Emily Lakdawalla
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drilling activities called the “sample playground,” and motorized inlet covers and spring-
loaded wind guards for the SAM and CheMin instruments.
5.2 ROBOTIC ARM AND TURRET
Curiosity’s arm is huge. It measures 2.2 meters long from its base to the center of the turret. The arm weighs 101 kilograms; the turret alone is 34 of that.1 Curiosity’s arm has five degrees of freedom, provided by individual motors. The motors power five joints, in order
of their position along the arm: the shoulder azimuth joint; shoulder elevation joint; elbow joint; wrist joint; and turret joint. The operation of most of these joints mostly mimics the flexibility of a human arm, except that Curiosity’s elbow is fully double-jointed. Curiosity’s arm was designed to be strong enough that an Earth copy, under Earth gravity, could support its own weight without any additional help, which makes testing motions using the
testbed rover substantially easier than it might otherwise be.
1 The arm is described in detail in Billing and Fleischner (2011)
© Springer International Publishing AG, part of Springer Nature 2018
185
E. Lakdawalla, The Design and Engineering of Curiosity, Springer Praxis Books,
https://doi.org/10.1007/978-3-319-68146-7_5
186 SA/SPaH: Sample Acquisition, Processing, and Handling
Figure 5.1. Parts of Curiosity’s Sample Acquisition, Processing, and Handling (SA/SPaH) system. Top image is the John Klein self-portrait from sol 177 (NASA/JPL-Caltech/MSSS); bottom image was taken during testing at Kennedy Space Center on August 13, 2011 (NASA
release KSC-2011-6470), annotated by Emily Lakdawalla.
5.2 Robotic Arm and Turret 187
5.2.1 Arm mounts
The arm exerts significant loads on the rover whether it is extended or stowed. While
stowed, caging mechanisms restrain the arm’s motion. On the top of the shoulder bracket
are three mechanisms that securely hold the turret when the rover is driving (and held it
during launch and landing). A forward-projecting (“+X”) parapet captures the turret, and
then the turret rotates 50°, pushing two hooks on the turret into two “duckbill” clamps on
the bracket, whose flaring mouths guide the hooks into place. See Figure 5.1 for the locations of all these components.
Although the turret is tightly restrained to the rover’s left shoulder when stowed, the
arm’s elbow joint only rests passively on its tripod-shaped bracket on the rover’s right
side. The elbow has to be able to slide back and forth along the bracket because the front
panel of the rover is made of aluminum and the arm’s tubular structure is titanium.
Aluminum’s coefficient of thermal expansion is almost three times higher than that of
titanium, so the front panel expands and contracts by millimeters more than the arm does
over the 180°C range of temperatures that Curiosity experiences over the Martian sea-
sons. The aluminum shoulder bracket that supports the arm incorporates flexures that
allow the bracket to accommodate the differing thermal expansion of the bracket and the
titanium shoulder motor.
5.2.2 Cabling
Running all the signals needed to monitor and control the arm’s motors and instruments to
the rover’s computer was a major challenge. There are 920 different signals being moni-
tored on the robotic arm, of which 555 are at the very end of the arm on the turret, including 300 within CHIMRA. The signals travel to the avionics through 10 meters of flex
cable, 63 millimeters wide and 5 millimeters thick, strapped to the outside of the rover
arm. To allow freedom of motion, the flex cable wraps several times around each of the
five actuators in large spools. The flex cable from the arm debouches into a rover bulkhead on the rover’s left shoulder, where its signals transfer to a huge bundle of Kapton-wrapped round wires.
5.2.3 Turret
The turret is about 60 centimeters in diameter. The centerpiece of the turret is the drill.
Attached to it are CHIMRA, the dust removal tool (brush), MAHLI, and APXS (Figure 5.2).
The science instruments are separated from the drill by vibration isolator mounts to miti-
gate the effects of vibration from CHIMRA and drill percussion.
5.2.4 Using the arm
The arm was tested and qualified to be able to reach targets within a cylinder-shaped
region called the “primary workspace” (often referred to as the “magic cylinder”), shown
in Figure 5.3. 2 But it can readily reach targets beyond that, in a wider region called the 2 Use of the arm for sample collection is described in Anderson et al (2012)
188 SA/SPaH: Sample Acquisition, Processing, and Handling
Figure 5.2. Parts of Curiosity’s robotic arm turret, including the drill, dust removal tool or brush (DRT), Collection and Handling for In situ Martian Rock Analysis (CHIMRA), and two science instruments, the MAHLI camera and APXS elemental analyzer. Navcam image
NLA_400335692EDR_F0040000NCAM00107M, taken during the first turret checkout on sol
32. NASA/JPL-Caltech/Emily Lakdawalla.
“work volume.” The arm functions slowly and deliberately, its tip moving at a maximum
speed of 1 centimeter per second. The arm’s heft imposes limits on the accuracy of its
placement: gearbox backlash, thermal expansion and contraction, and the massive weight
of the turret combine in difficult-to-predict ways with the tilt of the rover to make its position in space uncertain within about a centimeter.3 The uncertainty is slightly larger for the instruments because the amount and direction that they sag on their wire rope vibration
isolators depends on the orientation of the turret.
Once the arm has been deployed to a location, it can be repositioned to the same pose
over and over again with surprising precision. The precision holds even when a different
tool is selected on the turret. Rover planners take advantage of this precision to get closer to targets than the arm’s initial placement inaccuracies allow. The rover planners
commonly use the contact plate on APXS to test exactly where a target is, by advancing
APXS toward the target and waiting for the contact plate to record contact. In circum-
stances where the APXS can’t be used in this way (soils, loose rocks, or very uneven
3 Kuhn (2013)
5.3 The Drill 189
Figure 5.3. The “magic cylinder” is centered 105 centimeters in front of the rover body, 100
centimeters tall and 80 centimeters in diameter. If the rover is on a level surface, the workspace extends 20 centimeters below the surface. Modified from Billing and Fleischner ( 2011 ).
The CAD model of the rover shown here predates several design changes.
surfaces), they can use MAHLI. MAHLI’s autofocus distance is very sensitive to the dis-
tance to the target, so arm engineers can use the MAHLI autofocus distance to upgrade
their knowledge of the position of the target to get closer on a later sol. Rover planners can even use APXS in “proximity mode” (see section 9.3.2) as a test of where the surface lies.
5.3 THE DRILL
Curiosity’s drill is a percussion instrument that hammers its rotating bit, boring holes 1.6
centimeters wide and up to 6.5 centimeters deep. 4 The drill has four motors: a drill feed mechanism for moving the drill bit up and down; a drill spindle mechanism to rotate the
bit; a percussion mechanism; and a drill chuck mechanism that can release the drill bit
assembly and exchange it for a new one from one of two bit boxes located on the front of
the rover (Figure 5.4).
4 The drill is described in detail in Okon (2010)
190 SA/SPaH: Sample
Acquisition, Processing, and Handling
Figure 5.4. Parts of the drill. Images from Okon (2010 ), annotated by Emily Lakdawalla.
5.3.1 Drill bit assembly
The drill bit assembly consists of a drill bit, collection sleeve, and sample chamber (Figure 5.5).
The spade-shaped steel bit is a commercial off-the-shelf component that has been modified,
with two deep flutes machined into it to help move powdered rock up the collection sleeve and into the sample chamber (Figure 5.6). The steel collection sleeve covers all but the last 1.5
centimeters of the drill bit. After 16 drill sites, Curiosity is still using the original drill bit.
Although it is not as shiny as it once was, it has not dulled dramatically (Figure 5.6).
5.3.2 Drilling
To prepare to drill, Curiosity places two projecting contact sensors against the rock, and
then continues to drive the arm motors even after the contact sensors are in contact with
the rock target (Figure 5.7). This is called “preloading”; the arm can press onto the rock with up to 300 newtons of force. Front Hazcam images taken before and after preloading
usually document a tiny upward motion of the rover body as the arm pushes against the
rock. Once the arm is placed and preloaded for drilling, the arm doesn’t move; all drilling motion is performed by the drill itself using the drill feed mechanism.
5.3 The Drill 191
Figure 5.5. Parts of the drill bit assembly. Photos from Anderson et al ( 2012 ), annotated by Emily Lakdawalla.
Figure 5.6. Condition of Curiosity’s drill bit over time as observed using ChemCam RMI. Top row: sol 172, before the first drill site at John Klein. Bottom row: sol 1528, after drilling at Sebina. NASA/JPL-Caltech/CNES/CNRS/LANL/IRAP/IAS/LPGN.
192 SA/SPaH: Sample Acquisition, Processing, and Handling
Figure 5.7. Drill use on Mars. The two contact sensors/stabilizers are pressed against a rock, and the drill feed has extended to place the drill in contact with the ground. Mastcam image 0174ML0006380000105184E01, NASA/JPL-Caltech/MSSS/Emily Lakdawalla.
Once the drill feed mechanism has advanced the drill bit to contact the rock, drilling
begins with percussion from a 400-gram mass striking a spring-loaded anvil rod 30 times
per second. The team can select the initial energy of the blows within a range from 0.05 to 0.81 joules. At first, the drill uses no rotation, only percussion, to strike a small asterisk-shaped divot in the rock at the location of the desired drill hole – like a carpenter using a nail or awl to set the starting location for their drill. These initial taps dig no more than 0.8
millimeter into the rock.5
For the first 1.5 centimeters of drilling, the powdered material piles up around the drill
hole, making a tailings pile. After the first 1.5 centimeters, the collection tube contacts the surface, and powdered material that passes by the spade tip of the bit climbs up the auger
into a two-chambered sample collection area within the drill bit assembly. After drilling,
the feed retracts and the arm lifts the drill off the rock. The rover then photographs the drill bit with Mastcam to verify that it is none the worse for wear after the drilling activity.
5 Supplementary material to Grotzinger et al (2014)
5.3 The Drill 193
At many sites, Curiosity performs a “mini-drill” test before the full drill, penetrating
less than the unsleeved 1.5 centimeters into the rock, in order for the tactical team to assess rock and rock powder properties before committing to gathering powdered rock sample. A
rock with an extremely unusual water-rich mineral composition could liquefy under the
vibration of the drilling mechanism, which would be catastrophic for the ability to acquire samples. The team can choose to skip mini-drilling to save time if they determine from a
rock’s appearance (from Mastcam, MAHLI, and ChemCam RMI) and composition (from
ChemCam LIBS and APXS) that it is similar to previously drilled rocks.
5.3.3 Drill bit assembly replacement
What if the drill bit gets worn out, or worse, stuck in a rock? If the rover slips during drilling, it could leave the drill bit stuck. To avoid the situation, before drilling, the rover drivers make sure that the rover is in a stable position, with all wheels firmly in contact with the ground, and no small rocks under the wheels. If there is any question of wheel stability, they may sequence a set of MAHLI wheel images in order to be sure the wheels are stable.
If the rover should slip, binding the drill bit, the drill feed mechanism is capable of pulling upward with a force of nearly 10,000 newtons. 6 If the drill remains stuck, they can try pulling the feed while percussing and/or rotating, which would reduce the friction between the
drill and the rock but could also result in the loss of the acquired sample. If the drill bit remains stuck after that, they can try motion of the arm to counteract whatever motion of
the rover had caused the drill bit to bind.
If all of these efforts fail, the rover can detach its bit and leave it behind in the rock, exchanging it for one of two more bits located in bit boxes on the front of the rover
(Figure 5.8; another good view of a drill bit box is in Figure 5.18). Because the drill bit has not yet needed swapping, the drill chuck mechanism has not been used since a brief test
wiggle in the first weeks after landing. 7
5.3.4 Drill problems
Several issues have affected the drill both before and after launch. One was the potential
contamination of the drill bit that caused the reclassification of MSL’s planetary protection status (see section 1.7.4). The others are: the possible presence of Teflon debris in drilled samples; a short in the percussion mechanism; and a serious problem with the drill feed
mechanism. A new problem was diagnosed in the drill chuck mechanism as this book was
going to print around sol 1800. It is similar in character to the drill feed problem. It will not be further discussed here.
5.3.4.1 Teflon debris
Shortly before launch in November 2011, engineers doing testing of drilling operations
found that seals inside the engineering model of the drill bit assembly were slipping during 6 Limonadi D (2012b)
7 Ashwin Vasavada, personal communication, email dated February 9, 2017
194 SA/SPaH: Sample Acquisition, Processing, and Handling
Figure 5.8. A test of the positioning of the turret for drill bit replacement. If the drill bit really were being replaced, Curiosity would already have used the drill chuck mechanism to release the drill bit it is holding. Four round-tipped posts surrounding the drill bit assembly would advance into four funnels at the corners of the bit box, aligning the drill with the replacement drill bit assembly. To the right of the turret is the sample playground. Navcam image
NLA_400696022RAS_F0040000NCAM00110M1 taken sol 34. Inset: a view of the interior of
the bit box taken by MAHLI from a similar position. Image 0036MH0000490010100065E01.
NASA/JPL-Caltech/MSSS/Emily Lakdawalla.
drilling, which generated Teflon debris that mixed with the drilled rock powder. 8 This was a potentially serious source of contamination that could compromise SAM’s ability to
detect organic materials within Mars’ rocks. Although the possibility of Teflon contamina-
tion of the drilled material had been recognized early in development, when the drill was
actually tested, it generated more debris than expected. It was too late in the process to
effect any kind of design change, of course. The mission ultimately determined that the
amount of contamination was small enough that it would not likely affect SAM results,
and suggested that the mission avoid using the drill in a way that generated the most
debris – “minimizing the low-rate-of-penetration operations.” No si
gn of Teflon contami-
nation has been noticed in drilled samples since landing.
8 JPL (2014) Lesson Learned: Recognize that Mechanism Wear Products May Affect Science Results
http://llis.nasa.gov/lesson/10801. Article dated June 8, 2014, accessed October 14, 2015
5.3 The Drill 195
5.3.4.2 Battle short and the sol 911 percussion anomaly
Another potentially serious problem was discovered during Earth testing of a testbed ver-
sion of the drill mechanism in 2011. A broken bushing caused a short circuit in the test
drill that could have fried the rover’s motor controller if engineers had not acted swiftly.
The consequences of such an event happening on Mars would be dire. It was too late to
make any changes to the flight drill. Engineers in Florida opened the belly pan of the rover to install a “battle short” that would route half of the excess current to ground if such a short circuit developed in flight. 9
On sol 911, sensors detected current flowing through the battle short as Curiosity was
using drill percussion to transfer sample from the drill to CHIMRA, halting the opera-
tion.10 There is no way to know if the cause is the same as the problem discovered on Earth, but the effect is similar. The shorts have recurred since sol 911, but are intermittent and extremely brief. If they remain that way, the battle short adequately protects the electronics. The engineers have instructed the rover to tolerate very brief shorts without fault-
ing and terminating the drilling process.11 At the same time, the mission has shifted to relying less on the percussion mechanism. They now avoid using drill percussion for sample transfer, relying on CHIMRA vibration. They have also changed the way they operate
the drill: originally, they began drilling with a medium percussion level and made adjust-
ments according to the penetration rate, but they now begin with very light percussion and
only increase the rate as needed. Engineers have also developed a new rotary- only drilling technique, made possible by the softness of the rocks within Gale crater, but rotary-only