Emulating bistabilities in turning to devise gain tuning strategies to actively damp them using a hardware-in-the-loop simulator

Abstract

Bistabilities in turning are characterized by the process being stable for small perturbations and unstable for larger ones. These bistabilities occur due to nonlinearities in cutting force characteristics. Characterizing these bistabilities can guide selection of cutting parameters to lie outside these zones of conditional instabilities. However, if cutting is targeted in the bistable regions, or in regions that are globally unstable, active damping methods may need to be pursued to improve the stability envelopes. Since gain tuning for active damping of chatter vibrations is usually based on linear stability analysis, it would be useful to know if those gains are adequate for damping chatter in the presence of bistabilities that occur in processes prone to strong perturbations. However, experimentation on machines to investigate these bistabilities and/or tune gains to meet targeted productivity levels with active damping is difficult due to the destructive nature of chatter. This paper hence discusses the use of a hardware-in-the-loop (HiL) simulator to emulate bistabilities and to serve as a test bench for gain tuning in the presence of bistabilities. The HiL simulator has a hardware layer comprising a flexure representing a flexible workpiece, and two actuators. One emulates the cutting force calculated in real-time in the software layer. Another serves as the active damper operating with a velocity feedback control law. Bistabilities for three different nonlinear force models are experimentally illustrated on this HiL simulator. We show that active damping can stabilize these bistable regions. Since the width of the bistable regions depend on the nonlinear force characteristics, our investigations reveal that gains must be tuned for the force model and the static chip thickness under consideration. These results are useful and can instruct active damping strategies during more realistic cutting processes with nonlinear force characteristics that exhibit conditional instabilities in the presence of strong perturbations.