Rotary encoders are very frequently used in industry to measure the angular displacement of rotating shafts. Typically an encoder consists of a disc, which is coupled to the shaft, in which there are a number of features arranged around the periphery.
A sensor is positioned to detect the features as they pass. As the disc rotates a signal is generated each time a feature passes the sensor, creating a flow of signals. If the disc has 100 features, 50 signals would signify that the shaft had performed half a revolution, and so on. The number of the signals measures the angular displacement of the shaft, while the frequency of the signals measures the rotational speed. There are several types of encoder, for instance those based on the optical sensing of slots in a disc, the magnetic sensing of stripes recorded on a drum, and those based on toothed wheels, but the principle on which they work is the same.
The advance achieved in the IME invention is to increase the number of detector units in a standard rotary encoder to a total of three. Signals from each detector are accurately time stamped, and all the signals are processed.
At a given shaft speed, a signal arriving early, or late at a detector shows that there is a mechanical displacement of the encoder wheel. The displacement tangential to the encoder wheel at the detector point is proportional to the time difference of the signals. With three detectors arranged around the encoder disc, the displacement can be calculated, and resolved into two axes.
The run out of shafts rotating at 20,000 rpm can be measured to plus or minus one micron. The system still works on shaft speeds as low as 15 rpm.
