Page 323 - Sensors and Control Systems in Manufacturing

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S i x
Cha p te r

detects rotation of a shaft or an object. The shaft rotation information
can be fed back into a computer or controller mechanism for control-
ling the velocity or position of the shaft. Such a device has application
in robots and numerical control machine tools and for precision mea-
surements of strip advancement to generate a closed-loop feedback
actuation for displacement compensation.
The two classes of digital encoder sensors are:
• Encoder sensors yielding at the output a digital version of the
applied analog input. This class of encoder sensors includes
position encoders.
• Encoder sensors that rely on some physical oscillatory
phenomenon that is transduced by a conventional modulating
sensor. This class of sensors may require an electronic circuit
acting as a digital counter in order to yield a desired digital
output signal.

No sensors exist where the transduction process directly yields a
digital output. The usual process is to convert an analog input quan-
tity into a digital signal by means of a sensor without the requirement
to convert an analog voltage into its digital equivalent.

6.2.1 Position Encoder Sensors in Manufacturing
Position encoder sensors can be categorized as linear and angular
position encoder sensors. The optical encoder sensor can be either
incremental or absolute. The incremental types transmit a series of
voltages proportional to the angle of rotation of the shaft or object.
The control computer must know the previous position of the shaft
or object in order to calculate the new position. Absolute encoders
transmit a pattern of voltages that describes the position of the shaft
at any given time. The innermost ring reaches from dark to light
every 180°, next ring every 90°, the next every 45°, and so on, depend-
ing on the number of rings on the disk. The resulting bit pattern out-
put by the encoder reveals the exact angular position of the shaft or
object. For an absolute optical encoder disk that has eight rings and
eight LED sensors, and in turn provides 8-bit outputs, (10010110).
Table 6.1 shows how the angular position of the shaft or object can be
determined.
The incremental position encoder sensor suffers from three major
weaknesses:
• The information about the position is lost whenever the elec-
tric supply fails or the system is disconnected, and when
there are strong perturbations.
• The digital output, to be compatible with the input/output
peripherals of a computer, requires an up/down counter.