatures of - 2 5 ° C and +125°C. The complete package of components shown in Figure 4 - 1 4 had a volume of 25 cu In and weighed approximately 2 . 5 pounds.

TAPCO GROUP A^ Thompson Kamo WooldridgsIne.

TORQUE MOTOR CHARACTERISTIC

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TAPCO CROUP ^ ^ Thompson Samo Wooldridge Inc.

5.0 CONCLUSIONS

Analytical, experimental, and simulation studies were conducted and resulted In a thorough understanding of the problems associated with the operation of the SNAP ! space power system.

Based upon the system specifications originally established for the SNAP I power conversion system and upon the specifications established from the computer studies relative to control components, all requirements were satisfactorily met.

Test and prototype hardware wa> designed and fabricated to satisfy all requirements. Pre-liminary breadboard test and later systems tests demonstrated the capability of the SNAP 1 turbo-machinery, controls hardware, and auxiliary equipment to function satisfactorily as a system for periods much longer than the 60 day requirement originally specified.

TAPCO GROUP

A Thompson Kamo Wooldridge Inc.

BIBLIOGRAPHY

The reports listed in the bibliography are not available for genera! distribution. Any i n -quiries concerning the a v a i l a b i l i t y of this information should be directed to the AEC. TM 1333 Discussion of LC Discriminators and

Transistor Amplifiers as Related to

the VIP Control 1/8/59 Secret TM 1320 Magnetic Amplifier for VIP Speed Control 12/11/58 Secret ER 3694 Revised Simulation of the SNAP ! System 1/30/59 Secret ER 3234 Dynamic Characteristics and Equations

for a Control System Analysis of a