Hydraulic power steering system

A hydraulic power steering system that assists a steering operation by supplying hydraulic fluid from a hydraulic pump to a power cylinder, which is coupled to a steering mechanism of a vehicle, via a hydraulic control valve that is not mechanically coupled to a steering member, comprising:

• a valve driving motor that is used to control an opening degree of the hydraulic control valve;
• a pump driving motor that is used to drive the hydraulic pump;
• opening degree command value setting means for setting an opening degree command value that is a command value of the opening degree of the hydraulic control valve;
• valve driving motor control means for controlling driving of the valve driving motor based on the opening degree command value that is set by the opening degree command value setting means;
• rotation speed command value setting means for setting a rotation speed command value that is a command value of a rotation speed of the hydraulic pump;
• rotation speed command value correcting means for correcting the rotation speed command value, which is set by the rotation speed command value setting means, based on an amount of change in the opening degree command value that is set by the opening degree command value setting means,
• wherein the rotation speed command value correcting means includes rotation speed command value increasing means for, when the opening degree command value that is set by the opening degree command value setting means changes so as to decrease a pressure of the hydraulic fluid, making a correction such that the rotation speed command value, which is set by the rotation speed command value setting means, increases based on the amount of change in the opening degree command value;
• and
• pump driving motor control means for controlling driving of the pump driving motor based on the rotation speed command value that has been corrected by the rotation speed command value correcting means.