Roving over Operating nearly 125 million miles from home, the rover had to be very reliable and highly mobile to forge a path

along the Martian surface. By Donald Bickler Downloaded from http://asmedigitalcollection.asme.org/memagazineselect/article-pdf/120/04/74/6381472/me-1998-apr6.pdf by guest on 29 September 2021

To fit in a small space aboard Pathfinder on its trip to Mars, the rover must squat down by breaking the rocker links where they pivot on the body. The locking spring that will enable the Sojourner rover to reach its full operational height is shown in the bent position.

HROUDED IN AIR BAGS that formed its landing sys­ travels-12 times longer than the ve hicle was designed tem, the M ars Pathfinder hit the surface of the to las t-the rover explored the vicinity of the lander in SRed Planet-hard-on July 4, 1997. Some 90 the Ares Planitia, probing the Martian surface and pho­ minutes after bouncing around and finally coming to tographing its rocky environs. It analyzed the chen1.ical rest, the craft removed its cocoon to reveal a lander and makeup of the soil and various rocks. It dug into the the Sojourner rover vehicle. Back on Earth, millions of surface, measuring soil strength and the abrasive charac­ people tuned in to watch the rover's exploration of an­ teristics. The 630-miUi meter-long, lO.S-kilogram other world on television and World Wide Web sites Sojourner has done all that was expected of it and more, dedicated to the nussion. paving the way for future missions to Mars. The Pathfinder lander and Sojourner have completed In designing the vehicle's mobility system to handle the their successful mission. During its three months of harsh Martian surface, engineers at the Jet Propulsion Laboratory (jPL) in Pasadena, Cali f. , had to provide it Donald Bickleris technical-group supervisor at the J et with a mobility far in excess of conventional al l-terrain Propulsion Laboratory in Pasadeua, Calif. vehicles. Because command signals from Earth would

