THE UNIVERSITY OF MICHIGAN COLLEGE OF ENGINEERING Department of Electrical Engineering Instrumentation Report ELECTROSTATIC PROBES FOR ISISA SATELLITE Tuck B. Lee D. F. CrosSby ORA Project 08210 under contract with: NATIONAL AERONAUTICS AND SPACE ADMINISTRATION CONTRACT NO. NAS5-9360 WASHINGTON, D.C. administered through: OFFICE OF RESEARCH ADMINISTRATION September 1967 ANN ARBOR

TABLE OF CONTENTS Page LIST OF FIGURES iv 1.0 INTRODUCTION 1 2.0 SYSTEM DESCRIPTION 2.1 Probe Assembly5 2.1.1 Collector Electrode5 2.1.2 Guard Electrode5 2.1.3 Probe Mount5 2.2 Electronics Unit5 2.2.1 The dc-dc Convertor 9 2.2.2 Dual Range Current Detector 9 2.2.3 Ramp Voltage (AV) Generator 11 2.2.4 AV Limiting Circuit 15 2.2.5 Logic and Associated Circuits 18 3.0 SYSTEM CALIBRATION 21 4.0 DATA CONSIDERATIONS 23 5.0 PACKAGING 24.iii

LIST OF FIGURES Figure Page 1. Electrostatic probe experiment. 2 2. Electronics unit. 3 3. Cylindrical probe. 4 4. Probe assembly drawing. 6 5. System block diagram. 7 6. System schematic diagram. 8 7. Dc-dc converter characteristics. 10 8. Detector schematic diagram. 12 9, Typical output waveform with dummy resistor. 13 10o AV generator and limiter. 1411. Master timer and AV capacitor discharge circuit. 16 12. AV waveform. 17 13. Timing sequence. 19 14. Relay driver and 23 counter circuits 20.15 Calibration scheme. 22 16. Circuit modules. 25 17. Electronics packageo 26 iv

o10 INTRODUCTION This report describes a Langmuir Probe System which has been designed and built to be carried aboard the ISIS-A Satelliteo In this experiment9 the current collected by a cylindrical probe mounted on the satellite is measured as the probe potential is swept through a, range of negative and positive voltages with respect to the spacecraft, as shown in Fig, 1o The voltage output of the system, which is proportional to the collected current, is interpretable in terms of ionospheric electron temperature and density.* In most respects9 the system is the same as those successfully flown on earlier rockets and satellites However, several improvements were incorporated in this system, including: (1) a new ramp voltage generator which is limited if the system output exceeds a specified value, (2) a latching relay driver, and (3) various logic and control circuitry. The system consists of an electronics package, Figo 29 mounted on the interior of the spacecraft and two probe assemblies, one of which is shown in Fig 35, mounted on the exterior of the spacecrafto Coaxial cables connect the probes electrically to the electronics package. *Lo Ho Brace, "The Dumbbell Electro-Static Ionosphere lrobe, 1962

SATELLITE SURFACE GUARD COLLECTOR r A CURRENT DETECTOR IZF -- ----- - 0 OUTPUT. ~lz I L/~~~~~~~~~ 1 RAMP VOLTAGE (AV) GENERATOR Fig. 1. Electrostatic probe experiment.

Fig. 2. Electronics unit. 3

g.o O yr rc pc roob. g 3.'y *

2.0 SYSTEM DESCRIPTION 2.l PROBE ASSEMBLY The probe assembly consists of a probe electrode guard electrode and the probe mount', -which can be attached to or removed as a unit from the spacecraft at any time before the shroud is attached, without disturbing any of the other spacecraft assemblieso The collector and guard electrodes are mounted on a connector with a concentric spring arrangement that allows the probe assemblies to be folded out of the way for stowing a.nd to return to their desired position at shroud ejection, Figure 4 shows details of the probe assembly. 2olol Collector Electrode The collector electrode is a stainless steel rod 9 ino long and 0,022 ino in diametero 2 12 Guard Electrode The guard electrode is a stainless steel tube 0o065 ino O oD and 9 ino longo It is concentric with the collector elecectrode and insulated from it with teflon tubing. The resistance between the two electrodes is on the order of 1010 ohms or greater at room temperatureo The guard electrode is driven -with the same voltage as the collector and hence places the collector beyond the sheath of the satellite surface and provides a continuous sheath at the collector-guard boundaryo 2olo3 Probe Mount The probe mount provides a. base for mounting the probe assembly and also contains two rf filters, one for the collector and one for the guard 9 to prevent conduction of rf interference to the electronics packageo 2.2 ELECTRONICS UNIT The electronics unit contains a dc-dc convertorr two dual range current detectors, a ramp voltage generator^ a master timer, calibration circuits, and logic to control detector sensitivities9 probe switching and other functions. A block diagram of the system is shown in Figo 5 and the schematic is shown in Fig. 60 5

