The Mars Environmental Compatibility Assessment electrometer is an instrument intended to fly on a future Mars lander mission. The electrometer was designed primarily to investigate (1) the electrostatic interaction between the Martian soil and five different types of insulators attached to the electrometer, which are to be rubbed over the Martian soil. The MECA Electrometer is also capable of measuring (2) the presence of charged particles in the Martian atmosphere, (3) the local electric field strength, and (4) the local temperature. We have tested and evaluated the measurement capabilities of the MECA Electrometer under simulated Martian surface conditions using facilities located in the Electromagnetic Physics Testbed at KSC. The results of the study have demonstrated that rubbing an insulator over the Martian soil simulant does triboelectrically charge up the insulator\u27s surface. However, the charge buildup on an insulator was found to be as low as 1% of the current maximum range of the electrometer when it is rubbed through Martian soil. This indicates that the overall gain of the MECA Electrometer could be increased by a factor of 50, if measurements at the 50% level of full-range sensitivity are desired. The ion gauge, which detects the presence of charged particles, was also evaluated over the pressure range 13 - 533 mbar, and results will be presented

Buehler, M. G.

Calle, C. I.

Gompf, R. H.

Groop, E. E.

Linville, A. W.

Mantovani, J. G.

English

Embry-Riddle Aeronautical University

The Space Congress® Proceedings 2001 (38th) A Space Odyssey - The Next 50 Years May 2nd, 2:00 PM Paper Session II-B - Performance Status of the Mars Environmental Compatibility Assessment Electrometer J. G. Mantovani Florida Institute of Technology - Melbourne C. I. Calle NASA Kennedy Space Center E. E. Groop NASA Kennedy Space Center A. W. Linville Wilkes University R. H. Gompf NASA, Kennedy Space Center See next page for additional authors Follow this and additional works at: https://commons.erau.edu/space-congress-proceedings Scholarly Commons Citation Mantovani, J. G.; Calle, C. I.; Groop, E. E.; Linville, A. W.; Gompf, R. H.; and Buehler, M. G., "Paper Session II-B - Performance Status of the Mars Environmental Compatibility Assessment Electrometer" (2001). The Space Congress® Proceedings. 11. https://commons.erau.edu/space-congress-proceedings/proceedings-2001-38th/may-2-2001/11 This Event is brought to you for free and open access by the Conferences at Scholarly Commons. It has been accepted for inclusion in The Space Congress® Proceedings by an authorized administrator of Scholarly Commons. For more information, please contact commons@erau.edu. Presenter Information J. G. Mantovani, C. I. Calle, E. E. Groop, A. W. Linville, R. H. Gompf, and M. G. Buehler This event is available at Scholarly Commons: https://commons.erau.edu/space-congress-proceedings/proceedings-2001-38th/may-2-2001/11 Performance Status of the Mars Environmental CompatibilityAssessment ElectrometerJ.G. Mantovani, Florida Institute of TechnologyC.I. Calle, NASA Kennedy Space CenterE.E. Groop, NASA Kennedy Space CenterA.W. Linville, Wilkes UniversityR.H. Gompf, NASA Kennedy Space CenterM.G. Buehler, NASA Jet Propulsion LaboratoryAbstractThe Mars Environmental Compatibility Assessment electrometer is an instrument intended to fly on afuture Mars lander mission.  The electrometer was designed primarily to investigate (1) the electrostaticinteraction between the Martian soil and five different types of insulators attached to the electrometer,which are to be rubbed over the Martian soil. The MECA Electrometer is also capable of measuring (2)the presence of charged particles in the Martian atmosphere, (3) the local electric field strength, and (4)the local temperature. We have tested and evaluated the measurement capabilities of the MECAElectrometer under simulated Martian surface conditions using facilities located in the ElectromagneticPhysics Testbed at KSC. The results of the study have demonstrated that rubbing an insulator over theMartian soil simulant does triboelectrically charge up the insulator's surface. However, the charge buildupon an insulator was found to be as low as 1% of the current maximum range of the electrometer when it isrubbed through Martian soil. This indicates that the overall gain of the MECA Electrometer could beincreased by a factor of 50, if measurements at the 50% level of full-range sensitivity are desired. The iongauge, which detects the presence of charged particles, was also evaluated over the pressure range 13 -533 mbar, and results will be presented.1. IntroductionThe Mars Environmental Compatibility Assessment (MECA) Electrometer was designed jointly atNASA's Jet Propulsion Laboratory and Kennedy Space Center to be a flight instrument for a futureunmanned Mars lander mission [1]. The MECA Electrometer was designed primarily to investigate theelectrostatic interaction between the surfaces