Encoder is a device which is made by logical gates for selection of input to proceed in operation through output. There are various ways to find which input is selected but the best way is by using output results.
If output is ( 0,0) then Y0 is selected and operation is done for Y0. For Output (0,1), Y1 is selected. For Output (1,0), Y2 is selected and for output (1,1), Y3 is selected as shown in figure below:
The picture below shows priority encoder where two inputs are applied but only that input is selected which has higher value order. The outputs and inputs are shown as shown below…
But in Industrial automation we give priority on optical encoder disk. Encoders use rotating disk with optical windows. The encoder contains an optical disk with fine windows etched into it. Light from emitters passes through the openings in the disk to detectors. As the encoder shaft is rotated, the light beams are broken. The encoder shown here is quadrature encode.
There are two fundamental types of encoders; Absolute encoder and Incremental encoder.
An absolute encoder will measure position of shaft for single rotation. The same shaft angle will always produce the same reading. The output is normally a binary or grey code number.
An incremental or relative encoder will output two pulses that can be used to determine displacement. Logic circuits or software is used to determine displacement. Logic circuits or software is used to determine direction of rotation, and count pulses to determine the displacement. The velocity can be determined by measuring the time between pulses to determine displacement. The velocity can be determined by measuring time between pulses.
The encoder disk has two rings, the outer ring is most significant digit of encoder, the inner ring is least significant digit. The incremental encoder has two rings, with one ring rotated a few degrees ahead of the other, but otherwise its same. Both rings detect positions to the quarter of disk. To add accuracy in absolute encoder more rings, emitter and detectors must be added to the disk. To add accuracy in incremental encoder we only should add windows to the existing two rings. Typical encoders will have from 2 to thousands of windows per ring.
When using absolute encoders, the position during a single rotation is measured directly. If the encoders rotate multiple times then the total number of rotations must be counted separately.
When using relative or incremental encoder, the distance of rotation is determined by counting the pulses from the one rings. If the encoder only rotates in one direction then a simple count of pulses from one ring will determine the total distance. If the encoder can rotate in both directions a second ring must be used to determine when to subtract pulses. The signals are set up so that one is out of phase with other. Notice that for different directions of rotation, input B either leads or lags A.
Popular encoders and its signaling used in automation industry is shown below:
The below example is of HP HEDS-5505-A14 relative or incremental encoder. Check its properties in chart below.
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