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NASA Technical Reports Server (NTRS) 19990014049: Electrostatic Charging of the Pathfinder Rover PDF

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Preview NASA Technical Reports Server (NTRS) 19990014049: Electrostatic Charging of the Pathfinder Rover

AIAA 96-0486 Electrostatic Charging of the Pathfinder Rover Mark W. Siebert University of Toledo Toledo, OH Joseph C. Kolecki NASA Lewis Research Center Cleveland, OH 34th Aerospace Sciences Meeting & Exhibit January 15-18, 1996 / Reno, NV For permission to copy or republish, contact the American Institute of Aeronautics and Astronautics 370 L'Enfant Promenade, S.W., Washington, D.C. 20024 AIAA 96-0486 ELECTROSTATIC CHARGING OF THE PATHFINDER ROVER Mark W. Siebert University of Toledo Toledo, OH Joseph C. Kolecki NASA Lewis Research Center Cleveland, OH ABSTRACT conditions simulating the surface conditions expected at Mars. Tests The Mars Pathfinder mission will send a showed that a rover wheel, driven at lander and a rover to the martian typical rover speeds, will accumulate surface. Because of the extremely dry electrical charge and develop significant conditions on Mars, electrostatic electrical potentials (average observed, charging of the rover is expected to 110 volts). Measurements were made occur as it moves about. Charge of wheel electrical potential, and wheel accumulation may result in high capacitance. From these quantities, the electrical potentials and discharge amount of absolute charge was through the martian atmosphere. Such estimated. An engineering solution was discharge could interfere with the developed and recommended to operation of electrical elements on the mitigate charge accumulation. That rover. solution has been implemented on the actual rover. A strategy was sought to mitigate this charge accumulation as a precautionary INTRODUCTION measure. Ground tests were performed to demonstrate charging in laboratory A free ranging, solar powered surface rover will be delivered to Mars as part of the Pathfinder mission. Pathfinder Copyright © 1995 by the American Institute of will be launched in December, 1996, Aeronautics and Astronautics, Inc. No and will land on Mars on July 4, 1997. copyright is asserted in the United States The rover will travel about on the under Title 17, U.S. Code. The U.S. " Government has a royalty-free license to martian surface, conducting technology exercise all rights under the copyright claimed experiments and serving as an herein for Government purposes. All other instrument deployment mechanism rights are reserved by the copyright owner. (Ref. 1). Because of the extremely dry conditions on Mars, electrostatic electrostatic probe, and multiplying the charging of the rover is expected to result by the wheel capacitance occur as it moves about. Experiments (approximately 74 pico-farads). The conducted in a simulated martian charge is believed to arise during environment at the NASA Lewis compaction of the ALS by the moving Research Center have shown that a wheel. rover wheel, driven at typical rover speeds, will accumulate electrical To dissipate accumulated electrical charge on the order of 8X 10_ C, and charge, a metal point was attached to develop electrical potentials averaging the wheel. On the average, the wheel 110 V with transients 2- to 3-times that without any point developed an value. average steady-state electrostatic voltage of 110 V. Adding a discharge Voltages of 100 V and greater are point resulted in electrical potential believed sufficient to produce electrical reductions of up to 15 - 20% of the discharge in the Martian atmosphere. steady state value. The reduction in Also, with an accumulated net charge of 8 potential is believed due to the X 10_ C, and average arc time interval of dissipation of image charge from 1 _s, arcs can occur with estimated arc conducting surfaces of the wheel. currents reaching almost 10 mA. Charge connected with adherent dust Discharges of this magnitude could grains bleeds off very slowly due to the interfere with the operation of sensitive low conductivity of the dust. electrical elements on the rover (Ref. 2, 3,4). As a result of the tests conducted at Lewis, and recommendations made to In laboratory tests, a typical wheel was JPL, the Pathfinder rover will driven in Arizona Lunar Simulant (ALS). incorporate small wire discharge points This material was chosen because of its on the rover at the base of the rover availability (from NASA/JSC), and its antenna. This component is electrically relatively low outgasing rate, enabling continuous with the other parts of the chamber pumpdown to be rover. accomplished within a single day. About a pound of the material was EXPERIMENT DESCRIPTION used. In December, 1993, a memo was sent When the wheel was driven at typical to the Pathfinder Office at the Jet rover speeds (.76 - 1.5 cm/s) in the Propulsion Laboratory (JPL) which ALS, electrical charging occurred. The stated the result of a preliminary charge accumulated was determined by calculation made of rover potential and measuring the electrical potential of the charging rate. While the calculation was wheel using a capacitively coupled only an estimate, it pointed to possibilities that were of interest to the background pressure of 8x10 6 torr, Pathfinder team. As a result of this then filled with Martian atmospheric communication, NASA Lewis was simulant (approximate composition: requested to perform laboratorytests to 0.03% water, 0.153% carbon determine whether chargingwas a real monoxide, 0.336% oxygen, 1.36% issue,and whether it could be mitigated argon, 2.52% nitrogen, and balance by addition of a dischargepoint. carbon dioxide) to a pressure of 7 mbar. The wheel was mounted in a A Mars simulator facility had already rotary tray filled to a depth of 15 mm been set up to conduct tests for the with ALS, and driven with a variable- Wheel Abrasion Experiment (WAE, Ref. speed motor, external to the chamber. 5). This facility was adaptedto examine A xenon arc lamp provided a collimated the problem of electrostatic charging.A light source to the wheel through a capacitively coupled electrostatic probe quartz window. A computer connected was installed, andtests were runwith a to an analog-to-digital converter simulated Mars surface atmosphereand continuously recorded data during the Arizona Lunar Simulant (ALS). tests. (Fig. 1). Experiments were conducted with two different wheels, hereafter referred to Procedure as the test wheel and the System Integration Model (SIM)wheel. The test The test wheels were run at speeds of wheel was a crude mock-up of the 0.6 to 5.4 rpm. (The speed of the flight rover wheel, made to approximate unit rover wheel on Mars will be 0.6 to dimensions. The SIMwheel was a more 1.2 rpm). All outputs from wheel detailed model of the flight wheel. Both diagnostics were recorded. Test were driven at typical rover speeds. duration was typically > 1000 s. The Charging was found to occur. electrostatic probe was initially zeroed to chamber ground prior to each test, The rover wheel, driven at typical rover then repositioned and held speeds, accumulated an electrical perpendicularly to the wheel at 2 mm charge of about 8 X 10_ C, and during the test. All tests were developed electrical potentials on the conducted at room temperature, 23 °C. order of 110 V with transients 2- to 3- times that value. Two different wheels were used: the test wheel and the SIM wheel. Two Facility Description types of discharge points were attached to each of the wheels: a 12.7 The Mars simulator facility is a 62 cm pm radius stainless steel wire cut at the diameter by 62 cm long vacuum end, and a tungsten wire chamber. For charging tests, the electromachined at its end to a point of chamber was initially pumped to a radius 1 pm (Fig. 2). 3 3. The weight of the wheel was For the test wheel, the 12.7 gm changed. Electrostatic charging was stainless steel point and the tungsten measured with and without a point were soldered to a braided copper counterbalance. (In its usual wire. The braided copper wire was configuration, the wheel was wrapped around the wheel and held counterbalanced to a net load of 500 down with Kapton tape. The braided gm, the weight on one wheel of the copper wire with the discharge point rover in Mars gravity. When the was brought to the wheel axle, and counterbalance was removed, the load extended outward 5 cm. (Fig. 3). on the wheel increased to 1588 gm.) A 20% increase in wheel electrical For the SIM wheel, the tungsten and potential was observed with the higher 12.7 ]_m steel points were soldered to a load. copper wire. The copper wire was held in contact with the wheel by one of the These simple experiments implied that cap screws on the rim. The copper was the dominant mechanism of charge brought to the wheel axle, and generation was associated with dust extended 5 cm as before. compaction. Small grains assumed a positive charge, larger grains, a