Basic Research on Precise Position Control Robot System Applying New Direct Drive Motor

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It is well known that squirrel-cage induction motors with special stator and rotor slot combination produce harmonic synchronous torque at a specific slip; such a motor has never been designed practically because of crawling. We took note of the fact that the harmonic synchronous torque has very large magnitude at low speed, and made basic research to apply it to the direct drive robot system required precise position control.It was made clear by theoretical investigation how to design such a special motor, and manufactured a motor for experiment, whose torque was 42.4 N-m at 112.5 rpm under the supply of 150 V, 60 Hz.As the slip is 0.75 or more where harmonic synchronous torque is produced, too heavy current flows in the winding when driven normally at the slip. We devised a new control scheme which enable to drive even at such a slip with appropriate current depending on the load; it is based on the idea controlling the internal phase angle of harmonic synchronous torque.The motor is driven by switched transisto inverter with current control as follows: its phase is controlled to hold the value calculated from the detected rotor position and the reference of the internal phase angle, and its effective value is done to make no difference between the reference and detected speed.It was verified by the experiment that the application of this control scheme to the motor desiged especially resulted in smooth acceleration and deceleration and stable slow speed drive with reasonable current depending on the load.We have also concluded from the study of static torque that it is proper to apply the step motor torque produced by the stator and rotor slot combination to the position control, and made clear what is the optimum design for the purpose.