PROBLEM TO BE SOLVED: To improve characteristics in a very low speed range. SOLUTION: This controlling circuit is made up of a speed amplifier 2, which amplifies speed deviation, a load torque observer 31 for controlling disturbance, which presumes a load torque from the output of this amplifier and speed signal to output two-inertial system torque commands, a resonance proportional control circuit 51 for controlling vibration, which multiplies the torque command from this observer in proportion to resonance and presumes a torque from the signal multiplied in proportion to the resonance and speed signal to output a motor torque command to two-inertial system 8, and a speed detector 91 , which detects four edge signals of the A phase and B phase signals with the rotary encoder and outputs pulses overlapped with these signals as a speed signal. The shortened cycle of this speed signal makes it possible to restrain the vibration in a very low speed range. Even if the speed information is interrupted in the very low speed range, the circuits 31 , 51 presume the speed using a model speed so that speed can be controlled smoothly.
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