# Encoder

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#### Claudio Pires

I've been familiar with process automation and now I'm facing machinery automation and, so, I've been introduced to an "unknown" device entitled encoder.
Could you send me helpful information and describe basic concepts concerning encoder functioning and its PLC programming interface?
Thanks a lot,
Claudio Pires.

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#### Steve Myres, PE

An encoder is essentially a device which produces a string of pulses according to some mechanical motion, to allow your instrument to understand the position, velocity, etc., of the motion.

In practice, the encoder is usually a glass disk with lines printed in a radial pattern, or a metal disk with slits, through which a photo eye is arranged to look. As the center shaft of the disk is rotated, the photo eye output is a square wave, with the frequency proportional to the angular velocity.

A couple of added features are the addition of a second photo eye out of phase with the first one by 90 degrees electrical, and a third photo eye that gives a signal once per revolution. The second eye gives a square wave output like the
first one, and by comparing the phase relationship between this and the first one, you can determine direction of rotation. This is called a "quadrature" signal. The other, once per revolution, signal, is usually called an index, reference, or home signal, and can be used to keep track of absolute position. The encoders I have described are sometimes called "incremental" encoders, and can only register differential movement from a known point.

There is another type of encoder called an "Absolute" encoder, and this type is similar in construction, but with more photo eyes, and a disk constructed in a binary system, so that any position on the disk is unique. Thus, the true
position is know immediately on powering up the encoder, rather than having to home first.

Encoders also sometimes output a sinusoid or other non-square wave signal, to increase effective resolution to less than one-fourth of the natural resolution of the encoder.

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#### david mertens

As some encoders generate up to 2048 pulses per revolution and two channels with information allowing to detect the direction in which the
motion occurs, special modules are often required in PLC's to connect the encoders. (frequency's up to 200kHz or more may be required, this
is not possible with standard digital inputs) Some encoders come with profibus interface or serial link to remove the need for such fast
counter cards, they give the actual speed or position or both directly on the bus. Good luck.