74 APRIL 1998 M EC H AN I CAL ENG I NEERI NG take more than 11 minutes to travel the 123.5 nullion ground level, resulting in another moment that tends to lift miles when the probe landed, Sojourner had to be a the fi·ont. The friction at the rear raises the vehicle with a senuautonolTl.ous vehicle that would not need se rvicing. moment arm. equal to half the wheel base n"linus the rear­ That reliability invol ve d more than just not breaking wheel railius. The center of gravity continues to resist \ovith down: The device also had to be able to negotiate the a moment arm of half the wheel base minus the railius. terrain without getting jammed or hung up. At the de­ By increasing th e complexity 50 percent- in other sign stage, this involved the planet's actual makeup and words, by adding two wheels-a vehicle can c]jmb vertical any potentially haza rdous situations that might occur. walls on both sides when th e coeffi cient of friction is 0.8. For example, the design took into account smooth, The six-wheeled Sojourner rover can climb in the forward glassy volcanic rock with ]jttle or no friction and talcum­ direction w ith coefficients of 0.5, 0.68, and 0.6 for the powder-like dust at d epths greater than the vehicle three wheel positions in order, and in reverse with coeffi­ height. To our deli ght, the rover never encountered cients of 0.78, 0.75, and 0.82. (Eight wheels would per­ Downloaded from http://asmedigitalcollection.asme.org/memagazineselect/article-pdf/120/04/74/6381472/me-1998-apr6.pdf by guest on 29 September 2021 these hazards; the vehicle was overdesigned because we form even better, but the added complexity and associated didn't know exactly what we would be up against, but steering problems make the vehicle impractical. ) Six this helped the vehicle last so long. wheels are better than four because when two wheels are climbing over a rock, say, th ey get pushed against it by the SPIDER LEGS AND WHEELS other four. Tlus results in more climbing traction; at the In the 1980s, well before the Sojourner design was cho­ same time, a lesser fraction of the weight is being lifted. sen, contractors to ]PL performed numerous studies re­ To increase traction even more, the wheels have cleats garding the various types of vehicle configurations. One that protrude 10 millimeters and ilig into the sand. Formed proposal involved very complex legged designs intended to climb and leap like spiders and mountain goats. We all want­ ed to work on the challenging mechalucal problems involved with such designs, but the control problems were overwhelming. In addition, hav ing the device "see" ahead and plan foot placem ent proved very difficult. Another option was simple legged vehicles. These variations on the walkers used by the physically impaired place one set of feet forward as they stand stabl y on another set. Weig ht is then transferred to the forward se t to form a new stable confi guration, and the process is repea ted. With three legs, however, the vehicle tips easily; more legs than that and it teeters. Also, the simple legged design The flexibility of the mobility system enabled Sojourner to explore the surface and investigate rocks such as Yogi, which appeared to be similar to common basalt found on Earth. ca nnot be set on a rock, is a problem to steer, and limits instrument access to rock faces. from 0. 127 -nullimeter-tluck stainless steel, these rather On the basis of simplicity, efficiency, light weight, and sharp cleats will grip on the slightest irregularity. This fea­ ease of control, a wheeled vehicle was chosen. Four­ ture helps the rover climb right up the face of a rock, but it wheeled vehicles would be the ideal choice using these cri­ also complicates the autonomous navigation by changing teria, but they do not climb obstacles very well, especially the effective rolling radius over both soft and hard smfaces, obstacles that are larger than the iliameter of the wheels. A a problem. that was not fully overcome. configuration with a wheel base equal to three-and-a-half The 79-millimeter-wide w h eels enable the rover to wheel iliameters and a center of gravity nudway between "float" over the Martian surface. They sink to their design the wheels can be used as an example. If we place a vertical depth fo r soft material with a ground pressure of only 1.65 wall in front of both front wheels of such a ve hicle, the kilopascals. (A low ground-pressure requirement means fi'ont will rise when the coefficient of friction is 0.778 or the vehicle can traverse very soft soil .) This is far less than greater. The bottom of the rear wheel drives at ground the average ground pressure for an automobile, with ap­ level and the wall resists at axle height. This results in a proximately 240 kilopascals, and an Army tank, with 48. moment that ]jfts the fi·ont. The fi:iction at the £i'ont raises Conilitions on Mars never required such a low pressure, the vehicle with a moment arm equal to the wheel base however, because the ground was never that soft. plus the railius of the front wheel. The center of gravity re­ OBS TACLE S TO OVERCO ME sists with a moment arm equal to half the wheel base. When the wall is placed in front of the rear wheel, the The abi]jty to climb obstacles is not primarily a fun ction coefficient of fi:iction required becomes 1.4 or greater. of the wheels, however, but of the rover's suspension sys­ T he wall reacts at axle height and the front wheel pulls at tem. T he system's unique configuration, w hich has been

