The Design and Engineering of Curiosity

by Emily Lakdawalla

  To move the sample into CHIMRA, the rover tilts the drill and uses either drill percussion or CHIMRA vibration to shift the powder from the forward sample chamber to the aft sample chamber. Then CHIMRA vibration and a rolling motion of the arm guides the sample from the aft sample chamber out the sample exit tube on the drill and into the sample inlet tube on CHIMRA (Figure 5.10). With a combination of vibration and back-and-forth rocking motions, the sample moves through the sample inlet tube, past an elbow in the tube, and into the CHIMRA sample reservoir.

  Figure 5.10. Parts of the 150-micron sample pathway within CHIMRA. Photos from turret checkout before and after scooping at Rocknest on sols 64 and 65. NASA/JPL-Caltech/MSSS/Emily Lakdawalla.

  The CHIMRA reservoir is divided in two by an internal partition, called the thin wall, which has a slot on one side. When sample enters the reservoir, it pools in the corner of the upper half of the reservoir, away from that slot. To visually inspect the drilled material before it is sieved, rover planners can tilt toward the slot and use vibration to transfer the material to the lower half of the reservoir. From there it can be slid through the rabbit hole on the secondary thwack arm and into the scoop. Then they can open the scoop and take photos of the sample with the Mastcams, close the scoop, and tilt to return the sample back through the rabbit hole and into the reservoir.

  5.4.2.2 Scoop to CHIMRA reservoir

  To acquire a scooped sample, the rover opens the scoop and positions it over the sample site. The secondary thwack actuator closes the scoop, dragging it through the sand to a depth of about 35 millimeters, usually acquiring a big mound of sand in the scoop. The secondary thwack actuator can apply a huge amount of torque to overcome resistance from buried pebbles, if they exist. After acquiring the sample, the scoop tilts slightly downward and CHIMRA vibrates in order to spill material from the scoop until it has reached a level that corresponds to the desired 12 cubic centimeters of sample (see Figure 3.​6). Then the scoop can optionally be leveled out and vibrated within view of the Mastcams, which can take movies to watch the particles move around inside the scoop, performing a search for very large particles. Because this requires human inspection, the rover has to wait at least one night (until the next tactical planning sol) to proceed. If the sample passes muster, the scoop is closed and the material in it gets transferred through the rabbit hole to the CHIMRA sample reservoir.

  5.4.2.3 150-micrometer sieving

  To sieve, engineers rotate the turret to turn the reservoir topside down, which places the sample on the 150-micrometer sieve. Then CHIMRA vibrates and the arm wrist rocks the turret gently back and forth to encourage the sample to spread out across the sieve. Initially, engineers expected it to take as much as an hour to produce enough sieved sample, but experiments on Earth and Mars have yielded a standard 20-minute time of sieving operations to produce approximately 12 cubic centimeters of sieved material. The post-sieve (fine) material accumulates in the sample tunnel, while pre-sieve (coarse) material remains in the sample reservoir.

  5.4.2.4 Inspecting sieve efficiency

  Once sieving is complete, CHIMRA rotates and vibrates to move the sieved sample down the sample tunnel ramp and into the 150-micrometer portion box. This motion also moves the coarse pre-sieve material (the sample that did not pass through the 150-micrometer sieve) through the rabbit hole and into the scoop. At this point, engineers can peek into the portion box to assess how much material passed through the sieve, and can open the scoop to see how much material did not pass through the sieve. Comparing the two volumes gives an estimate of sieving efficiency.14 The engineers changed this behavior following the development of a problem with the primary thwack arm on sol 1231 (see section 5.4.6).

  5.4.2.5 150-micrometer portioning

  To prepare a portion, CHIMRA uses a series of small rotations and vibrations to walk the sieved sample around the interior of the portion box until it is all piled up on top of the portion hole. (These motions may also move the coarse pre-sieve material that was in the scoop through the 1-millimeter sieving pathway, where it stays until it is dumped.) A very small amount of vibration encourages sample to enter the portion hole – not much, to avoid packing the hole and potentially clogging it. The hole has an inverted funnel shape, opening wider toward the outside, to prevent clogging. For a single 75-cubic-millimeter aliquot, the rover tilts CHIMRA again to move the extra sample away from the portion hole, back under the “top shelf” of the interior of the portion box. For a “portion plus” aliquot (used only for dropping a larger sample to the observation tray), CHIMRA skips the step of sliding the excess material off of the top shelf.

  5.4.2.6 Delivering a 150-micrometer portion to SAM or CheMin

  Before delivery, Mastcam turns and takes photos of the sample inlet. Then the mast head rotates 180° away from the sample inlet, to keep any blowing dust away from the cameras. The rover moves the turret over a sample inlet, opens the inlet cover, moves the turret to within about a centimeter of the inlet, opens the portion door, and vibrates to make sure the portion drops. The portion door closes, the arm moves away, the sample inlet closes, and CHIMRA can repeat the sample preparation and drop-off process again. After all of the sample dropoffs have been completed, Mastcam takes another picture of the sample inlet to document successful closure of the inlet door.

  5.4.3 CHIMRA 1-millimeter sample pathways

  This pathway generates a single aliquot with a volume of 45 to 130 cubic millimeters; all the rest of the sample is lost during the portioning activity. Therefore, the mission hasn’t used the 1-millimeter pathway on precious drilled sample, only with scooped samples, although it is theoretically possible to create a 1-millimeter-sieved portion from a drilled sample. Only SAM can accept this coarser sample size; it’s unsafe for delivery to CheMin.