TYPE "HO" FIBER GLASS SLEEVING. 9.000 Fig. 4. Probe assembly drawing.

I'g ~ RY3 ~~_ _ _ n I RY4 RYI RY2 RY3 L.H. R.H. L. H. R. H. SIZE DESCRIPTION PART NO. N NAME NO. ROQ'D. DASH NO. MATERIAL DESIGNED BY TBL APPROVED BY SPACE PHYSICS RESEARCH LABORATORY DRAWN BY JRP SCALE CHECKED BY DATE 12- 2 - 65 DEPARTMENT OF ELECTRICAL ENGINEERING CHECKED BY DATE 2-2-65 THE UNIVERSITY OF MICHIGAN BLOCK DIAGRAM ANN ARBOR, MICHIGAN ESP 141 PROJECT UNLESS OTHERWISE SPECIFIED TOLERANCES ARE: DIM. ENDING.00 ~.030 ANGULAR DIM. DWGG.NO. A E368 DIM. ENDING.000.010 ~ 30MIN. j Fig. 5. System block diagram. 7

15.2L I0OK 10W 11101.2K lOOK RIO 0 RIO! _ lRY3 RY2 Y4 It R1 UPPER PROS_ i3' -C2:'S- ^1 _ I > ^ C Tl ~ ]S o r A, 2 Cs _. PROEB CAIA R.0I1 |...SC D 3GUARD: 80 D D I.II A 2OO~N~l5 K2K - N3S CA!~ <" 33K REG 3c REG - 70 C17 - 5246 R -— 24 —<3_. TIMER_________ DISCHARGE _________ R16 NFll I^- " ----- -^ 1^ -- -4 ^ ^ _________ _ R 6.8/35 2 - DT-DiI 0 1 GUARD. — - ---- - ^' — f -- ^ ------ 49U.41116 —----,1 1 A —--' -— ] OUT OWr -^E " ^ -^-ICs 69V.01 RI R41 | -_ —K t.;0 w 1/ *1 6cR3 C'3 9 H OKA | - ~.'~,l - C4.0< M7A -- 044 (^ \ L 1t ~< o -" A CRII r'2 - 9i II. L -- 1 -- 10 K' K D3 \ -. OK YS-I: oIIgOK o AMP 2 ~'1 R328,C- P 32- *__ri SO- YnTI - (-E. T, I4 s6.9 R24A R2:4J DISCHA- GE6 11 2 KY$1 IOKYSI- C7 - R44C R24 D-' 2.2K!N645,~ _LIN645 33K 1N6A45.01 K3.4K.0_ It 42 ml23 159 50DIK,2, I -AAA L _AAA_ 122 ~ I -,YY~7- YIY7'0 ) 2wP -T 1 ISIG RIET 1 IOUTPUT22 OUTPUT 2A sm mRET 2 AV CIRCUIT GND AV COMMO 2 AV COMMD ONITO &V MONITOR K VT VV ___ 30 Lt] Wio 20 680K AMP 3 1N645 20i R3- G7 R23 - 00 - IN 2 _ 227K( 33u 2N936^ -— ^W"3 ---- ^-. i- D1 -- 021. V - 4 S R2 4 - t3 2 -—' o4v D I4 23 RY4 Y2 R3 ONE ZER O DRIVER ZERO ON ZRIERO ON ZER Q I 5 7 CND~ 1.6 22K -3 V -3V -2*;V'' 26: v'w +.v +v,, V +t *l AV/L F2_N956 1N64 5a DDZ~~~~1 ~10K0 R36 K i 033 Cl1 TI I -1~~ r7 R40;..=-_ 4.7 K 2_ 4.7K. R39 < 10K 131'AA I o FLAG NO.2 7c.01 (DETECTOR) C25 999 L.osour OU s -------. -— ) Pos POWER -V W O., INPUT 1N645.1 IN C26 PUT D22 -,o,PC-DC t -- - 1CONVERTOR -T 33/35 c15 T22/15 oN N 15-4 — P —--— C —-- -— @OWER 01 RETURN N1 ~ FLAG NO.3.01 (PROE) 2N956 C2 (PROS - -- + ) Fl c -I Z _.01 I (RANGCE);)T C29 - SCHEMATIC DIAGRAM ESP 142 T/n'/S Fig. 6. System schematic diagram.

