Design and control of a long stroke fast tool servo

Abstract

In this thesis, I detail the design and control of a linear long stroke fast tool servo (FTS) with integral balance mass. The long stroke fast tool servo consists of an air bearing stage driven by a unique three phase oil cooled linear motor. The linear FTS has a travel range of 25 mm and is capable of 100 m/s² accelerations. The FTS is mounted to a T-base diamond turning machine (DTM). The FTS is attached to a hydrostatic bearing supported in-feed stage which is driven by a second linear motor. The in-feed stage is allowed to move in response to the FTS actuation forces and thus acts as an integral balance mass. We have developed a unique control structure to control the position of both the FTS and the reaction mass. The FTS controller employs a conventional lead-lag inner loop, an adaptive feedforward cancelation (AFC) outer loop, and command pre-shifting. For the FTS controller, the AFC resonators are placed in the forward path which creates infinite gain at the resonator frequency. The controller for the hydrostatic stage consists of a conventional lead-lag control inner-loop and a base acceleration feedback controller. The acceleration feedback controller consists of a high-pass filter, a double integrator for phase compensation, and an array of AFC resonators. For the base acceleration controller, the AFC resonators are placed in the feedback path and thus act as narrow-frequency notch filters. The notch filters allow the hydrostatic stage/balance mass to move freely at the commanded trajectory harmonics thus attenuating the forces introduced into the DTM. The AFC control loops are designed using a new loop shaping perspective for AFC control. In this thesis, we present two extensions to AFC control.

(cont.) The first extension called Oscillator Amplitude Control (OAC) is used to approximate the convergence characteristics of an AFC controller. The second extension termed Amplitude Modulated Adaptive Feedforward Cancelation (AMAFC) is designed to exactly cancel disturbances with a time varying amplitude.