Tire-Road-Friction-Estimation-based Braking Force Distribution for AWD Electrified Vehicles with a single Electric Machine

Braking force distribution (BFD) for electrified vehicles for maximizing energy regeneration has been a challenging research topic, due to the complex operating conditions and tradeoff among different vehicle performance measures. It is known that the level of tire-road friction has a significant impact on the braking force boundaries that define the locking conditions of front and rear wheels. However, conventional BFD strategies for electrified vehicles have not taken full advantage of tire-road friction and generally preferred conservative algorithms. These suggest a potential for exploiting tire-road friction estimation for BFD so as to enhance energy regeneration of electrified vehicles while retaining vehicle stability. This study tackles this challenge by proposing a tire-road-friction-estimation-based BFD for all-wheel-drive (AWD) electrified vehicles with a single electric motor. The specific topology considered in this study is a plug-in hybrid electric vehicle that is powered by an internal combustion engine and a single electric machine. The AWD capacity is provided by a propeller differential shaft connecting the front and rear axles, which imposes a constraint on the ratio of front/rear regenerative brake forces, which is always equal on both axles for the vehicle topology considered here. For the proposed BFD strategy, a fuzzy-logic based tire-road friction estimation algorithm is developed, which uses the longitudinal wheel slip estimated from sensor measurements of vehicle acceleration and wheel speeds. The tire-road friction estimation algorithm is accordingly integrated within the braking controller for front and rear braking force generation and allocation. Simulation analyses are conducted, and the results and discussions demonstrate the effectiveness of the proposed tire-road friction estimation algorithm, and that the tire-road friction estimation-oriented BFD strategy can help to improve the braking energy recovery.