Hardware Reference
In-Depth Information
position, and pressure. The data returned from the sensors together with con-
trol inputs from the operator (where appropriate) will subsequently be used to
determine the behaviour of the system.
Any practical industrial process control system will involve the use of a
number of devices for sensing a variety of physical parameters. The choice of
sensor will be governed by a number of factors including accuracy, resolution,
cost, and physical size. The following table covers the range of sensors and
inputs most commonly encountered in industrial process control systems. The
list is not exhaustive and details of other types of sensor can be found in most
texts devoted to measurement, instrumentation, and control systems.
Physical
Type of
parameter
sensor
Notes
Angular
Resistive rotary
Rotary track potentiometer with linear law produces
position
position sensor*
analogue voltage proportional to angular position.
Limited angular range. Analogue input port required.
Optical shaft
Encoded disk interposed between optical transmitter and
encoder*
receiver (infra-red LED and photodiode or phototransistor).
Usually requires signal conditioning based on operational
amplifiers. Digital input port required.
Differential
Transformer with fixed E-laminations and pivoted I-laminations
transformer
acting as a moving armature. AC source, rectifier, and filter
required. Analogue input port required.
Angular
Tachogenerator
Small DC generator with linear output characteristics. Analogue
velocity
output voltage proportional to shaft speed. Requires an
analogue input port.
Toothed rotor
Magnetic pick-up responds to the movement of a toothed ferrous
tachometer
disk. May require signal conditioning (typically an operational
amplifier and a Schmitt input logic gate). Some sensors contain
circuitry to provide TTL-compatible outputs. The pulse repetition
frequency of the output is proportional to the angular velocity.
Digital input port required.
Optical shaft
Encoded disk interposed between optical transmitter and receiver
encoder*
(infra-red LED and photodiode or phototransistor). Usually
requires signal conditioning based on operational amplifiers.
Digital input port required.
Flow
Rotating vane
Turbine rotor driven by fluid. Turbine interrupts infra-red beam.
flow sensor*
Pulse repetition frequency of output is proportional to flow rate.
A counter/timer chip can be used to minimize software
requirements. Digital input port required.
Linear
Resistive linear
Linear track potentiometer with linear law produces analogue
position
position sensor*
voltage proportional to linear position. Limited linear range.
Analogue input port required.
(
continued
)
