Sliding mode cutting force regulator for turning processes

Continuous sliding mode control is applied to turning processes for cutting force regulation. The motivation of the use of the slide mode control scheme is to solve the nonlinearity problem caused by the feedrate override command element in the commercial CNC machine tool. When the adaptive control algorithm is applied to the commercial CNC machine tool, it is one of the practical methods that the programmed feedrate is overridden after the control algorithm is carried out. However, most CNC lathe manufacturers offer limited number of data bits for feedrate override, thus resulting in nonlinear behavior of the machine tools. Such nonlinearity brings ‘quantized' or discrete effect so that the optimal feedrate is rounded off before being fed into the CNC system. To compensate for this problem, continuous sliding mode control is applied. Simulation and experimental results are presented in comparison with those obtained from applying adaptive control which is a widely used approach in cutting force regulation. Adaptive control loses its effectiveness in the presence of nonlinearity since it generally requires linear parametrization of the control law or the system dynamics. Experiments are conducted under various machining conditions, subject to changes in spindle speed, material of work-piece, and type of machining process. The suggested slide mode controller shows smoother cutting force fluctuation, which cannot be achieved by the conventional adaptive controller. The experimental set-up reflects the emphasis on the practicality of the sliding mode controller. In order to avoid the use of a dynamometer in the course of measuring the cutting force, the indirect cutting force measuring system is used by means of feed drive servo-motor current sensing.