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

Abstract

Energy saving has been a prominent concern of ground vehicle Original Equipment Manufacturers and research agencies for decades. The search for technological advances that can increase energy efficiency of vehicles has been a relentless quest. 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 incoming 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 work 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 in experimental research of the vehicle on deformable terrain. The article presents a terramechanics-based IWM UGV mathematical modeling that integrates both 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 work, including lab tests at the UAB 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.It was found that maximum IWM UGV slip energy efficiency is achieved if the electric motors are controlled to provide the same tire slippages at all four wheels by zeroing the kinematic discrepancy between the theoretical linear velocities of the 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 slippage value of 11.1% on Norfolk Sandy Loam (NSL) soil.