Traverse Performance Characterization for the Mars Science Laboratory Rover

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<ul><li><p>Traverse Performance Characterization for the MarsScience Laboratory Rover</p><p>Matt Heverly, Jaret Matthews, Justin Lin, Dan Fuller, Mark Maimone, Jeffrey Biesiadecki, and John LeichtyJet Propulsion Laboratory, California Institute of Technology, Pasadena, California, 91109</p><p>Received 14 December 2012; accepted 19 July 2013</p><p>It is anticipated that the Mars Science Laboratory rover, named Curiosity, will traverse 1020 km on the surface ofMars during its primary mission. In preparation for this traverse, Earth-based tests were performed using Marsweight vehicles. These vehicles were driven over Mars analog bedrock, cohesive soil, and cohesionless sand atvarious slopes. Vehicle slip was characterized on each of these terrains versus slope for direct upslope driving.Results show that slopes up to 22 degrees are traversable on smooth bedrock and that slopes up to 28 degreesare traversable on some cohesive soils. In cohesionless sand, results show a sharp transition between moderateslip on 10 degree slopes and vehicle embedding at 17 degrees. For cohesionless sand, data are also presentedshowing the relationship between vehicle slip and wheel sinkage. Side by side testing of the Mars ExplorationRover test vehicle and the Mars Science Laboratory test vehicle show how increased wheel diameter leads tobetter slope climbing ability in sand for vehicles with nearly identical ground pressure. Lastly, preliminary datafrom Curiositys initial driving on Mars are presented and compared to the Earth-based testing, showing goodagreement for the driving done during the first 250 Martian days. C 2013 Wiley Periodicals, Inc.</p><p>1. INTRODUCTION</p><p>When driving the rover on Mars, a sequence of motion com-mands is sent to the vehicle roughly once per Martian day,otherwise known as a sol. Various levels of rover autonomycan be invoked to drive the rover anywhere from a few me-ters to 100 meters or more per sol. The final location of thevehicle is typically dictated by the science objectives, butthe traverse path to reach the final location is constructed tomaximize vehicle safety. This process of selecting a sciencetarget, evaluating the terrain, and constructing a sequenceof motion commands must be done in only a few hoursin order to allow new commands to be sent to the vehicleeach sol. It is therefore important to have accurate and con-cise metrics to be able to evaluate vehicle safety and predictvehicle performance on a given terrain. One such metric isvehicle slip. While slip is not inherently unsafe, it does resultin deviation from the planned path, which could result inthe vehicle encountering unsafe terrain such as large rocks.In soft soil, excessive slip can lead to vehicle embedding.The Mars Exploration Rover (MER), named Opportunity,encountered a soft sand ripple on its 446th day of surfaceoperations. High wheel slip and sinkage resulted in embed-ding, and it took more than five weeks to extricate the rover.</p><p>Formerly of the Jet Propulsion Laboratory, currently at SpaceX,Hawthorne, CA.Direct correspondence to: Matt Heverly, e-mail: Matthew.C.Heverly@jpl.nasa.gov</p><p>Due to these safety concerns, it is important to characterizethe Curiosity rovers slip potential in a variety of terraintypes.</p><p>The ability to predict terrain traversability as a functionof slope was not only helpful in tactical operations, it alsoaided in selecting a safe landing site for the rover. Sinceterrain slopes can be measured from orbital data and terraintype can be generally classified by orbital images, the abilityof a vehicle to traverse can be mapped to first order priorto its arrival on Mars. This allowed for the selection of alanding site that was tolerant to uncertainty in the preciselanding position based upon the probability of landing ontraversable terrain.</p><p>Sections 2 and 3 of this paper describe the Curiosityrover as well as the Earth test vehicle known as Scare-crow, shown in Figure 1. Section 4 presents a GlobalPositioning System (GPS) -based method as well as animage-based motion-tracking method used to measurerover motion during terrestrial tests. Section 5 describesthe vehicle test venue at the Jet Propulsion Laboratory (JPL)known as the Marsyard, as well as naturally occurring Marsanalog terrains used for testing in the Mojave Desert of Cal-ifornia. Section 6 presents slip versus slope data for variousterrains, as well as slip versus sinkage data in cohesionlesssand. Section 7 compares terrestrial test results for the MarsScience Laboratory test rover with results from identicaltesting done with the smaller Mars Exploration Rover testvehicle, showing the impact of wheel diameter in soft sand.The paper concludes in Sections 8 and 9 with a discussion</p><p>Journal of Field Robotics 30(6), 835846 (2013) C 2013 Wiley Periodicals, Inc.View this article online at wileyonlinelibrary.com DOI: 10.1002/rob.21481</p></li><li><p>836 Journal of Field Robotics2013</p><p>Figure 1. The Mars weight Scarecrow rover during traversetesting at Dumont Dunes in the Mojave Desert of California.