Encoders - How do they work?

 I will explain the term Quadrature a little later. This is a type of Incremental encoder, so I should first talk a little about how incremental encoders work.

Most encoders are based on a wheel of some sort. It might be made of some opaque material with a series of holes around the edge, or perhaps some transparent material with a series of opaque regions separated by transparent regions. Or it might be a printed circuit board with specially shaped traces to provide several different channels of electrical contact.

As the wheel turns, some sort of sensor, optical or electrical, marks the presence or absence of signal, and converts that information to some usable form. What is happening is that a mechanical device is generating digital data, which can be processed to give us the information we want.

                                   http://frontrangerobotics.org/Jan05/EncoderForRMS.jpg

In this picture, we see a code wheel and an optical sensor. It can detect the presence of the opaque sections of the encoder wheel, or the "see-through" areas. Sensors like this are used a lot in printers, to signal the presence/absence of a sheet of paper. I'll be showing you later how I used some to build my own encoder! This is an example of an Incremental encoder.

                                            http://en.wikipedia.org/wiki/Rotary_encoder

In this picture, we see a 13 track Absolute encoder. There are 13 concentric tracks of conductive areas on a non-conducive disk. 13 small metal spring contacts will ride on each of those 13 rings

and generate electrically a binary bit. All 13 of those bits will be used to generate a binary number,

that corresponds to one and only one position on the wheel.  And notice that the tracks don't go all the way around. We don't WANT an absolute encoder disk to turn more than one revolution.

We connect that wheel in some manner to something we want to monitor, like a rotating wheel, or the stem of a valve. Now, in the case of the tank level gauge, we are converting the linear motion of the float, riding on the surface of the liquid, into a measurement of distance.

Our little encoder wheel is revolving, being driven by a gearing system tied to the float by a flexible metal tape. Is it a rotary encoder? Or a linear encoder? Note there is a distinction made between how the encoder works and what sort of measurement is being made.

Also, consider that the encoder used in this tank level gauge is Absolute...it generates a unique digital pattern for each position on the wheel. That should tell you that we have to scale one revolution of this encoder wheel to the entire length that the float can travel, because if the wheel turns more than 360 degrees, we'd be generating these unique codes twice. This is done mechanically, with gearing.