Fuzzy-logic control of cutting forces in CNC milling processes using motor currents as indirect force sensors

A fuzzy-logic controller (FLC) is designed to automatically adjust feed rate in order to regulate the cutting force of milling processes in a vertical machining center. The FLC has a double-loop structure, consisting of the inner PD (proportional-derivative) velocity-control loop for a feed servo and the outer fuzzy-logic force-control loop. Reference cutting forces are well maintained in both numerical simulation and experiments when the cutting-depth profile of an aluminum workpiece (Al6061-T6) is varying step-wise or continuously. In order to replace expensive and impractical tool dynamometers, ac-induction-motor currents in the feed system and the spindle system are analyzed, and then compared as a cutting-force sensor. The bandwidth of both systems are not high enough to sense the cutting dynamics for common spindle speeds. Thus, quasi-static quantities (i.e., average or maximum resultant cutting force per spindle revolution) are compared instead. The spindle-motor current is chosen because quasi-static sensitivity is much higher (i.e., 5.415 × 10−3 vs. 2.128 × 10−4 A/N). Reference cutting forces (230 and 330 N) are well maintained when the depth of cut is less than 4 mm.