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DESIGN OF A HIGH-MOBILITY LOW-WEIGHT LUNAR ROVER

A. Winterholler, M. Roman, T. Hunt, and D.P. Miller

University of Oklahoma, 865 Asp Ave, Rm 212, Norman OK, 73019 USAalois@ou.edu, bellx-1@ou.edu, dpmiller@ou.edu & timhunt@ou.edu

ABSTRACT

This paper reports on a set of experiments using a newhighly mobile rover prototype that was tested underlunar-like operational conditions (i.e., time-delay andreduced communications bandwidth) in the lava fieldsof the Mojave desert near Amboy, CA. In addition todescribing the mobility system we will describe someresults from the field test and observations about the linkbetween mobility confidence and operational speed.

Key words: Lunar rover; mobility.

1. INTRODUCTION

In early 2004, NASA announced its new vision for spaceexploration [12] which described a new thrust in Lunarexploration. The plan outlines numerous roles for mobilerobots on the Moon. One of the more challenging aspectsof the plan is exploration of parts of the Lunar terrainsuch as the Lunar South Pole, where there is a possibilityof finding ice, or exploring the central peaks and craterrims of major craters such as Copernicus, where mantlematerial may be exposed from the impact, giving insightto the origin of the Moon and the formation of the planets.

In order for NASA to carry out their Lunar objectivesthey will need to be able to traverse the Lunar surfacewith robots through all sorts of terrain and conditions.These robots will probably be tele-operated. One of thekey factors is the speed and distance that can be coveredby a tele-operated robot that has to work with time-delayis operator confidence that the robot will not break or be-come immobilized if the operator issues a slightly lessthan optimal set of commands. This paper describes themechanics of the Copernicus Rover (Figure 1) – a tele-operated rover with dozens of kilometers of traverse ex-perience in Lunar analog terrain.

This type of exploration will involve relatively large tra-verses (many tens or even a few hundreds of kilometers)

Figure 1. The Copernicus rover at a small volcanic craterrim

to get to the various destinations needed to make a properscientific survey. Because of the extreme temperaturesand lack of power available during the lunar night, sur-vival through the night by a lone rover is a risky strat-egy, and would require engineering aspects that wouldraise the overall mission costs. An alternative would beto achieve the primary mission within a single lunar dayrequiring the capability to traverse a number of kilome-ters per day. Such speeds are several orders of magnitudegreater than NASA’s Mars rovers from the Pathfinder [6]and MER missions. These slow traverses are due both tothe Mars time delay and to the way the rovers are oper-ated [10].

Lunar rovers have a great advantage over those on Mars– they are less than three light seconds away (round trip)from Earth, while Mars rovers suffer light-time delays ofmany minutes. This allows lunar rovers to be directlytele-operated. But previous lunar rovers, the Lunokhods,still had traverse speeds well below a kilometer a day [3].

We believe these low traverse speeds are due to two mainfactors:

1. Poor tele-operation interfaces, and

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2. Lack of confidence (perhaps well justified) in the ro-bustness of the mobility system of the rover.

The first of these factors is covered in [11]. The secondis the subject of the remainder of this paper.

2. THE COPERNICUS ROVER MOBILITY SYS-TEM

The Copernicus rover is a 4-wheel drive 4-wheel steervehicle featuring a springless suspension system and avery high ground clearance. Mechanically, the front andrear halves of the rover are completely identical. The twohalves are connected at the main body (which holds thebatteries and all of the electronics) by a passive differen-tial. The differential allows the front axle to roll with re-spect to the rear axle (see Figure 2). This allows the fourwheels to maintain continuous contact with the ground,and a relatively even weight distribution on each tire,even in situations where the four contact points are noteven remotely co-planar.

Figure 2. The rover’s central differential allows all fourwheels to remain on the ground in uneven terrain

2.1. Mechanical Layout

Starting at the central differential and moving forwardwe immediately come to the front motor compartment(Figure 3, inset A). Axially aligned within the center ofthis compartment is the front-wheel drive-motor and itsplanetary gear-box. Off center in the compartment isthe front-wheel steering-motor and its first-stage plane-tary gear-box. By locating all of the motors internallynear the central body of the rover, the wiring and thermalcontrol of the robot are greatly simplified. The motorsand the gear-train are also further isolated from dirt anddust, simplifying the tribology for the mechanical systemin future flight versions.

