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

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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
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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
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Figure 1: Schematic (al and photograph {b} of test facility
Figure 2: Electron microscope photograph of tungsten discharge point (scale
on photo)