High Performance Milling Torque Sensor

Abstract

Machine tool supervision and control algorithms require reliable and effective sensor signals to operate properly. In effort to satisfy this need, a high stiffness, wide bandwidth torque sensor for use in milling has been developed which directly measures the torque applied to a milling cutter during operation. The sensor is designed to fit between the tool and holder on conventional tooling with very little effect on the cutting process. The sensor is strain gage based and provides a virtually distortionless torque measurement over a bandwidth from DC to 2000 Hz when using a 100 mm diameter face mill on a commercial machining center. High torsional stiffness was achieved to provide a wide measurement bandwidth while allowing enough material strain, in the sensing element, to provide sufficient resolution of the milling torque. The radial stiffness of the sensor was also designed to be large enough not to compromise the stability and accuracy of the machine tool. The sensor is designed to house the critical electronic components which amplify the small voltage strain gage signal and convert the measurement into digital samples. These samples are continuously transmitted from the rotating spindle, in all positions, to a stationary receiver. Because the sensor is part of a structural system which also includes the spindle, tool holder and tool, the frequency response has distortions associated with the vibrational modes of the system. In order to obtain a wide undistorted bandwidth, a compensation filter having the reciprocal response of the sensor has been designed and implemented on a digital signal processor (DSP). The combined system of the sensor cascaded with the DSP provides a flat magnitude and linear phase frequency response.