Operating instructions

Principles of Rotary Encoders
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Introduction
In automation applications, rotary encoders are used as
sensors for angle, position, speed and acceleration. By using
spindles, gear racks, measuring wheels or cable pulls, linear
movements can also be monitored by a rotary encoder. Rotary
encoders convert a mechanical rotation value into an electrical
signal that can be processed by counters, tachometers, logic
controllers and industrial PCs.
Rotary encoders use a glass or plastic disc with alternating
transparent and opaque fields, with a light source on one
side and a light-sensitive sensor on the other. As the disc
rotates, the light source is alternately blocked and revealed
to the sensor. Whenever the light source hits the sensor, the
encoder transmits an electric pulse that can be interpreted
by a controller. The pulse ends when an opaque field on the
disc blocks the light source. Rotation of the disc results in a
square-wave pulse output.
Most rotary encoders use an infrared light-emitting diode as a
light source, and photodiodes or phototransistors as receivers.
If no other functions are added to the encoder, the only
output is a square wave that indicates that the disc is rotating.
The direction of rotation and absolute position cannot be
determined from a square wave output alone. Therefore,
additional components are added to many rotary encoders to
provide additional data about the rotation.
Types of Encoders
Incremental Rotary Encoders
Incremental rotary encoders supply a certain number of pulses
for each shaft revolution. Measuring the cycle duration, or
counting the number of pulses during a pre-determined unit of
time determines rotational speed. If the pulses are measured
after a reference point is added, the calculated value
represents a parameter for a scanned angle or the distance
covered.
Two-channel encoders (those with a phase shift of 90°) enable
the controller to determine the direction of rotation and can
enable bi-directional positioning. Three-channel incremental
encoders provide a “zero signal” for each revolution, giving a
fixed point of reference.
For more information, please refer to the section titled
“Operating Instructions for Incremental Rotary Encoders” on
pages 14-15.
Absolute Rotary Encoders
Absolute encoders provide a uniquely coded numerical value
for each shaft position. Absolute rotary encoders eliminate
the need for expensive input components in a positioning
application because they have built-in reference data. In
addition, reference runs after a power failure or when the
machine is switched off are not required because the encoder
provides the current position value immediately.
Single-turn absolute encoders divide the shaft into a defined
number of steps. The maximum resolution is 16 bits, which
means that up to 65,536 positions can be defined.
By using a multi-step gear, multi-turn absolute encoders not
only provide the angular position within a revolution, but also
the number of revolutions. Multi-turn encoders have a 14-bit
resolution to indicate the number of turns, which means that
up to 16,384 revolutions can be identified. Overall resolution
is 30 bits (16 bits per turn + 14 bits for the number of turns) or
1,073,741,824 measuring steps.
Parallel absolute encoders transmit the position value to
external analyzing electronics through multiple wires, one for
each bit.
In the case of serial absolute encoders, the output data can
be transmitted by means of standardized interfaces and
protocols. In the past, point-to-point wiring was used for
serial data; today, fieldbus systems are becoming increasingly
popular.
For more information on encoder operation, please refer to the
section titled “Operating Instructions for Absolute Encoders”
on page 16.
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