The basic principle of the quadrature technique is that some measuring mechanism, such as magnetic coupling between coils or a pattern of metal slots and bars on a glass plate produce two sine or square waves as the scale moves past the reading head. The waves are arranged to be out of phase by 90 degrees. This means that as one wave is exactly at its peak, the other is exactly at its 50% level. This 90 degree relationship allows the direction of travel to be determined.
Some terms related to these encoders are not universally used or defined, and this can cause problems. The cycles of the waves should be referred to as cycles per revolution for shaft encoders, and cycles per inch (or mm) for the linear encoders. Now, where the confusion occurs is that when a quadrature encoder is read by a couner that 'counts all transitions', it will count 4 counts as the encoder moves through a full cycle of the pattern or wave. So, a 1000 cycle/rev. encoder will produce 4000 counts per revolution. Some people call this a 4000 count /rev encoder, which is still correct. But, if you abbreviate this as 4000 CPR, does the reader understand this as 4000 Counts, or 4000 Cycles?
Many encoders, especially the rotary type, have a third channel, in addition to the two quadrature channels, called the index channel. This generally produces a logic 'high' level for only a small portion of a rotation, maybe about 1/2 of a quadraure cycle, once per rotation. it can be used with a home position switch to accurately return a machine to a particular reference position.
A glossary of related terms -