Line pressure control apparatus and method for belt-drive continuously variable transmission

In a line pressure control apparatus of a belt-drive CVT, a shift is achieved by varying V-groove widths of primary and secondary pulleys by a differential pressure between primary and secondary pulley pressures, originating from line pressure. The differential pressure is created by bringing a shift actuator, such as a step motor, into a control position corresponding to a target transmission ratio. The target transmission ratio is calculated based on a theoretical transmission ratio based on engine-and-vehicle operating conditions, taking account of a hardware response delay and a disturbance. The target transmission ratio is converted into a reference-model operative position, corresponding to a reference-model number-of-steps of the shift actuator. An actual transmission ratio is converted into an actual-transmission-ratio related operative position. Line pressure is corrected in a manner so as to reduce the positional deviation between the reference-model operative position and the actual-transmission-ratio related operative position.