On-board the Mars Pathfinder spacecraft, launched in December of 1996, is a small roving vehicle named Sojourner. On Sojourner is an experiment to determine the abrasive characteristics of the Martian surface, called the Wheel Abrasion Experiment (WAE). The experiment works as follows: one of the wheels of the rover has a strip of black anodized aluminum bonded to the tread. The aluminum strip has thin coatings of aluminum, nickel and platinum deposited in patches. There are five (5) patches or samples of each metal, and the patches range in thickness from 200 A to 1000 A. The different metals were chosen for their differing hardness and their environmental stability. As the wheel is spun in the Martian soil, the thin patches of metal are abraded away, exposing the black anodization. The abrasion is monitored by measuring the amount of light reflected off of the samples. A photodetector was developed for this purpose, and that is the subject of this paper

Jenkins, Phillip P.

Scheiman, David A.

Wilt, David M.

English

NASA Technical Reports Server

NASA Technical Memorandum 113123
Photodetector Development for the Wheel
Abrasion Experiment on the Sojourner
Microrover of the Mars
Pathfinder Mission
David M. Wilt
Lewis Research Center, Cleveland, Ohio
Phillip P. Jenkins
Essential Research, Inc., Cleveland, Ohio
David A. Scheiman
NYMA, Inc., Brook Park, Ohio
Prepared for the
32nd Intersociety Energy Conversion Engineering Conference
cosponsored by AIChE, ANS, SAE, AIAA, ASME, and IEEE
Honolulu, Hawaii, July 27--August 1, 1997
National Aeronautics and
Space Administration
Lewis Research Center
October 1997
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PHOTODETECTOR DEVELOPMENT FOR THE WHEEL ABRASION
EXPERIMENT ON THE SOJOURNER MICROROVER OF THE
MARS PATHFINDER MISSION
David M. Wilt, Phillip P. Jenkins', David A. Scheiman 2
NASA Lewis Research Center
21000 Brookpark Rd, M.S. 302-1
Cleveland, Ohio
Phone (216) 433-6293, Fax (216) 433-6106
1Essential Research, Inc.
23811 Chagrin Blvd. #220
Cleveland, Ohio
(216) 831-0177
=NYMA, Inc.
2001 Aerospace Parkway
Brook Park, Ohio 44142
ABSTRACT
On-board the Mars Pathfinder spacecraft, launched in
December of 1996, is a small roving vehicle named
Sojourner. On Sojourner is an experiment to determine
the abrasive characteristics of the Martian surface, called
the Wheel Abrasion Experiment (WAE). The experiment
works as follows: one of the wheels of the rover has a
strip of black anodized aluminum bonded to the tread.
The aluminum strip has thin coatings of aluminum, nickel
and platinum deposited in patches. There are five (5)
patches or samples of each metal, and the patches range
in thickness from 200A to 1000A. The different metals
were chosen for their differing hardness and their
environmental stability. As the wheel is spun in the
Martian soil, the thin patches of metal are abraded away,
exposing the black anodization. The abrasion is
monitored by measuring the amount of light reflected off
of the samples. A photodetector was developed for this
purpose, and that is the subject of this paper.
INTRODUCTION
The photodetector is attached to the arm or "bogie"
which supports the rover center-right wheel (fig. 1). The
constrains placed upon the design of the photodetector
were as follows: mass budget of lOg, physical size of lcm
x lcm x lcm, must operate in the photovoltaic mode
(unpowered), imaging limited to one sample (1.5cm x
2.5cm), able to withstand 60g landing load, able to
withstand launch vibrational environment, able to operate
at -50°C, and obviously able to detect small changes in
the reflectivity of the abrasion samples.
Several photodetector designs were considered. The
initial development focused on a detector which utilized a
black anodized honeycomb aluminum extrusion for
restricting the field of view of the detector. Several
extrusions were found with suitable cell sizes (~l.5mm),
unfortunately the black anodization did not produce a
surface which was optically black at high angles of
incidence. We observed that multiple reflections within
the honeycomb increased the acceptance angle of the
detector, making this design unsuitable for the mission.
