Slip controller design and implementation in a continuously variable transmission

Continuously variable transmissions (CVT) can be used to operate a combustion engine in a more optimal working point. Unfortunately, due to the relatively low efficiency of modern production CVT´s the total efficiency of the driveline is not increased significantly. This low efficiency is mainly caused by losses in the hydraulic actuation system and the variator. Decreasing the clamping forces in the variator greatly improves the efficiency of the CVT. However, lower clamping forces increase the risk of excessive belt slip, which can damage the system. In this paper a method is presented to measure and control slip in a CVT in order to minimize the clamping forces while preventing destructive belt slip. To ensure robustness of the system against torque peaks, a controller is designed with optimal load disturbance response. A synthesis method for robust PI(D)-controller design is used to maximize the integral gain while making sure that the closed loop system remains stable. Experimental results prove the validity of the approach.