Accurate tracking control of a linear fast tool servo unit for noncircular cutting

In this paper, an accurate position tracking control scheme is proposed for a moving-coil-type linear DC motor driven fast tool servo unit for noncircular cutting application. A sliding mode tracking controller is designed to ensure the system has a fast tracking characteristic to the position command. Moreover a disturbance observer is used to estimate and compensate exogenous disturbance to improve robustness and stability of the fast tool servo unit. Because the reference input is approximately periodic in noncircular cutting process. Therefore, the periodic error can he further reduced by augmenting an iterative learning controller to the existing sliding mode controller for repetitive position tracking. The experimental results of noncircular cutting adopting the proposed control method, including tracking performance and robustness of the system, are much improved.

I. Introduction

It is well-known that many important products have noncircular profile, such as the profile of an engine piston, disk cam, multilateral axis, and so on. Noncircular turning is a single-point cutting process that produces noncircular cross section shape by synchronizing the reciprocating motion of the linear fast tool servo (FTS) with the spindle rotation. Traditional lathes are not suitable for cutting a noncircular profile because of their low system bandwidth. FTS technology can enable precisely manufacturing complicated surfaces with high accuracy requirement. In this paper a FTS unit based on moving-coil-type linear DC motor (LDM) is proposed to drive the cutting tool for machining noncircular workpieces on a lathe.