| Shaft encoders are rotating
disks with holes in them.
Light shines through the holes and is detected by a photosensitive device or devices. These convert pulses of light into electrical pulses. The pulses can be counted to determine how far the disk has rotated. The speed of the disk in RPM can be determined by counting the number of pulses each second and dividing the result by the number of windows to get revs per second. Multiply this by 60 to get
revs per minute.
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The digital shaft encoder is divided into 16 equal angles. Each angle has 4 sectors. Some sectors are transparent allowing light to shine through. Other sectors are opaque to light. The angle with 4 opaque sectors represents binary 0000. The next angle clockwise has 3 opaques and 1 transparent representing 0001 binary. The highest number is the one with 4 transparent sectors representing 1111 binary (15 denary). Four photosensitive devices detect the position of the disk giving an output in binary code. A more accurate output can be obtained by using the Gray code.
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Copyright Graham Knott 2004