Engaging-force control apparatus and method

1. An engaging-force control apparatus for a friction-engagement element, adapted to control a slip rotational speed between an input-side rotational speed and an output-side rotational speed of the friction-engagement element by increasing or decreasing an engaging force of the friction-engagement element which is determined by a target engaging pressure for the friction-engaging element, the engaging-force control apparatus comprising:

an engaging-force feedback control section configured to control the engaging force of the friction-engagement element to bring the slip rotational speed of the friction-engagement element to 0 by way of feedback control; and

an engaging-force restricting section configured to restrict the engaging force of the feedback control of the engaging-force feedback control section to prevent the engaging force of the feedback control from exceeding a minimum engaging-force value necessary to maintain the slip rotational speed at 0, by

calculating the target engaging pressure so as to bring the detected slip rotational speed to 0 during the feedback control of the engaging-force feedback control section, if the target engaging pressure is lower than or equal to a minimum engaging-pressure value necessary to maintain the slip rotational speed at 0, and by

setting the target engaging pressure at a value equal to the necessary minimum necessary engaging-pressure value, if the target engaging pressure becomes higher than the necessary minimum engaging-pressure valve during the feedback control.

2. The engaging-force control apparatus as claimed in claim 1 , wherein the engaging-force restricting section is configured to set the necessary minimum engaging-force value at a greater value as an input torque to the friction-engagement element becomes greater.

3. The engaging-force control apparatus as claimed in claim 1 , wherein the necessary minimum engaging-force value is set to have a margin against a maximum engaging-force value given within a controllable range for the friction-engagement element.

4. The engaging-force control apparatus as claimed in claim 1 , wherein the engaging-force feedback control section is configured to stop the feedback control of the engaging force during a time period during which the engaging-force restricting section is restricting the engaging force.

5. The engaging-force control apparatus as claimed in claim 1 , wherein:

the engaging-force feedback control section is configured to start the feedback control to bring the slip rotational speed of the friction-engagement element closer to 0, when a predetermined condition of the friction-engagement element becomes satisfied;

the engaging-force restricting section is configured to fix a target value for the engaging force to the necessary minimum engaging-force value, from a time point when the target value for the engaging force becomes greater than the necessary minimum engaging-force value until a time point when the predetermined condition becomes not satisfied; and

the engaging-force feedback control section is configured to stop the feedback control of the engaging force, from the time point when the target value for the engaging force becomes greater than the necessary minimum engaging-force value until the time point when the predetermined condition becomes not satisfied.

6. The engaging-force control apparatus as claimed in claim 1 , wherein:

the engaging-force feedback control section is configured to carry out the engaging-force feedback control for the friction-engagement element in accordance with a value of integral of a deviation between a target slip rotational speed equal to 0 and an actual slip rotational speed of the friction-engagement element; and

the engaging-force feedback control section is configured to stop the integral of the deviation during a time period during which the engaging-force restricting section is restricting the engaging force.

7. The engaging-force control apparatus as claimed in claim 1 , wherein the friction-engagement element is a lockup clutch adapted to restrict a relative rotation between input and output elements of a torque converter.

8. An engaging-force control apparatus for a friction-engagement element, comprising:

means for controlling an engaging force of the friction-engagement element to bring a slip rotational speed between an input-side rotational speed and an output-side rotational speed of the friction-engagement element to 0 by way of feedback control, wherein the engaging force is determined by a target engaging pressure for the friction-engagement element; and

means for restricting the engaging force of the feedback control to prevent the engaging force of the feedback control from exceeding a minimum engaging-force value necessary to maintain the slip rotational speed at 0, by

calculating the target engaging pressure so as to bring the detected slip rotational speed to 0 during the feedback control, if the target engaging pressure is lower than or equal to a minimum engaging-pressure value necessary to maintain the slip rotational speed at 0, and by

setting the target engaging pressure at a value equal to the necessary minimum engaging-pressure value, if the target engaging pressure becomes higher than the necessary minimum engaging-pressure valve during the feedback control.

9. An engaging-force control method for a friction-engagement element, comprising:

controlling an engaging force of the friction-engagement element to bring a slip rotational speed between an input-side rotational speed and an output-side rotational speed of the friction-engagement element to 0 by way of feedback control, wherein the engaging force is determined by a target engaging pressure for the friction-engagement element; and

restricting the engaging force of the feedback control to prevent the engaging force of the feedback control from exceeding a minimum engaging-force value necessary to maintain the slip rotational speed at 0, by

calculating the target engaging pressure so as to bring the detected slip rotational speed to 0 during the feedback control, if the target engaging pressure is lower than or equal to a minimum engaging-pressure value necessary to maintain the slip rotational speed at 0, and by

setting the target engaging pressure at a value equal to the necessary minimum engaging-pressure value, if the target engaging pressure becomes higher than the necessary minimum engaging-pressure valve during the feedback control.