User guide

4
Avago Technologies Motion Control Encoders in Electrical Motor Systems: Design Guide
2.2 Basic Encoder Operations
2.2.1 Operations of a Transmissive Encoder
A transmissive encoder consists of an emitter, detector,
codewheel or codestrip. The emitter and detector are
located on the opposite sides of the codewheel or
codestrip. (See Diagram 1)
Diagram 1. Diagram of a transmissive encoder
A
Comparators
Index Processing
Circuitry
Photo
Diodes
Bar
Resistor
Emitter Section
Code
Wheel
Detector Section
V
cc
Ch. A
Ch. B
5
GND
Ch. I
Lens
LED
Window
Signal Processing
Circuitry
A
B
B
I
T
When light from the light source, is projected onto the
moving codewheel or codestrip, the bars act as a “gate”
that enables and then prevents the light penetration at
intervals. This sequential action creates moving shadows
that fall on the photodiodes located on the detector.
The photodiodes then generates output or signal
(See Diagram 2).
Diagram 2. Light on the photodiode
DECODE
A
B
LED
Lens
Codewheel
Photodiodes
Detector IC
2.2.2 Operations of a Reective Encoder
The basic elements of reective encoders are similar to
the transmissive encoder. In a reective encoder, the
emitter and detector are located on the same side of the
codewheel or codestrip (See Diagram 3).
Diagram 3. Reective encoder process
Emitter
Codewheel/Codestrip
Window
Bar
Detector
Processing
Circuitry
When light from the light source is projected onto the
moving codewheel or codestrip, the bars act to reect
or not reect the light, at intervals. This repeated action
creates moving shadows that fall on the photodiodes
located on the detector. The photodiodes then generates
output or signal.
It is important to note that the reective technology is
currently conned to Avago’s incremental optical encoders,
specically the AEDR series in modular form and housed
HEDR and HRPG (Rotary Pulse Generator) series.
2.2.3 Operations of a Magnetic Encoder
Magnetic encoders are a viable alternative to optical types
because they are inherently rugged and operate reliably
under shock and vibration, at high temperature, and in
contaminated and humid areas. Hall eect and magneto
resistive (MR) devices are two sensors commonly used
for these conditions. Motor speed and position accuracy
dictate which of the two is better suited for an application.
Avago’s magnetic encoder is based on the Hall sensing
technology. A diametrically magnetized magnet rotates
o-axis over a single or multiple pairs of linear Hall sensor
arrays. Magnetic ux variations are detected by the Hall
sensors and converted to analog sinusoidal voltage,
where there will be one sine wave per revolution. (The
Hall sensing principle allows for compact and miniature
encoders with small dimensions and high resolutions).
After signal processing and digitization, the absolute
information is available as a serial communication data
output. Thanks to the built-in compensation electronics,
the magnetic encoder is very robust against temperature
change as well as axial and radial shaft displacements.