The present invention relates to a control system for a continuously variable transmission.
JP-A No. 63-43837 discloses a conventional control system for a continuously variable transmission. This control system is constructed and arranged so that a target reduction ratio is achieved by a feed-back control of a shift actuator. When a motor vehicle is not driven, viz., an engine brake is applied, a feed-back gain becomes larger. This ameliorates the followability and the responsibility of shifting to the maximum reduction ratio. Thus, even if quick deceleration takes place, shifting up to the maximum gear ratio can be achieved before the motor vehicle comes to a standstill.
A problem encountered in such a conventional control system for a continuously variable transmission is that, since the feed-back gain is kept constant regardless of change in an input torque to the transmission, viz., an engine torque, time is needed to bring a current reduction ratio to the target reduction ratio when the engine torque is large, and it is difficult to keep the target reduction ratio corresponding to the current reduction ratio even during normal operation of the motor vehicle. Specifically, with a continuously variable V-belt transmission, the reduction ratio is determined by a balance between a hydraulic pressure on a movable conical member of a pulley and a tension of a V-belt. If the engine torque increases out of a predetermined balanced reduction ratio, for example, the tension of the V-belt also increases. In this event, the hydraulic pressure on the pulley increases in a delayed manner, the reduction ratio is deviated to the maximum reduction ratio. Further, with a continuously variable traction roller transmission which is disclosed, for example, in JP-U No. 63-84451, shifting takes place in a controlling position of roller support members by the hydraulic pressure. Thus, when the engine torque increases, the hydraulic pressure within a hydraulic servo apparatus for position control of the roller support members changes in a delayed manner, resulting in an occurrence of a deviation between the current reduction ratio and the target reduction ratio. Even during normal operation of the motor vehicle, the hydraulic pressure for holding the roller support members has to increase as the engine torque becomes larger. In this event, the hydraulic servo apparatus may not be held in a predetermined position due to increased leakage from seal portions, resulting in the occurrence of a deviation of the current reduction ratio to the maximum reduction ratio.