Calibration and error definition for rotary motion instrumentation using an incremental motion encoder (IME).
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 (!ME), which is based on the time interpolation of the digital
signals produced by an optical encoder. Experiments have shown that the !ME 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 1ME. However, further work was needed to assess the accuracy of the
measurements, to analyse the principles of the 1ME, 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 1ME
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
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
1ME 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 1ME.
An 1ME 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.