Control of a CVT in a Flywheel Assisted Driveline 2000-05-0132

Hydraulic control of a Continuously Variable Transmission (CVT) is the focus of this paper. This CVT is part of a driveline, which incorporates a compact flywheel assisting the engine in high-speed transients. Controlling the CVT boils down to realizing desired gear ratios at maximum transmission efficiency, while preventing belt slip. These wishes are conflicting in that maximizing efficiency is performed by minimizing operating pressures, while preventing belt slip for all-partly unpredictable-situations calls for a safety margin in the applied pressures. Preferably, outputs-to-be-regulated would be the gear ratio and some quantity representing the margin for belt slip. Because of the highly nonlinear behaviour of the CVT and its hydraulic actuation circuit (valve saturations, nonlinear differential equations, etc.) it seems useful to consider nonlinear control theory. In the implementation, severe model uncertainties suggested to incorporate a less model-based controller. The resulting compensator is a cascade of two SISO PI pressure controllers, and a model based feed forward for the pressure set points. The feed forward is determined from the desired speed ratio, and constrained by the belt slip criterion besides operational limits. The controller design is experimentally validated at a 78 [kW] test stand, showing promising results.