by The Design
sent the pre-sieve sample through the 1-millimeter pathway. CHIMRA created a single
aliquot as described above, and then delivered it to SAM.
5.4.4 Cleaning and thwacking
After the first drill at Yellowknife Bay, scientists expressed a desire to use the contact science instruments (particularly APXS) to examine the drilled sample, and the rover plan-
ners quickly developed a way to oblige them, dumping the sample in neat piles for APXS
analysis (see Figure 3.7). If the pre-sieve material is not dumped before 150-micrometer portioning, it can end up in the 1-millimeter sample reservoir. Material dumped from there
drops less neatly than material dumps from the scoop, because it falls off the secondary
thwack arm around the bee trap on all sides, which tends to disperse it over a broader area.
A tighter pile is better for APXS, because then it is more likely that the APXS field of view will contain only sample material and not “windows” of whatever material the pile was
204 SA/SPaH: Sample Acquisition, Processing, and Handling
Figure 5.12. A medium-grained (150-micron to 1-millimeter) aliquot in the scoop, ready for delivery to SAM. Left Mastcam photo 1228ML0036640000503705E01. NASA/JPL-Caltech/MSSS.
dumped on. Pre-sieve (coarse) material is most often dumped before departing a sample
site, while post-sieve (fine) material is usually held for longer as cached sample.
To prepare for a new sample, CHIMRA goes through a process to make the interior as
clean as possible. First, it dumps any remaining pre-sieve sample by opening up the scoop
wide, then rocking and vibrating to encourage very last bit of sample that may have been
lurking within the reservoir to exit the scoop. Then it opens the sample tunnel and
150-micrometer sieve and uses rocking and vibration to empty all of the loose material
from that side. At that point, it’s time for thwacking.
During ordinary operation of the primary thwack actuator, the sample tunnel opens on
its own for the first few degrees. After 5° of opening, a cam inside the mechanism engages
the primary thwack arm, which holds the 150-micrometer sieve. As the sample tunnel
continues to open, the primary thwack arm follows it, maintaining the 5° separation and
winding a spring. Toward the end of its range of motion, continued opening of the sample
tunnel door passes a “point of no return,” shortly after which its continued motion
5.4 CHIMRA: Collection and Handling… 205
disengages the latch holding the primary thwack arm to the sample tunnel. The coiled
spring unwinds, slamming the primary thwack arm against the sample reservoir at very
high speed, dislodging any material that may have been stuck in the holes of the sieve. The force of the thwack also dislodges material that was stuck to other interior surfaces of
CHIMRA. CHIMRA follows a primary thwack with vibration and rocking to encourage
any loosened material to exit out through the open scoop.
Secondary thwack works similarly. As the secondary thwack actuator opens the scoop,
the secondary thwack arm follows the scoop for its first 10° of motion. Then a latch
engages, holding the secondary thwack arm in place while the scoop continues to open,
winding a spring. As the scoop reaches the end of its range of motion, the latch disengages, and the thwack arm slams against the scoop, dislodging any material that may have been
stuck in its sieve and grate. Further rocking and vibrating activity cleans out any powder
dislodged by the secondary thwack.
Note that the two thwack operations are designed to encourage material that may be
wedged in a sieve to exit the sieve in the direction from which it arrived.
Before and after primary and secondary thwacks, the rover uses Mastcam to examine the
interior surfaces of CHIMRA (Figure 5.13). Table 5.1 lists Mastcam images documenting
the interior and exterior of CHIMRA. The only times that the cameras can get a clear look at the back sides of the two sieves, or of the interiors of the sample tunnel and bee trap, are after primary and secondary thwacks. After photographing the sieves and other parts, the primary
and secondary thwack actuators close up CHIMRA, re-engaging the latches that hold the
thwack arms close to the CHIMRA doors, and CHIMRA is ready to accept a new sample.
Figure 5.13. Pre-thwack (left column) and post-thwack (right column) views of the primary (top) and secondary (bottom) thwack mechanisms. Photos taken by Mastcam on sol 840.
NASA/JPL-Caltech/MSSS.
Table 5.1. Sols of Mastcam and ChemCam RMI photos documenting CHIMRA activities to sol 1800.
Images were taken with Mastcam unless ChemCam is specified.
