FORCE CONTROL IN TURNING BASED ON ROBUST PI CONTROLLER-DESIGN

Real-time cutting force control is a very straightforward way to impro ve safety, performance and quality in rough turning operations. The ma in problem in controller design for force control is to achieve robust ness. Several time-variable process parameters, such as the depth of c ut, the spindle rotational speed and the specific cutting force, direc tly affect the closed-loop system gain and time constant. In this pape r, two approaches to robust PI (proportional integral) controller desi gn based on the internal model control (IMC) method are presented. The first approach treats the cutting process as a first-order system wit h a time-variable gain. By designing for highest process gain, the con troller becomes robust, but perhaps a bit sluggish for the lower range of process gain. In the second approach, the cutting process is linea rized by introducing non-linear transformations. In this way, the desi gn may be based on a constant-gain system and the time-variable parame ters may be considered as additive disturbances. This controller is so mewhat faster than the first approach and shows almost identical behav iour over the entire operating range. Both controllers incorporate, in addition, simple parameter adaptation by gain scheduling with respect to spindle speed variations, in order to handle variations in the pro cess time constant. The work presented in this paper shows that it is possible to achieve robust force control in turning using the common P I controller.