Instrumentation techniques and improved control of stepper motor driven machinery

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Stout, A.J., 2000. Instrumentation techniques and improved control of stepper motor driven machinery. PhD, Nottingham Trent University.

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Abstract

Continuous path, multi-axis stepper motor driven machinery frequently suffers from performance limitations. This performance limitation manifests itself as microscopic rough motion. It has been hypothesised that rough motion present m stepper motor systems generally arises from the excitation of the machine dynamics through interpolation. The interaction of acceleration with interpolation severely compounds the problem. The 'art of stepper motor system design' suggests that motors typically having a torque capability of double the load requirement should be sufficient to ensure reliable operation. However when full account of interpolation and ramping are considered, yet larger motors may be required. Steiger has shown theoretically that the resulting rough motion of the machine, causes the limitation in performance through an increase in load being reflected back to the motor. The aim of the current research is to confirm the principal causes of this performance limitation through experimentation, and further, to develop control techniques to minimise the effect.

Building upon the work of Steiger, a test platform representing a single axis of a conventional CNC machine was built. Sensors in the form of incremental encoders where attached to the rig at all the motion transfer points. The sensors include a high-resolution linear encoder able to measure the movement of the end effector to a resolution of 0. 2 microns, whilst the shaft encoder mounted on the stepper motor can measure angular movement to a resolution of l/50''' of a full motor step. Experiments under a range of variable conditions have been closely examined in order to characterise the resulting continuous path motion of a stepper motor driven machine. A special purpose data acquisition system was enhanced to allow the synchronous capture of pulse timing data over multiple channels. Validation of the performance hypothesis mentioned above was obtained through the instrumentation of a production CNC machine provided by Pacer systems Ltd., a manufacturer of CNC machinery collaborating in this research.

The information obtained from the test rig, and later from the CNC machine substantiated the theoretical findings of Steiger. However other features of the mechanical system and electronic drives were shown to have an equally important effect on the overall performance. These factors have been examined in detail and ranked as to their significance in typical application.

Experimental and simulation work confirmed the validity of the approach suggested by Palmin concerning maximum torque utilisation for single axis systems. The experimental results supported the solution implemented, a solution to smooth the motion on the secondary axis. Palmin's principles are extended to the continuous path multi-axis case through a processor based velocity smoothing algorithm minimising system excitation and corresponding path errors. This solution was realised in the form of a novel inverse velocity filter algorithm implemented on a DSP. The research then continued towards developing a technique of measuring the true stepper motor's performance. This method of deducing the static and dynamic rotor position can form the basis of calculating the torque/speed graph for the stepper motor driven system under test. This method was used to measure the effectiveness of the filtering solution.

Thus the significant performance limitations of continuous path stepper motor driven machinery have been demonstrated. These experiments concerning the motion of CNC machinery have resulted in new knowledge being developed within the field of engineering. The work has successfully shown the validity of both Steiger and Palmin. Steps to overcome the performance limitation have led to the development of a novel inverse velocity control algorithm. This solution was realised in hardware and reduces the excitation of the machine dynamics significantly. The research has verified the path following accuracy of the solution through step error measurement demonstrating increased performance together with increased accuracy. To summarise the research has shown that the engineer can achieve a higher level of performance in terms of path accuracy and top speed if a technique such as the one presented is utilised in the control system.

Item Type:

Thesis

Creators:

Stout, A.J.

Date:

2000

ISBN:

9781369313161

Identifiers: