Design of Rover Wheel Testing Apparatus

Daniel FlippoUniversity of OklahomaJeepfleet@POBOX.Com

Abstract

A Rover’s wheel performance is one of thelimiting factors in its ability to successfully traverseother planets. More research focusing on wheelmaterials, design, and optimized tread patternsneeds to be conducted so that a rover can reachmore of its potential. A wheel test bed is beingfabricated to test new wheel designs and to collectdata which will be used to map the relationshipbetween a wheel’s design and its performance ondifferent planetary terrains in different missionroles. The Suspension and Wheel Evaluationand Experimentation Testbed (S.W.E.E.T.) canmeasure traction, sinkage, lateral force, com-pliancy, and rolling efficiency for each wheeltested. The data collected by the testbed will beused to understand and fabricate wheels that arespecifically suited for particular planetary missiontasks. These tasks will go beyond the currentmissions of exploration and include such roles asregolith excavation, equipment transportation, andequipment manipulation.

1. Introduction

America has had a successful rover presence onthe Moon and Mars. The latest two Mars rovershave started their forth year of exploration on thered planet. Their mission is to take pictures andanalyze Martian terrain. These two rovers, dubbedSpirit and Opportunity, have vastly exceededNASA’s expectation of 90 day missions. They havesent back some amazing pictures of the Martianlandscape and terrain, but their exploration isonly scratching the surface of Mars’s geographicalfeatures. Mars features Olympus Mons, which isthe largest mountain in the solar system, toweringat a height of 27 km, and is also home to VallesMarineris, a network of canyons that wind 4000km along the planet’s terrain and range from sevento ten kilometers deep [14] .

Why is NASA not exploring these features in-stead of the flat plains of sand and small to medium

sized stones it currently examines? The realityis that NASA is understandably squeamish abouttaking their 400 million dollar rovers to such dan-gerous locations for fear the equipment may getdamaged or immobilized. Even in the current mis-sion, NASA’s fears proved to be valid when Oppor-tunity became mired in a sand dune for five weeks[3]. Engineers eventually inched it out, but if NASAhad wheels that gripped better in sand they mighthave avoided this mishap entirely.

It is a simple design change to make a wheel per-form better in sand, but most likely that changewill be at the expense of performance in other ar-eas such as rolling efficiency and turning ability.This leads one to ask what is the best wheel de-sign for every mission? An optimized tread, wheelsize, and overall design should vary according to themission’s specific goals. The wheel tread for a roverdoing long distance exploration through sand dunesshould be different than the tread for a rover thatworks in a local arena digging samples or clearinglanding sites. A "one tread fits all" approach limitsthe rover’s potential.

Very little research has been done in the area ofinterplanetary wheel tread and wheel design as op-posed to the tires used on Earth that don’t havethe material and weight constrictions. As NASAand other agencies expand their endeavors to otherworlds, and establish a presence on Mars and againon the Moon, they must pay close attention to thebasics of wheel and tread design and wheel soil in-teraction. At some point in the future wheels willnot be the only form of locomotion on other planets.Legs and other robotic forms will be used to over-come many of the transverse problems inherent innatural terrain, but until that time, rover designerswill need wheeled rovers that will traverse to moreplaces of interest on Mars, as well as do the workof preparing the planet for human occupation. Asexpectations for rover performance rise, rovers willbecome bigger and more complex and it will be crit-ical that wheels have optimized performance withrespect to their missions.

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To fill the gap in the understanding of rover wheeldesign and wheel to soil interaction, testing ma-chines have been designed by various institutions.In 1971, NASA tested the Lunar Rover Vehicle’swheels on a testing device called a dynamometersystem which measured load, sinkage, pull, torque,horizontal, and angular velocity [10]. NASA nowuses devices such as the variable terrain tilt plat-form (VTTP), at JPL, to gain a better understand-ing of entire rover systems in a sloped environ-ment. The VTTP is a 16 x 16 ft table that can tiltup to 25 degrees and can be left bare or coveredwith terrain [8]. At the Massachusetts Institute ofTechnology a testing device dubbed the "Field andSpace Robotics Laboratory terrain characterizationtestbed" tests a single driven wheel through dif-ferent mediums to better understand wheel to soilinteraction [5]. A similar device is used at TohokuUniversity to refine rover steering and other param-eters [15]. Other comparable devices test wheels forEarth’s terrain [9, 12, 2]

This paper describes a testbed that aids in ourunderstanding of wheel to soil interaction, as wellas tests new wheel designs. The following sectionswill describe the testbed’s design, testing mediums,and proposed experiments, while the final sectionwill summarize the information and give a smalloutline of how the experimental data will be used.