Moving forward from the motor compartment, the out-put from the drive motor’s first stage 43:1 reduction gear-box is connected to a solid drive shaft. The speed of the

drive shaft at this point is still relatively high, with a cor-respondingly low amount of torque being passed – reduc-ing the chance of mechanical failure.

Figure 3. The Copernicus rover mechanical system

The steering output is connected through a spur gear tothe steering tube – a concentric tube that is mounted onbearings around the drive shaft. The steering tube, withthe drive shaft inside of it, travels the length of the frontbody tube till the front body tube joins with the front axletube. At that point, the steering tube is capped with apinion gear which drives a gear rack in a standard rackand pinion steering system (Figure 3, inset B).

The drive shaft continues past the steering pinion gearand into the constant traction differential located insidethe junction of the body and axle tubes. The constant trac-tion differential consists of a standard differential bodybut contains four parallel plates rather than a set of spidergears. The plates contain interlocking teeth on one sideand the center two have springs and guide pins on theother face. They are arranged so that the sides with guidepins on two of the plates meet at the center, and are forcedapart by a central pin that is mounted to the differentialcase, whenever that case is turned. Each of these platesthen faces another plate with interlocking teeth, and thoseplates are attached to the left and right axles, respectively.When the drive shaft turns the differential housing, thepin forces the two center plates apart, causing them lockagainst their respective left or right axle plates, causingthose axles to turn. If one of the wheels is turning faster,and has no force on it, the housing pin will no longer bepushing against that side’s plate, and it can slip towardsthe center, disengaging that wheel from the drive motor,and putting all of the torque into the wheel that is turningmore slowly. In other words, this differential does almostthe opposite of a standard differential in a slipping situ-ation. Where a normal differential will put all of the en-gines energy out through the wheel with the least traction,the differential on the Copernicus rover puts all of the en-ergy out through the wheel with the most traction. If onewheel is on ice and the other is on sandpaper, then both

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wheels will move at the same speed and the full force ofthe drive system goes into the wheel on sandpaper. Thisarrangement increases the tendency of the rover to trackstraight, increases its climbing capability and allows it tomaintain traction moving down bumps as well as goingup.

The left and right drive shafts, connected via an elas-tic coupler to the constant traction differential, travel thelength of the front axle tube, go through a U-joint andthen enter the second stage 5:1 reduction gearbox (Figure3, inset C). At this gear box the drive shafts are slowedto their final speed, increasing the torque at the wheels.The gearboxes are arranged on the body to give the roveradditional ground clearance. The output of the gearboxesis mounted directly to the wheels. This assembly is con-nected to the rover through a pivot that also contains thesteering arm, which is linked to a pushrod that attaches tothe end of the steering rack.

The entire assembly, starting with the motor housings andending at the front wheels, is mirrored for the rear half ofthe rover. This means that the rover has exactly the samemobility characteristics going forwards or backwards, al-lowing it to get out of anything it has gotten into. It alsoprovides complete redundancy in the steering and drivesystem. The robot can fully move and steer even withthe failure of a drive or steering motor, with only a slightchange in turning radius in the case of a steering failure,and a more substantial, but acceptable hit in the climbingperformance in the case of a drive system failure.

2.2. Rover Limits

When first designing the Copernicus rover we decided tofollow ‘the rule of 30’ – be able to travel at speeds inexcess of 30cm/sec, be able to climb extended slopes ofat least 30o, be able to roll up and down 30cm obstaclesof arbitrary shape and have a ground clearance of at least30cm.

To meet the ground clearance requirement we decided ona wheel diameter of 74cm. This size wheel matches thelargest consumer bicycle wheel (the 29” bicycle wheel)which allowed us to speed construction of the rover. Itis also very close to the size wheel (81cm) that was usedby the Apollo Lunar Rover [7]. The ground clearancewas further enhanced by the placement of the secondarygearbox (Figure 3, inset C) which yielded a total groundclearance below the lowest point of the chassis of justover 40cm.