MEC HANI CA L ENGINEErUNG APRil 1998 75 developed over the las t several years, is known as the fe rent on the hori zontal than on the obstacle. T he most rocker-bogie system because the fro nt and center wheels popular set was a coefficient of 0.8 on the obstacle and 0.3 are j oined on each side to form bogies. These bogies on the horizontal, for climbing rocks in sand. All of these pivot freely at the front of rocker links. The rockers each analyses assumed unlimited torque at every wheel. T his have a rear wheel at the other end, and are pivoted freely assumption allows the wheel traction to be limited by the at a point near their middle. These pivots are where the soil fri ction coeffi cient only. The actual fli ght design has body attaches . more than enough torque to turn th e wheels under the The body is controlled in the pitch direction by a set of most severe conditions. In fact, it has so much torque that links that form a differential, keeping the body at an aver­ it is capable of a tangential force on each w heel equal to age angle between the two rockers. The system has no half the weight of the entire vehicle on Earth. springs or intentionally elastic members, which improves The rover can easily climb obstacles that are more than Sojourner's traction. When a wheel is suspended with an 30 percent of its length. These obstacles can be overrun elastic system, the downward force increases as the wheel both straight on and while turning at will. By compari­ Downloaded from http://asmedigitalcollection.asme.org/memagazineselect/article-pdf/120/04/74/6381472/me-1998-apr6.pdf by guest on 29 September 2021 is raised according to the spring rate of the system. T he son, the average family sedan could drive unconstrained greater force makes it more difficult to raise the wheel as it over obstacles measuring 1.5 meters. takes downward force away from the remaining wheels, reducing their traction. STEERING THE MARTlAN WAY Stability is key to the vehicle's overall effectiveness . T his Climbing is not the only function of JPL's rover mobility includes the ability to stop at any point and proceed or system; steering is just as important. The final rove r de­ reverse, even while climbing a ve rtical wall. For this rea­ sign steers the wheels at all four corners, and each of son, the first analyses were done to optimize the ability those wheels has an independent steering system. The to climb vertical bumps that are axle-high or larger. vehicle performs well in a crabbing m aneuve r-w hen Bumps are obstacles that fit between the wheels, so that the vehicle moves sideways like a crab-even though the Sojourner spent three months traveling over Mars, 12 times longer than originally designed. all wheels not climbing are on a single plane. Steps, center wheels aim forward. With all four corner wheels which carry the vehicle from one level to another, repre­ aimed diagonally to o ne side, the center w heels sc uff se nt another type of obstacl e. C limbing steps is easier sideways, contributing only forward thrust. As the velu­ than climbing bumps, because the rear wheels are hoist­ cle crabs, it m oves to the side without changing heading. ed by the rest o f the ve hicle; also, the lifting force is On Mars, Sojourner was able to perform this maneuver downward on the w heels on the step, increasing their successfully to sidestep obstacles. traction. Bumps, on the other hand, cause the forward Experimental models were m ade with all six wheels wheels to be dragged ba ckwards as the rear climbs. The steerable, which allowed crabbing without scuffing and downward force of the climbing wheel in this case tends increased versa tility. This would improve the ability to to lift the forward w heels, reducing traction. This diffi­ crab as the rover climbed obstacles. Better versa tility culty had to be overcome since bumps are common on would also raise weight, cost, and volume of the increase Mars and steps are rare. from the additional steering m otors and gearb oxes, so The process of optimiza tion began by solving for the such a design was vetoed. coeffi cient of friction needed to climb vertical surfaces at N ormal steering requires the axes of all wheels to coin­ each of six positions (in front of and behind each pair of cide at a single point, a geometry called Ackerman. The w heels, front center, and rear). This m ade it a two­ single point is the center of rotation. Because each wheel is dimensional problem by taking both sides of the vehicle steered independently, this center can be situated anywhere up the obstacle at once (it is considerably less difficult to along the centerline of the middle wheels. When the climb an obstacle on one side only). A seventh position wheels are turned in a crossways manner (the right front was added later to analyze the four-wheeled situation in wheel is turned to the left, the left front to the right, and so w hich the rover " pops a wheeli e," raising the fro nt on), this center of rotation can be at the vehicle's center. wheel(s) because of resistance at the rear. The linkage This turns out to be the most popular maneuver because proportions were traded-changed in a given direction so the navigation system. does not have to calculate a new po­ as to favo r one wheel over another-in a way to mini­ sition for a change in direction. In fac t, after Soj ourner mize the limiting coeffi cient of fri ction. We later changed backed down the lander's rai11p, its fi rst such maneuver was the fro nt of the bogie link, lengthening it so the front to ro tate in place, so the rover could turn without losing wheels can back out easier. T his configuration was com­ the known position. Sojourner fre quently performed this pared with the results of other analyses, which solved for m ove to turn and climb rocks at the same time. the steepest angle that can be climbed when the coeffi­ T he ability to cl ear rocks and other obstacl es, rather cients of friction are known. than just going over them, is also an important aspect of In this set of analyses, the coefficient of friction was dif- design not o nly fo r Soj Olirner but for any all - terrain