2. S.W.E.E.T. design

The purpose of this testbed is to test newwheel designs and collect data that will be usedto map the relationship between a wheel’s designand its performance on different terrains. Thetestbed (Figure 1) has a 10 x 10 ft footprint andis fabricated from modular aluminum. A weighteddrop down test leg, incorporating a driven wheeland a six axis, force torque sensor, stays stationaryin the X and Y directions but allows movementalong the Z-axis via a counterbalance system.

S.W.E.E.T. differs from those discussed earlierin that the table can move in the X and Ydirections underneath the test stand, as well asrotate about the Z-axis. This added advantagegives the apparatus the unique ability to measureforces and torques in a true turn. This testbed canmeasure traction, sinkage, lateral forces, turningefficiency, compliancy, and rolling efficiency foreach wheel tested. The table can move along theX and Y -axis at velocities faster than 20 cm

sec . Thisis more than needed considering Spirit or Oppor-tunity but it will allow for testing the emerging,faster rover concepts [11]. S.W.E.E.T. is also large

enough to be used to test other assemblies such asa suspension system or an entire rover.

Figure 1: Testbed

2.1. Motion

To simulate motion on the testbed, the ta-ble moves under the test leg. This motion isfacilitated by three DC geared omni-directionalwheels that are offset 120 degrees from each other(Figure 2)

Figure 2: Motor configuration

To transform the desired cartesian table motionof X, Y , and Θ to the angular speed of each of thethree motors, a transformation equation (1) wasused [13]:

ξ = R(Θ)−1J−11f J2φ (1)

In this equation, ξ is a vector containing the de-sired table motion parameters of X,Y , and Θ, rep-

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resented as:

ξ =

X

Y

Θ

(2)

RΘ is the rotational transformation matrix that isdependent on the angular position of the table Θ.

R(Θ) =

cos(Θ) −sin(Θ) 0sin(Θ) cos(Θ) 0

0 0 1

(3)

Jf is a matrix made up of three constraint equa-tions, one for each wheel. Jf transforms the desiredmotion values of ξ into the control parameter, φ,which is a vector containing the three motor angu-lar speeds. Figure (3) shows how Jf is dependenton the three angles and the distance, l, the wheelis away from the center of the rover.

Figure 3: A Swedish wheel and its parameters [13]

J1f = sin(α1+β1+γ1)−cos(α1+β1+γ1)−lcos(β1+γ1)(4)

For the configuration in this apparatus, β and γare zero, making Jf dependent only on α. If allthree wheels are taken into account and set up inmatrix form, the result is equation (5):

Jf =

sin(α1) −cos(α1) −lsin(α2) −cos(α2) −lsin(α3) −cos(α3) −l

(5)

J2 is a matrix holding the radius values of eachwheel. Since each radius is the same J2 can besimplified to a scalar value r.

J2 =

rad1 0 00 rad2 00 0 rad3

(6)

Taking into account equations (2) and (6), wecan simplify equation (1) into equation (7) which

maps a relation between the table motion (X, Y , Z)and the motor angular velocities (ω1, ω2, ω3). ω1

ω2

ω3

=1

rwheelJfR(Θ)

X

Y

Θ

(7)

2.2. Test leg

The test leg (Figure 4) is a fully adjustableassembly that hangs from the center of the pyra-mid shaped apparatus, and is free to move alongthe Z-axis using linear rod bearings. The testwheel is powered by a FaulHaber DC motor geareddown to (43:1) and then again geared down (2:1)via a bevel gear set. An encoder is used on themotor for PID control. The test leg also holds theforce torque sensor, discussed in section 2.4.

Figure 4: Test leg

2.3. Electrical

Most of the electrical system is housed entirelyin the control box shown in Figure (5). Three240 Watt power supplies along with three 40 AmpPWM controlled H-bridges provide power andcontrol for the main motors. The control boxis also equipped with a 200 Watt power supplythat powers the encoders, testing wheel, and force

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Figure 5: Control box

torque sensor. Another H-bridge controls thetest wheel and has a current sense that is usedto record the test wheel wattage. The apparatusutilizes two National Instruments data acquisitionboards linked to a computer. A PCI-6602 DAQboard handles all the digital and counter, PWMsignals to the H-bridges, while a USB-6020 DAQboard handles all the force torque analog signals,as well as encoder signals of the four motors andthe Z-axis displacement

2.4. Force torque sensor

Like the devices at MIT and Tohoku Uni-versity [5, 15], the main sensor used in the Testbedis located on the test leg, along with the sensor’sadjustable gain amplifier, just above the wheel(Figure 7). It is a six degree of freedom forcetorque sensor machined from 2024 aluminum,incorporating 32 strain gages networked together[4]. This sensor gives us a full view of the forcesand torques being applied to the test wheel. Boththe sensor and the amplifier were made at theUniversity of Oklahoma.