DETECTOR DESIGN
The final photodetector design (fig. 2) utilized
geometrical optics and included three (3) small (2mm x
4mm) gallium arsenide (GaAs) solar cells wired in parallel
and connected to a load resistor. Geometrical optics
limited the field of view to slightly smaller than one
abrasion sample (1.5cm x 2.5cm) as viewed at an angle
of incidence of 30 ° and a separation distance of 19.2mm
(center of sample to front of detector). The photodetector
ray tracing design is shown in fig. 3. This approach was
utilized to determine the size and location of the GaAs
cells and the openings in the aperture plate.
Alignment of the photodetector optics was accomplished
by machining a step in the bottom of the photodetector
housing. This step was used to locate the circuit board
which held the GaAs solar cells. Black anodized
aluminum baffle plates were incorporated to optically
isolate each of the three detectors. The aperture plate
(not shown) was also located by means of a step
machined in the top of the photodetector housing. All of
the components of the photodetector housing were
fabricated from 7075-T6 aluminum which was black
anodized (MIL-A-8625-F, type II, class 2).
NASA TM-113123 1
Bogie/_ )/
P1
_ _Phatodefector
Abrasion
Sample
Reference
Sample
Cleats
FIGURE 1 - OVERVIEW OF WAE EXPERIMENTAL
SET-UP.
The following materials were used in the construction of
the detector:
- Gallium arsenide (GaAs) on germanium (Ge)
solar cells (4.25mm x 2.0 mm)
- Kovar solar cell interconnects
- 0.021", 1 oz. MIL-S-13949H-A1/4D circuit
board, tinned
- DC 93-500 silicon adhesive (cover glass)
- DC RTV 566 silicon potting compound
- 1.26 kohm surface mount resistor, MIL-R-
55342, 0.1% tolerance, 0.01%/1000 hours Life
Failure Rate.
- 28 AWG Teflon insulated wire, Type ET-MIL-W-
1687F-J6
- 7075-T6 aluminum, black anodized to MIL-A-
8625-F, type II, class 2
- Hysol EA 9394 epoxy
- G10 fiberglass mount with (2) gold plated
bifurcated terminals (supplied by JPL)
PHOTODETECTOR TESTING
The photodetector design was tasted using a prototype
detector, fabricated with available LPE GaAs solar cells,
and secured to a fixture which maintained the correct
sample-detector spatial and angular separation. The
fixture also permitted the abrasion sample to be curved to
the appropriate radius.
Output
Signal
Surface Mount
Bus
Cell
+z out from
thepaper x
0_
Baffle Plates Bus
Top View with Aperture Plate Missing
FIGURE 2 - TOP VIEW OF PHOTODETECTOR WITH
APERTURE PLATE MISSING.
This fixture was mounted on a sun tracker which
contained direct and global solar insolation measurement
equipment. The prototype fixture also contained a
rotation stage which permitted the incidence angle of the
sun to be varied about three (3) axis'.
TOP VIEW
Abrasion GaAs Cells
Sample
Ga_ _teCto/_ rrture
VIEW
h \ \
Abras_
Sample
FIGURE 3 - RAY TRACING DESIGN OF
PHOTODETECTOR
Several different abrasion samples were used to
determine both the max. response and the angular
dependence of the WAE experimental set-up. The first
measurements were performed using aluminum foil, since
AI has the highest reflectivity of the three (3) selected
abrasion materials, in the wavelength range of interest.
NASA TM-113123 2
Once the actual flight samples became available they
were also used to further define the characteristics of the
WAE experiment. A set of measurements were also
made with palaginite dust coated samples. This data is
shown in figure 4. The rotational axis are: alpha - rotation
about the rover centerline (angle of incidence); beta -
rotation about the wheel axle; gamma - rotation about the
normal to the sample directly under the detector. The y
axis units represent the response to the direct component
of illumination (i.e. measurements made on a clear day,
>90% direct insolation, <10% diffuse).
