Incremental rotary encoder

An incremental rotary encoder also known as a relative rotary
encoder or a quadrate encoder, has two outputs called quadrate outputs. They can be either mechanical or optical. In
the optical type there are two gray coded tracks, while the
mechanical type has two contacts that are actuated by cams on
the rotating shaft. The mechanical types requires denouncing and
are typically used as digital potentiometers on equipment
including consumer devices. Most modern home and car stereos use
mechanical rotary encoders for volume. Due to the fact the
mechanical switches require denouncing, the mechanical type are
limited in the rotational speeds they can handle. The
incremental rotary encoder is the most widely used of all rotary
encoders due to its low cost: only two sensors are required.

The fact that incremental encoders use only two sensors does
not compromise their accuracy. One can find in the market
incremental encoders with up to 10,000 counts per revolution, or
more.

There can be an optional third output: reference, which
happens once every turn. This is used when there is the need of
an absolute reference, such as positioning systems.

The optical type is used when higher RPM’s are encountered or
a higher degree of precision is required.

Incremental encoders are used to track motion and can be used
to determine position and velocity. This can be either linear or
rotary motion. Because the direction can be determined, very
accurate measurements can be made.

They employ two outputs called A & B which are called
quadrate outputs as they are 90 degrees out of phase.

The state diagram:

The two output wave forms are 90 degrees out of phase, which is
all that the quadrate term means. These signals are decoded to
produce a count up pulse or a count down pulse. For decoding in
software, the A & B outputs are read by software, either via an
interrupt on any edge or polling, and the above table is used to
decode the direction. For example if the last value was 00 and
the current value is 01, the device has moved one half step in
the clockwise direction. The mechanical types would be denounced
first by requiring that the same (valid) value be read a certain
number of times before recognizing a state change.If the
encoder is turning too fast, an invalid transition may occur,
such as 00->11. There is no way to know which way the encoder
turned; if it was 00->01->11, or 00->10->11.