Exploration Rover Mobility Simulation of Traverses on , , Mars Gabrielle L. Coutrot, Raymond E. Arvidson, Feng Zhou Department of Earth & Planetary Sciences, Washington University in St Louis – Contact: [email protected]

Principle of Terramechanics Traverses at Matijevic Hill The shear & normal stresses from Stresses & forces along a driven wheel Rover traverse 200 m ω angular velocity plate tests are given in the red box. R radius of the θ θ ω Segments modeled f V V longitudinal speed The shear stresses are the main source θr 3112 3101 wheel R of driving traction and thus the main Soil properties on Slip velocity = tangential 50m Vcosθ source of slip i , defined as follows: Rω θ velocity – wheel speed  d modeled segments

V V = Rω – Vcosθ id = Vs0 / Rω id = 1- V/ Rω 3090 τ s are estimated using soil Slip is the parameter being modeled in 3053 Rω images, geologic map V Vs slip velocity this study. Negative slip is called skid. and visual odometry Vs0 σ For a wheel this equation is mapped 50m (slip checks). into a cylindrical geometry. The soil t n j At the bottom  x shear displacement becomes: Walkabout loop jx= Vs dt   z    of the wheel:   kx  0   c  ck bk   tan 1 e j  R(( –  ) – (1 – i )(sin – sin )), i  0  c    x f d f d Whitewater Front Vs0 = Rω – V  b  Big Nickel    (sin – sin ) Lake j  R( –  – f ), i  0 Shear stress σ normal stress x f d sol 1 – id Broken 3051 Soil properties γ c φ kc kφ n kx ky b z Hammer Longitudinal Internal Reece Reece Pressure- Lateral shear Estimated as very Estimated as very Soil weight Soil shear Wheel Description friction cohesion friction sinkage deformation sinkage Slip checks hard soil, slip hard soil density cohesion deformation width angle modulus modulus exponent modulus reported ≤ 10% reported <3% modulus

Unit kg m-3 kPa Degree N/mn+1 N/mn+2 N/A mm mm cm cm Navcam sol 3104 Navcam sol 3098 Estimated to be softer Simulations: results Study area is divided into two main soil, slip reported up to types of terrain: 10% ARTEMIS: principles Soil Density c (kPa) φ (deg) kx (mm) ky (mm) kφ kc n Initial simulations run with Hard soil 3%

results of the real drives. After several iterations run by adjusting parameters c, φ, kx, kφ, kc and n, only c and k were found to noticeably influence shear stress and therefore slip. These two Future work x Opportunity plans to spend the winter at parameters were then adjusted until simulated drives matched actual drives. The best fit Solander Point because it has steep parameters are given below. However, the simulated drive for BN3112 produced uphill skid on north facing slopes(~20 °). the second part of the drive: this led to defining a validity field for the ARTEMIS models at a What is the best given φ. The Navcam image below shows two types of soil, consistent with simulation results. approach for driving Soil Density c (kPa) φ (deg) k (mm) k (mm) k k n Navcam mosaic sol 3113 x y φ c on such slopes? Up Hill (UH) 1600 1.6 38 5 5 800 100 1.1

Big Nickel 3112 soil 1 1600 2.5 38 5 5 800 100 1.1

3112 Big Nickel Big Nickel 3112 soil 2 1600 1 30 10 10 800 100 1.2 HiRISE perspective view – (BN) 3112 2.5 exaggeration Pancam super resolution sol 2720 average 0m ARTEMIS is 15m 3101 0m 1.9% slip software that 6m average ARTEMIS will be used to average 3.7% slip assigns soil 0m 1.4% slip 4m 3090 simulate driving on these steep properties to a Soil 1 BN3112 Soil 2 BN3112 UH slopes with different soil 3053 DEM and Skid (actually run ) Slip (actually run ) properties (i.e., c and kx) to models the 100%Slip (actually run ) evaluate ways to traverse rover wheel interactions with the soil. Max slip trend for given c 50m Min slip trend for given c Solander Point. For example, Traverse on hard soil Three terrain models were built to simulate Traverse on very hard soil Soil 2 BN3112 the rover could approach the uphill area by following a contour the three drive paths shown in yellow. c HiRISE – traverses modeled k x validity field line and use a switch-back pattern to go downhill.