RESULTS AND DISCUSSION negative charge. The smaller grains were light enough to adhere to the Mechanisms assumed responsible for wheel surface, and carried their net wheel charging included dust positive charge with them. The compaction, and friction of the wheel in electrostatic probe registered the the ALS. These assumptions were corresponding electrical potential. The tested in simple experiments. entire metal surface of the wheel became charged through image charge 1. The wheel axle was deliberately generation. The addition of a point jarred while the wheel was charged. removed some of the image charge, Large quantities of dust were observed allowing the wheel potential to drop to fall from the wheel, and the potential proportionally. The track behind the immediately dropped from 70 V to < moving wheel became charged with an 12V. equal charge of opposite sign. This charge was also observed with the 2. A braking force was applied to the electrostatic probe. tray increasing slippage of the wheel from a nominal 10% to close to 100% Test Wheel in the ALS. No substantial changes in potential were noted. A plot of electric potential vs. time for the test wheel is shown in Fig. 4. The initial transients and the steady state 4 region are labeled. These data were SlM Wheel taken with no discharge point on the wheel. The potential settles to its The steady-state potentials of the test steady-state value within six wheel, with 500 gm load, no discharge revolutions. point, a 12.7 ]_m stainless steel point, and a tungsten point are shown in Fig. Comparison of the steady-state 7. The initial voltage transient of the potentials with no discharge point, a wheel may be as large as three times 12.7 _zm stainless steel discharge point, the steady-state voltage. The reason for and the tungsten point are shown in this transient is not clear. A chamber figs. 5. Each of the two types of points related effect might be the most produced about the same reduction in reasonable first assumption. But since voltage. Increasing the number of 12.7 demonstrating rover charging and _m radius stainless steel points from 1 possible mitigation schemes were the to 7 gave no further reduction in primary objectives of these tests, and potential. since time was short, a good deal of additional science was left undone. The effects of wheel loading and of adding a discharge point to the wheel Discharge Curves are shown in Fig. 6. A wheel with a load of 500 gm and seven 12.7 _zm The SIM wheel was initially charged by radius stainless steel discharge points running it in the ALS. The wheel was shows a reduction in steady-state then stopped and allowed to discharge potential by 14% 1over a wheel with no into the test environment. Wheel point at comparable speeds. Retaining potential was recorded during the seven points on the wheel, and discharge. The resulting data were increasing the wheel load from 500 gm fitted to a decaying exponential and to 1588 gm produced an increase in time constants were inferred. These steady state potential by 16%. With time constants are: no discharge point, increasing the wheel load from 500 gm to 1588 gm 3555+430 s, no discharge point increased the steady state voltage by 2898+386 s, 12.7 ]_m point 25% at similar speeds. 2553+340 s, 1 ]_m point A very slight dependence of voltage on Data are averaged over the range of wheel speed is also seen in Fig. 6. speeds, 0.6 to 5.4 rpm. Comparison with the case of no point shows that 'o the time constant is lower by ==20% when either the 1 i_m or the 12.7 ]_m z Percentages quoted are obtained by discharge point is present. The averaging over alldata points. discharge points have a significant 20 _m (Fig. 10 a), compared to the effect on dischargerateof wheel. 100 ]_mof the bulksample(Fig.10 b). DustCharging Capacitanceof SIMwheel The negative charge left behind by the The capacitance of the SIM wheel SIM wheel after its passage over the (without the discharge point attached} ALS was measured with the was measured with the chamber electrostatic probe appropriately backfilled with 7 mbar Martian repositioned. This charge was recorded atmosphere simulant and Arizona Lunar for all three point configurations of Simulant. A capacitance meter, with a points (above), and different wheel range of 1 pF to 200,000 mF, was speeds. Results are shown in Fig. 8. connected through an electrical The dust charged negatively to feedthrough between the wheel and approximately the same absolute value the chamber (ground). The