UGV with a distributed electric driveline: Controlling for maximum slip energy efficiency on stochastic terrain

Authors: SALAMA, MOSTAFA - University Of Alabama; VANTSEVICH, VLADIMIR - University Of Alabama; Way, Thomas - Tom; GORSICH, DAVID - Us Army Engineer Research And Dvelopment Center

Submitted to: Terramechanics Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/4/2018
Publication Date: 6/14/2018
Citation: Salama, M., Vantsevich, V., Way, T.R., Gorsich, D. 2018. UGV with a distributed electric driveline: Controlling for maximum slip energy efficiency on stochastic terrain. Terramechanics Journal. 79:41-57.

Interpretive Summary: Important aspects of agricultural tractors equipped with pneumatic tires and other self-propelled off-road vehicles equipped with tires are (a) the tractive performance of the vehicle on soil and (b) the amount of soil compaction caused by the vehicle tires. Traction is important for efficient propulsion of the vehicle across the soil surface. Minimizing soil compaction is typically desirable in agriculture, to promote root growth of plants, and ensure there is sufficient void space in soil, to provide good water-holding capacity of the soil. A small four-wheel vehicle weighing 40 kg (90 lb) and equipped with pneumatic tires, was operated on a loose sandy loam soil. Each of the four wheels was powered by its own electric motor. The slip energy efficiency was maximized when the electric motors were controlled to provide the same tire slip at all four wheels. At this condition, the total electric current drawn by all four DC motors was minimum. The maximum slip energy efficiency of 89.3% was achieved at a mean tire slip value of 11.1% on the sandy loam soil.

Technical Abstract: Energy efficiency has been a prominent concern of ground vehicle manufacturers and researchers for decades. The framework of research on energy efficiency improvements has been considerably extended after the introduction of fully electric vehicles with electric motors that individually drive each wheel, i.e. In-Wheel Motors (IWM). Although newly developed IWM vehicles can significantly decrease driveline power losses and, thus improve vehicle energy efficiency compared to conventional mechanical driveline systems, one technical problem related to the vehicle-tire-terrain interaction needs to be addressed in fully electric terrain vehicles. These vehicles are still lacking strategies to manage power distribution between the drive wheels, which are not connected by a driveline system anymore, with the purpose to minimize slip power losses at all tires and maximize vehicle slip energy efficiency. Inappropriate power delivered to each of the wheels, which run in different stochastic terrain conditions, can deteriorate slip energy efficiency of a vehicle with four individually driven wheels. The research presented in this article addresses the problem of wheel power distribution for an unmanned ground vehicle (UGV) with four IWMs. The goal of this study was to analytically establish and determine conditions for the wheel power split that corresponds to the maximum slip efficiency of the IWM UGV, design a control algorithm that implements the conditions, and verify the effectiveness of the control through experimental research of the vehicle on deformable terrain. The article presents terramechanics-based IWM UGV mathematical modeling that integrates the vehicle, electric-motor, and wheel dynamics with stochastic soil characteristics. The control algorithm is based on the inverse dynamics approach to control the wheel angular velocity to overcome stochastic terrain load torque and to satisfy a required program of UGV motion given by a UGV velocity profile. Each wheel angular velocity is controlled by varying the wheel torque using a control-by-acceleration principle that provides each wheel with the required angular velocity and the tire rolling radius in the drive mode. Experimental research, including lab tests at the University of Alabama at Birmingham Vehicle and Robotics Engineering Lab and soil bin experiments of the IWM UGV at the USDA-ARS National Soil Dynamics Lab, was conducted to validate the analytical models and control algorithm. A small four-wheel IWM UGV weighing 40 kg (90 lb) and equipped with pneumatic tires, was operated on a loose sandy loam soil. The maximum IWM UGV slip energy efficiency was achieved when the electric motors were controlled to provide the same tire slip at all four wheels. At this condition, the total electric current drawn by all four DC motors was minimum. The maximum IWM UGV slip energy efficiency of 89.3% was achieved at a mean tire slip value of 11.1% on the sandy loam soil.