Cutting process modeling and adaptive force control of the turning process

Abstract

In order to improve the productivity in turning operations, feed force is regulated by adjusting feedrate so that cutting forces can be kept constant to some extent. To achieve the above obiective, main efforts are concentrated on two basic problems: accurate modeling of the turning process and design of a controller suitable for the derived model. A mathematical model relating feedrate command as an input with the feed force as an output is presented for a turning process with the machine tool compliance taken into consideration. Effects of consideration of the machine tool compliance in the process modeling are investigated through stability analysis of PI control system and comparison study between the proposed and conventional model. It is found that inclusion of the machine tool compliance into control model makes the predictions ofr stability get closer to the behavior of real process and, hence, is desirable. In addition, the stable region by the proposed model is far bigger than that by the conventional one. A fixed gain PI control is applied to the turning process to control the cutting force. It is shown that the fixed gain PI control scheme tuned at a specified condition might become either unstable or sluggish depending on the variation of cutting conditins., In order to overcome the limitations of the fixed gain PI control, an adaptive PI and an adaptive pole assignment control algorithms are formulated. As a parameter estimation algorithm, a recursive least squares with variable forgettin factors is introduced so as to keep abrest of time-varying nature of the cutting process. The adaptive PI control using the idea of self-tuning gain is found to have some shortcomings that the transient characteristics could not be maintained constant despite of better performance than the fixed gain PI control from the viewpoint of stability. It is shown that the adaptive pole assignment control algorithm can be successfully implemented to the cutting force cont...