</p><p>on how these data were used in selecting a landing site forCuriosity, and slip versus slope data are presented from thefirst 250 sols of operations on Mars.</p><p>2. BACKGROUND</p><p>The Mars Science Laboratory (MSL) rover, named Curios-ity, is a mobile geochemistry lab that successfully landed atGale Crater near the equator of Mars on August 5th, 2012.The missions objective is to assess whether the planet everhad an environment that was able to support life. To dothis, the rover has a suite of seven science instruments, 17cameras, and a robotic arm for sample inspection and acqui-sition. The 899 kg vehicle measures 2.8 m from the outsideof the front wheels to the outside of the rear wheels, is 2.8 mwide, and 2.2 m from the ground to the top of the imagingmast. It uses a six-wheel-drive rocker bogie system similarto the Mars Exploration Rovers (MER) Spirit and Oppor-tunity launched to Mars in 2003, as well as the Sojournerrover launched to Mars in 1996. The rocker bogie systemis a passive suspension with a differential connecting theleft and right arms of the rocker. On the forward end of therocker arms are the left and right front wheels. The aft endof the rockers connects to a freely pivoting bogie with theforward end of the bogie connecting to the mid wheels andthe aft end connecting to the rear wheels. This suspensionsystem allows for nearly equal weight distribution acrossall six wheels independent of pose when articulated overobstacles or undulating terrain (Bickler, 1998; Harrington,2004). It also allows for articulation over obstacles in excessof one and a half wheel diameters (75 cm) in height.</p><p>The four corner wheels of the rover are steerable, butthe mid wheels are fixed. This dictates that with Ackermansteering, the rover always drives in a constant curvature arc(for a given set of wheel steering angles) pivoting about apoint along the line connecting the middle wheels. The cur-vature of the arc can range from zero, resulting in straight-</p><p>line motion, to infinite, where the rover pivots around itsorigin (otherwise known as a point turn). The top speed ofthe vehicle is 4.2 cm/s, so all motions are quasistatic. Addi-tionally, each drive actuator has sufficient torque such thatthe rim thrust is never the limiting factor for driving overobstacles or up slopes.</p><p>For the MER rovers, testing was done prior to the ve-hicles arrival on Mars to characterize traverse performanceusing a Mars weight vehicle on a tilt table (Lindemannet al., 2005). This provided valuable data on vehicle slipas a function of slope. These data are still in use by thosecommanding the Opportunity rover to predict the vehicleperformance and to plan the safest and most efficient routespossible. This vehicle slip testing is also complementary tothe computer modeling of traverse routes for predicted per-formance (Iagnemma et al., 2011; Trease et al., 2011).</p><p>Other terrestrial work has been performed characteriz-ing the slope climbing performance of potential planetaryvehicles (Sutoh et al., 2012). Experimental testing has beenperformed at the single wheel level to determine the influ-ence of several physical wheel parameters such as diameter,width, grouser size, and spacing (Ding et al., 2011). Simi-lar experiments have been performed to characterize soilflow under a wheel in granular materials (Moreland et al.,2012; Senatore et al., 2012). A single wheel test apparatushas also been used in an attempt to characterize the soilproperties of planetary bodies based upon wheel soil inter-action (Sullivan et al., 2011). The effect of gravity has beencharacterized at the single wheel level using aircraft duringvariable gravity maneuvers (Kobayashi et al., 2010).</p><p>Due to the size of the Curiosity rover, and a desire todo vehicle level performance characterization, laboratory-based experiments similar to those mentioned above werenot feasible. This necessitated the use of the outdoor testfacility at JPL known as the Marsyard. There was also adesire to test in naturally occurring terrain analogous towhat was expected on Mars. Testing in the wind-blownsand dunes and desiccated lake beds of the Mojave Desert inCalifornia ensured that the terrain properties were as closeas possible to those expected on Mars. This paper presentsthe methods and results of this testing.</p><p>3. TEST PLATFORMS</p><p>The Curiosity rover, shown in Figure 2, was assembled in aclass 10,000 cleanroom to ensure that the mechanisms andinstruments were free of debris, and to ensure that Earthcontaminants were not inadvertently taken to Mars. Insidethis cleanroom, the vehicle only drove a few tens of meterson flat ground and over gentle ramps to do the most basicof mechanism checkouts.</p><p>A second identical copy of the vehicle was built to fa-cilitate software and hardware testing. Initially, this vehicleonly had the rover chassis, wheels, and suspension system.At this point, the vehicle had a mass of 436.8 kg, which on</p><p>Journal of Field Robotics DOI 10.1002/rob</p></li><li><p>Heverly et al.: Traverse Performance Characterization - Mars Science Laboratory Rover 837</p><p>Figure 2. Traverse testing of the flight rover in the assembly cleanroom prior to launch.