  5.4.3.1 1-millimeter sieving

  Figure 5.11 shows parts of CHIMRA relevant to the 1-millimeter grain-size sample pathway. To pass material from the scoop through the 1-millimeter pathway, the turret tilts in the opposite direction to the one it uses to move material from the scoop into the reservoir. The material lands on the 4-millimeter grate, and whatever passes through lands on the 1-millimeter sieve behind the grate. Whatever passes that sieve falls into the bee trap, a funnel that opens into the 1-millimeter reservoir. Anything that passes the 4-millimeter grate but not the 1-millimeter sieve exits the space between the two through a slot, returning to the scoop. Curiosity dumps remaining coarse material before proceeding. The shape of the bee trap prevents the sieved material from being lost as any coarse stuff is dumped.

  Figure 5.11. Parts of the 1-millimeter CHIMRA sample pathway. Photos from turret checkout on sol 229. NASA/JPL-Caltech/MSSS/Emily Lakdawalla.

  5.4.3.2 1-millimeter portioning

  To prepare a 1-millimeter aliquot, the arm tilts so that the material in the bee trap piles up on the 1-millimeter portion tube. This portion tube, unlike the 150-micrometer portion hole, is a blind tube, closed at one end. With the portion tube filled, CHIMRA cracks open the scoop and secondary thwack arm. All the remaining sieved sample that was in the reservoir slides out of it and onto the ground, leaving behind the material collected in the 1-millimeter portion tube. Then CHIMRA closes up the scoop again, tilts to spill the material that was in the portion tube back into the reservoir, and then angles the aliquot into a bypass hole on the secondary thwack arm. While CHIMRA is closed, the bypass hole is closed at one end by a wide lip on one side of the scoop. Then CHIMRA angles the scoop like a cup and cracks the scoop open slightly, allowing the material to spill out of the bypass and into the scoop. A little chute cut into the side of the scoop encourages material to fall neatly from the bypass into the scoop. Once the portion is in the scoop, it can be inspected before delivery.

  Because this process drops all of the rest of the 1-millimeter-sieved material that had been held in CHIMRA, only one portion can be created from each sample. To get the double or triple portion that SAM now prefers (see section 9.​5.​2.​5), Curiosity
has to start over with a new scoop of sand for each portion.

  5.4.3.3 Delivering a 1-millimeter sieved aliquot

  Dropping the sample is a delicate operation because the width of the scoop is similar to the width of the sample inlet. To deliver a 1-millimeter sieved aliquot to an instrument, the scoop is tilted and vibrated to slide the portion into one of the corners of the scoop’s tip. Then a SAM sample door is opened and the scoop tip placed over the sample inlet. As the scoop opens, the rest of the turret rotates in order to keep the position of the scoop tip motionless, dumping the sample into the inlet.

  5.4.3.4 Medium-grain-size fraction portioning

  The 150-micrometer and 1-millimeter pathways can be combined to create a single aliquot with an intermediate sample size. This capability was first used at Namib dune on sol 1228 (Figure 5.12). CHIMRA acquired a scooped sample, passed it to the reservoir, and sieved it through the 150-micrometer sieve. Then Curiosity dumped the post-sieve sample and sent the pre-sieve sample through the 1-millimeter pathway. CHIMRA created a single aliquot as described above, and then delivered it to SAM.

  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.

  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 planners 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 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 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.

  Turret checkout

  Sieve checkout

  ChemCam RMI sieve checkout

  Scooping activity

  Pre- and post-sieve volume inspection

  Drill bit assembly checkout

  32 (all)

  51 (all)

  65 (all)

  73 (all)

  81 (all)

  128 (thwackless)

  173 (thwackless)

  486 (thwackless)

  576 & 578 (all)

  581 (thwackless)

  704 (all)

  781 (thwackless)

  840 (all)

  894 (all)

  954 (thwackless)

  1048 (all)

  1089 (all)

  1132 (thwackless)

  1133 (all)

  1202 (all)

  1226 (secondary thwack only)

  1231 (secondary thwack only)

  1293 (primary thwack only)

  1327 (all)

  1359 (all)

  1418 (thwackless)*

  1419 (all)

  1457 (thwackless)*

  1459 (secondary thwack only)

  1460 (all)

  1491 (thwackless)*

  1493 (secondary thwack only)

  1494 (all)

  1533 (thwackless)*

  1534 (secondary thwack only)

  1535 (all)

  576

  840

  1064

  1123

  1133

  1142

  1293

  1535

  172 (partial)

  564 (focus test)

  578

  704

  840

  894

  1048

  1089

  1133

  1202

  1293

  1327

  1359

  1419

  1460

  1494

  61

  64

  66

  67

  69

  70

  73

  74

  93

  411

  1224

  1228

  1231

  1651

  79

  193

  194

  279

  464

  623

  762

  884

  922

  1061

  1121

  1139

  1224

  1228*

  1323**

  1334**

  1362**

  1425**

  1465,1466***

  1495,1496***

  128

  173

  180

  182

  229

  486

  581

  617

  621

  704

  726

  7
56

  759

  762

  781

  840

  867

  881

  882

  894

  908

  954

  1048

  1059

  1060

  1089

  1116

  1119

  1132

  1133

  1134

  1137

  1202

  1226

  1231

  1293

  1321

  1327

  1332

  1359

  1361

  1418

  1419

  1420

  1422

  1457

  1459

  1460

  1464

  1491

  1493

  1494

  1495

  1533

  1534

  1535

  1536

  1537

  1541

  1542

  1543

  1757

  1780

  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.