Based on this data, we estimate:
Max. Response = 5 pAcm2/mW (@0°C)
Max. Illumination = 40 mW/cm 2 - Based on
projected operation at 1400 hr. on a
clear day (Tau = 0.5) (JPL data)
Temp. Coe. (Isc) = 0.067%/°C - Based on
actual SR vs. Temp. measurements of
ASEC GaAs/Ge cells.
Operating Temp. = -30°C (JPL Data)
Sample = AI film with light dusting
Iscmax = 196 pA
To be conservative, in sizing the load resistor we chose to
limit the max. voltage to 250mV thus,
R = 0.25 V/1.96e-4 A
= 1.275 kohm
The data in figure 4 shows several interesting features.
First of all, the aluminum foil demonstrates a higher
specular reflectance characteristic than the 900A AI film.
Secondly, the addition of the palaginite (Mars simulant)
dust to the AI film reduces the specular component of the
reflection but has little effect on the diffuse component as
noted by the reduction in peak height and increase in
peak width. Based on this detector characterization, a
pointing tolerance of +/- 15 ° has been established for the
mission.
PHOTODETECTOR TEST PLAN
The following test plan was developed to qualify the
photodetector design (qualification testing) and to verify
the flight worthiness of the actual flight hardware (flight
acceptance testing).
Vibration Testinq
Qualification Testing
1) Epoxy test photodetector unit to aluminum test fixture
adapter plate.
2) Test assembly at Qualification Levels for 3
minutes/axis. (JPL supplied profile)
3) Check response of detector to standard input (He-Ne
laser).
Flight Acceptance Testing
1) Secure detector Flight Unit (FU) to test fixture with
temporary hold-down (clamp)
2) Test assembly at Acceptance Levels for 1 minute/axis.
(JPL supplied profile)
3) Check response of Detector to standard input (He-Ne
laser).
14
_ t2
E l0
i °
_ 4
2
0
0
14
12
4
2
0
o
AJ Foll
_LAI Film - C_81li'l
lO 20 30 40 50 60 70 80
AI Foil
...i .... i .... i .... i .... i .... i....I .... i
40 30 -20 -I0 0 10 20 30 40
Bet= (')
14
/d Foil]0
gooA AI Film - Cle_l_'n
8
2
o_£.......................................30 -20 -10 0 1O 20 30 40
Gammt (*)
FIGURE 4 - PHOTODETECTOR CHARACTERIZATION
PROFILES FOR ROTATION ABOUT ALPHA (ROTATION
ABOUT THE ROVER CENTERLINE, I.E. ANGLE OF
INCIDENCE), BETA (ROTATION ABOUT THE WHEEL
AXLE) AND GAMMA (ROTATION ABOUT THE NORMAL
TO THE SAMPLE DIRECTLY UNDER THE DETECTOR)
FOR DIFFERENT ABRASION SAMPLES.
Low FreQuency Load Testing
Qualification Testing
1) Epoxy test Photodetector unit to aluminum test fixture
adapter plate.
2) Test assembly at Qualification Levels (60g) in the +z
direction and +/- x direction.
NASA TM-113123 3
3) Check response of Detector to standard input (He-Ne
laser).
Flight Acceptance Testing
1) Secure Detector Flight Unit (FU) to test fixture with
temporary hold-down (clamp, etc.)
2) Test assembly at Acceptance Levels (60g) in the +z
direction. (see fig. 2)
3) Check response of Detector to standard input (He-Ne
laser).
Thermal Testing
Qualification Testing
1) Epoxy test Photodetector unit to aluminum test fixture
adapter plate.
2) Test assembly at Oven Levels:
Ambient: 760 torr dry nitrogen
Low temp.: -110°C, 1 hour dwell
High temp.: +40°C, 1 hour dwell
Transition Time: 1 hour
Cycles: 3
Detector shall be illuminated during
transition from low to high temp. to
check continuity.
3) Check response of Detector to standard input (He-Ne
laser).
4) Test assembly at Life Cycle Levels:
Ambient: 760 torr dry nitrogen
Low temp.: -1D0°C, no dwell
High temp.: +30°C, no dwell
Transition Time: 1 hour
Cycles: 35
5) Check response of Detector to standard input (He-Ne
laser).
