Emily Lakdawalla

by The Design

  drilling has not yet been used on Mars because of a different drill anomaly.

  5.3.4.3 Sol 1536 drill feed anomaly

  On sol 1536, the engineers attempted rotary-only drilling at a site called Precipice. The

  operation did not complete, because the drill feed mechanism stalled immediately. Current

  flowed to the drill feed motor, but the motor produced no motion. Like the problem with

  the percussion mechanism, it is intermittent, so has been difficult to troubleshoot, but it appears to reside in the drill feed brake mechanism. As of sol 1800, the rover hasn’t done

  any drilling.

  The drill feed motor has a power-off brake: when no electricity is flowing to the brake, a

  disk (the “moveable brake”) is pressed against another disk (the “fixed brake”) by a set of springs. The pressure holds the drill feed firmly in position even when percussion, vibration, and rotation mechanisms are operating. Energizing a solenoid pulls the moveable

  brake away from the fixed brake, allowing the drill feed motor to spin a worm drive that

  slowly translates the drill feed out or in. The brake has two solenoids for redundancy.12

  9 Manning and Simon (2014)

  10 James Erickson, interview dated April 10, 2015

  11 Ashwin Vasavada, interview dated May 1, 2015

  12 Steve Lee, interview dated September 1, 2017

  196 SA/SPaH: Sample Acquisition, Processing, and Handling

  Engineers troubleshooting the issue found that energizing either solenoid with the nor-

  mally commanded current failed to produce any feed motion. Commanding with tweaked

  parameters (like higher current, energizing both solenoids instead of one, multiple attempts to disengage the brake, and so on) produced some motion, but not reliably. The team

  strongly suspects that a displaced component or piece of foreign debris is interfering with motion of the movable brake, preventing it from fully disengaging when commanded.

  From December 2016 through March 2017, engineers tested and performed diagnos-

  tics in an attempt to recover the full use of the drill feed. After developing several innovative techniques, they achieved the full range of feed motion, albeit at speeds too high to

  drill into rocks. However, after using CHIMRA to sieve a sand sample at Ogunquit Beach

  on sol 1651 (March 29, 2017), engineers found that the behavior of the drill feed had

  deteriorated.

  As of this writing, the engineering team is pursuing a new drilling and sample delivery

  approach that does not require using the drill feed. They successfully extended the feed to its full 110-millimeter distance on sol 1780. On Earth, they are working on developing the

  ability to perform feed-extended drilling (FED), using arm motion instead of feed motion

  to advance the drill bit into the rock. Initial testing of feed-extended drilling began on Mars on sol 1848. While this can recover the ability to drill, not using the feed also prevents

  transfer of sample material to CHIMRA (see section 5.4.2.1). Future feed-extended sample transfer (FEST) may involve reverse augering material from the sample chamber out

  through the bit and directly into SAM and CheMin.

  5.4 CHIMRA: COLLECTION AND HANDLING FOR IN SITU MARTIAN

  ROCK ANALYSIS

  CHIMRA (pronounced “chimera”) is a labyrinth of chambers that can sieve and portion

  out samples for delivery to the science instruments. 13 The main parts of CHIMRA are shown in Figure 5.9. There are two main paths by which sample moves around inside CHIMRA: one with a 150-micrometer sieve, and another with a 1-millimeter sieve.

  Curiosity can acquire sample material either through drilling or through scooping loose

  material with the CHIMRA scoop. CHIMRA uses a combination of gravity and vibration

  to move sample around: the rover rotates the turret into a direction where the desired direction of sample motion is downward, and then uses its vibration mechanism to encourage

  the powder to move. CHIMRA’s labyrinthine interior is difficult to imagine even for the

  engineers who interact with it on a regular basis. Four 3D-printed models of CHIMRA

  located throughout mission operations enable engineers to twist and turn it and open and

  close its doors to simulate its movements physically.

  13 The main published source for information on CHIMRA is Sunshine (2010). Cambria Hanson and Louise Jandura explained its intricacies and some last-minute design changes to me in great detail in an interview on June 3, 2016

  5.4 CHIMRA: Collection and Handling… 197

  Figure 5.9. Parts of CHIMRA. Left Mastcam image 0032ML0000830000100870E01 of turret from initial checkout on sol 32. NASA/JPL-Caltech/MSSS/Emily Lakdawalla.

  5.4.1 CHIMRA tour

  Engineers designed CHIMRA to avoid clogging. Its interior spaces are as wide open as

  possible, without sharp corners. Wherever possible, the design avoids forcing sample to

  move through a narrower space than it has already passed through. The mechanism was

  also designed to allow engineers to visually inspect every surface within CHIMRA repeat-

  edly over the course of the landed mission.

  CHIMRA has four motorized mechanisms: the vibration actuator, the portion door

  actuator, and the primary and secondary thwack actuators. The vibration actuator is a self-

  contained mechanism that rotates an off-center tungsten mass to generate vibrations. It

  generally vibrates at a speed that encourages the CHIMRA mechanism to resonate, which

  efficiently shakes the 8-kilogram CHIMRA on its mount while not wasting much energy

  vibrating the rest of the 34-kilogram turret. The portion door mechanism is a very small

  motor that rotates a lever that presses up against the open end of the hole out of which

  CHIMRA drops 150-micrometer-sieved portions. The thwack mechanisms both serve

  multiple functions. Each of the two thwack mechanisms is connected to a door that opens

  up CHIMRA for inspection, sample dumping, and cleaning, and a “thwack arm” that

  198 SA/SPaH: Sample Acquisition, Processing, and Handling

  carries a sieve. The primary thwack mechanism is connected to parts of the 150-microm-

  eter sieve path (section 5.4.2). The secondary thwack mechanism is connected to parts of the 1-millimeter sieve path, including the scoop (section 5.4.3). Both can be wound up with a spring to slam the sieve against the rest of the mechanism to clear stuck sediment,

  hence the “thwack” moniker (section 5.4.4).

  5.4.2 CHIMRA 150-micrometer sample pathways

  This pathway can generate individual sample aliquots amounting to about 75 cubic milli-

  meters each for delivery to SAM or CheMin, or a single “portion plus” aliquot of (very

  approximately) three times that size.

  5.4.2.1 Drill to CHIMRA reservoir

  After Curiosity has drilled a sample, the sampled powder sits in the forward sample cham-

  ber, immediately above the drill bit. The drill reservoir is two-chambered so that the drill can be used at angles of up to 20° without sample spilling out of the sample exit tube

  prematurely, regardless of drill orientation. Once the drill feed is fully retracted, the drill bit assembly sample exit tube aligns with the CHIMRA sample inlet tube. In the aftermath

  of the drill feed anomaly described in section 5.3.4.3, this is an important detail. If the drill feed is not available, the only way to transfer material from the drill to CHIMRA will be

  by dumping the drilled material somewhere and picking it up again with the scoop, a dif-

  ficult or perhaps impossible proposition.

  To move the sample into CHIMRA, the rover tilts the drill and uses either drill percus-

  sion or CHIMRA vibration to shift the powd
er 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.

  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.

  5.4 CHIMRA: Collection and Handling… 199

  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.

  200 SA/SPaH: Sample Acquisition, Processing, and Handling

  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 opera-

  tions 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, engi-

  neers 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

  14 Steven Kuhn, personal communication, email dated August 14, 2015

  5.4 CHIMRA: Collection and Handling… 201

  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.

  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

  202 SA/SPaH: Sample Acquisition, Processing, and Handling

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

  5.4 CHIMRA: Collection and Handling… 203

  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