76 AP RI L 1998 M EC HA N ICA L ENG I N EERIN G vehicle as well. The ground clearance is set at one-and­ fiber-reinforced-plastic composite body heated by three a-half wheel diameters. Should the rover initially clear an radioisotope heating units. Dry lubricants prevent the object but then get stuck while going over it, the body's vehicle being pasted with congealing oil. The wheels belly pan can support the ve hicle 's entire weight. This drives use ball bearings with plastic balls, aluminum belly pan is considered part of the mobility system. races, and no lu brica tion. An X-type bearing design, Once on Mars, the rover climbed obstacles as efficient­ large enough for the motor and gears to fit inside, takes ly as our field tests and calculations predicted. The big all wheel loads and the final stage of the planetary gear­ unknown was encountering a soil type we were not fa­ ing. This type of bearing takes radial, axial, and moment rniliar with, but appears to be very similar to loads, all with a single row of balls. that on Earth. Furthermore, joints had be designed to accommodate the differential expansion over the temperature extremes. In ABOARD PATHFINDER

addition, the motors have heaters on them because some Downloaded from http://asmedigitalcollection.asme.org/memagazineselect/article-pdf/120/04/74/6381472/me-1998-apr6.pdf by guest on 29 September 2021 Once the specific six-wheel configuration was chosen, lubricant had to be used on the commutators. These mo­ JPL engineers had to make the vehicle squat down so it tor heaters were not used, so they proved to be another would fit economically onboard Pathfinder for the trip to exanlple of Martian overkill. Mars. Originally, the plan was for the rover to be on the One key element of Soj ourner's design had nothing to base petal of the lander and straddle some of the electron­ do with the environment on Mars but everything with ics under its body in the ground-clearance space. In the final Pathfinder design, how­ ever, the rover had to be mounted to a side panel where it only straddles the lan­ der's solar cells. To fit in that small space, the vehicle squats down by "breaking" the rocker links where they pivot on the body. The rover later stands up by driving the rear wheels forward with the remaining wheels locked. After the rocker links are arched to their full height, a l11.echanism on each side snaps into place, locking them into position forever. This mechanism is a coil spring tightly wound to its solid height and bent over to where the opposite ends almost touch when the rover is down. With Pathfinder's air·bag cocoon removed, its "petals" are unfurled to expose Sojourner to the After full height is reached, the spring Martian air. Scientists used this image to decide whether ramps for the rover could be deployed. snaps straight to become a ridged com­ pression member. The differential is active in the standing­ the financial climate back home: maintaining low costs. up process, and keeps the body parallel to the panel as it is part of NASA 's Discovery Program, an stands. To do this, the differential arm on the rocker is initiative for planetary missions with a maximum three­ joined to the front of the right side and to the rear of the year development cycle and a cost cap of $150 million left side. With the vehicle folded, the differential behaves (in 1992 dollars) for developlTlent. To this end, the as if a very large obstacle is at the right front and another at motors and gears are slightly modified, off-the-shelf the left rear. As the vehicle stands, both these obstacle an­ commercial products. In addition, the rover is the fifth gles decrease and the differential keeps the body parallel to generation of an idea from the 1980s; the first four gen­ the point where it locks in place. To pull this off, the rock­ erations were funded as research and development. This er links must be broken at a point where the front and the cut down on engineering costs, which are greater than back parts rotate through the same angle, which involved hardware costs. putting a kink in the front half of each rocker link to clear The success of Sojourner's mission means that more the center wheel when fo lded. Because the rover can rovers will be used in the future to carry scientific instru-' stand up without tipping, the instruments on each end are ments to other sites on Mars. What we learned in de- not smashed against the lander bed. , signing this rover's mobility system will be applied to other missions, such as for the nus­ THE COST OF COLD DAYS sion. Other rovers may someday be large enough to carry Another key design factor was the Martian temperature: manned missions and nonrobotic drivers. By designing On the afternoon of July 30, for exanlple, temperatures these reliable, lughly mobile planetary rovers, we can ex­ reached· a balmy -13°C, and the forecasted low for the pand the reach and scientifi c value of exploratory nus­ next day was - 73°e. We· designed the rover for a cold sions to other worlds . • winter night of - 100°e. Sojourn er's batteries and elec­ This research ",as perforllled by the CalifonJia [lIStitllte of Teclmology's Jet Proplll­ tronics, for example, are in a high- tech aerogel-insulated, sio ll Laboratory ,,,,dcr a col1tract with NASA,

MECH AN ICAL ENG I NEEIl. I NG APR I L 1998 77