Figure 6: Force torque sensor device

Figure 7: Force torque sensor installation

2.5. Programming

Since the Testbed uses two National In-struments data acquisition boards, all the controland measurement programming was done inLabview 8.2. The programming incorporates PIDcontrol algorithms for each of the motors.

When executed, the program initially reads froma test procedure file into an array that controls theprogram during a test, allowing for different exper-iments. The test procedure file also can direct theprogram to control motion by force feedback, simu-lating dragging and other parameters that the usermay wish to incorporate. While the test is run-ning, data from the test procedure file is appendedto sensor readings and written to a test result file.Shown in Figure (8) is one of the control screensin the program. This particular screen is used tovisually inspect and adjust the PID coefficients forbetter control.

Figure 8: Labview interface control screen

3. Testing medium

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Different terrain mediums will be used fortesting. Those of particular interest are mediumsthat mimic the terrain of Mars and our Moon.Sand will be the most common and will come inseveral grain sizes, densities, and slopes. Rockswill also be used as a medium in varying sizesand orientations depending on the test. Each ofthe differing terrains will be housed in bins thatare easily rolled in and out of the machine for thedifferent tests being done.

4. Experiments

Several tests are possible for the apparatusin such categories as traction, sinkage, turningefficiency, compliancy, and rolling efficiency invarying terrain. The following are a few of thetests that are possible with this apparatus.

The first, is intended to test static traction ineach wheel and is the simplest to perform. Withthe wheel weighted down, the test motor is rampedup over a certain period of time, from zero to fullspeed while the table is motionless. As the power isincreased to the wheel, the measured force increasesalong its line of motion. At some point, the forceovercomes the frictional force between the wheeland the media, causing the wheel to slip. This pointis apparent in the results when the force in the di-rection of the line of motion levels off. This can beused to determine the static coefficient of frictionbetween the terrain and the wheel being tested.

The second test is similar to [1] and is concernedwith wheel sinkage on loose terrain. For this test,the table will be moving along with the wheel butwill simulate a drag by using a force feedback loop.This loop will move the table along the line of mo-tion as long as there is a set amount of force felt bythe wheel. The force, as well as the speed, are allprescribed in the test procedure file. As the wheelspins, it will sink in the loose terrain and the Z-axisencoder will measure the displacement incurred. Avariation to this test, which is similar to [7, 6] is toorient the terrain on a slope simulating a hill. Thisapparatus is large enough that a sloping bin can beinstalled on the table and the test carried out asbefore.

Steering is a very important part of a rover’s de-sign, so another test would be to discover how ef-ficiently a wheel can turn in varying terrains [15].Several variations to this experiment are possibledepending upon what type of turning mobility isbeing tested. The test apparatus can simulate Ack-erman steering by moving the table beneath thestationary wheel in a curving motion and measur-

ing the forces and moments incurred. If a skid steerwheel is being tested, the table is programmed to goat an angle, dependent upon the rover’s geometry,while the test wheel rotates at a set speed.

A third experiment is to test for compliancy witheach wheel. The test wheel can be driven off a setheight and dropped onto varying terrains and withvarying weights. The Z displacement and all accel-erations can be measured through this test to seewhich wheels do better under varying conditions.

The final initial experiment proposed, with thistestbed, measures the rolling efficiency of a roverwheel. This is done by recording the wattage usedby the test wheel as it traverses different terrains,at different weights, and with opposing forces.This will give us a better understanding of howtread and overall design affects efficiency.

5. Conclusions

In this paper, a testbed for rover wheelswas described. The physical parameters of themachine as well as the electrical system and con-trol program were explained. Uses and proposedtesting experiments designed for this apparatuswere also detailed. The data obtained from theseexperiments, in the form of performance values inseveral categories, will allow attempts at linking awheel’s design using a neural network. This linkwill be used by a genetic algorithm to evolve anoptimized wheel for a given mission. The testbedhas been completed and preliminary results will bereported at the conference.

6. Acknowledgments

This work was supported in part by a grantfrom Malin Space Science Systems, Inc. Theauthor would also like to acknowledge the supportand help of Dr. David Miller and those at theUniversity of Oklahoma robotics lab.

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