capacitance of potential as the wheel. Grain size of the wheel relative to the chamber, measurements (see below) showed that averaged over three sets of small grains adhered to the wheel, large measurements, was found to be 74 pF. grains did not. The negative values of Thus at a mean potential of 110 V, the the ALS potential reveal the charge absolute charge of the wheel may be separation between these differently calculated as 8.1 X 10.9 C. sized grains. CONCLUSIONS Electron Microscopy of Arizona Lunar Simulant The tests reported indicate that the Pathfinder rover, in its traverses over The scanning electron microscope was the martian surface, may become used to determine the size and the sufficiently charged to raise its electrical shape distributions of the Arizona Lunar potential to within the neighborhood of Simulant. Energy Dispersive X-ray 110 V. Voltages of 100 V and greater are Analysis (EDX) spectra were also taken believed sufficient to produce electrical to confirm its chemical composition. A discharge in the Martian atmosphere. typical EDX spectrum, with elemental Also, with an accumulated net charge of 8 peaks labeled, is shown in Fig. 9. (The X 104 C, and average arc 5t of 1 ps, arcs Au peak is due to a 200 A thick sputter can occur with estimated arc currents coating to discharge the ALS particle reaching almost 10 rnA Discharges of surfaces). Electron microscop.e this magnitude could interfere with the photographs were taken of the material operation of sensitive electrical that adhered to the wheel. The average elements on the rover. grain diameter of the simulant that adhered to the wheel is on the order of In the nominal mission on Mars, the closest analogy is a Van de Graft rover will travel approximately two generator in which the moving rover is meters per day. During that time, the the source of static charge, the point is wheel will make at least five complete the brush, the wind blown dust is the revolutions in the martian regolith. belt, and the martian surface to which Experience with fine, dry clays in the dust eventually, returns is the terrestrial laboratories demonstrates accumulator. Perhaps an analog circuit that charging is commonplace. It is not model of this mechanism may be unreasonable to expect that the rover developed once more is known of the on Mars may charge more strongly electrical characteristics of the actual than the wheel did in the laboratory. martian surface material that will be Discharges between active encountered. components, or between the rover and its surroundings may become very REFERENCES likely. Since actual martian conditions are unknown, discharge points will be 1. "Mars Pathfinder Fact Sheet, Mission added to the Pathfinder rover antenna Summary", October 20, 1994, JPL base as a precaution against 400-538. electrostatic charging. Future missions should seriously consider including 2. Haberle, R. M. and Greeley, R. components to measure vehicle "Sand and Dust on Mars", NASA CP charging. Possibilities might include an 10074, February, 1991, pg. 39. electroscope, or an optical means for detecting luminosity from electrical 3. Hillard, G. B. and Kolecki, J. C., discharges taking place in and around 'q'he Interaction of High Voltage the rover. Systems with the Environments of the Moon and Mars", NASA TM 106107, FINAL REMARK AIAA-93-0704, January, 1993. During our investigation, the question arose whether a circuit model exists for 4. Kolecki, J. C. and Hillard, G. B., charging/discharging the Pathfinder "Electrical and Chemical Interactions at rover on Mars. Speculation is all that is Mars Workshop, Final Report, Parts 1 possible at the moment. The rover will and 2", NASA CP 10093, November, accumulate its charge from surface 1991. dust, then deliver some of that charge into the atmosphere through a point 5. Layman, W. E, Matijevic, T. R., with a high electric field. Dust blowing. Mishkin A. M. and Sorota A. R., by the rover in the martian surface "Microrover and LMRE Technical wind will accumulate this charge and Baseline", Feb. 6, 1995, JPL MPF eventually fall back to the martian ROVER DFM 93- 006S. surface completing a circuit loop. The 7 _JBCUUm Tank ---.... Ground plane Linear Feedthroughs E_ectrostattcProbe Photoce_,\ Arc -X 4- • ,_bl=_ Rover Whee ArSiizmoun\aanLtunar ! Figure 1: Schematic (al and photograph {b} of test facility Figure 2: Electron microscope photograph of tungsten discharge point (scale on photo)

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