Technical Briefs

Control-Oriented Modeling Requirements of a Direct-Drive Machine Tool Axis

[+] Author and Article Information
Michael A. Stephens

 ANCA Motion, 1 Bessemer Road, Bayswater North, Victoria 3153, Australia

Chris Manzie, Malcolm C. Good

Department of Mechanical Engineering,  The University of Melbourne, Victoria 3010, Australia

J. Dyn. Sys., Meas., Control 134(5), 054503 (Jun 05, 2012) (6 pages) doi:10.1115/1.4006216 History: Received November 22, 2010; Revised January 06, 2012; Published June 05, 2012; Online June 05, 2012

The high performance demands on commercial computer numerical control (CNC) machine tools have led to the widespread adoption of direct-drive servo axes. In industrial machines, where the workpiece is manipulated by the axis, the plant dynamics seen by the control system may vary widely between different workpieces. These changing plant dynamics have been observed to lead to limit-cycle behavior for a given controller. In such a situation, conventional modeling approximations used by practitioners may fail to predict the onset of instability for these axes. This work demonstrates the failure of conventional modeling approximations to predict the observed instability in an industrial CNC servo axis and investigates the model fidelity required to replicate the observations. This represents an important consideration when designing model-based controllers for direct-drive axes in CNC machines.

Copyright © 2012 by American Society of Mechanical Engineers
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Figure 1

Headstock servo axis of a tool and cutter grinding machine loaded with both small (left) and large (right) workpieces. The small workpiece is a 10 mm end mill, while the large workpiece is a 300 mm side-and-face cutter mounted to the headstock via a 32 mm arbor (shaft).

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Figure 2

Experimentally observed limit-cycle behavior during stationary operation of the axis loaded with the large workpiece. The position reference is the dashed line and the measured position is the solid line. The motor current saturation limits are also shown. The frequency of the observed oscillations is approximately 410 Hz.

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Figure 3

Machine tool servo drive control loop structure

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Figure 4

Finite element modal analysis of the servo drive axis loaded with the large workpiece

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Figure 5

Closed-loop poles of the motor position loop for the discrete model of the servo drive axis loaded with both small and large workpieces

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Figure 6

Closed-loop poles of the motor position loop for the discrete model of the servo drive axis loaded with both small and large workpieces—the model includes the fast sample-rate elements for (a) and (b), and additionally, a notch filter for (c) and (d)

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Figure 7

Friction model experimental data (dots) and model fit for both small (dashed line) and large (solid line) workpieces

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Figure 8

Root locus for closed position loop with default tuning for (a) and (b), and with default tuning and a notch filter for (c) and (d), for both small and large workpieces in each case. The loci show the effect of varying the effective viscous friction coefficient.




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