A Model Predictive Control of Electronic Limited Slip Differential and Differential Braking for Improving Vehicle Yaw Stability

This article presents a model predictive control design for improving the yaw stability of a rear-wheel-drive vehicle equipped with an electronic limited slip differential (ELSD) and differential braking capability. It first develops a model for an ELSD system for predicting its torque distribution dynamics, then uses the model in designing an intelligent ELSD control that prevents unwanted oversteering yaw moments through direct control of the ELSD clutch pressure. Since differential braking degrades the vehicle's longitudinal motion and driver comfort, two control actuations are prioritized: 1) ELSD clutch pressure and 2) differential braking. An appropriate stability limit is defined for the vehicle yaw rate, and two control objectives: 1) enforcing the yaw rate stability limit and 2) tracking the desired yaw rate are defined and prioritized. The actuation priorities and the objective priorities are combined within a model predictive control structure with particular soft constraints such that the low priority actuation is activated when the high priority objective demands it. Additionally, optimum corner braking forces are calculated by geometrically analyzing tire force vectors. The performance of the proposed controller, implemented in a Cadillac CTS vehicle, is experimentally evaluated for a variety of driving maneuvers.