The unit measures 6055 x 4o18 x 1o24 in,, and weighs 1o78 lbo It is designed to operate with input voltages between 18 and 50 v over a temperature range of -400C to +60~C at a constant input current of about 85 ma.o 2o201 The dc-dc Convertor The dc-dc convertor converts the spacecraft supply voltage to suitable voltages for the various electronic subcircuitso It is a, standard convertor circuit utilizing a Zener reference diode for primary regulationo Since the primary voltage is clamped with a reference diode and the load current is kept constant, the operating frequency of the convertor remains constanto To insure that this constant frequency'would meet. the spacecraft requirement of 1800 hz + 35, the characteristics of the transformer core were measured th:rough a pilot run and used for operating frequency computations. A positive temperature coefficient resistor in the convertor starting circuit allows the convertor to turn on over the required temperature range of -40 to +6(C at the minimum supply voltage of 18 Vo Figure 7 shows the test results for the dc-dc convertor of the prototype system; ESP141l The convertor starts its voltage regulation at approximately 18 v input over the required temperature range Since it employs simple primary regulation, the power input is proportional to the input voltage a.nd the input current remains constant (approximately 85 ma) within the regulation range 2o2o2 Dual Range Current Detector A prime instrumentation requirement of a Langmuir Probe experiment is a floating input current detector, as indicated in Figo lo A dc amplifier employing a diode ring modulator* is used in this caseo The diode ring modulator is a proven, reliable system, which is well suited to space instrumentation because of its small size and low power consumptiono It also has a high common mode input voltage capability which is utilized in this systemo Two completely isolated dual range current detectors a.re used to increase the reliability of the systemo Each detector consists of a diode ring modulator, a modulator driver, an ac amplifier and a demodulatoro To insure the accuracy of the current detectors, the following steps were taken; (1) The diodes utilized in the demodulator section were matched over the entire operating temperature range, (2) a temperatu.re compensating resistor network was added to the ac amplifier section to ensure constant amplifier gain, *Lo Ho Brace, "Transistorized Circuits for Use in Space Research Instrumentation," 1959o 9

CONVERTER FREQUENCY *OUTPUT25 xxxX —--— 2C ____ x-_ —----- +60~C X ----- -------— X X K POSITIVE OUTPUT VOLTAGE - 26v 25.5v 1850 + 1800 4 0 * * 0 F -— * V —-------- -— 0 - -—. —--- +60~C ---- -25~C -25v REQUENCY H 0 1750 +. 1700 - I I I I I I I I a I I I I --- - I~~~~~~~~~~ 17 18 I8 19 19 I I I I 20 21 22 23 INPUT VOLTAGE Fig. 7. Dc-dc converter characteristics. 2 24 2 25 I I 26

