Emily Lakdawalla
Page 22
4.6.1 Rocker-bogie suspension system
The front wheels attach to a long rocker arm. The middle and rear wheels are linked
together to form a bogie, which connects to the back end of the rocker arm through a pas-
sively rotating pivot that can tilt forward and back by as much as 45°. The rocker arm is
connected to the rover body at another passive pivot, which can tilt forward and back by
about 20°. (In practice, much tighter limits are usually set on these pivots such that the
rover will autonomously stop driving if unexpectedly higher angles are reached.) If that
were the end of it, the rover body would flop forward or backward on the two rocker piv-
ots, but a differential mechanism connects the left and right sides of the rocker-bogie
suspension system to keep the rover body nearly level. A vertical swingarm connected to
the rocker rises above the rocker pivot and connects through a link assembly to a
34 Edwards et al (2013a)
35 There is no publication by an engineer that describes the rocker-bogie suspension system in detail.
Sources for description of the mobility system include Heverly (2012) and Arvidson et al (2017)
4.6 Mobility System 163
Figure 4.11. Engineers demonstrate the obstacle-climbing capability of the rocker-bogie suspension system on the “Scarecrow” test bed rover, June 19, 2007. Scarecrow is a stripped-down model designed to exert the same force on Earth’s surface that the actual rover does on Mars under lower Martian gravity. Note that Scarecrow’s body is nearly level and all wheels are in contact with the ground despite the fact that three of the wheels are scaling obstacles similar in height to a wheel. Photo by Emily Lakdawalla.
horizontal swingarm that crosses the back of the rover. The horizontal swingarm is attached to the rover body at the center differential pivot, another passive pivot.
If one front wheel climbs an obstacle, it pushes the horizontal swingarm backward on
that side, resulting in an equal and opposite downward motion of the front wheel on the
other side. The opposing vertical motions of the front wheels ensure that they maintain
contact with the ground, and the rover body stays level. Meanwhile, the passive bogie
pivot allows the middle wheel on the same side as the obstacle to drop, staying on the
ground, as the front wheel climbs.
The rover is robust to local tilt, designed to be stable on a slope of up to 45°. For safety, rover drivers set tight limits on rover tilt based upon their expectations for the terrain. They rarely set limits above 7° of tilt for the rockers and 17° for the bogies, which are the angles they expect when traversing a 40-centimeter-tall obstacle sitting on flat terrain. 36
36 Matt Heverly, personal communication, email dated March 11, 2017
164 How the Rover Works
The rocker-bogie suspension system actually performs better rolling bogie-first than
rocker-first. Curiosity, the Mars Exploration Rovers, and Sojourner have all been designed
to drive rocker-first so that if a forward drive gets the rover into a hazardous situation, it is more likely to be able to back straight out of the problem terrain. Curiosity sometimes
drives backwards, but when facing backward the MMRTG obstructs the Navcams’ view of
the nearby terrain, preventing the rover drivers from obtaining the images they need to
plan future drives. So backwards drives have to finish with a turn or at least a wiggle to one side or the other to allow the Navcams to see the terrain ahead.
4.6.2 Motors
Curiosity’s actuators consist of a motor, a gearbox, a brake, and an encoder; in this book,
“motor” typically applies to a whole actuator assembly (Figure 4.12). The motors are very powerful. A single drive motor has enough torque to drive the rover up a vertical wall. The rover’s top speed, 4.2 centimeters per second (151 meters per hour), is so slow that the
motion is quasi-static. There is no freewheeling, and all wheel rotation is commanded
wheel rotation. When the wheels aren’t rolling, they are braked.
Because only the four corner wheels are steerable, the rover can’t “crab” (drive side-
ways), but it can turn in place, allowing it to pick its way safely among a field full of
obstacles provided that the obstacles are separated by more than the width of the rover. The Figure 4.12. One of the high-torque drive motors for Curiosity’s mobility system. The motor end is at the right side; its output passes into a four-stage gearbox that rotates the plate at left.
From Cook (2009).
4.6 Mobility System 165
steering motors are positioned above the wheel’s centers, connected by U-shaped brackets
to the motorized wheel hubs, so that the wheels steer in place about a vertical axis. A turn in place of 60° or more draws a complete circle of wheel tracks on the ground, leaving
telltale “donuts” about 2.75 meters in diameter along the rover’s tracks (Figure 4.13).
Figure 4.13. Donuts along tracks document rover turns. Top: Right Navcam photo taken after a drive on sol 527 (NRB_444289916RADLF0260000NCAM00252M1) showing marks of two
turns in place: “B” is a complete donut, and “A” is not. Credit: NASA/JPL-Caltech. Bottom: Mars Reconnaissance Orbiter HiRISE image ESP_035350_1755 taken sol 538, including
donuts A and B as well as tracks of several arcing turns. NASA/JPL-Caltech/UA/Emily
Lakdawalla.
