Development of a Slip Control System for a Lock-Up Clutch

Lock-up operation of an automatic transmission is known as one good method of improving fuel economy. However, locking up the transmission at low vehicle speeds can often cause undesirable vibration or booming noise. 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 value. This control system has to overcome large changes in the parameters of the lock-up system at low vehicle speeds and also changes with regard to the speed ratio in a continuously variable transmission (CVT). In this study, this complex non-linear system has been modeled as a first-order linear parameter varying (LPV) system. A robust control algorithm was applied taking various disturbances into account to design a new slip lock-up control system. First, the feedback controller was designed using μ synthesis to attain robust stability and the desired performance for all the operating conditions considered, and then a feedforward controller was added to obtain better response. As a result, a two-degree-of-freedom control system was implemented, which enables slip control to start at a very low vehicle speed and the transmission is completely locked up before reaching a vehicle speed of 20 km/h. In this paper, the results of simulations and driving tests are presented to demonstrate the effectiveness of this slip control system.