of insulating materials and the soil on the surface of Mars.The insulating materials were selected for their use on previous space missions. The five insulatorsselected for the MECA Electrometer were: Fiberglass/Epoxy, Polycarbonate (known as LexanT M),Polytetraflouroethylene (TeflonTM), Rulon JTM, and Polymethylmethacrylate (LuciteTM or PMMA).It was not the purpose of the research described in this paper to study the electrostatic properties ofthe five types of insulating materials mounted on the electrometer. Rather, the goal of the research was totest and evaluate the performance of the MECA Electrometer and the four types of measurement sensorsit has onboard: (1) triboelectric sensor array, (2) ion gauge (charged particle sensor), (3) local electricfield sensor, and (4) temperature sensor.Testing the triboelectric sensor array (tribo) involved measuring the degree to which the insulatorsurfaces became electrically charged when rubbed over Martian soil simulant (JSC Mars-1) either at lowCO2 pressure inside a vacuum chamber, or under low humidity conditions at atmospheric pressure usingdry air in a glovebox. Ottawa quartz sand and, at the end of the study, Lunar soil simulant were alsostudied as abrasive mediums that could triboelectrically charge the surfaces of the insulating materials.The ion gauge (IG) was tested using a reliable generator of atmospheric ions. The IG output voltage wasmeasured as a function of CO2 pressure inside a small vacuum chamber. We tested the electric fieldsensor (ELF) by applying a fixed voltage to a flat metal plate that was placed above the ELF electrode.These measurements were conducted at various CO2 pressures inside a vacuum chamber. Finally, thetemperature sensor located inside the MECA Electrometer could not be studied because the Mars vacuumchamber that provides temperature control down to typical Martian surface temperature was still underconstruction in the Labs and Testbeds Division at KSC. However, the temperature sensor was observed towork properly at room temperature, where all of the measurements were conducted. The data and resultsof the testing of the MECA Electrometer are presented in the following sections. We conclude with asummary of our evaluations of the MECA Electrometer, and our recommendations for how the MECAElectrometer’s performance might be enhanced.2. Results and DiscussionIn this section, we describe the four types of measurement sensors contained within the MECAElectrometer. Data taken with the sensors is presented, and each sensor's performance is discussed.2.1 Evaluation of the Triboelectric SensorsThe triboelectric sensor array consists of five insulating materials placed above metal electrodes thatare connected to five independent electrometer circuits, one circuit for each type of insulator. The tribosensors are housed inside the MECA Electrometer, whose case is made of titanium of volume ~50 cm3,total mass of ~50 g, and power consumption of <250 mW. Figure 1 contains a picture of the electrometer,and shows how it would be mounted to the heel of the Mars lander’s scoop.The five small circular patches shown in Figure 1 are the five types of insulators. Below these patchesare the electrometer circuitry (not shown) that measure the amount of electric charge that develops on theinsulator surfaces after the scoop drags the electrometer through the Martian soil. The two openingsshown above the five insulators in the electrometer photo are the local electric field sensor (ELF) on theleft, and the ion gauge (IG) on the right. The temperature sensor is a dedicated integrated circuit chip thatis mounted inside the case, and is not shown in the photo of the MECA Electrometer.The electronics contained within the MECA Electrometer housing have been described elsewhere [2],and will only be discussed as needed in this paper. The tribo sensor circuit has an output voltage, Vout, thatis proportional to the electric charge that develops on the surface of the insulator. The circuit is shownschematically in Figure 2. The overall gain of the tribo circuit (not shown) is 4x. Thus, Vout = 4Q/C,where the fixed capacitor C = 1 nF. So, the charge Q on the surface of the insulator must be given byQ = Vout / (4x109 F-1). Hence, the tribo circuit’s gain is 0.25 nC/V as measured at the output.  