166 How the Rover Works
The rover’s motor controller can only run eight motors at a time, so the rover cannot
steer and drive simultaneously. Thus drives alternate between straight drive segments and
arcing turn segments. Wheel rotation rates are adjusted for arcing turns so that the wheels on outer edges of turns rotate faster than inner wheels.
4.6.3 Wheels
Curiosity’s wheels presented a design challenge because they had to serve as both landing
gear and running gear. 37 As landing gear, they had to absorb the mechanical shock of touchdown, protecting the wheel motors from harm. After landing, the wheels needed to
provide good traction over Martian terrain, including floating the heavy rover over sand.
They needed to be as lightweight as possible, and to fit within the narrow confines of the
aeroshell. The final design represents a compromise among all these competing require-
ments. Surviving the landing trumped all other requirements, and most of the design effort
focused on ensuring Curiosity could drive away from any imaginable landing scenario.
Unfortunately, that proved shortsighted.
The wheels are 50 centimeters in diameter at their centers (including the height of the
treads), with a crowned profile such that they are 46.5 centimeters in diameter at their
outer edges (Figure 4.14). They are 40 centimeters wide. They consist of an aluminum tire and a titanium hub-and-spoke assembly. The spokes have a complex shape that makes
them springy in all directions, allowing them to do their job of absorbing a landing jolt
even if they landed on slopes or rocks.
Each wheel was machined from a single block of aluminum. The wheel skin is incred-
ibly thin – at just 0.75 millimeters, as thin as it was possible to machine – in order to limit the wheels’ total mass. The wheels are stiffened by three circumferential rings: two at the inner and outer edges, and a third ring located about a third of the way inside the outer ring to provide a place for the spokes to attach. Together, all these design elements enabled the wheels to deform dramatically under the force of a landing and return to their original
shape (Figure 4.15).
Other design elements had to do with surface operations. A black anodized coating
prevents the wheels from th
rowing glints into camera images. For traction, the wheels
have treads or “grousers”. The height and spacing of the grousers represent a compromise
among several factors. The grousers had to be spaced close enough that they would cog
with features on rock faces, about 65 millimeters apart. Their height is relatively short.
Through laboratory tests of different tread designs, the mobility team found that most of
the improvement in wheel traction came with treads whose height was comparable to the
particle size of the material the wheel drove on. They settled upon a tread height of about 3% the wheel radius, or about 7 millimeters. After the challenging Opportunity experience
of driving a rover across sloping crater walls, in which the rover tended to skid downslope, they added a chevron pattern to the wheel treads in order to prevent the same from happening to Curiosity.
37 Haggart and Waydo (2008)
Figure 4.14. Parts of Curiosity’s wheel. Curiosity wheels are crowned, 50 centimeters tall at center, 46.5 centimeters diameter at sides, and 40 centimeters wide. NASA/JPL-Caltech/Emily Lakdawalla.
Figure 4.15. Spring-like deformation of a rover wheel during testing. In this test, two of the spokes have “bottomed out” on the inside surface of the wheel. After this test, the wheels sprang back to their original shape. From Lee (2012 ).
168 How the Rover Works
If the ground were perfectly flat and rigid, the crowned shape of the rover wheels would
touch it only at one point. In reality the weight of the rover drives it in to the ground, so to approximate the ground pressure of rover wheels on the surface, engineers defined the
contact area as being the wheel width times the wheel radius. (This effectively assumes
that 57° of the wheel’s full circumference is in contact with the surface.) In operation, the wheels do not generally touch the ground over so much of their radius (Figure 4.16).
The wheels have twelve holes cut into them, part of an asymmetric tread feature that
interrupts the otherwise regular pattern of the wheel treads (Figure 4.17). This feature makes marks at regular intervals (about 1.5 meters apart) in rover wheel tracks. The track
markings can be directly compared to the expected distance traveled in order to measure
Figure 4.16. Wheel performance on different substrates. Upper left: small rocks over regolith, the substrate encountered by most previous missions. The wheels dig slightly into the surface, but only a small area of the wheel is in contact. Upper right: a jagged, rocky surface. At times, the rocks contact the surface at only one point, as the right rear wheel does here. The wheel skin is thick enough that the rover’s weight merely resting on a pointy rock does not puncture a wheel. Lower left: a well-packed sand ripple on which the wheels are getting good traction, similar to that in the upper left image. Lower right: a ripple made of fluffier sand into which the wheels are embedding as they slip. MAHLI images 0504MH0002610000200627E01, 0506MH0002610000200672E01,
0529MH0002610000201142E01, and 0711MH0002610010204346E01, NASA/JPL-Caltech/
MSSS.
4.6 Mobility System 169
Figure 4.17. Asymmetric tread features in the rover wheels mark the rover’s tracks every time the wheel has rotated once, about once every 1.5 meters. They also spell out “J P L” in Morse code. J is · - - - ; P is · - - · ; L is · - · · . Mosaic of two left Navcam images taken on sol 535.
