1. Field of the Invention
This invention relates to a system for controlling the operation of a slipping clutch for use in an automatic transmission. More particularly, this invention pertains to such a system for controlling the operation of a lockup clutch in accordance with a clutch slip command signal applied to the winding of a solenoid by an electronic computer control system.
2. Description of the Prior Art
In an automatic transmission having either a startup clutch or a torque converter lockup clutch, the clutch must be applied smoothly to assure imperceptible engagement. After the lockup clutch is applied, vibrations can occur in the driveline at various road speeds because the damping effect of the torque converter in the driveline is eliminated after the torque converter is locked. Sudden changes in the application of the engine throttle, called tip-in and backout, can produce large, unacceptable driveline torque disturbances that are uncomfortable to the passengers of the vehicle, and, if the engine speed is low, lugging can occur.
Conventional drivelines include a single stage or multiple stage damper located in the transmission in series with the engine to suppress tip-in and backout and other noise, vibration and harshness (NVH) disturbances. Although dampers for this purpose have been successful in dealing with these problems, their presence adds cost, complexity and weight to the system.
In electronically controlled transmissions that produce discrete drive ratios or continuously variable, stepless drive ratios, a closed loop control circuit can be employed to smoothly lockup the clutch and control slip across the clutch. It has been found that controlling the slip, can eliminate tip-in and backout NVH and lugging problems. When a control of this type is used, mechanical dampers can be eliminated and the cost and complexity of the system can be reduced.