| 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
transducer 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 feedback. Until a few years ago, tachometers
provided the speed and limit switches provided the position
information. However, with new requirements of higher accuracies,
fast machine speeds, and greater reliability combined with technological
breakthrough in the field of electronics, a variety of new designs of
position transducers emerged. These transducers made it possible
to know the machine position at all times, rather than waiting for a
limit switch to give position indication 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 limitations. 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.
Optical encoders and resolvers are
available in two major categories: Absolute and Incremental.
The incremental encoder, when it rotates, generates pulses, which are
counted to give position information relative to a know point, where as
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 application where it takes
several revolutions of the encoder shaft to complete one machine cycle. |
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Optical
Encoders
The Optical Encoders typically
consist of a rotating and a stationary 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.
Optical absolute encoders
use a coded disc to give shaft position in gray-code,
which is then converted to BCD or Binary.
The incremental optical encoders use a much simpler 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 information. The pulse rate indicates shaft speed.
An additional LED pair can also determine the direction of
rotation. Some modules 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
configurations are available, but the most commonly known are the
quadrature encoders, where two square wave pulses 90 degrees apart from
each other are generated. |
