Model Reference Adaptive Force Control in Milling

This paper describes the design and implementation of a Model Reference Adaptive Controller (MRAC) for force control in milling. First, previous work in this area is discussed. Results from previous work on the performance of fixed gain process controllers is summarized. The design of an MRAC for force control in milling is described, including a discussion of the implementation issues of noise and com­ putational speed. The adaptive controller was found to perform more satisfactorily than fixed gain controllers, but is difficult to implement and tune, primarily because of the unmodeled dynamics or measurement noise resulting from runout on the mill­ ing cutter. In this problem there was sufficient separation between the noise and the signal frequency that the noise could be filtered. However, the addition of the filter added additional dynamics to the system which reduced the overall performance from that expected from digital simulations. Increasing labor costs and the increasing need for machined parts with high precision, complex geometries has caused a dramatic increase in the number of Numerically Controlled (NC) and Computer Numerically Controlled (CNC) machine tools being used in production. These NC and CNC machine tools have greatly reduced operator input, resulting in signifi­ cant improvements in productivity. However, it has been demonstrated that further improvements can be made by on­ line manipulation of the programmer specified feedrates [1]. This can be done by implementing a process controller which uses some measure of the process (typically cutting force) to adjust the feedrate. The use of such process controllers has resulted in substantial improvements in productivity [2]; however, they have introduced some serious new problems resulting from variations inherent in the cutting process. For­ tunately, these problems can be reduced or eliminated by using ideas from adaptive control theory in the controller design. The purpose of the research reported in this paper is to ex­ amine the application of a model reference adaptive force con­ troller to the milling process. This paper is organized as follows: in the next section previous work is discussed, in­ cluding the development of a process model and a discussion of the effects of process parameter variations on the perform­ ance of fixed gain process controllers. A subsequent section describes the design of a model reference adaptive controller (MRAC) for milling. The implementation of the MRAC, and experimental results are then presented and discussed. Finally, the research is summarized, major conclusions are drawn, and topics for future research are indicated. Previous Research •«. This section discusses the literature pertinent to the applica­ tion of adaptive control to milling. Before a detailed discus­ sion of the literature can be presented, a point of terminology must be clarified.