JPL Stories, September 28, 2000

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28 Sep 2000 (The shape of the vehicle wheels resembled narrow automobile tires, The design has a master link on each side, but there was a problem

The Retelling of

    “Romancing the Rover (How Sojourner Came to Be in the Late 1980s and Its Journey to


    JPL Stories, September 28, 2000

    Presenter: Don Bickler

    Reteller: Marilyn Morgan with Don Bickler

The sixth in the Library’s series of JPL Stories attracted a roomful of enthusiasts, including some

    of the presenter’s team members ―to see,‖ he asserted, ―if I make a mistake.‖ The room buzzed

    with anticipation, and even before Don Bickler began his talk, we could perceive a familiar JPL

    gestalt the energetic demeanor, the ebullient manner, the general look of someone who is

    about to share something that is really terrifically exciting.

Sojourner had gone to Mars, fulfilled all expectations, captured the public imagination, even

    became a toy, and finally emerged as an icon of JPL ingenuity. We were about to hear from

    someone who was present at the creation and not only that, he was wielding the wrench and screwdriver.

I’m convinced that engineers, despite their sober and serious reputation, have just about all the

    fun there is to be had in the world. They grab onto a problem and address, define, analyze, and

    solve it intellectually. That’s exhilarating in itself, but then they get to build things: structures,

    bridges, vehicles, rockets. Rocketry engineers have even more fun: they get to build things and

    then fire them into the sky or (accidentally of course) blow them up. If you have seen the old

    films of Robert Goddard and his crew jubilantly experimenting with liquid-fueled rockets in the

    New Mexico desert, you know what I’m talking about.

Engineers who build robotic vehicles also have a bit of that maniacal gleam in the eye. They

    experience the joyfulness of creating things that scurry, scoot, roll, reach, climb, scramble, pivot,

    and peregrinate, while carrying out instructions on command or, even better, these neoteric

    critters decide things on their own through software created by their human designers. Like

    symphonies, sophisticated robots are art forms. But the robots that seem to operate so easily and

    beautifully are not born so much as bred, through generations of robots, some of which never get

    beyond the drawing board, and some of which are magnificent failures. Sojourner, which seems

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    the perfect realization of a compact robotic rover, evolved at JPL over quite a few years. Her distant lumbering ancestor is the Surface Lunar Roving Vehicle, circa 1967.

The Bogie Man

    Don Bickler was introduced by Barbara Amago of the JPL Library storytelling team. We learned that Don graduated from Northwestern University in 1956 with a degree in mechanical engineering, and eventually ended up at JPL in 1975. He is now supervisor of the Advanced Mechanical Systems Group in the Mechanical Engineering Section (352). Bickler’s invention of the rocker-bogie suspension system earned him a nickname The Bogie Man.

(An aside According to the Oxford English Dictionary, a golfing ―bogey‖ derives from a

    popular 1899 song describing a dreaded person, devil, or goblin, but a mechanical ―bogie‖ originated in 19th-century northern English dialect, describing a low truck on four small wheels used by masons to move large stones. Eventually ―bogie‖ came to describe a truck running on two or more pairs of wheels supporting the forepart of a locomotive engine on which it swivels freely in passing curves.)

Rough-Terrain Vehicles

    A before-talk exchange between Don and an audience member confirmed that the bogie design is commonly used in tandem-wheeled trucks, military trucks, and cranes to provide stability and mobility on uneven terrain. Could Sojourner, so refined looking, be related to these rough characters?

    In the 1960s, we were told, the military developed several types of articulated vehicles. Don showed photographs of these strange vehicles, including a six-wheeled ―bent jeep‖ that was

    supposed to be able to climb over low obstacles, and the Lockheed Twister, bearing the appearance of several vehicles in a state of post-collision. Another, the sturdy-looking Gamma Goat, was classed as a 1-1/2-ton truck and sported articulated six-wheel drive. All these were designed to traverse rough terrain such as might be encountered in war-fighting situations or military logistical enterprises. Early rover designers studied these types of vehicles for clues to creating mobile machines that would be successful in planetary excursions.

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Lunar and Planetary Roving Vehicles

    Sojourner’s pedigree begins with a Surface Lunar Roving Vehicle (SLRV) delivered to JPL about

    1967. The segmented vehicle was supposed to fold up into an ―S‖ shape, and could climb

    obstacles 1.5 times its wheel diameter. The SLRV was never launched, but provided a study

    object for early JPL rover engineers. The vehicle was invented by Mieczyslaw G. Bekker, whose

    team designed the Lunar Roving Vehicle used in the Apollo 15, 16, and 17 missions (19711972).

    Bekker’s first book on the theory of land locomotion and terrain–vehicle interactions

    (―terramechanics‖) was published in 1956, and by 1963 he was proposing Moon rover designs.

    Bekker tried lots of things in his designs, including variations on Archimedes’ screw.

    In those days the heady Apollo days NASA was thinking big, and cost was not a problem. The Apollo LRV was about 10 feet long by 6 feet wide (to wheel centers). Each spring-steel,

    wire-mesh wheel was driven by a 1/4-hp electric motor, and the front and rear wheels had

    separate steering systems. Either of the front or rear wheels could be turned while the other pair

    was locked straight, or both could be used simultaneously, so the rover could turn completely

    around within its own length. It could climb over obstacles 1 foot high and negotiate 25-degree

    slopes. All these were desirable characteristics in a roving vehicle. Additionally, the Apollo LRV

    had to fit into the Lunar Descent Module for stowage, all four wheels folded inward and the vehicle folded double and be easily deployed on arrival.

Another idea for a lunar rover was the MOLAB or mobile laboratory: a 3-ton, closed-cabin

    vehicle with a range of about 100 kilometers. The MOLAB was an articulated vehicle with

    independent suspension and 1.5-meter-diameter wheels, but was far too large for what NASA

    ultimately required for Apollo.

Big Mars Rover Concepts

    In the 1980s, interest in Mars spurred new thinking at JPL about what we would want in a robotic

    rover, should the opportunity arise for a journey. The SLRV, the baseline vehicle, morphed into

    the Little Blue Rover. The Blue Rover had an elastic (spring) chassis and could clamber over 1-

    1/2-wheel-diameter obstacles. Don showed us a vintage-1986 drawing of a rover concept that

    illustrated a six-wheeled articulated vehicle of three sections: a forward portion with two science-

    instrument arms, a middle portion with camera, and an aft portion that seemed to be solely RTGs

    on two wheels. (The shape of the vehicle wheels resembled narrow automobile tires, and this

Sojourner by Bickler: FINAL FINAL page 3

became an issue when different types of terrain were considered.) Bickler began thinking about

    bogie linkages to overcome the undesirable characteristics of using springs.

The next illustration Don showed us was ―Fig. 3‖ from his 1989 patent (no. 4,840,394) for an

    articulated, springless suspension that maintains uniform distribution of weight and traction on

    the wheels. Fig. 3 showed a six-wheeled vehicle with front and rear linkages coupling the front

    and rear wheels to the middle wheel. It was assumed that equal weight on each wheel is best, but

    the trick was to make three linkages work (―like clapping with three hands‖). The design has a

    master link on each side, but there was a problem you can’t steer the wheels. The next drawing

    was a wire frame of a six-wheeled vehicle based on a pantograph design. (A pantograph is the

    drawing instrument made of four rigid bars joined in parallelogram form to maintain scale while

    copying a drawing.) The linkages allow steerable struts. Now we’re getting somewhere.

In a JPL review discussing rover designs, a CAD of the pantograph design drew pointed criticism

    from John Casani ―too complicated,‖ ―we don’t know if it will work,‖ and ―why not four wheels?‖ That shout from the back of the room was Bickler’s: ―How about a model?!‖ Don’s 1/8-

    scale table-top model for the pantograph was favorably received at the next review.