and (3) the modulator section was prebiased to assure linearity over the entire measured current range. Figure 8 is the schematic diagram of one detectoro The detector input resistance is dominated by a, shunt resistor across the ring modulator. The input resistance of the modulator itself is approximately 100k, The shunt resistances are chosen such that the connection of either detector to the probe will cause not more than 1% deviation at the detector output. The driving power for the modulator is obtained from a, separate winding of the dc-dc convertor transformer. The operating frequency of the transformer is near optimum for modulator operation. The frequency response of the detector is limited primarily by the low pass LC filter of the demodulatoro The detector rise and fall times are 2 and 5 msec, respectively. Figure 9 shows a typical detector output with a dummy probe resistor connected to the input. Detector No. 1: Range: Input Impedance: Output Impedance: Output Voltage: 20 4a, and 2 pa. full scale lot kQ for 20 Ia, 19 kQ for 24a, 3.1 kO for both no. 1 and noo 1A 0-6.8 v (normal mode) 0-5.5 v (AV cmd mode) Detector No. 2: Range: Input Impedance: Output Impedance: Output Voltage: 0.' pa and 0.02 ta. full scale 55.8 kQ for 0,2 pa 93.2 kQ for 0 02 4a 351 kO for both noo 2 and no, 2A 0-6.8 volts (normal mode) 0-5.5 volts (AV cmd mode) o2o23 Ramp Voltage (AV) Generator The ramp voltage (AV) generator produces a continuous sawtooth wave form which is applied to the probe. Two ramp slopes are available and are controlled by ground command. Figure.10 shows the operational amplifier integrating circuit used to generate the ramp voltage. The output voltage of the ramp generator, Er is expressed as follows: 11

DIT8 - hIIIc —- v \n LI HU ~~~~~~~~DEPARTMENT OF ELECTRICAL ENGINEERING ESP 14 +4 I | UNIVERSITY OF MICHIGAN 4.7,- R I 3R9 ~~-22V ENGINEER TBL DRAFTSMAN JRP SPACE PHYSICS RESEARCH LABORATORY AMPLIFIER AND DEMODULATOR DEPARTMENT OF ELECTRICAL ENGINEERING ESP 14 UNIVERSITY OF MICHIGAN ANN ARBOR, MICHIGAN B29018901 Fig. 8. Detector schematic diagram.

Fi g. 9. T.ypical. output wavef-orm -wth dummy res:i stor. 15

r- - -- -- - ---- ---- I ON NORMAL OP MODE 0 1 OFF Af COM'D MODE I O )5 I H-u 4-^ FROM DET. #1 FROM DET. #2 - 26V + 26V Fig. 10. AV generator and limiter. r Ir r% I m - - U - ENGINEER BL I SPACE PHYSICS RESEARCH LABORATORY DEPARTMENT OF ELECTRICAL ENGINEERING UNIVERSITY OF MICHIGAN ANN ARBOR, MICHIGAN DRAFTSMAN JRP *a..-.. -- J I AV AND LIMITER 11-13- 65 5-25-65 ESP 14 B-E301 018-907 DATE

Er - E2 - (E1 - E2) t/rc The starting point is set by the fixed voltage E2 and the slope is determined by (E E1 E2) t/rco The integrating capacitor is periodically discharged by transistor switches which are controlled by the master timer9 as shown in Fig. 11. For normal operation the AV command side of resistor R24 is grounded producing the low level ramp voltage, Lo AV. When the AV command is initiated this groundccrnn.ction is removed thus increasing the voltage levels of El and E2 and producing the higher level ramp voltage. Hi AV. Both wave forms are shown in Fig. 12. Since the measuring accuracy of the system depends upon the accuracy of the ramp voltage applied to the probe, several steps are taken to insure the reliable operation of this portion of the system, among them are the following: a, very stable silver mica capacitor is used for the integrating capacitor; a zero temperature coefficient zener diode is used to stabilize the AV input voltages, El and E2 over the operating temperature range; and the operational amplifier used is a very stable, low offset current device9 manufactured to Milo Specso Also, to increase the reliability of the ramp generator, a series parallel combination of four transistors is utilized for the capacitor discharge circuit, requiring the failure of two transistors to degrade the operation of this critical circuito AV Ramp Generator Data Hi AV Lo AV Magnitude: -4 to + 20 v -2 to +10 v Slope: 12 v/sec 6 v/sec Period: 2 sec 2 sec 2,024 AV Limiting Circuit Due to the unpredictibility of the spacecraft potential, Hi and Lo AV ranges are provided to ensure that the probe voltage will be able to sweep positive enough to accelerate electrons in the plasma, In. the normal operating mode the ramp generato~r is held on Lo AV and the AV limiting circuit is inoperativeo If the Lo AV mode does not supply sufficient probe voltage, due to excessive spacecraft potential, the ramp voltage can be commanded to the Hi AV modeo During this AV command mode, the AV limiting circuit becomes operativeo In this mode the AV ramp voltage is limited to sweep no higher than is required to cause full scale detector output, (approximately 5~5 v) for the duration of the sweepo The purpose of this circuit is to be sure that the positive sweep is no higher than necessary to obtain measurements while still allowing the flexibility 15

