A predictive Wheel-Soil Interaction Model for Planetary Rovers validated in Testbeds and against MER performance data

Transcription

1 In Proceedings of the 9th ESA Workshop on Advanced Space Technologies for Robotics and Automation 'ASTRA 6' ESTEC, Noordwijk, The Netherlands, November 8-3, 6 A predictive Wheel-Soil Interaction Model for Planetary Rovers validated in Testbeds and against MER performance data Lutz Richter 1, Alex Ellery, Yang Gao, Stephane Michaud 3, Nicole Schmitz 1, Sebastian Weiß 1 1: DLR Institute of Space Simulation, Cologne, Germany : University of Surrey, Guildford, UK 3: Contraves Space, Zurich, Switzerland

2 Planetary rovers then and now Lunokhod MFEX/Sojourner Apollo LRV MER

3 Planetary rovers the future MSL (NASA) ExoMars (ESA) 3

4 Rover design issues careful assessment of terrain relevant for vehicle mission predictions of mobility performance for rational trade-off s on choice of locomotion concept and sizing principal issues driving the chassis design: stress-strain properties of the planetary surface distribution of rocks in the terrain representing potential obstacles to movement gravity level on celestial object in question 4

5 Reported here: traction prediction models for wheel sizes and wheel loads relevant to current and nearterm robotic planetary rovers wheel diameters being between ~ and 5 mm vertical quasistatic wheel loads in operation of roughly 1 to N indispensable for sizings of future rovers to analyse rover mobility across soils 5

6 Evolution of DLR wheel-soil model MIDD SOLERO MER based on Bekker- and Wong-type semi-empirical analyses 1st version: developed for MIDD ESA TRP Study (4 kg vehicle, 4-wheeled w/o rigid suspension); had to be compatible with ~4 N wheel load and 16 mm diameter, narrow wheels experimentally validated for MIDD wheel intermediate version: for RCET Study for ESA: augmentation of original model with distinct features experimentally investigated for rigid wheels in new single wheel tester developed in RCET Solero vehicle demonstrator wheel (15 mm dia. x 9 mm width, 1- N wheel load); MER wheel (5 mm dia. x 16 mm width, ~11 N wheel load) to span range of near-term rovers in Europe 6

7 Initial model tailored to MIDD wheel k b * c * k = + kϕ A W.4 7

8 Single wheel measurements: SOLERO & MER wheel 8

9 Solero wheel: measurements (SWT) vs. predictions R [N] 4,5 4 3,5 3 dependency on translational speed,5 DP [N] poor agreement! 1,5 1,5 Polynomial Fit Test Series 1 Polynomial Fit Test Series 1 W=15N (measured) W=15N (predicted, classical theory) v [mm/s] i [%] H [N] poor agreement! W=15N (measured) W=15N (predicted, classical theory) i [%] 9

10 MER wheel: measurements (SWT) vs. predictions R [N] 6 dependency on translational speed (W=1N, Test Series 1) (W=1N, Test Series ) Polynomisch ((W=1N, Test Series 1)) Polynomisch ((W=1N, Test Series )) v [mm/s] i [%] poor agreement! DP [N] W=118N (measured) W=118N (predicted, classical theory) poor agreement! H [N] W=15N (measured) W=15N (predicted, classical theory) i [%] 1

11 Further model evolution driven by SWT wheel measurements and poor matching with intermediate model predictions principal measure: new, non-linear description of wheel slipsinkage z s 1 zs = 1.hb i => zs = 1. hb i i ( ) refined assignment of value for slip coefficient K (dependent on wheel and soil) 11

12 Present version wheel-soil model, rigid wheel Single wheel (tandem, triplet): W = b u k * z z n D ( z z) D ( z z) ( z z) d n n n ( ) ( ) 1 Wtandem = b u k* Dz z + z z + z z 3 n n n ( ) ( ) 1 Wtriplet = b u k* Dz3 z + z + z3 z + z + z3 z3 3 Drawbar pull, specific energy, and drive power: DP = H R j= m * vehicle,tandem = j vehicle,tandem j= 1 DP H R j= m * vehicle,triplet = j vehicle,triplet j= 1 DP H R R Q = 36 M P = mm ω = mm ω, total ω, total v D ( 1 i) z 1 Sinkage, restistance terms, torque: τ z s ( θ ) [ c + p( θ ) tanφ] M ω, total ( 1 ) = h i i θ = r b τ b 3 1 e ( θ ) dθ + F r + h ( r )[ θ θ ( 1 i )( sinθ sinθ )] K = 1 F = b γ shb Nφ + qhb Nφ + chb Nφ ( α' φ) ( γ γ t c s t ) z t b sin + * n R= b k z dz+ z c K + z K sin α'cosφ

13 MER & SOLERO wheel: comparison with evolved model DP [N] W=15N (measured) W=15N (predicted, classical theory) W=15N (predicted, theory adaptation) i [%] DP [N] i [%] W=1N (predicted, classical theory) W=1N (predicted, theory adaptation) W=1N (measured) 13

14 System-level application of model: Solero vehicle 3 1 DP [N] Slip [%] tests with v< mm/s MSS-D predix 14

15 System-level application of model: MER Slope Capability [ ] MER-A, Columbia Hills, Husband Hill, Sols (soil and rocky debris) MER-A, Columbia Hills, Champagne, Sols (soil) MER-B, Eagle crater egress, Sol 55 (soil) MER Mission Data predicted (Dry Sand) predicted (Sandy Loam),,4 i [-],6,8 1 Slope Capability δ: MER-A NAVCAM Laguna Hollow DPvehicle = Mg sinδ 15

16 -,6 Ongoing work analysis of flexible wheels Bridget wheel Torque [Nm] -1,4 -,6 softer flexible wheel ,8 t [s] Torque [Nm] -1-1,4 Messung1 Messung -1,8 Messung1 -, t [s] Messung 16

17 Summary & outlook wheel-soil model: by now gone through several stages of validation and improvement consistently takes into account measurements on single wheels and complete vehicles under controlled conditions in testbeds in use to derive soil properties on MER future work: expand model to describe the observed effects of translational speed on traction performance complete validation of flexible wheels performance predictions (as opposed to rigid ones) -> in parallel with pending development of ExoMars rover flexible metallic wheels 17