NASA/JPL-Caltech.
170 How the Rover Works
how much the rover wheels have slipped during a traverse. In initial wheel designs, these
features were the letters “J P L” machined into the pattern of the treads (see Figure 4.11), but NASA objected to JPL labeling the wheels in this way. So the design was changed to
one that incorporated bland rectangular holes. Mischievously, the wheels’ designers made
those holes spell out “J P L” in Morse code in the tracks.
4.6.4 Wheel degradation
The wheels performed perfectly upon landing; the only visible damage from the landing
event was a tiny crack in the left middle wheel. During the first 500 meters of the rover
traverse from Bradbury Landing to Yellowknife Bay, the wheels suffered little additional
damage. The team surveyed the wheels with MAHLI on sol 411, noticing a puncture in
the left front wheel. Re-imaging the wheels on sol 463, they observed that the tear had
grown dramatically worse. From that sol forward, the team commanded numerous wheel-
imaging sequences, shooting photos of all wheels with the MAHLI camera and the right-
side wheels with Mastcam in between every drive. Periodically, they would devote an
entire drive to full surveys of the wheels by driving the rover short distances between four or five wheel surveys in order to image the entire wheel surface. Wheel imaging is summarized in Box 4.1.
Box 4.1. Sols with MAHLI and ChemCam RMI wheel imaging to sol 1800.
Sols in bold indicate full wheel imaging. Sols in italic indicate ChemCam RMI imaging. Sometimes the full wheel imaging included images taken on multiple consecutive drive sols.
34
527
589
744
1260
60
528
591
803
1269
177
529, 532
595
834
1287
275
537
597
840
1313, 1315
411
540
601
842
1355
463
542
605
939, 940
1380
469
546, 547
631
955
1386
472
548
633
958
1403
476
549
635
962, 963
1407
488, 490
552
636
971
1416
493
553
637
989
1434, 1435
502
554, 555
640, 641
1046
1444
504
559
646
1057
1459
506
560
653
1065
1471
508
561
660
1076
1482
510
562, 563
667
1087
1512
512, 513, 515
564
679
1102
1591
518
566, 568
695
1127
1681, 1682
519
569
706
1157
1729, 1730
520
571
708
1178, 1179
1798
521
574
711
1182
524
587
713
1214
526
588
729
1245
Over time, more punctures and tears appeared in the middle and front wheels, while the
rear wheels remained relatively unscathed. Figure 4.18 through Figure 4.22 document the condition of all six wheels at three points in the mission. As of sol 513, the rover had
driven 4.7 kilometers; as of sol 708, 8.7 kilometers; and as of sol 1513, 15.1 kilometers.
Improved
understanding of how to save the rovers’ wheels slowed the progression of
4.6 Mobility System 171
Figure 4.18. MAHLI survey of right front wheel on sol 513 (left column), 708 (middle), and 1313–1315 (right). See text for discussion. NASA/JPL-Caltech/MSSS.
172 How the Rover Works
Figure 4.19. MAHLI survey of right middle wheel on sol 513 (left column), 708 (middle), and 1313–1315 (right). See text for discussion. NASA/JPL-Caltech/MSSS.
4.6 Mobility System 173
Figure 4.20. MAHLI survey of rear wheels on sols 513 and 1313–1315. See text for discussion. NASA/JPL-Caltech/MSSS.
damage after sol 708. Fewer new punctures formed, but dents and cracks progressed. The
first wheel images to reveal broken grousers, on the left-middle wheel, were taken on sol
1641, after about 16 kilometers of driving. As of sol 1800 the rover has driven 17.5 kilo-
meters, with no further broken grousers.
The punctures were caused by two factors: metal fatigue and forces intrinsic to the
rocker-bogie suspension system. 38 Fatigue is a consequence of the flexibility of the wheels.
38 The investigation of causes of wheel damage is described in Arvidson et al (2017)
174 How the Rover Works
Figure 4.21. MAHLI survey of left middle wheel on sol 513 (left column), 708 (middle), and 1313–1315 (right). See text for discussion. NASA/JPL-Caltech/MSSS.
4.6 Mobility System 175
Figure 4.22. MAHLI survey of left front wheel on sol 513 (left column), 708 (middle), and 1313–1315 (right). See text for discussion. NASA/JPL-Caltech/MSSS.
176 How the Rover Works
They flex back and forth with each wheel rotation. Stress concentrates at the tips of the
chevron shapes in the grousers. Eventually the skin cracks near chevron points, and over
time the cracks grow and merge. Once the cracks propagate entirely across the width of the
wheel, the grousers are unsupported by skin stretching between them, so they flex even
more with each wheel rotation. Eventually, repeated flexing fatigues the grousers and they
also begin to snap.
At first, it was difficult to understand why only the left and middle wheels seemed to be
getting damaged. When at rest on level ground, the front, middle, and rear wheels on each
side bear weights of 564, 636, and 458 newtons, respectively. Experiments in the Mars