ChemCam
Pre- and
Drill bit
Sieve
RMI sieve
Scooping post-sieve volume assembly
Turret checkout
checkout checkout
activity
inspection
checkout
32 (all)
576
172 (partial) 61
79
128
1321
51 (all)
840
564 (focus
64
193
173
1327
65 (all)
1064
test)
66
194
180
1332
73 (all)
1123
578
67
279
182
1359
81 (all)
1133
704
69
464
229
1361
128 (thwackless)
1142
840
70
623
486
1418
173 (thwackless)
1293
894
73
762
581
1419
486 (thwackless)
1535
1048
74
884
617
1420
576 & 578 (all)
1089
93
922
621
1422
581 (thwackless)
1133
411
1061
704
1457
704 (all)
1202
1224
1121
726
1459
781 (thwackless)
1293
1228
1139
756
1460
840 (all)
1327
1231
1224
759
1464
894 (all)
1359
1651
1228*
762
1491
954 (thwackless)
1419
1323**
781
1493
1048 (all)
1460
1334**
840
1494
1089 (all)
1494
1362**
867
1495
1132 (thwackless)
1425**
881
1533
1133 (all)
1465,1466***
882
1534
1202 (all)
1495,1496***
894
1535
1226 (secondary thwack only)
908
1536
1231 (sec
ondary thwack only)
954
1537
1293 (primary thwack only)
1048 1541
1327 (all)
1059 1542
1359 (all)
1060 1543
1418 (thwackless)*
1089 1757
1419 (all)
1116 1780
1457 (thwackless)*
1119
1459 (secondary thwack only)
1132
1460 (all)
1133
1491 (thwackless)*
1134
1493 (secondary thwack only)
1137
1494 (all)
1202
1533 (thwackless)*
1226
1534 (secondary thwack only)
1231
1535 (all)
1293
Turret checkout: Photos of the interior and exterior of CHIMRA taken in initial checkout or after sample dumping. “All” indicates both primary and secondary thwacks were performed, and CHIMRA components imaged before and after each. Where only primary, only secondary, or no thwacking was performed, some components will not be visible. *After the primary thwack actuator anomaly on sol 1231, “thwackless” inspections performed after post-sieve sample dumping did not include motion of the primary thwack arm. Scooping activity includes scooping and manipulation of sample within the scoop. Pre- and post-sieve volume inspection: After sieving, the pre-sieve material (coarse fraction) is directed to the scoop for visual inspection, and the tunnel ramp cracked open to view the post-sieve material (fine fraction) in the portion box. *Sol 1228 includes inspection of a medium-size fraction). **After the problem with the primary thwack arm at Namib dune, they switched to examining both fractions in the scoop. ***Near Murray buttes, wind blew the dumped pre-sieve sample before APXS had a chance to examine it. They switched to dumping the sample and immediately performing an overnight APXS observation, following up with examination of the post-sieve sample in the morning.
5.4 CHIMRA: Collection and Handling… 207
5.4.5 Cached sample operations and doggie bagging
At Rocknest it quickly became apparent that if the rover couldn’t drive while CHIMRA
held sample, the mission could be stuck with a prolonged delay. The SAM team wanted to
take many deliveries of sample, running their experiment in different ways each time. One
option would be to perform many dropoffs to SAM sample tubes before driving away, a
procedure called “doggie bagging.” Curiosity does doggie-bag samples, but not many; the
SAM team has to strike a balance between holding options open for future analyses and
consumption of clean sample tubes.
The engineers came up with a workable solution: driving with cached post-sieve sam-
ple in the stowed turret. The clamshell happens to be positioned perfectly for long-term
sample storage (with its opening upward) when the turret is stowed. There is sufficient
room in the clamshell for 12 cubic centimeters of sample without spilling, provided that
the rover’s tilt does not exceed 20°. The limitations exist because sieved sample must not
be allowed to fall onto the back side of the 150-micrometer sieve. Thwacking is a one-way
operation designed to motivate particles out of the sieve toward the reservoir. Any particle that falls on the back of the sieve and clogs a hole would be further embedded by thwacking and likely stuck forever. After performing any cached-sample contact science with the
arm, CHIMRA does a recovery sequence of rocking and vibrating to ensure that any sam-
ple that may have escaped the clamshell returns to it before the arm is stowed.
The limits for cached sample operations were developed very quickly, early in the mis-
sion, with many time pressures on the engineering team. Cached-sample operations
required lots of extra arm motions to move the sample to different locations depending on
the desired orientation of the turret, always preventing the sample from falling on the back side of the sieve. Later, the engineering team developed a new set of operational rules
called evolved cached sample operations, which they first used on Mars on sol 1064.
While protecting the safety of the CHIMRA system, the new rules allow some sample to
fall on the back side of the sieve, requiring fewer arm motions and therefore less energy
and time to run. This increases the flexibility of cached-sample operations.15
5.4.6 CHIMRA concerns and anomalies
CHIMRA has mostly functioned as designed – quite a coup for such a complicated piece
of equipment, the likes of which has never before been sent to another planet. There are
two issues affecting its use: concern about its 150-micrometer sieve, and a problem with
the primary thwack actuator.
