Specifications

Even though the position transducers are the most critical
parts in a motion control system, many times they are left
towards the end to be designed in. A control design engineer
can save himself a lot of headache by considering the transduc-
er features and trade-offs at the beginning of the project. The
following discusses various types of position transducers and
the factors to consider when making a selection.
Many industrial control systems need position and speed feed-
back. Until a few years ago, tachometers provided the speed
and limit switches provided the position information.
However, with new requirements of higher accuracies, faster
machine speeds, and greater reliability combined with techno-
logical breakthrough in the field of electronics, a variety of
new designs of position transducers emerged. These transduc-
ers made it possible to know the machine position at all times,
rather than waiting for a limit switch to give position indica-
tion at a predetermined point. This permitted faster machine
operation and increased throughputs.
In the initial stages, the position transducers consisted of
potentiometers, brush encoders, magnetic encoders and rarely
optical encoders and resolvers. Each device had certain limita-
tions. The potentiometers and magnetic encoders had limited
resolution. The brush encoders required frequent maintenance.
The optical encoders used incandescent lamps, which were
large in size and had limited life expectancy. The resolvers
could offer better resolution and accuracy, but were very
expensive due to the decoding electronics required.
The recent technological developments have brought some
improvements in the initial models. Today optical encoders and
resolvers are more commonly used in industry. The magnetic
and magnetoresistive encoders find applications less frequently.
We will focus on two of the more popular types of position
transducers: Optical Encoders and Resolvers.
Optical encoders and resolvers are available in two major cate-
gories: Absolute and Incremental. The incremental encoder,
when it rotates, generates pulses, which are counted to give
position information relative to a known point, whereas an
absolute encoder provides a unique value at each position and
retains actual shaft position even if power fails. Multi-turn
units with built-in gear trains are available for linear applica-
tion where it takes several revolutions of the encoder shaft to
complete one machine cycle.
Optical Encoders
The Optical Encoders typically consist of a rotating and a sta-
tionary member. The rotor is usually a metal, glass, or a plastic
disc mounted on the encoder shaft. The disc has some kind of
optical pattern, which is electronically decoded to generate
position information. The rotor disc in absolute optical
encoder uses opaque and transparent segments arranged in a
gray-code pattern. The stator has corresponding pairs of LEDs
and phototransistors arranged so that the LED light shines
through the transparent sections of the rotor disc and received
by phototransistors on the other side. See figure below.
Depending upon the shaft position, the phototransistor output
is modulated in a gray-code pattern, which can be converted
internally to binary or BCD. Typically CMOS, TTL-, PNP-,
and NPN-type outputs with 8- or 10-bit Gray-code, binary, or
BCD formats are available.
The incremental optical encoders (figure 1) use a much sim-
pler disc pattern. This slotted rotor disc alternately interrupts
the light beam between the LED transmitter-receiver pair and
thus produces a pulse output. The number of pulses depends
on the number of slots on the disc. The pulses are then fed to
a counter, where they are counted to give position informa-
tion. The pulse rate indicates shaft speed. An additional LED
pair can also determine the direction of rotation. Some mod-
ules also provide a marker pulse output, which is generated
once every revolution at a fixed shaft position and can be used
to mark a zero reference point. Many different pulse configu-
rations are available, but the most commonly known are the
quadrature encoders, where two square wave pulses 90 degrees
apart from each other are generated.
SELECTING THE RIGHT POSITION
TRANSDUCER FOR YOUR APPLICATION
A-3
800-TEC-ENGR
www.avg.net
Position Transducers/Sensors
SECTION
A