</p><p>Earth gave it a weight of 128% of what the flight vehicleweighs on Mars. This was the lowest mass configurationpossible with the flight-identical components, and in thisconfiguration the vehicle was known as the Dynamic TestModel (DTM). The DTM rover was used to do a subset oftraverse testing to quantify performance while in this lowmass configuration. The rover was then populated with theremainder of the mechanical components, science instru-ments, cameras, and computing elements to make it a trulyidentical copy of the flight vehicle. The test rover in thisfinal configuration is known as the Vehicle System Test Bed(VSTB). Since Mars gravity is approximately 3/8 of Earthgravity, the VSTB weighs 260% of what Curiosity weighson Mars. This increased weight and ground pressure meansthat the vehicle is not a good analog for traverse character-ization and the VSTB is used primarily for software andautonomous navigation testing.</p><p>The Scarecrow rover is a 340 kg vehicle that weighs onEarth what Curiosity weighs on Mars. The center of gravity(CG) of the vehicle is within 5 cm of the CG location on theflight rover. It has identical wheels and suspension system,but it has a minimal chassis with commercial electronics.The vehicle has onboard batteries and is commanded viaa wireless network, so it does not require a tether. Teleme-try, similar to the flight vehicle, including motor currentand roll/pitch/yaw from an onboard inertial measurementunit (IMU), and suspension angles are all recorded onboardthe vehicle at 2 Hz. Additionally, the vehicle has ultrasonicrange finders mounted to the axle of each wheel to mea-sure wheel sinkage. Table I summarizes the vehicles of theMars Science Laboratory program as well as their mass andweight in their operating environments.</p><p>The Mars Exploration Rover project had a similar strat-egy for vehicle testbeds. A development version of the Spiritand Opportunity rovers known as the Surface System TestBed (SSTB)-Lite was often configured to weigh on Earthwhat the rovers weigh on Mars. The 66 kg version of the ve-hicle was used extensively for traverse performance charac-terization both during the early MER mission developmentand again in the years following the landing of the MERvehicles in 2004. It was used in 2012 to perform head-to-head vehicle comparisons in identical terrain quantifyingthe impact of the wheel diameter for vehicles with similarground pressure. Table II summarizes the vehicles of theMars Exploration Rover program as well as their mass andweight in their operating environments.</p><p>All of the flight Mars rovers to date have had rigidwheels. Wheels for the Sojourner Rover, Mars ExplorationRovers, and Mars Science Laboratory Rover are shown inFigure 3. The MSL wheels are 50 cm in diameter and 40 cmin width and are machined from a single piece of aluminum(Haggart et al., 2008). The six wheels have chevron-shapedgrousers with a 15 degree spacing. The grousers protrude7.5 mm radially from the 0.75-mm-thick wheel skin. In themajority of the terrains that the vehicle encounters, theground deforms to allow for a contact patch that distributesthe weight of the vehicle over a nominal area. Since thecontact area is highly terrain-dependent, a metric has beendeveloped at JPL to define the effective ground pressure(EGP), shown in Eq. (1). The total vehicle weight is dividedby the number of wheels to provide the average forceF of the wheel on the ground. An assumption of wheelsinkage gives a contact patch area equal to the width of thewheel, w, multiplied by the wheel radius r. In the case of</p><p>Journal of Field Robotics DOI 10.1002/rob</p></li><li><p>838 Journal of Field Robotics2013</p><p>Table I. Mars Science Laboratory rover mass/weight comparison.</p><p>Vehicle Curiosity DTM Scarecrow</p><p>Mass 899 kg 437 kg 340 kgWeight on Earth 4,287 N 3,329 NWeight on Mars 3,336 N </p><p>Table II. Mars Exploration Rover mass/weight comparison.</p><p>Vehicle MER SSTB-Lite</p><p>Mass 177 kg 66 kgWeight on Earth 643 N + 10 N of unsupported tetherWeight on Mars 655 N </p><p>noncylindrical wheels where the outer diameter of thewheel is convex, the largest radius, usually at the midplane,is used. When the wheels periphery has cleats, lugs, orgrouser bars, a determination is made as to the amountof their height added to the wheel diameter, based on theprojected area ratio of these tractive elements. This simplemodel gives an easy metric to compare a nominal groundpressure for vehicles with rigid wheels independent ofterrain,</p><p>EGPF</p><p>r w (1)</p><p>Table III shows the effective ground pressure for theMSL and MER rovers. This table makes the simplifyingassumption of a cylindrical wheel, but it does not include theheight of the grousers in the wheel radius. Since the Earthtest rovers (Scarecrow and SSTB-Lite) have identical wheels</p><p>to the flight vehicles and ground pressure scales directlywith gravity, the ground pressures of the Earth-based...</p></li></ul>

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