In the search for balance among the trade-offs, considerations of weight on the wheels, size of the

    wheels, number of wheels, type of wheels, steering and articulation, surface speed, ground

    clearance, and how high the vehicle could climb (size of the obstacle to overcome) were all part

    of the picture. Four wheels were not enough; six was the optimum number of wheels for stability

    and obstacle-crossing capability. The idea was that a rover would have to be able to climb over

    obstacles 1.5 meters high almost 5 feet. Objects of this size could be seen from orbit. Don’s photograph of a person holding a 1.5-meter-long measuring stick showed us that such a rover

    would have to be really large, and indeed, the next photo showed an experimenter in the east

    Mojave Desert, standing, for scale, next to wheels of the requisite size: huge, 1-meter wheels on a

    single axis. These experimental wheels were narrow in shape. Wheel shape is something to think

    about; you don’t want your rover to start off on a Martian traverse and get ignominiously stuck in

    a ravine into which the wheels precisely fall. Don recalled riding a bicycle as a kid in Chicago

    along streetcar tracks and realizing that ―if you get stuck in the tracks, you’re dead.‖ This insight,

    he declared, ―really made an impression on me.‖

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between the wheels. As the trailer is pulled by the tractor, the trailer’s wheels rise vertically in

    attempting to negotiate the bump. But the trailer’s wheels lock up and are restricted from going

    forward; at the same time, they pull the tractor backwards. The tractor wants to keep going, and

    its second set of wheels have a tendency to drive the vehicle up under itself, raising the front

    wheels. This was nicknamed ―popping a wheelie.‖

The problems for a rover, expected to meet plenty of bumps on Mars, were evident. If you could

    maintain the wheel load distribution without resorting to the use of an oscillation-producing

    spring suspension, you would increase the stability and terrain-negotiating performance of the

    vehicle. Through optimizing the geometry of a bogie system, Bickler produced the (now-

    legendary) rocker-bogie suspension design. No one is exactly sure who named this bit of

    engineering history: a six-wheeled chassis with a springless, articulated system that maintained

    even weight and traction on all the wheels the simplest mechanism to satisfy the requirements

    without pantograph linkages

The Iterations of Rocky Rover

    Don showed us photographs of the iterations of Rocky, as the robotics team experimented with

    electronics, batteries, materials, and loads. They even went to a farm equipment show to learn

    about struts. Rocky I, with 9.25-centimeter (3.66-inch) diameter wheels, could traverse steps and

    gaps over two wheel diameters, and climb over rocks about twice its size. The Rocky II design

    was abandoned before realized in hardware, but Rocky III was another improvement on the

    rocker-bogie design, with more mobility and lower surface pressures. Rocky III had 13-

    centimeter (5-inch) wheels, the same dimensions as the 1/8-scale table model of yore. (Bigger

    wheels work better in sand; smaller wheels sink.) Don showed us a photograph of a ―dramatically

    overloaded‖ Rocky III being tested in the Mojave Desert, and here we observed the rover

    surrounded by robotics team members bearing the bemused expressions one sometimes sees on

    parents’ faces as they watch a tottering but determined offspring engage the uneven terrain of the

    world. The vehicle, with all its apparatus, was so heavy that the metal wheels turned in the rubber

    tires the experimenters had to use Crazy Glue to attach the tires to the wheels.

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Sojourner Rover

    The slimmed-down, sophisticated Rocky IV was a flight prototype version of Rocky III, with all-

    metal (rubber is not a desirable material on the cold surface of Mars) flexible wheels to get

    around better in sand and other soft materials. But the slotted wheels had a tendency to break and

    protrude, locking against the struts, so the next wheel design was solid metal. An iteration or two

    more, including Rocky VII, a close-coupled design intended to have one motor drive two wheels,

    and finally Rocky VIII the elegant little rover that became Sojourner.

