Calibration and error definition for rotary motion instrumentation using an incremental motion encoder (IME)

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Hatiris, E., 2001. Calibration and error definition for rotary motion instrumentation using an incremental motion encoder (IME). PhD, Nottingham Trent University.

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Abstract

Condition based monitoring is widely used for the determination of the health of machines. The Nottingham Trent University Computing Department has developed a new system, the Incremental Motion Encoder (IME), which is based on the time interpolation of the digital signals produced by an optical encoder. Experiments have shown that the IME can be used as a condition based maintenance sensor as it is possible to detect rolling element defects, an unbalanced shaft and oil contamination of a bearing. The system uses a geometrically configured optical device to scan a precision encoder disc and Digital Signal Processing technology is used to interpret the signals. Previous work has demonstrated the qualitative usefulness of the IME. However, further work was needed to assess the accuracy of the measurements, to analyse the principles of the IME, to validate the performance of the existing device and to develop methods for error definition and error compensation.

Testing and experimentation on the existing experimental system have been carried out by the Candidate and an understanding gained of the device. The sources of error of the IME have been identified, which had not been quantified previously. Measuring and compensating for the three main sources of error, read head position, eccentricity of the encoder disc and encoder abnormalities are the three major tasks of the project. Modifications to the experimental rig have been developed in order to allow these tasks to be addressed.

The Candidate has developed three different types of techniques to measure the position error of the read heads. A pattern recognition method was developed and is successful for IME systems that use an encoder disc with significant grating line errors. A second method using Fast Fourier Transform (FFT) has been developed to exploit the fact that the difference in the phase angles, obtained using a FFT, gives the angle between the read head positions. The new experimental system is now able to obtain the angular position of the read heads by using the index grating line. The third method relies on the presence of the index grating line on the encoder disc which may not be present in all systems.

Eccentricity of disc centre relative to the centre of rotation affects the correct calculation of the angular position of the encoder disc. Algorithms have been developed by the Candidate in order to compensate for this type of error. Experimental results have shown that angular position error can be corrected successfully.

The Candidate has developed methods for detection of small abnormalities of the encoder disc by using a multiple averaging technique. Computational algorithms have been developed to correct the encoder disc abnormalities by using individual information from each read head, promising results have been obtained from the experimental IME.

An IME device can be tailored to fulfil the desired requirements of resolution, bandwidth and accuracy. A self calibration instrument can be developed by using the previously mentioned techniques in order to self calibrate and increase the accuracy and reliability of an IME's results.

Item Type:

Thesis

Creators:

Hatiris, E.

Date:

2001

ISBN:

9781369316117

Identifiers: