Emily Lakdawalla
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Yard showed that wheels were robust to punctures at these forces. It takes 800 newtons for
a sharp metal cone to puncture wheel skin, and 1500 newtons for sharp rocks collected
from the Mojave Desert to do so.
How could such high forces on wheel surfaces be generated? Earth tests revealed that
the answer lay in kinematics of the full rocker-bogie suspension system. The motors drive
all six wheels at fixed rotation rates. When one wheel encounters an obstacle that does not move aside or press into the ground under the weight of the rover, that wheel must travel
a longer distance than the other wheels as it rolls over the obstacle. But its motor drives it at the same fixed rate as the other wheels. So the wheel encountering the obstacle is
dragged as it travels.
Additionally, when the rover drives forward, four of its six wheels are on forward-
projecting legs. If one of these wheels climbs an obstacle, some of its rotation rate goes
toward vertical motion, and its horizontal motion is slowed. The remaining wheels con-
tinue moving horizontally at full speed, shoving the blocked wheel on its forward-
projecting leg toward the obstacle with considerable force. If the obstacle is strongly
cemented into the ground and pointy, it can open a hole. Consider a rolling suitcase: when
you drag it behind you, you exert an upward force as you pull on the wheels, helping it to
climb an obstacle. When you push it in front of you, you exert a downward force as you
push, and the suitcase’s motion is easily and often stopped by small obstacles, jarring your arm. In tests in the Mars Yard of driving forward over sharp cones, the front and middle
wheels punctured easily, while the rear wheels remained whole.
The mission dramatically reduced the rate of damage by:
• Picking local drive paths carefully among potentially damaging rock, consequently
ending the use of autonav (which would blithely drive the rover over pointy rock
patches).
• Mapping the terrain ahead using orbital data (including HiRISE images and
Odyssey THEMIS thermal inertia maps) and seeking out less “pointy” terrain dur-
ing long-term traverse planning.
• Avoiding turns on sharp terrain.
• Sometimes driving backwards.
After sol 660, the engineers decided that the turns-in-place required at the ends of back-
wards drives in order to do drive-direction imaging held more potential for wheel damage
than was saved through backwards driving.
4.6 Mobility System 177
As of the mission’s second landing anniversary, when the rover had driven about
8 kilometers, the engineers estimated the following remaining lifetime for the
wheels:39
• Bedrock with lots of rocks: 8 kilometers.
• Lots of rocks, not on bedrock: 13–14 kilometers.
• Bedrock with few rocks (like flagstones): 30–40 kilometers or more.
• Smooth or sandy, with few or no rocks: indeterminate (causes no damage).
Furthermore, Mars Yard testing suggests that, on average, once three grousers have
broken on a wheel, about 60% of its life has been consumed.40 The rover’s wheels are now expected to survive as long as the mission does, although they may look much the
worse for wear by the time the mission ends. Curiosity should be able to achieve at
least 28 kilometers total mission odometry unless there is a dramatic change in the
terrain.
On sol 1646, in response to the observation of broken grousers on sol 1641, the mission
tested new traction control ability for the first time.41 Traction control was turned on by default on sol 1678. The rover senses when a wheel is climbing an obstacle by monitoring
tilts of rockers and bogies. The rover responds by slowing the turn rate of the wheels that are not climbing obstacles, allowing the climbing wheel to rotate faster, thereby reducing
the likelihood of punctures and widening cracks.
Even with “failed” wheels the rover may continue to be able to drive. The wheels fail
when all the grousers have snapped, leaving the inner two-thirds of the wheel diameter
flapping, connected to the rest of the wheel only at the locations of the asymmetric
tread features (Figure 4.23). This is hazardous to the rover, because sharp edges on the broken wheels can scrape against the cable that runs to the wheel motors. Slicing into
a cable could not only jeopardize the functioning of that wheel’s motors, it could also
potentially cause a short circuit that would risk the motor controller – which also con-
trols the motion of all other moving parts on the rover. Driving the rover with a wheel
in this condition on Mars could be hazardous, but it would still be better than not driv-
ing at all. In the Mars Yard, driving on such wheels has been tested; eventually the inner
two-thirds of the wheel snaps off completely, and the rover is able to drive quite effec-
tively on the remaining third of the wheel surface that is still attached to the inner
stiffening ring. 42
39 Lakdawalla (2014)
40 Steve Lee, personal communication, review dated August 13, 2017
41 Herkenhoff (2017)
42 James Erickson, personal communication, interview dated September 18, 2014
178 How the Rover Works
Figure 4.23. Wheel tested to failure. The rover can still drive effectively on this wheel, but the sharp edges of the broken grousers and webbing present a hazard. Photo taken in the JPL
Mars Yard on October 13, 2014 by Emily Lakdawalla.
