Hydraulic control apparatus for vehicle

1. A hydraulic control apparatus for controlling an engagement hydraulic pressure provided to a frictional engagement element that is arranged between a driving source and a driving wheel and that connects/disconnects a torque transmitted between the driving source and the driving wheel in a vehicle, the hydraulic control apparatus comprising:

a target engagement hydraulic pressure calculation section configured to calculate a target engagement hydraulic pressure of the frictional engagement element;

a slip control section configured to control the frictional engagement element such that a revolution speed at a driving source side of the frictional engagement element is a higher revolution speed than a revolution speed at a driving wheel side of the frictional engagement element;

a command current output section configured to output a command current to a solenoid valve that controls the engagement hydraulic pressure provided to the frictional engagement element based on a map that has a relationship between the target engagement hydraulic pressure and the command current;

a hydraulic pressure decrease section that, when a vehicle speed is equal to or less than a predetermined vehicle speed at which the vehicle is judged to be at a vehicle stop during execution of slip control, is configured to decrease the engagement hydraulic pressure provided to the frictional engagement element; and

a correction section that, when the hydraulic pressure decrease section decreases the engagement hydraulic pressure, is configured to correct the map such that a variation of the command current with respect to a variation of the target engagement hydraulic pressure is smaller than a prior-to-correction variation of the command current with respect to a prior-to correction variation of the target engagement hydraulic pressure.

2. The hydraulic control apparatus as claimed in claim 1 , wherein:

before the hydraulic control apparatus is mounted in the vehicle, the solenoid valve is operated and a relationship between the engagement hydraulic pressure and the command current is recorded as an operating point within a predetermined range of the hydraulic pressure, and a plurality of maps are prepared by a plurality of measurements and records,

the map is selected from the plurality of maps as having a characteristic closest to a characteristic of a plurality of operating points among the plurality of the maps,

a hydraulic pressure that is a minimum value in the predetermined range at a measurement is set as a first threshold value, and

the correction section makes the correction of the map in a range that is lower than the first threshold value.

3. The hydraulic control apparatus as claimed in claim 2 , wherein:

the map is formed by lines connecting a plurality of inflection points,

a hydraulic pressure, which is higher than an inflection point that is a smallest hydraulic pressure among the inflection points that also is higher than the first threshold value, is set as a second threshold value, and

the correction section makes the correction of the map using the variation of the command current with respect to the variation of the target engagement hydraulic pressure at the second threshold value.

4. A method for controlling an engagement hydraulic pressure provided to a frictional engagement element that is arranged between a driving source and a driving wheel and that connects/disconnects a torque transmitted between the driving source and the driving wheel in a vehicle, the method comprising:

calculating a target engagement hydraulic pressure of the frictional engagement element;

controlling the frictional engagement element so that a revolution speed at a driving source side of the frictional engagement element is a higher revolution speed than a revolution speed at a driving wheel side of the frictional engagement element;

outputting a command current to a solenoid valve that controls the engagement hydraulic pressure provided to the frictional engagement element based on a map that has a relationship between the target engagement hydraulic pressure and the command current;

decreasing the engagement hydraulic pressure provided to the frictional engagement element when a vehicle speed is equal to or less than a predetermined vehicle speed at which the vehicle is judged to be at a vehicle stop during execution of slip control, and

correcting the map such that a variation of the command current with respect to a variation of the target engagement hydraulic pressure when decreasing the engagement hydraulic pressure is smaller than a prior-to-correction variation of the command current with respect to a prior-to correction variation of the target engagement hydraulic pressure.