Y__ I_ ____ _ __ K& 19-1153 1-65.'IOM IN645 IN645 IO 6 R3 D5 D6 R4 OG r(I r3 oi-^-1 ^- r — 2N2900 I 2 SECOND TIMER _6 C5 5 I I SECOND TIMER -26V | I AV CAPACITOR DISCHARGER 5.6K2N956' R34 ~Y I \TO 3V REGULATOR IN645 D21 J 23 "22K " COUNTER. _ R35 I |ENGINEER TBL DRAFTSMAN JR P SPACE PHYSICS RESEARCH LABORATORY 2 SEC. TIMER & AV CAPACITOR DISCHARGEI DEPARTMENT OF ELECTRICAL ENGINEERING ESP 143 11 UNIVERSITY OF MICHIGAN. 4ANN ARBOR, MICHIGAN B-E295 018-904 R -13- 65 -26-65 DATE Fig. 11. Master timer and AV capacitor discharge circuit.

Fig,:12. AV wave:form.

of increasing the sweep voltage if the spacecraft potential were more negative than anticipated. The following is a description of the circuit operation: Referring to Fig. 6, the ac carrier of either amplifier 1 or 2 is rectified by a diode, filtered by R29, Cll and C12, and applied to the voltage divider consisting of R27 and R28. R27 is chosen such that when the demodulated amplifier output is approximately 5.5 v, the voltage at the junction of R27 and R28 rises to -4.5 v and D18 begins to conduct, stopping the AV sweep. When not in the AV command mode, the AV command 1 line renders the AV limit circuit inoperative by grounding the output of the filter through diode D20. While in the AV command mode, limiting is prevented during the negative portion of the AV sweep by the combination of D19, D23, R41, and R42 which clamp the limiter voltage at -3.5 v, thus rendering the limiter inoperative. 2.2.5 Logic and Associated Circuits The logic circuits, shown in Fig. 5, consist of a master timer, a pulse shaping circuit, a three-stage ripple counter and the latching relay drivers. All sequencing functions of the system, including probe switching, detector range changing, and system calibration, are controlled by the logic circuit and are synchronized with the master timer period as shown in Fig. 13. 2.2.5.1 Master Timer. A simple Shockley diode relaxation oscillator is used for the master timer and is shown in Fig. 11. The timer is actually two sychronized timers, paralleled for reliability. Either timer is capable of operating the system independently. Ihe 2-sec timer period, although not critical, is held towithin ~2%over the operating temperature range by means of a resistor temperature compensating network. The output of the timer drives the capacitor discharge circuit of the ramp generator and the pulse shaping circuit for the ripple counter. 2.2.5.2 Ripple Counter. The ripple counter is a standard 23 counter utilizing three Texas Instrument SN 510 integrated circuit flip-flops. The SN 510's were chosen because of their small size and minimal power consumption. 2.2.5.3 Relay Drivers. Latching relays are used throughout the system for various functions such as probe switching, calibration, and detector range changing. The relay driver circuit, as shown in Fig. 14, utilizes a transistor switch to discharge a, capacitor through a, relay coil to latch the relay in either of its stable states. Thus, the relay does not require the steady power drain of the conventional relay. The control signals for the relay drivers are obtained from the flip-flops of the counter chain. Figure 14 is the schematic diagram of counter chain and the relay drivers.

KO Wfles 1 9 I I I I I II ~ I I _ S 2 SEC 23 COUNTER oI i - i 1. i 6 |- 4 SEC - 23 Io vTER,4 oI O. 0 CO -UL ZOA UAEROI ZERO ______ ~-32 SEC 5 ---- 64 SEC,/1/! 12 —----— S12 SEC ------ ---- E ok- I I OUTPUT Hi _ _ -- - NO. I LO2UA R ZERO OUPUT Ni _____ --- CAL ----. ZUA/ LO2ZERO R ZERO R _ _ _ __ _ _ NO. I UPPER PROBE o LOWER NO2 ENGINEER TBL DRAFTSMAN JRP SPACE PHYSICS RESEARCH LABORATORY TIMING SEOUENCE DEPARTMENT OF ELECTRICAL INGINEERING UNIVERSITY OF MICHIGAN ANN ARMOR, MICHIGAN B- E36 1DAT Fig. 13. Timing sequence.

