Analysis of Power Monitoring on AC Induction Drive Systems

The performance of AC induction motor drive systems as torque sensors is evaluated. Their potential for monitoring tool wear or breakage as well as machine component failure is discussed. A bond graph model of the motor and drive train is developed to determine the relation between electric motor power consumption and the applied torques and to identify the machine components that dominate this dynamic relation. The model, as applied to a CNC milling machine spindle system, shows (1) that the rotor input power is linearly related to both static and dynamic cutting torques under normal process conditions and (2) that the static sensitivity of the spindle system as a sensor increases and the bandwidth of the sensor decreases as the spindle speed is increased. The sensitivity and bandwidth changes are due to changes in system parameters caused by altering the gear ratios between the motor and spindle. Experiments show that the signal-to-noise ratio is affected by the motor torque vibration induced by a geometric irregularity in the drive belt. The vibration, however, does not affect the bandwidth of the sensor system. Power monitoring appears to be a viable torque sensing technique.