CONTROL DEVICE FOR CONTINUOUSLY VARIABLE TRANSMISSION FOR VEHICLES AND METHOD OF CONTROLLING THE SAME

In a continuously variable transmission of the type in which a speed change ratio is changed in a manner that the radius of rotation of a belt wound around a drive pulley and a driven pulley is varied by changing the width of a groove between the pulley parts of each pulley with an oil pressure. A control means for controlling an oil pressure in various control modes shifts the control mode from a hold mode to a start mode, when a driver manipulates a vehicle carrying the continuously variable transmission with his intention to start the vehicle, and an amount of discharge flow of an oil pump for generating an oil pressure exceeds a preset value. The safety factor in a stationary state is calculated using the effective tension of the CVT belt, secondary side oil pressure, and the speed change ratio. Using the calculated stationary state safety factor, a manipulated variable previously gained, and a changing rate of the primary oil pressure, a safety factor in a speed change state (transient state) is calculated. A manipulated variable of the speed change control means is limited so that the safety factor in a speed changing state exceeds a predetermined value. When the speed change performance may be damaged with the limitation of manipulated variable, a necessary quantity of the line pressure or the CVT input torque is corrected so as to increase the limit value.