Sensorless cutting force estimation for full-closed controlled ball-screw-driven stage

Process monitoring technology has been studied in order to realize higher efficiencies and greater automation of the machining process. The cutting force is widely regarded as being the most valuable physical quantity to be gathered when monitoring a metal-cutting process. However, a practical, wideband, and indirect means of measuring the cutting force has not yet been attained for ball-screw-driven machine tools. In recent years, full-closed ball-screw-driven stages mounting linear encoder have been widely used for high-end machine tools. Considering indirect cutting force estimation using servo information from a full-closed controlled ball-screw-driven stage, three pieces of information are available as servo signals of the feed drive: the motor current command, the rotation angle of the motor, and the displacement of the stage. In this study, a high-precision and wideband sensorless cutting force estimation method was proposed for full-closed controlled ball-screw-driven stages using a multi-encoder-based disturbance observer (MEDOB). The MEDOB was originally proposed for estimating the external force on the load side of flexible robots, and was used for resonance ratio control, which was intended to suppress low-frequency vibration. This study aimed to develop an estimation technique for high-frequency cutting forces that exceed the bandwidth of current control loops for the highly stiff ball-screw-driven stage. The influence of extra phase lag on the control signals was considered in order to apply MEDOB to an actual ball-screw-driven stage. The validity of the proposed method was verified using both an analytical simulation and cutting experiments.