Slip control for a lock-up clutch with a robust control method

Abstract

Lock up of a clutch operation of an automatic transmission is known as a good method of improving fuel economy. However, the vehicle can often cause undesirable vibration or booming noise when the clutch is locked up at a low velocity. Slip control of the lock-up clutch can resolve these problems, but the speed difference of the lock-up clutch needs to be controlled at a certain circumstance. This control system has to overcome large changes in the parameters of the lock-up clutch system at low velocities and also changes with regard to a gear ratio of a continuously variable transmission (CVT). In this study, the complex non-linear system was modeled with a simple first-order model by treating it as a linear parameter varying (LPV) system. A robust control algorithm was applied to this plant model taking large changes in the parameters and various disturbances into account, and a new slip lock-up clutch control system has been designed. The feedback controller was designed using /spl mu/-synthesis to attain robust stability and desired performance for all operating conditions considered, and then a feedforward controller was added to obtain better response. Thus, a two-degree-of-freedom control system was implemented, which enables precise slip control over a wide range of the low velocity area. We confirmed this control system has sufficient gain margin and phase margin through driving tests. The test results showed that this control system solves undesirable vibration or booming noise when the clutch is locked up at low velocity. Moreover, this control system enables slip control to start at very low velocities with low throttle opening angles and it also enables the transmission to completely lock up the clutch before reaching a velocity of 20 km/h. Test results have confirmed that it contributes to an appreciable improvement of fuel economy, while satisfying the requirements for drivability and clutch durability at the same time.