5.4.6.1 150-micrometer sieve edge weld separation
Originally, three identical CHIMRA devices were built. One is now on Mars, one is on the
testbed rover, and one was tested to failure on Earth in the Qualification Model Dirty Testbed (see section 4.7.4). After performing about 130 primary thwacks with the test unit, the edge 15 Vandi Verma, personal communication, email dated April 1, 2017
208 SA/SPaH: Sample Acquisition, Processing, and Handling
Figure 5.14. ChemCam RMI inspection of the 150-micrometer sieve performed on sol 1048.
NASA/JPL-Caltech/CNES/CNRS/LANL/IRAP/IAS/LPGN/mosaic by William Rapin.
welds that hold the 150-micrometer sieve onto the primary thwack arm began to pop apart,
creating a gap through which larger particles could leak into the sieved sample. 16
To prevent the degradation of the edge welds on the flight unit of the 150-micron sieve,
engineers now command primary thwacks only when preparing for a new sample. After
some primary thwacks, they use ChemCam to perform a detailed inspection of the edge
welds and the sieve, including angling the sieve to direct a specular reflection at the cameras in order to search for any deformation (Figure 5.14). Table 5.1 lists all sols on which
ChemCam inspection of the sieve was performed.
5.4.6.2 Drill sample cross-contamination
Transfer of material from drill to CHIMRA was originally intended to be done with some
drill percussion. Following the sol 911 drill percussion anomaly (section 5.3.4.2), engi-
neers developed a method to perform sample transfer with limited percussion. However,
16 Dan Limonadi, personal communication, email dated February 2, 2013
5.5 DRT: Dust Removal Tool 209
continued testing suggested that this new method did not empty the drill sample chamber
as effectively as before, increasing the risk that sample material from a previous site might cross-contaminate a new sample. Indeed, CheMin results suggest Buckskin sample cross-contaminated the Big Sky sample, although there could be other explanations (section
9.4.4). Engineers experimented with a new non-percussion sample transfer method on
Mars on sols 1460 and 1494 to reduce cross-contamination risk.17
5.4.6.3 Primary thwack actuator anomaly
On sol 1231, during routine sample processing, the primary thwack arm stalled. After sieving, Curiosity typically cracks open the primary thwack arm to peek into the portion box to estimate how much sample is inside. This time, it stalled after opening only 1°. Cautious testing since then has seen the actuator operate fairly normally without stalling. (They tested cautiously because if the primary thwack arm were to fail in a wide-open position, it would no longer be possible to sieve a fine fraction, making it difficult to prepare samples for delivery to CheMin.) Testing included extra imaging of primary thwack arm motion on sols 1237–1243.
Engineers have reduced use of the primary thwack arm in order to
avoid faults. They no
longer crack open the primary thwack arm to inspect the post-sieved sample. Since
Lubango (drilled sol 1324), they drill, inspect the pre-sieve (coarse) fraction of the sample in the scoop, immediately dump this fraction, transfer the post-sieve (fine) fraction to the scoop, and visually inspect it there, thereby shifting the workload to the secondary thwack actuator rather than the primary thwack actuator.
5.5 DRT: DUST REMOVAL TOOL
The dust removal tool is a brush for cleaning rock surfaces before they are studied with
MAHLI, APXS, or Mastcam’s narrowband filters. It consists of two wire brushes, driven
by a single motor (Figure 5.15). 18 When Curiosity landed, functions related to the use of the brush had not yet been tested on Earth, so its first use was on sol 150. Initially, rover planners inspected the brush only after brush operations. On sol 291, following the third
brushing operation, at Cumberland, they discovered that one set of bristles was bent
(Figure 5.15, middle row), leading to concern that the bent wire bundle could wrap around the brush’s central spindle during brush operations. The mission halted use of the brush
and began a period of extensive Earth testing. The brush was not cleared for use on Mars
until arrival at the next drill site, Windjana, and didn’t see routine use until arrival at Pahrump Hills. Since then, they have imaged the brush both before and after each operation, and no further degradation in the brush bristles has been observed (Figure 5.15, bottom row). Table 5.2 lists all brush sites up to sol 1514.
17 Ashwin Vasavada, personal communication, email dated November 17, 2017
18 There is no published paper about the DRT hardware. Information in this section comes from a paper mentioning the DRT software by Kim (2013) and personal communication with Ashwin
Vasavada (email dated February 9, 2017).
210 SA/SPaH: Sample Acquisition, Processing, and Handling
Figure 5.15. Condition of the dust removal tool (DRT) over time, as seen in standard right Mastcam engineering support imagery. Top row: after its first use on sol 150. Middle row: after the third use, at Cumberland, when one wire bristle was discovered to be bent. Bottom row: after more than 50 uses, on sol 1416, at Chibia. NASA/JPL-Caltech/MSSS.
The motor speed can be changed during a single brush operation, and the robotic arm
can be moved while the brush is running to sweep an elongated area. If Curiosity were to
hold the brush in one position during use, the brushing action would leave an unbrushed
