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Linear encoders have been refined so that they are very useful. This type of encoder operates similarly to the rotary absolute encoder.
A linear encoder has two identical rectangular pieces of glass that are both etched with opaque and transparent segments. One of the pieces of glass is fixed, and the other moves by a sliding arm that is attached to the movable part of a machine or robot. When the robot or machine moves, the arm moves the sliding piece of glass past the fixed piece of glass. At each point along its movement, the opaque and transparent segments of glass will create unique light patterns (on and off segments) that are decoded into a binary number that indicates a position. The advantage of this type of encoder is that the size of the glass plates will be the same size as the total distance of the machine travel. Hence the robot or machine will know precisely where it's at every point along the travel distance, even if the machine is powered down and power is returned. Diagram 1 shows some linear encoders. This type of encoder has resolution down to 0.0000001 inch (0.1 microns).
Above: Diagram 1: Examples of a linear encoders, which
have a fixed piece of rectangular glass and a movable piece of glass.
The two pieces of glass lineup at every position to provide a unique pattern
of on and off bits that are converted into position.
Note that this type of encoder is specifically designed where the travel is linear rather than rotary. These types of encoders are necessary for precision machining, welding, and applications using lasers. The latest technology requires highly polished or finished surfaces that are held to very tight tolerances and older technology can't provide the amount of resolution required.