-~~ --— I - ss s — a9ra* - ~-~ —--- a b O 1i! TI I L___________ _~_v_ __ SN456......... RY 4 DRIVER ] ENGINEER rT r DRAFTSMAN irp SPACE PHYSICS RESEARCH LABORATORY 2' COUNTER & RELAY DRIVER DEPARTMENT OF ELCTICAL ENGIN N ESP 42 UNIVERSITY OF MICHIGAN 6 -65 ANN ARBOR, MICHIGAN B-E302 018-903 DATo a I _ - I - II s, I,,, Fig. 14. Relay driver and 23counter circuits. la

3.0 SYSTEM CALIBRATION One of the systems two detectors is used for current measurements at all timeso The detector not engaged in measurement at any given time is calibratedo The calibration sequence consists of a zero current mode and a simulated current mode for each detector range. During the calibration sequence, the probe is disconnected from the detector. For the zero current mode, only a capacitor, equivalent to the probe cable capacitance, is connected to the detector. For the simulated current mode, a. known resistor, appropriate in value for the associated detector sensitivity, is connected to the detector. The equivalent probe cable capacitance is also connected during this mode. The a.ccura.cy of the detector calibration is limited only by the accuracy of the calibrate resistors and the slope of the ramp voltage. For this reason, the calibration resistors are chosen to with 1% and the AV slope is kept within.1% over the entire operating temperature rangeo To reduce the size of the calibration resistor for the most sensitive detector range (002 pa), this calibration resistor is not returned to ground, but to a point which is at 9/10 of the AV voltageo This allows the use of a. calibration resistor of only 1/10 the value it would normally be if it were returned to ground. Figure 15 shows the scheme. 21

CALIBRATION RESISTANCE Rc.9AV')O ro R ~ Rc EFFECTIVE CAL. R z IORc 9R Fig. 15. Calibration scheme.

4.0 DATA CONSIDERATIONS Due to the great amount of data, obtained during the spacecraft orbital lifetime it is almost mandatory to rely on automated means of data processing. For this rea.son the various modes of system operation are monitored in such a, manner as to lend themselves easily to computer methods of data reduction. Three system monitor flags are brought out of the system. Flag No. 1 indicates the high or low current detection ranges. Flag No. 2 indicates which system is performing the measurement, and Flag No. 3 indicates which probe is being utilized for measurements. If we denote the high (6 v) flag output as "1" and the low (0 v) output level as "0O" the following table describes the operation of the system except for Hi-Lo AV changes. AV will change only on command and will be obvious from the data during calibrate. Flag Output System Operating Mode 1 2 3 Output 1 Output 2 (R) (D) (P) 20 & 2 4a 0.2 & 0.02 va. O 0 0 20 pa measure 0.2 ia calibrate with upper probe and zero reference 1 0 0 2.0 4a. measure 0.02 ia calibrate with upper probe and zero reference O 1 0 20 ta, calibrate 0.2 [a measure and zero reference with upper probe 1 1 0 2.0 Ma calibrate 0.02 ya measure and zero reference with upper probe O 0 1 20 Ma measure 0.2 pa calibrate with lower probe and zero reference 1 0 1 2.0 ia measure 0.02 Ia, calibrate with lower probe and zero reference 0 1 1 20 Ma calibrate 0.2 Ma, measure and zero reference with lower probe 1 1 1 2.0 Ma calibrate 0.02 ya measure and zero reference with lower probe 23

5 0 PACKAGING To accommodate all the components necessitated in the ISIS-A mission, in an existing enclosure design, it was decided to use a high-density packaging module approach. Cord-wood techniques are used for several modules with a three layer system utilized for the latching relay driverso Figure 16 shows some of the individual modules and Fig. 17 shows the inside of the assembled package 24

Fig. ]16. Circuit mnodules. 25

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UNIVERSITY OF MICHIGAN 39III III02 0111 3 9015 03527 0126