Unmanned machine tools as part of an automated factory require reliable inexpensive sensors to provide machine and process information. The electric current in the DC motor of a CNC machine tool can be inexpensively measured and used to calculate loads on the drive system. To characterize the bandwidth, sensitivity and accuracy of current monitoring on a DC field controlled spindle drive of a CNC lathe, a dynamic lumped parameter model of this sensor system is developed. The model is used to identify the system components that have a dominant effect on the behavior of the sensor when the spindle system is operated above the base speed. Tests were conducted to determine the model’s parameters and to verify the model. The bandwidth and sensitivity of this sensor are shown to be spindle speed dependent. High speeds improve sensitivity and reduce bandwidth. Sensitivity and bandwidth vary by factors of 4.3 and 18.5, respectively, over the speed range. The signal to noise ratio is limited by external load induced variations in the spindle system friction. Recommendations concerning machine designs and process operating conditions to improve current monitoring on spindle drives are presented.

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