Line pressure control device and method for belt type continuously variable transmission

A line pressure control method for a belt type continuously variable transmission in which a belt is wrapped around a primary pulley connected to an input side of a vehicle drive system and a secondary pulley connected to an output side, a groove width of the primary pulley and the secondary pulley is varied in accordance with an oil pressure supplied by a hydraulic pump, and a speed ratio is varied in accordance with the groove width, the transmission comprising:

• a first pressure regulating valve which comprises a solenoid and regulates the oil pressure supplied by the hydraulic pump as a line pressure by varying a valve opening in accordance with a solenoid current supplied to the solenoid;
• a second pressure regulating valve which regulates an oil pressure supplied to the primary pulley on the basis of the line pressure;
• and
• a third pressure regulating valve which regulates an oil pressure supplied to the secondary pulley on the basis of the line pressure, and
• the method comprising:
• calculating a target line pressure which is equal to or greater than the larger of the oil pressures supplied to the primary pulley and the secondary pulley;
• calculating the solenoid current supplied to the solenoid required to realize the target line pressure by looking up a table showing a relationship between the solenoid current and the target line pressure, the table being set such that the target line pressure decreases as the solenoid current increases and the table has an inflection point in which a decrease rate of the target line pressure in a high current value region beyond the inflection point is smaller than a decrease rate of the target line pressure in a low current value region below the inflection point;
• feedback-controlling the line pressure to equal the target line pressure by controlling the solenoid current;
• determining that a divergence between the target line pressure and the actual line pressure occurs when the target line pressure has fallen below a first low pressure which is a line pressure at the inflection point during the feedback-controlling;
• and
• restricting the target line pressure, when the divergence is determined to have occurred during the feedback-controlling, such that the target line pressure does not fall below a lower limit value, the lower limit value being set to the line pressure at the time of a determination in which the divergence occurs.

A line pressure control device for a belt type continuously variable transmission in which a belt is wrapped around a primary pulley connected to an input side of a vehicle drive system and a secondary pulley connected to an output side, a groove width of the primary pulley and the secondary pulley is varied in accordance with an oil pressure supplied by a hydraulic pump, and a speed ratio is varied in accordance with the groove width, the transmission comprising:

• a first pressure regulating valve which comprises a solenoid and regulates the oil pressure supplied by the hydraulic pump as a line pressure by varying a valve opening in accordance with a solenoid current supplied to the solenoid;
• a second pressure regulating valve which regulates an oil pressure supplied to the primary pulley on the basis of the line pressure;
• and
• a third pressure regulating valve which regulates an oil pressure supplied to the secondary pulley on the basis of the line pressure, and
• the device comprising:
• target line pressure calculating means for calculating a target line pressure which is equal to or greater than the larger of the oil pressures supplied to the primary pulley and the secondary pulley;
• solenoid current calculating means for calculating the solenoid current supplied to the solenoid required to realize the target line pressure by looking up a table showing a relationship between the solenoid current and the target line pressure, the table set such that the target line pressure decreases as the solenoid current increases and the table has an inflection point in which a decrease rate of the target line pressure in a high current value region beyond the inflection point is smaller than a decrease rate of the target line pressure in a low current value region below the inflection point;
• line pressure control means for feedback-controlling the line pressure to equal the target line pressure by controlling the solenoid current;
• divergence determining means for determining that the target line pressure has fallen below a first low pressure, when a divergence between the target line pressure and the actual line pressure occurs, the first low pressure being a line pressure at the inflection point during the feedback-controlling performed by the line pressure control means;
• and
• line pressure restricting means for restricting the target line pressure such that the target line pressure does not fall below a lower limit value when the divergence is determined to have occurred during the feedback-controlling performed by the line pressure control means, the lower limit value being set to the line pressure during the determining performed by the divergence determining means.