A
quadrature encoder is a way of building a
shaft encoder capable of
determining speed and direction. They have two
channels, each of which is, at
any moment, a
logical one or a
logical zero. Thus, the two channels divide
a
full circle into four pieces, hence
quadrature.
The output of a quadrature encoder looks something like this:
1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4
+ +---+ +---+ +---+ +---
A: | | | | | | | |
+---+ +---+ +---+ +---+
--+ +---+ +---+ +---+ +-
B: | | | | | | | |
+---+ +---+ +---+ +---+
Thus, the
circuit/
program tending to the encoder must notice transistions
from one state to another. When it sees a 1->2 transition, for instance, it
knows that the shaft has just rotated forward; when it sees a 3->2 transition,
for instance, it knows the shaft has rotated backwards. What it does when it
encounters, say, a 4->2 transition is left as an
exercise for the reader.