Centralized Torque Controller for a Nonminimum Phase Phenomenon in a Powersplit HEV

Torque controls for the engine and electric motors in a Powersplit HEV are keys to the success of balancing fuel economy, driveability, and battery power control. The electric variable transmission (EVT) offers an opportunity to let the engine operate at system-optimal fuel efficient points independently of any load. Existing work shows such a benefit can be realized through a decentralized control structure that translates the driver inputs to independent engine torque and speed control. However, our study shows that the decentralized control structures have a fundamental limitation that arises from the nonminimum phase (NMP) zero in the transfer function from the driver power command to the generator torque change rate, and thus not only is it difficult to obtain smooth generator torque but also it can cause violations on battery power limits during transients. Additionally, it adversely affects the driveability due to the generator torque transients reflected at the ring gear. This study investigated a novel centralized torque controller to enhance smart coordination between the engine and generator. We first analyzed the theoretical limitations of the decentralized control and then presented a centralized strategy with a theoretical proof. Simulation results also verified that the centralized control can effectively smooth the generator torque and eliminate the inverse response of the generator torque during rapid vehicle accelerations/decelerations. Further it prevents battery power spikes by reducing generator torque transients, thus enhancing the battery durability. Finally it smoothes the mechanical torque spikes transmitted to the planetary ring gear, and therefore helps reduce NVH.