Patent ID: 7789780

Claim:
A continuously variable transmission comprising: an input side primary pulley which varies a groove width according to a first oil pressure; an output side secondary pulley which varies a groove width according to a second oil pressure; a belt looped around the primary pulley and the secondary pulley, having a contact radius with the pulleys which varies in accordance with the groove width, in which a rotary force transmitted from an output side of an engine of a vehicle via a frictional engagement element is transmitted from the primary pulley to the secondary pulley via the belt, and a speed ratio determined according to a relationship of the contact radius varies continuously; a speed change actuator; a speed change control valve which controls the first oil pressure by switching between a position for supplying the first oil pressure to the primary pulley, a position for discharging the first oil pressure, and a neutral position in accordance with a physical positional relationship between a position of a variable sheave of the primary pulley and a position of the speed change actuator; a pressure adjusting mechanism which adjusts the second oil pressure; a sensor which detects the second oil pressure; and a controller, which: when the vehicle is in a low return control state such that the vehicle is stationary and the frictional engagement element is released, shifts the speed ratio toward a lowest speed ratio by controlling the speed change control valve so that the first oil pressure reaches substantially zero and reducing the contact radius between the belt and the primary pulley and by controlling the pressure adjusting mechanism so that the second oil pressure rises to a predetermined oil pressure and increasing the contact radius between the belt and the secondary pulley while the primary pulley and the secondary pulley are in a non-rotational state; stores a speed ratio immediately prior to when the vehicle stops as a pre-stop speed ratio; integrates a speed ratio variation determined on the basis of the detected second oil pressure during the shifting of the speed ratio toward the lowest speed ratio, when the first oil pressure is decreased and the second oil pressure is increased; and estimates, at a first time interval, the speed ratio which is shifting toward the lowest speed ratio on the basis of adding the pre-stop speed ratio to the integrated speed ratio variation and estimates, at time intervals subsequent to the first time interval, the speed ratio on the basis of adding a previously estimated speed ratio to the integrated speed ratio variation.