CVT downshift control strategy to minimize slip at the drive pulley

A method of downshifting a continuously variable transmission, of the type having adjustable radius primary and secondary pulleys that are connected by an endless drive element and are hydraulically controlled by primary and secondary forces, to avoid slip between drive the element and the primary pulley by controlling the drive element ratio and primary force comprising the steps of:

• a) initiate a downshift,
• b) calculate (108) a desired primary pressure that corresponds to the downshift,
• c) select (116) a ratio controller duty cycle limit corresponding to the downshift;
• d) calculate (120) a desired secondary pressure that corresponds to a desired primary force,
• e) compare (122) said desired secondary pressure to a known secondary pressure maximum,
• f) repeating steps (c) through (e) for values of secondary pressure that exceed the known secondary pressure maximum until an acceptable secondary pressure is achieved,
• g) measure the actual primary pressure in the continuously variable transmission,
• h) compare (202) the actual primary pressure that has been measured in the continuously variable transmission to the primary pressure set point to get an error signal,
• i) calculate adjustment terms (204) for the secondary pressure set point and the ratio controller duty cycle limit based upon the error signal,
• j) add the adjustment terms (206, 208) to the secondary pressure set point and the ratio controller duty cycle limit, and
• k) use the adjusted secondary pressure set point and adjusted ratio duty cycle limit in the continuously variable transmission.

A method of downshifting a continuously variable transmission, of the type having adjustable radius primary and secondary pulleys that are connected by an endless drive element and are hydraulically controlled by primary and secondary forces, to avoid slip between the drive element and the primary pulley by controlling the drive element ratio and primary force to substantially prevent slip between the primary pulley and the drive element comprising the steps of:

• a) initiate a downshift (101);
• b) calculate a new desired primary pressure (108);
• c) compare the new desired primary pressure to a known minimum primary pressure (110);
• d) calculate a new desired primary pressure for values of new desired primary pressure that are not equal or greater than the minimum primary pressure until the new desired primary pressure is equal to or greater than the minimum primary pressure, and calculate a desired primary force;
• e) establish a new shift rate (114) based upon the mode of operation of the continuously variable transmission, the engine throttle position, the chain-belt ratio and the primary engine speed;
• f) select from desired primary pressure and shift-rate data, the ratio controller duty cycle limit (116);
• g) select from desired primary force and selected ratio controller duty cycle limit data, the desired secondary force (118);
• h) calculate the desired secondary pressure that corresponds to the desired secondary force (120);
• i) compare the desired secondary pressure to the maximum secondary pressure (122), if the desired secondary pressure is larger then increase the shift time (123) and return to the step (116) where a new ratio controller duty cycle limit is selected;
• j) compare (126-128) the desired secondary pressure to the existing secondary pressure and use (130) the higher as the new secondary pressure set point;
• and
• k) use the selected ratio controller duty cycle limit and the new line pressure set point in the continuously variable transmission.

A method of downshifting a continuously variable transmission, of the type having adjustable radius primary and secondary pulleys that are connected by an endless drive element and are hydraulically controlled by primary and secondary forces, to avoid slip between the drive element and the primary pulley by controlling the drive element ratio and primary force to substantially prevent slip between the primary pulley and the drive element comprising the steps of:

• a) initiate a downshift (101);
• b) calculate a new desired primary pressure (108);
• c) compare the new desired primary pressure to a known minimum primary pressure (110);
• d) calculate a new desired primary pressure for values of new desired primary pressure that are not equal or greater than the minimum primary pressure until the new desired primary pressure is equal or greater than the minimum primary pressure, and calculate a desired primary force;
• e) establish a new shift time (114);
• f) select from a test data table a ratio controller duty cycle limit (116);
• g) select from a test data table the desired secondary force (118);
• h) calculate the desired secondary pressure that corresponds to the desired secondary force (120);
• i) compare the desired secondary pressure to the maximum secondary pressure 122), if the desired secondary pressure is larger then increase the shift time (123) and return to the step (116) where a new ratio controller duty cycle limit is selected;
• and
• j) repeating steps (e) through (i) for values of desired secondary pressure that exceed the maximum secondary pressure until an acceptable desired secondary pressure is achieved;
• k) compare (126-128) the desired secondary pressure to the existing secondary pressure and use (130) the higher as the new secondary pressure set point;
• l) measure the actual primary pressure in the continuously variable transmission;
• m) compare (202) the actual primary pressure that has been measured in the continuously variable transmission to the primary pressure set point to get an error signal;
• n) calculate adjustment terms (204) for the secondary pressure set point and the ratio controller duty cycle limit based upon the error signal;
• and
• o) add the adjustment terms (206, 208) to the primary pressure set point and the ratio controller duty cycle limit;
• and
• p) use the adjusted primary pressure set point and adjusted ratio controller duty cycle limit in the continuously variable transmission.