The motor and gearing selection for the drive wheelswas selected to give a no-load speed of the rover ofabout 60cm/sec and flat surface rolling speed in excessof 30cm/sec. The maximum torque output of each twostage gearbox is 137Nm, which is adequate for a singlewheel to pull the entire rover up a 30o slope, or for twowheels to be hitting a 30cm step while the vehicle climbsa 30o slope.

The rover’s tubular frame was designed to withstand 10gshocks. A fall from 30cm – if the rover should slip offfrom on top of a 30cm obstacle it was surmounting –would result in an impact velocity of about 245cm/sec.This velocity far exceeds the maximum rolling velocitythat the rover could achieve, and was therefore the worstcase impact we had to worry about (falls from greaterthan 30cm due to severe operator error could always bea possibility, but there is no limiting case for that situa-tion). The 30cm fall could be absorbed within the shocklimit by compressing the four cm of pneumatic tire downto a thickness of a single centimeter. The air-pressurein the tires was adjusted so that ten times the normalwheel loading would result in that level of compression.A spring-steel version of the tire, capable of functioningin the lunar environment, was also designed but was notbuilt.

3. ROVER CONTROL ELECTRONICS

The rover was outfitted with a variety of sensors. Theseincluded several ways of measuring the robot’s motion.The most basic of these were quadrature encoders on eachof the four motors (two drive and two steering). Theseencoders were fed into a custom decoder board contain-ing four 16-bit decoders and a HC6811 microcomputerwhich would poll, sum and log the encoder data at a suffi-cient rate to ensure that the decoders never wrapped. Thisboard also contained several A/D channels for analog in-put. These sensor lines were used to monitor the batteryvoltage and current drain. One line was also used for theremote kill switch, based on a COTS automobile keylessentry system.

DIO lines were used to monitor limit switches on the rackand pinion steering system. The limit switches were usedboth to calibrate the steering and to detect the rare casewhen an impact with an obstacle while turning wouldfurther back-drive one set of wheels beyond its normaloperational range of steering. In such a case the micro-processor would halt the motors. The robot would then bebacked away from the offending obstacle and the steeringre-centered.

The 6811 sent its data via a serial line to an Apple G3Lombard laptop. The laptop, running the tele-operationsoftware, would process the encoder data and relay itback to the operator over a 802.11g connection. The lap-top also gathered, processed and relayed data from theroll, pitch, heading sensor [5] and the Magellan GPS re-ceiver (used as a terrestrial substitute for an inertial navi-gation system (INS)).

The remaining sensors on the robot were a pair of Elphel313 network cameras [4] used to provide stereo imageryto the driver, and a USB webcam which was mounted ona servo. The webcam could be aimed at either of the twofront tires. This could be used as a driving aid in veryhazardous situations. The Elphel cameras could be soft-ware configured to generate JPEG images of a specified

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resolution and at a specified frame rate.

The laptop also processed pan/tilt [2], drive and steercommands from the user. The onboard software wouldconvert pointing commands into the proper serial formatfor the pan tilt head. In the case of drive or steering com-mands, the directions would be converted to the appropri-ate byte codes and sent over a serial port to a serial servocontroller [8] which generated the appropriate wave formto drive the motors through a H-bridge motor driver [1].The loop was closed on the steering commands througha PD loop running on the 6811, using the steering mo-tor encoder for feedback. Drive speed commands wereexecuted open loop on the rover, closed by the driver.

The onboard system could also accept joystick input froma USB joystick connected to the laptop. Connecting ajoystick to the rover would cause the laptop to ignore anycommands being sent to it via the wireless connection.

4. TELE-OPERATION INTERFACE

The tele-operator interface consists of a stereo displaywhich projects images and data from the rover, anda joystick interface which allows control of rover mo-tion, camera controls and control of the display overlays.The pan/tilt head is mounted approximately 1.5m off theground, giving the rover operator a roughly human auto-mobile driver perspective. The full interface is describedin detail in [11].

5. FIELD TESTS

In May of 2004 the Copernicus rover was loaded intoa motor home that also housed the control station, anddriven west to Amboy, California. Amboy is a 70km2

primarily pahoehoe lava field (see Figure 4) with a promi-nent 75m high cinder cone near the Northern edge [13].The terrain is littered with boulders, broken sheets ofrock, occasional ah ah lava flows, ridges and small (10min diameter and a couple meters deep) vent-hole craters.

Several days of mobility and driver testing were per-formed. During this period the rover was run over a halfkilometer course a number of times using different imageresolutions, frame rates and time delays. The drivers gota feel for what the rover was and was not capable of, andalso learned how to deal with the issues of time delay. Af-ter the second day the communications parameters werefrozen at a four second time delay (lunar time delay plus1.3 seconds to account for latency in the communicationsnetwork) and a 100kbit/sec data rate from the rover. Thisis a data rate easily achievable from a rover on the Moonequipped with a hemispherical antenna, broadcasting tothe 10M dish of the DSN. The data rate limit meant thatthe JPEG image quality level was set rather low, and theframe rate would bounce between 1.5 and 2 FPS depend-ing on the particular compressibility of the images. An

Figure 4. Copernicus in the middle of the Amboy lavafield

example image from the rover camera is shown in Figure5.

On the sixth day of testing the rover was driven to the cin-der cone and back – a round trip of almost four kilome-ters. During the trip the rover encountered and traversedslopes of up to 34o. The average speed (while moving)on the way out was 0.355m/sec1. The trip to the cone(1.9km) required 170 minutes. This means that includingstops for panoramas, rest breaks for the driver and otherdelays, the rover still averaged 0.186m/sec – a potentialrange of over 16km per day.

This speed with time-delay over rough terrain, for an ex-tended distance was achievable, we believe, in large partdue to the robustness of the rover.

The rover did not need to be coddled. The approachthe drivers took between the first day of testing and thelast differed greatly. The stereo images and slightlylower than normal height view point of the rover cam-eras tended to exaggerate the roughness of the terrain toa driver’s eyes. The view from the rover as it went overan embankment into a crater was often quite frightening.But experience showed that the rover could do it and thatthe speed that the rover went over a ridge or a rock, ordown a slope, was in most cases immaterial. By this lastday of testing, the rover was usually being driven at closeto full speed for a significant portion of the time. It wasstill necessary to pause on occasion to get a panorama,

1Schedule constraints on the return trip required the time-delay tobe eliminated for a portion of the return, so the speed during the returntrip is not considered valid.

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Figure 5. Image of the Amboy cinder cone taken fromthe rover camera at field test resolution

so as to plot out the next phase of driving, but when therover was moving it tended to be moving at full speed –and it was moving more than half the time.

The speed and distance results with Copernicus are sim-ilar to those reported in [9], with some important dif-ferences. First, Copernicus was run over rugged natu-ral terrain where the Rattler robot was run over a man-made course designed to look like Lunar terrain. Theslopes and obstacle sizes encountered and surmounted byCopernicus were far in excess of those handled by theRattler robot.

More importantly, the runs in [9] are not normal tele-operation. The rover is assumed to have an onboardwizard which can veto or modify in real-time the time-delayed commands coming from the operator. During thetrials reported, the Rattler sometimes used a human wiz-ard and sometimes one that was AI-based. However, ineither case the wizard had a similar psychological impacton the driver to what we experienced with the robust mo-bility of Copernicus – i.e., that the driver spent little timeworrying about safeguarding the rover and much moreeffort guiding the robot to where they wanted it to go.

6. CONCLUSIONS

Future lunar operations will require long traverses atspeeds far in excess of what has previously been donewith unmanned planetary rovers. We have shown that acapable rover with the proper operational interface canbe operated at speeds that will allow scientific and infras-tructure missions that will be required for NASA’s newexploration initiative. Our rover is also a demonstrationthat a moderate sized rover (with a footprint similar toMER but at a third the mass) can operate in terrain andbe tele-operated under conditions very similar to thosethat would be encountered in an actual lunar mission.

Our preliminary results give strong evidence that longrange lunar missions are feasible. In future experimentswith the Copernicus rover we plan to further quantifythe operator performance as a function of bandwidth andtime delay. This should allow us to further optimize theoperational parameters increasing performance, reducingmission telecom requirements, or both.

ACKNOWLEDGMENT

The authors wish to thank Mike Malin and Mike Ravinefor their support and direction on this project. We alsowish to thank Steve Bailey and Tomas Svitek for their as-sistance with telecom, and LMA for additional logisticalsupport.

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