PURPOSE: To position a hydraulic actuator with high accuracy by calculating the deviation of a displacement command signal on the basis of the output of a load displacement detecting means and control valve displacement detecting means so as to compensate the displacement command signal in accordance with the deviation.

CONSTITUTION: In a servo controller of an articulated mechanism such as a working robot arm, a displacement command drives a servo motor E through a characteristics compensator B and electric power amplifier, C to which a deviation controller A and a speed detector F return a signal, and further, a rotary control valve I and a rotary actuator K drives a load L via a deceleration gear G which receiving a mechanical feed-back. According to the detected displacement of the load L and to the detected rotation of the DC servo motor E, i.e., the detected displacement of the control valve, the deviation controller A detects a deviation of a displacement command signal 10 compensate the signal, thereby controlling the position of the rotary actuator K. Therefore, a highly accurate positioning and a stable control can be realized.