The present invention relates to a hydrostatic transmission system comprising: at least one pump (110), at least two wheel motors (120, 122; 140, 142) supplied by the pump (110) for the mechanization of a machine, characterized by the fact that it comprises means (130) designed to offset in time a change of displacement of the motors (120, 122; 140, 142) into several groups so as to have a progressive evolution of the apparent displacement of the motors.

A hydraulic circuit for a transmission includes a friction element that engages or disengages an operating element of the transmission, a pump that pumps oil to form preliminary pressure that allows the friction element to be maintained in a standby state in which the friction element is ready to operate, and a direct control valve that receives the oil that has passed through the pump and adjusts the preliminary pressure to form operating pressure that allows the friction element to operate, when operating the friction element. The friction element includes a hydraulic chamber into which the oil is introduced, a first supply hole through which the oil that has passed through the pump is supplied to the hydraulic chamber, and a second supply hole through which the oil that has passed through the direct control valve is supplied to the hydraulic chamber.

An object of the present invention is, even if an abnormality occurs in an oil pressure circuit of an automatic transmission, to avoid traveling performance deterioration by the abnormality when gear change is performed. The present invention provides a control device of an automatic transmission and a method of controlling the automatic transmission, each of which switches from a normal state to an abnormality diagnosis state at a predetermined change gear ratio to diagnose whether there is an abnormality in an automatic transmission, the normal state being a state where an oil pressure supply portion supplies oil pressure to a predetermined friction engaging element, the abnormality diagnosis state being a state where an abnormality diagnosis oil pressure supply portion including an oil pressure supply passage different from an oil pressure supply passage of the oil pressure supply portion supplies the oil pressure to the predetermined friction engaging element.

A gear change control system includes: a shift operation member configured to receive a shift range operation; a clutch configured to transmit a torque of an input shaft to an output shaft; a clutch actuator configured to control engagement of the clutch by using a working fluid; a manual valve configured to switch a supply destination of the working fluid to the clutch actuator in accordance with a decided shift range; a shift actuator configured to move the manual valve on the basis of a shift range changing operation of the shift operation member; and an oil pressure control actuator configured to reduce an oil pressure of the working fluid to a target oil pressure after the shift range changing operation received by the shift operation member is started and before the shift actuator moves the manual valve toward a shift range changing destination.

An object of the present invention is, even if an abnormality occurs in an oil pressure circuit of an automatic transmission, to avoid a case where a traveling performance deterioration by the abnormality when gear change is performed. The present invention provides a control device of an automatic transmission and a method of controlling the automatic transmission, each of which switches from a normal state to an abnormality diagnosis state at a predetermined change gear ratio to diagnose whether there is an abnormality in an automatic transmission, the normal state being a state where an oil pressure supply portion supplies oil pressure to a predetermined friction engaging element, the abnormality diagnosis state being a state where an abnormality diagnosis oil pressure supply portion including an oil pressure supply passage different from an oil pressure supply passage of the oil pressure supply portion supplies the oil pressure to the predetermined friction engaging element.