Operating instructions

Principles of Rotary Encoders
14
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Operating Instructions for
Incremental Rotary Encoders
Incremental rotary encoders emit pulses as the shaft is
rotated, and the number of pulses is used to calculate angular
position. The resolution (Z) of an incremental encoder is the
number of pulses per revolution. The signal frequency is used
to determine the angular speed (ω) and the change in position
for a given period of time is used to calculate the angular
acceleration (α).
Rotational Direction Monitoring in
Incremental Rotary Encoders
In order to determine the direction of rotation of a movement,
the scanning principle is used on both channel A and channel
B. The direction of rotation can be determined by evaluating
the two signals, which are phase-shifted by 90 degrees.
In the first figure below (I cw), channel A precedes channel
B. This indicates clockwise rotation. II ccw shows counter-
clockwise rotation. The direction of rotation is determined by
viewing the encoder shaft head-on.
Depending on the type of incremental rotary encoder, channel
A or channel B is the leading channel. For more information,
refer to the encoder data sheet.
Zero Signal
Determining the frequency of the pulses is the job of the
controller, PLC or tachometer. The zero signal is a pulse that
occurs once per revolution at a fixed point and is transmitted
using a third channel (often called channel 0 or Z). The zero
signal is usually used as a reference signal for positioning.
The diagram below illustrates the output of a 3-channel
incremental rotary encoder.
Inverted Channels
Inverted signals are transmitted in addition to channels
A, B and Z to improve signal quality. Inverted signals are a
standard feature in RS-422 interfaces and are optional on
push-pull outputs.
The advantage of normal and inverted signal transmissions is
that filtering of unwanted signals is possible. If a noise pulse
occurs, it will be induced equally on all channels. Subtracting
the normal and inverted encoder signals from each other
eliminates the noise pulse. The figure below illustrates how
this is done.
Pulse Multiplication
Pulse multiplication is used to increase the number of
measuring steps or to reduce the output frequency of an
incremental rotary encoder. The signals of an incremental
rotary encoder can be doubled or quadrupled by linking
channels A and B.
For example, an application may need 20,000 measuring
steps for each revolution at a speed of 3000 RPM. If the
control unit (PLC, counter or tachometer) provides the
option to quadruple the signal, then an inexpensive rotary
encoder with 5000 pulses can be used. In addition, the output
frequency of the incremental rotary encoder is reduced.
• Without quadruple multiplication: 1 MHz (this frequency is
too high for most control units)
• With quadruple multiplication: 250 kHz
A
B
A
B
90˚
I cw
II ccw
A
B
Z
A
A
B
B
Z
Z
5 V
A
0 V
5 V
0 V
A
A
5 V
0 V
Signal cable
Inverted signal cable
Filtered signal
Interference