Flight Acceptance Testing
1) Secure Detector Flight Unit (FU) to test fixture with
temporary hold-down (clamp, etc.)
2) Test assembly at Acceptance Levels:
Ambient: 760 torr dry nitrogen
Low temp.: -95°C, 1 hour dwell
Mid temp.:-60°C, 1 hour dwell
High temp.: +25°C, 1 hour dwell
Transition Time: 1 hour between settings
Cycles: 1
Detector illuminated at Mid temp. setting
to check continuity.
3) Check response of Detector to standard input (He-Ne
laser).
Vacuum Testin9
Qualification Testing
1) Epoxy test Photodetector unit to aluminum tes( fixture
adapter plate.
2) Subject test assembly to a pressure of less than 1 torr
for a minimum of 24 hours at a temperature of 40*C.
3) Check response of Detector to standard input (He-Ne
laser).
Flight Acceptance Testing
1) Secure Detector Flight Unit (FU) to test fixture with
temporary hold-down (clamp, etc.)
2) Subject test assembly to a pressure of less than 1 torr
for a minimum of 24 hours at 40°C.
3) Check response of Detector to standard input
Planetarv Protection
FU shall be baked at 110°C for a min. of 50 hours.
FLIGHT UNIT CALIBRATION
The flight unit (FU) utilized GaAs/Ge solar cells bonded
to the circuit card by Applied Solar Energy Corporation
(now TECSTAR). These devices demonstrated a
significantly higher responsivity that the LPE GaAs cells
used to size the load resistor. As such, the maximum
output voltage of the detector has been increased to
400mV. Testing has indicated that the detector begins to
saturate at 500mV (under room temperature testing), thus
we feel confident that that under actual operating
conditions (i.e. low temperature, dust obscuration, rover
misalignment) the detector will always be operating in its
linear region.
Figure 5 shows the actual calibration curve for the FU
detector with the FU wheel. This test was conducted
using a photographic test lamp (750W tota light) which
produced approximately 0.25 Mars solar insolation. The
sample pattern for each metal is: reference / 200A film /
reference / 300A film / 450A film / reference / 700A film /
lo00A film, as viewed in order of increasing wheel
rotational angle. This pattem was chosen so that each
abrasion sample would be adjacent to a blank reference
area, with the thinnest sample bounded on both sides by
a reference sample.
There are several features of note in the calibration
data. Firstly, none of the thin metal samples are opaque.
This is shown in the increasing reflectance as the sample
thickens. Secondly, the peaks demonstrate a fine
structure which is caused by variations in the abrasion
sample surface. These variations are attributed to the
process by which the samples were pre-bent and glued to
the wheel.
Finally, the blank samples do not produce a zero output
from the photodetector. Figure 6 shows both the spectral
reflectance from the black anodized aluminum and the
spectral response from a GaAs cell. The black
anodization has a fairly low reflectance (< 5%) up to
700nm, at which point it begins to reflect. By the
NASA TM-113123 4
bandedge of GaAs (870nm), the reflectance has
increased to > 40%. This reflectance accounts for the
non-zero reference value.
ACKNOWLEDGMENT
The authors would like to thank Frank DeAngelo and Dr.
Dennis Flood of NASA LeRC for their enthusiastic support
of this development and our involvement with the
Mars/Pathfinder project.
8O
>E
"_ 6o
O
,.,i ...............,.i .... 1...i .i .... i t., ,[
I , i!
iJl ii! i 'l;
:J J U L_ t,r ,,i .
.,_ .... J .... _ .... i .... i .... i .... , .... l..,
0 50 100 150 200 250 300 350 400
Rotat=onal Angle (°)
FIGURE 5 - CALIBRATION DATA FOR THE FLIGHT
UNIT WHEEL AND DETECTOR UNDER 0.25 MARS
ILLUMINATION. THERE ARE FIVE (5) ABRASION
SAMPLES FOR EACH METAL.