4.7 TESTBEDS
4.7.1 The Mars Yard
A rover as large as Curiosity requires a large area for testing purposes. The JPL Mars Yard is 66-by-36 meters in size, located at the top of the steep Pasadena campus (Figure 4.24).43
Most of it is flat and level, with the surface material made of beach sand, decomposed
granite, brick dust, and volcanic cinders. There are also lots of basalt rocks of different sizes that engineers can move around to simulate different driving conditions. One side of
43 JPL (2008)
4.7 Testbeds 179
Figure 4.24. Panoramic view of the Mars Yard at JPL. NASA/JPL-Caltech.
the Mars Yard is sloped at a range of angles for testing driving and arm operations on sloping surfaces. At one end is a small building that garages test rovers, associated equipment, and engineers (Figure 4.25).
4.7.2 The Vehicle System Testbed
The Vehicle System Testbed (VSTB), also known as “Maggie,” is the highest-fidelity copy
of the rover and is housed in the shed at JPL’s Mars Yard.44 It is used for testing driving, arm movements, and drilling using the same software and electronics that are on Mars, on
a suspension system that will put the rover in similar positions as experienced on Mars.45
It has the same body, suspension system, arm, sample handling system, mast, and other
motorized elements as the flight rover. Initially, it had the same wheels as the flight rover, but after degradation they were eventually replaced with wheels twice as thick as those on
Mars. (Their rapid degradation resulted in part from bearing nearly the full Earth weight
of the full-scale rover.)
The Vehicle System Testbed’s avionics are similar to those of the flight rover, but are
housed on a rack outside the rover’s body and connected to the rover’s body with an
umbilical to facilitate testing. There is no RTG, so the same umbilical carries power. The
umbilical is long enough to stretch the entire length of the Mars Yard. Like the flight rover, there are two complete main computer
systems. While there is a cooling system, it is different from the one used on Mars. Because it’s on Earth, there is flexibility to reconfigure the VSTB as needed to accommodate tests. For example, prior to landing, the arm was
removed and operated separately on a tiltable stand to allow engineers to do driving and
arm testing simultaneously.
44 A sign in the Mars Yard shed states that MAGGIE stands for “Mars Automated Giant Gizmo for Integrated Engineering,” but that is likely a backronym for the name, the original source of which is lost to history
45 Vandi Verma, personal communication, email dated February 9, 2017
180 How the Rover Works
The VSTB has a full complement of engineering cameras, but does not have many
flight-like science instruments. There is a flight-like MAHLI, which took the self-portrait in Figure 4.25. The APXS instrument is similar to the flight one but does not usually have its radioactive source (though the APXS team did once install a source for testing).
Figure 4.25. MAHLI self-portrait of the vehicle system testbed taken inside the Mars Yard shed, August 1, 2012. NASA/JPL-Caltech/MSSS.
4.7 Testbeds 181
Substitutes take the place of the ChemCam imager and Mastcams. There is no MARDI or
CheMin. SAM electronics are present, but not the rest of the instrument. However, there
are functioning SAM and CheMin inlet covers, and the engineers can collect sample mate-
rial dropped through them in order to measure volume.
4.7.3 Scarecrow
Scarecrow is a second engineering model of the rover. It consists of a full-scale mobility
system connected to a small body containing batteries and electronics. The whole model
has a mass of 340 kilograms, or 3/8th of the mass of the flight rover, so that it exerts the same ground pressure on Earth that Curiosity does on Mars. Its name derives from the
character in The Wizard of Oz: Scarecrow doesn’t have a brain. It does have force and torque sensors in its axles to measure wheel loading under Mars gravity. It can report motor current as well as roll, pitch, and yaw using onboard inertial measurement unit, much as the actual rover can. It has ultrasonic range finders on each wheel to measure sinkage.46 It is used primarily to test how well the rover traverses different types of terrain (Figure 4.26).
Figure 4.26. Scarecrow descending a slope in the Mars Yard, October 2007. NASA/JPL-Caltech image release PIA10014.
46 Heverly et al (2013)
182 How the Rover Works
4.7.4 The Qualification Model Dirty Testbed
Before and shortly after landing, the tricky operations of drilling and sample preparation
were worked out in the Qualification Model Dirty Testbed (QMDT). This had a non- flight-
like arm with a high-fidelity duplicate of drill and sampling system. It was operated in a
thermal vacuum chamber to mimic the Mars environment.
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5
SA/SPaH: Sample Acquisition, Processing,
and Handling
5.1 INTRODUCTION
Curiosity has unprecedented capability for interacting with the Martian surface using a
collection of hardware called the Sample Acquisition, Processing, and Handling (SA/
SPaH, pronounced “saw-spaw”) system (Figure 5.1). SA/SPaH includes the robotic arm and turret, the drill, and the sample scooping/sieving/portioning apparatus called Collection and Handling for In situ Martian Rock Analysis (CHIMRA, pronounced “chimera”). Also
included in SA/SPaH are the Dust Removal Tool (DRT, but usually just called the “brush”),
a variety of immobile hardware bolted to the front of the rover that supports sampling and