Abstract
M.Ing.
Shaft encoders, are devices generally used in speed and position control applications
for sensing position of rotational and linearly moving objects.
Most conventional shaft encoders operate on the principle of reading encoded
information off a disk, which is fitted to the shaft or using resolver units. Problems are often
experienced in fitting high-resolution encoder disks or resolvers to small motors or moving
objects. The resolution of an encoder system, using an encoded disk, is a function of the
maximum number of slots that are cut in the disk. If the disk itself has to be very small,
physical constraints limit the accuracy of the encoder.
However, any machined surface will have characteristic marks on it and it is
believed that these marks can be used to obtain a unique analogue signature for the shaft.
This analogue signature can then be used to gain further knowledge about the rotational
speed and possibly the position of the shaft. This project was an investigation into the
possibility of gaining accurate, absolute positional information for a rotating shaft.
An optical source- and detector system is used in the process of observing the
irregularities on the shaft surface. This signal data is gathered and processed using a
technique, which compensates for variation in shaft speed during the period in which data
is sampled. The entire algorithm, which calculates the absolute position of the shaft from
the data gathered, was implemented on a digital signal processor board.
Results were evaluated and the maximum non-linearity of the test system was found
to vary between 1.8% and 3.7% of full-scale deflection. The resolution of the sensor was
between 1.5 and 2.5 degrees with hysteresis between 1% and 1.5% of full scale deflection.
The test system allows for rotation in any direction.
Possible problems that might be encountered with this type of encoder system are
pointed out in this report. Future developments and possible improvements are also
discussed.