Year

2008

Degree Name

Doctor of Philosophy

Department

School of Electrical, Computer & Telecommunications Engineering - Faculty of Informatics

Abstract

The precision control of linear feed axes in machine tools is examined in this thesis. Al¬though high precision in machining has been a focal point for engineers for over 200 years, the traditional solutions have often been based on complex mechanical designs. In this thesis, two aspects of feed axis controller design are examined: i) the use of appropriate mathematical models and ii) the significance of three of the most common performance limiting factors that have traditionally affected precision in linear feed axes. The three particular performance limiting factors considered are: i) dynamic stiffness, ii) torsional vibrations and iii) backlash. The most effective way of obtaining knowledge about a control system is through appro¬priate mathematical modelling. A new two-body model for a simple motor-transmission¬load system is presented in this thesis. This new model is shown to provide a more accurate representation of both the total inertia and lowest natural frequency of a system, when compared with the two-body model that is traditionally used by researchers and system designers. A new model to represent backlash in a two-body system is also pre¬sented. These new models are then extended to provide accurate mathematical models of four common linear feed axis drive configurations: i) a rotary motor driving a rack and pinion transmission, ii) a rotary motor directly driving a ballscrew transmission, iii) a rotary motor driving a ballscrew transmission via a synchronous belt, and iv) a linear motor directly driving the axis. Different control solutions to the problems of dynamic stiffness, torsional vibrations and backlash are examined in this thesis, with each controller implemented on specially con¬structed test-beds. An approach using Quantitative Feedback Theory (QFT) is presented xxi for systems with inherently low dynamic stiffness. This QFT approach is shown to pro¬vide a transparent design process, which results in high dynamic stiffness. Different controllers for torsional vibrations are compared both theoretically and experimentally, with many previously published solutions shown to be theoretically equivalent. A new backlash controller is also presented, which is shown experimentally to provide dynamic stability and good tracking performance at both high and low velocities. The importance of treating these performance limiting factors simultaneously is also ad¬dressed in this thesis, with the control solutions developed to address some factors shown to also affect the other factors. The QFT approach is shown to provide a suitable inte¬grated design process, where the implications of any compromises, on the control of each factor, are clearly visible.

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.