A50
40
_3o
_<20
_ 10
60 .... _ .... = .... , .... , .... , .... , .... 0.6
" _ _ 0.3 i
_ R==_,_=_,, 0.2 -_
0 ' ' ' ' ' ' 0
300 400 500 600 700 800 900 1000
Wavelength (nrn)
FIGURE 6 - GAAS SPECTRAL RESPONSE AND
ANODIZED ALUMINUM SPECTRAL REFLECTANCE.
THE CROSS HATCHED REGION SHOWS THE REASON
FOR THE NON-ZERO REFERENCE VALUE.
CONCLUSION
A photograph of the flight unit detector is shown in figure
7. The detector had a final mass of 2.4g and met all of
the performance, size limitations and durability
requirements. A duplicate detector is being used for test
bed activities.
Several Operational Readiness Tests (ORTs) have
been conduced at JPL using a SIM rover with a flight
qualified detector. To date, the detector has performed
well and has provided excellent data even in poor
illumination conditions.
FIGURE 7 o PHOTOGRAPH OF THE FU DETECTOR.
THE BIFURCATED TERMINALS SHOWN IN THE
BACKGROUND WERE USED FOR INTERFACING THE
DETECTOR TO THE ROVER ELECTRONICS.
NASA TM-113123 5
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1. AGENCY USE ONLY (Leave blank) 2. REPORT DATE 3. REPORT TYPE AND DATE_ COVERED
October 1997 Technical Memorandum
4. TITLE AND SUBTITLE 5. FUNDING NUMBERS
Photodetector Development for the Wheel Abrasion Experiment
on the Sojourner Microrover of the Mars Pathfinder Mission
6. AUTHOR(S)
David M. Wilt, Phillip P. Jenkins, and David A. Scheiman
7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio 44135-3191
9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)
National Aeronautics and Space Administration
Washington, DC 20546- 0001
WU-632-1A-1A
8. PERFORMING ORGANIZATION
REPORT NUMBER
E-10889
10. SPONSORING/MONffORING
AGENCY REPORT NUMBER
NASA TM-113123
11. SUPPLEMENTARY NOTES
Prepared for the 32nd lntersociety Energy Conversion Engineering Conference cosponsored by AIChE, ANS, SAE, AIAA, ASME,
and IEEE, Honolulu, Hawaii, July 27--August 1, 1997. David M. Wilt, NASA Lewis Research Center; Phillip E Jenkins, Essential
Research, Inc., Cleveland, Ohio; and David A. Scheiman, NYMA, Inc., 2001 Aerospace Parkway, Brook Park, Ohio 44142 (work
funded by Contract NAS3-27186). Responsible person, David M. Wilt, organization code 5410 (216) 433-6293.
12a. DiSTRIBUTION/AVAiLABILITY STATEMENT
Unclassified - Unlimited
Subject Category: 18 Distribution: Nonstandard
This publication is available from the NASA Center for AeroSpace Information, (301) 621-0390.
12b. bi_iMiBUTION CODE
13. ABSTRACT (Maximum 200 words)
On-board the Mars Pathfinder spacecraft, launched in December of 1996, is a small roving vehicle named Sojourner. On
Sojourner is an experiment to determine the abrasive characteristics of the Martian surface, called the Wheel Abrasion
Experiment (WAE). The experiment works as follows: one of the wheels of the rover has a strip of black anodized
aluminum bonded to the tread. The aluminum strip has thin coatings of aluminum, nickel and platinum deposited in
patches. There are five (5) patches or samples of each metal, and the patches range in thickness from 200/_ to 1000/_.
The different metals were chosen for their differing hardness and their environmental stability. As the wheel is spun in
the Martian soil, the thin patches of metal are abraded away, exposing the black anodization. The abrasion is monitored
by measuring the amount of light reflected off of the samples. A photodetector was developed for this purpose, and that
is the subject of this paper.
14. SUBJECT TERMS
Photodetector; Mars; Pathfinder
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OF REPORT
Unclassified
18. SECURITY CLASSIFICATION
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19. SECURITY CLASSIFICATION
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Unclassified
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A03
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Photodetector Development for the Wheel Abrasion Experiment on the Sojourner Microrover of the Mars Pathfinder Mission

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