If the A/Dconverter has a 2mV per bit resolution, then the device can detect an amount of electric charge as small as0.5 pC, which is numerically equivalent to 3.1x106 electrons or protons.2.1.1 Electrostatic Rubbing ExperimentThe tribo sensors were tested using two methods. The first method described here used a “rubbing”machine that allowed the MECA Electrometer to be dragged through soil with the tribo sensors face downin the soil. The horizontal motion of the electrometer was under computer control that allowed the speedand travel distance to be set separately. A speed of 10 mm/sec was suggested as the speed at which theMars Lander will drag the electrometer through the Martian soil. The size of the rubbing machine did notallow us to perform this type of experiment in a low pressure CO2 atmosphere in the small vacuumchamber. However, we were able to conduct this rubbing experiment inside a plastic glovebox, into whichdry air flowed so as to reduce the humidity within the box. The relative humidity could be brought below30% by continuously flowing bottled dry air into the box. An air ionizer was used to neutralize thecharges that develop on the tribo materials after each measurement was made. Data was taken using theParallax Basic Stamp IITM controlled by a PC laptop running MS-Windows 95TM.Using this experimental setup, the MECA Electrometer could be rubbed over various types of soils,including quartz sand (SiO2), Martian soil simulant (JSC Mars-1) [3], and Lunar soil simulant [4]. OnlyMartian soil simulant was investigated using the rubbing machine. A typical graph of the tribo sensoroutput voltages versus time are shown in Figure 3. This data was taken at atmospheric pressure in dry airat a relative humidity of 43% with the MECA Electrometer being rubbed over the Martian soil simulant at10 mm/s. The time between points is 1.4 seconds.Figure 3Figure 3 indicates that only small amounts of triboelectric charging will occur when the Martian soilsimulant is used. The Rulon JT M insulator appears to charge up the most. The tribo voltage for thismaterial changed by -0.04 V, which corresponds to a charge buildup of –10 pC. This is close to the A/Dconverter resolution of 0.5 pC/bit.2.1.2 Electrostatic Rocking ExperimentSince the rubbing machine could only be operated in dry air at atmospheric pressure, an alternativemethod of triboelectrically charging the insulators was needed that would work at low CO2 pressures. Arotary feedthrough mounted on a vacuum chamber was used to rotate an aluminum box containingMartian soil simulant back and forth. The MECA Electrometer was mounted flush with the bottom of thebox, and the tribo sensors were facing upwards. As the box was manually rocked back and forth, the soilwas able to flow over the tribo sensors and cause the insulators to charge up. Using this experimentalsetup, it was possible to place 50 g of Martian soil simulant in the box. The vacuum chamber wasevacuated of air using a mechanical pump. When a pressure of 1 Torr was reached, CO2 from a gascylinder was allowed to flow into the vacuum chamber through a microvalve mounted on the vacuumchamber. This microvalve and a gas regulator attached to the CO2 cylinder allowed sufficient control ofthe flow of CO2 into the chamber that any desired CO2 pressure could be maintained. Figure 4 shows thetribo sensor responses (data acquired at 1.4 sec time intervals) for the five insulators at 7 Torr CO2pressure when Martian soil simulant is used in the rocking experiment. An air ionizer was placed in thevacuum chamber to later neutralize any charge buildup on the tribo material’s surfaces.Figure 4This rocking experiment was repeated at increasing CO2 pressures with the Martian soil simulant andlittle change in the tribo sensor responses was discerned. Different and larger responses were obtainedwhen Ottawa quartz sand was used. Whereas the Martian soil simulant tended to charge the tribo sensorspositively, the Ottawa sand charged the same materials negatively, or not at all. Lunar soil simulant wasalso used in the rocking experiment, but only in dry air at atmospheric pressure in the glovebox.2.2 Evaluation of the Ion GaugeThe ion gauge measures the presence of charged particles and atmospheric ions in the vicinity of theMECA Electrometer. The ion gauge circuit is basically a current-to-voltage converter as shown in Figure5.  In the actual circuit, the electrode is held at either a positive or a negative voltage. This causes negativeor positive charges, respectively, to accelerate towards the electrode, thus producing the electric currentthat the circuit detects. The ion gauge was tested using a weak Am 241 source located at a fixed distanceabove the electrode.The Am 241 source produces alpha particles that ionize any CO2 molecule they collide with. In orderto detect the presence of both positive and negative charges, the voltage at the electrode was cycledbetween the values +3V, 0V, -3V, and 0V. A typical ion gauge sensor response is shown in Figure 6 withthe data taken at 5 sec time intervals. From the actual ion gauge circuit, the overall circuit gain wascalculated to be 42.2 pA/V. In Figure 6, an output voltage of 420 mV would thus correspond to an ioncurrent of 18 pA. If we assume an A/D converter resolution of 2 mV/bit, the minimum current sensitivityof the ion gauge is found to be 84 fA, which exceeds the input bias current of the operational amplifier.The dependence of the deduced ion gauge current on CO2 pressure is shown in Figure 7. Ion currentvalues due to both positive and negative currents are shown.Figure 6Electrode (ion collector)V = I RIon Gauge (I-to-V conv.)R = 1GI+-Figure 5Figure 72.3 Evaluation of the Local Electric Field SensorThe electric field sensor (ELF) circuit is the same as the tribo sensor circuit, except that the insulatoris absent from the top of the electrode. To test the ELF, a metal plate at a fixed voltage is placed at aknown distance above the electrode. For the MECA Electrometer, the distance between the plates isapproximately 0.3 cm. The ELF sensitivity is calculated using the expression (Vplate/d) / VELF where VELFis the potential difference between the plate and the electrode. The ELF circuit had a voltage offset of+0.032 V. When +1000 V was applied to the plate, the ELF output voltage was measured to be +0.265 V.And when Vplate = -1000 V was used, Vout = -0.199 V. After the offset voltage is taken into account, wefind | VELF | = 0.231 V. The sensitivity of the ELF is then calculated to be  (1000V) / (0.3cm) / (0.231V) =14.4 kV/cm / V.2.4 Evaluation of the Temperature SensorThe temperature sensor was not specifically tested. All measurements were conducted at roomtemperature, which appeared to have been measured correctly by the temperature sensor. The Marsvacuum chamber, which will be able to simulate the range of temperatures and atmosphere, was underconstruction while this research was being conducted, but can it can now be used to test the entire MECAElectrometer under simulated Martian surface conditions.3. ConclusionsAlthough the rubbing experiment using Martian soil simulant (JSC Mars-1) could not be performed ina low CO2 atmosphere, the rocking experiment described in Section 2.1.2 could be conducted in either theglove box in dry air, or in the small vacuum chamber in a low CO2 atmosphere. We found that there wasnot a significant amount of difference observed in the tribo sensor output using the rocking chamber in alow CO2 atmosphere or in dry air at atmospheric pressure. This would suggest that if the rubbingexperiment had been performed in a low CO2 atmosphere, then the tribo sensor responses would not bemuch different than in the dry air environment. The observation that the tribo sensors developed suchlittle electric charge would indicate that the MECA Electrometer’s triboelectric sensor circuitry should bemodified to provide a larger gain (increased sensitivity) that would allow the charge buildup to be moreaccurately measured. From Figure 3, we see that when the maximum tribo output voltage change ismeasured to be 0.04 V for Rulon J TM, it appears that as little as (40 mV/ 4096 mV x100%) = 1% of theA/D converter’s range is being utilized. If a utilization of 50% of the full range is desired, then the overalltribo circuit’s gain should be increased by a factor of 50. Finally, the largest output voltage that could beobtained by vigorously rubbing the tribo materials with wool was 1.5 V, which corresponds to only 18%of the full range of the A/D converter.The performance of the ion gauge was found to be near ideal. The current sensitivity was calculatedto be 42.2 pA/V. That is, if the ion gauge output voltage was measured to be 0.002 V (2mV)corresponding to the bit resolution of the A/D converter, then the ion current would be 84.4 fA, which isvery close to the input bias current of the LMC 6044 op amp used in the current-to-voltage converter.The sensitivity of the local electric field sensor was measured to be 14.4 kV/cm / V. That is, at a plateseparation distance of 0.3 cm and with a 1 kV potential difference between parallel plates, the electricfield sensor produced an output voltage of 0.231 V.The temperature sensor consisted of a dedicated integrated circuit chip. Since all experiments thatwere conducted during this research were at room temperature, the temperature sensor’s performanceunder simulated Martian temperature conditions was not determined.AcknowledgementsThis research was supported in part by the NASA-ASEE 2000 Summer Faculty Fellowship Program,and by Undergraduate Space Research Program grants from the Florida Space Grant Consortium.References:[1] M. Buehler, L.-J. Cheng, O. Orient, M. Thelen, R. Gompf, J. Bayliss, J.Rauwerdink, “MECAElectrometer: Initial Calibration Experiments”, Inst. Phys. Conf., Ser. No. 163, pp. 189-196, Proceedingsof the 10th Int. Conf., Cambridge, 28-31 March 1999.[2] M.G. Buehler, et al., “From Order to Flight in Eighteen Months: The MECA Electrometer CaseStudy”, IEEE Aerospace Conference, March 2000.[3] C.C. Allen, et al., “Martian Regolith Simulant JSC MARS-1”, Lunar and Planetary ScienceConference XXIV, Houston, TX, 1998.[4] D.S. McKay, et al., “JSC-1: A New Lunar Regolith Simulant”, Lunar and Planetary ScienceConference XXIV, Houston, TX, 1998.

Paper Session II-B - Performance Status of the Mars Environmental Compatibility Assessment Electrometer

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Paper Session II-B - Performance Status of the Mars Environmental Compatibility Assessment Electrometer

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