Specifications
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Incremental encoders are available in two basic output types, single
channel and quadrature, shown below.
OUTPUT A
OUTPUT B
INDEX Z
180
O
MINIMUM
CLOCKWISE ROTATION
OUTPUT A
A single channel encoder, often called a tachometer, is normally
used in systems that rotate in only one direction and require simple
position and velocity information. Quadrature encoders have dual
channels (A and B), phased 90 electrical degrees apart. These two
output signals determine the direction or rotation by detecting the
leading or lagging signal in their phase relationship. Quadrature
encoders provide very high speed bi-directional information for
very complex motion control applications.
An encoder is a sensing device that provides feedback from the physical
world—it converts motion to an electrical signal which can be read
by some type of control device, such as a counter or PLC. The control
device can then use that signal to control a conditional event, such as
activating a print head to create a mark at a specific location. Encoders
use different types of technologies to create a signal. Some common
encoder technologies are mechanical, magnetic, resistive and optical.
Currently, the most common technology employed by encoders is
optical. Encoders may produce either incremental or absolute signals.
Incremental signals provide a series of high and low waves which
indicate movement from one position to the next; there is no special
indication provided by the encoder to show the specific position, only
an indication that the position has changed. Absolute encoders, on
the other hand, use a unique “word” for each position, meaning that
an absolute encoder provides both the indication that the position has
changed and an indication of the absolute position of the encoder. Each
encoder type has its advantages, however, for the sake of this article,
our discussion will be limited to the most common type of encoder
used today—optical incremental encoders.
The inset diagram outlines the basic construction of an incremental
encoder. A beam of light emitted from an LED
passes through a transparent disk patterned
with opaque lines. The light beam is picked
up by a photodiode array, also known as a
photosensor. The photosensor responds to
the light beam, producing a sinusoidal wave
form, which is transformed into a square
wave or pulse train. This pulse signal is then
sent to the counter or controller which will
then send the signal to produce the desired
function. The diagram is for a typical rotary
encoder. Incremental encoders can provide
a once-per-revolution pulse (often called the
index, marker, or reference) that occurs at the
same mechanical point of the encoder shaft
revolution. This pulse is on a separate output
channel (Z) from the signal channel or quadrature outputs. The
index pulse is often used to position motion control applications to a
known mechanical reference.
Resolution is a term used to describe the Cycles Per Revolution (CPR)
for incremental encoders. Each incremental encoder has a defined
number of cycles that are generated for each 360
degree revolution of the shaft. These cycles are
monitored by a counter or motion controller and
converted to counts for position or velocity control.
The diagram below is how the whole encoder
comes together.
If you still have questions as to how an encoder
works in your specific application, please feel
free to call a Customer Service Representative for
Technical Support.
enCoder basiCs
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