Abstract
While torque converters in automatic transmission systems provide benefits such as good riding quality, they also decrease transmission efficiency of engine torque. To improve the efficiency, most automatic-transmission systems are equipped with a lock-up clutch, which directly combines the engine and the driving shaft. In a low-speed range, however, the lock-up clutch must be slipped at a rate by controlling the hydraulic pressure on the clutch; otherwise low-frequency noises and vibrations are generated. This paper first proposes a first-order model of the dynamics from hydraulic pressure command to the slip rate. The model is obtained from the experimental result that the dynamics from the converter torque to the slip rate is described by a first-order state equation whose parameters depend on turbine revolution rate only. Using the linear-parameter-varying model, a two-degree-of-freedom-control system is designed. The feedforward controller is given as a desired model that is gain-scheduled as a function of the turbine revolution rate. Next, the feedback controller is designed by a robust control method, i.e., μ-synthesis, which makes the closed-loop system robust against uncertainties such as variation of operating condition and aging. The effectiveness of the model and the controllers is demonstrated through driving tests.
