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6.5
Encoders
Section 6 — Motion Control Tutorial
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The home search routine is a very important procedure for the posi-
tioning accuracy of the entire system and it requires full attention
from the controller. Do not interrupt or send other commands during
execution, unless it is for emergency purposes.
PID closed-loop motion control requires a position sensor. The most widely
used technology by far is the incremental encoder.
The main characteristic of an incremental encoder is that it has a 2-bit gray
code output, more commonly known as quadrature output (Figure 6.5-1).
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2
3
A
B
Figure 6.5-1 — Encoder Quadrature Output
The output has two signals, commonly known as channel A and channel B.
Some encoders have analog outputs (sine - cosine signals) but the digital
type are more widely used. Both channels have a 50% duty cycle and are
out of phase by 90°. Using both phases and an appropriate decoder, a
motion controller can identify four different areas within one encoder
cycle. This type of decoding is called ×4 (or quadrature decoding), meaning
that the encoder resolution is multiplied by 4. For example, an encoder
with 10µm phase period can offer a 2.5µm resolution when used with a ×4
type decoder.
Physically, an encoder has two parts: a scale and a read head. The scale
is an array of precision placed marks that are read by the head. The most
commonly used encoders, optical encoders, have a scale made out of a
series of transparent and opaque lines placed on a glass substrate or
etched in a thin metal sheet (Figure 6.5-2).
CAUTION
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