Patent ID: 6102825
Filing Date: 2000-08-15
Classification: F16H

Abstract:
A control system for an automatic transmission that has a high speed gear stage, a low speed gear stage, and an input rotation, comprising:a frictional engagement element engageable to establish the high speed gear stage;a rotational element that can be stopped from rotating with a brake to establish the low speed gear stage, a rotational direction of the rotational element when the high speed gear stage has been established being opposite to a direction of torque to the rotational element created when the rotational element is stopped from rotating to establish the low speed gear stage; anda control unit that controls hydraulic pressures applied to the frictional engagement element and the brake, wherein:the brake is formed of a band brake which has a difference of engagement force between a self-energizing operation and a de-energizing operation, and includes a hydraulic servo;the band brake is set so that a direction of the self-energizing operation is the same as the direction of the torque to the rotational element created when the rotational element is stopped from rotating to establish the low speed gear stage, and in order to provide a region in which a hydraulic pressure, which is needed to engage the rotational element when the low speed gear stage is established, is lower than a hydraulic pressure, which is needed to engage the rotational clement when the rotational element is stopped from rotating in the direction of the de-energizing operation, the region being an operational range such that the band brake is set so as to include the operational range; andthe control unit includes a disengaging device which disengages the frictional engagement element at a downshift from the high speed gear stage to the low speed gear stage, a synchronization determining device which determines that the input rotation of the automatic transmission is synchronized with the rotation at the low speed gear stage, a constant pressure maintaining device which maintains the hydraulic pressure applied to the hydraulic servo of the band brake with a waiting pressure, which is lower by a predetermined amount than a pressure stopping the rotation of the rotational element in the direction of the self-energizing operation and with which the input rotation is raced by a basic racing amount after a synchronizing point, until the synchronization is determined by the synchronization determining device, and a pressure increasing device which increases the hydraulic pressure applied to the hydraulic servo from the waiting pressure in order to stop the rotational element from rotating after determining the synchronization.