    The naming of the rover, Don explained, was Donna Shirley’s idea, and ―it had to be female.‖ (Donna Shirley was manager of the JPL Mars Exploration Program.) When informed that the

    rover would be named thusly, Don impishly proposed the name ―Trixie.‖ Donna affixed him with

    a look (and one can only imagine what that gaze conveyed), and proclaimed, ―You’ve got nothing

    to do with it, Bickler!‖ A playful name was not to be; instead, the rover was eventually named in

    honor of the remarkable 19th-century American abolitionist and champion for women’s rights,

    Sojourner Truth.


    During the question and answer portion of the storytelling, someone asked about optimum wheel

    size: how does one get there? It turns out that the wheel size is selected through exploring the

    solution space using a blend of human judgment and computer analyses. As Don pointed out, ―a computer programmed to overlook flotation in soft sand would say that zero-diameter wheels are

    best.‖ As to whether a Sojourner could be created in the ―faster-better-cheaper‖ environment,

    Don’s answer was ―Absolutely!‖ It’s engineering interest, not the prosaic limits of 40-hour weeks,

    that drives the process, and ―we have a very big LEGO kit‖ for experimentation.

Another questioner asked about the biggest threat to the rover, and that, as might be surmised, is a

    situation where the rover somersaults. The rover allows for more than a wheel diameter of

    obstacle traversal, and can be symmetrical in forward/backward motion capability. A hazard-

    detection system is designed to prevent an overtipping condition by sensing pitch and roll; if pre-

    set limits are exceeded, the rover stops. Sojourner can actually climb a 45-degree slope (if the soil

    can be piled that steep), and can scale a 20-centimeter-high rock. If Sojourner were somehow

    flipped over, though, it could not right itself. Information on traversable obstacles (and the

    allowed traversability numbers are conservative) is stored on board, and if Sojourner encounters a

    non-traversable object (a rock too high), it is supposed to turn away and find another path. An

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interesting example of Sojourner meeting and evaluating an obstacle on Mars is captured in a

    brief movie at <>. Open up the

    first item on the list, <rover_movie_sol24_S0050Q.gif>, and you’ll see the rover bump a

    pyramid-shaped rock, turn, rotate 35 degrees, back up, and then blithely move forward as its right

    set of wheels clambers right over the rock. (Sojourner is capable of climbing over rocks twice that

    size.) The movie is a neat little example of the rover’s climbing abilities, and provides a good

    view of how the suspension system and steering work.

Then there was the sand issue could Sojourner get stuck? Don gleefully told the story. The

    team had tested the rover in sand created to ―match Houston’s recipe‖ for lunar soil, so they knew

    the rover was capable of traversing the fluffy stuff. A (nameless) challenger, citing M.G.

    Bekker’s equations, maintained that the JPL rover was such a poor design that it could not handle a downhill slope. The challenger was politely invited to come to JPL, where a hole was dug in the

    simulant for the rover. Sojourner, of course, promptly climbed right out.

At the end of the too-short hour, Barbara Amago thanked Don and presented him with a small

    crystal globe, the customary gift that represents the jewels of insight brought to us by the JPL


Eyeing it appreciatively, The Bogie Man pronounced it ―one heck of a marble.‖


Web Sites of Interest

    Timeline of JPL Mars rover development. Mars Microrover photo

    gallery. Here are many pictures of various rovers as well as photos from the Pathfinder mission.

    Includes a composite image comparing the size of Sojourner with the Apollo Lunar Roving

    Vehicle. The 1989 rocker-bogie patent. Type in ―Bickler‖ and a list of patents comes up, including no. 4,840,394 for the 1989 articulated suspension system.

Sojourner by Bickler: FINAL FINAL page 8 Information on Sojourner (Mars Microrover). Detailed description of Sojourner rover. ―Movies‖ of Sojourner on Mars. How Sojourner rover got its name. The Sojourner Truth Institute Web site. Sojourner Truth’s famous 1851 speech. An Australian hobbyist’s palm-sized Sojourner model built of LEGO blocks, with a miniature rocker-bogie system.


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