Pneumatic transmission
of signals has been used for a long time in industrial instrumentation systems,
and still find use in today’s plant processes. They find common use in
applications where electrical signals or sparks can ignite combustible materials.
Signal Conversion
You can also read: How Digital Communication is used in Industrial Control
A linear pneumatic amplifier or booster can be used to increase the pressure from a low-level pressure signal to a high-pressure signal for the actuator control.
Most plants
have replaced or reduced the usage of Pneumatic instruments in signal
transmission due to various reasons: Pneumatic transmission of signals over
long distances requires an excessively long settling time for modern processing
needs, when compared to electrical signal transmission. Pneumatic signal lines
are also bulky, inflexible and costly compared to electrical signal lines, and
they aren’t microprocessor compatible. As we move to the future their
application will remain in critical applications mentioned above, where
electrical signals or sparks may pose a danger.
Pneumatic
transmission pressures are standardized into the range: 3-15 psi (20 to
100 kPa). Zero is not used for the minimum of the ranges, since the low
pressures do not transmit well. The zero level can be used to detect system failure.
Signal Conversion
Both
Pneumatic and electrical signals are used to control actuators. Signal
conversion is required between low-level signals and high-energy control
signals for actuator and motor control.
Electrical control
signals can either be digital, analog voltage or analog current. In some
instances, it is necessary to convert electrical signals to pneumatic signals
for actuator control.
Example of a
current to pressure transducer is shown below:
The spring
tends to hold the flapper closed, giving a high pressure output (15 psi). When
the current is passed through the coil, the flapper moves towards the coil,
closing the air gap at the nozzle and increasing the output pressure. The
output air pressure is set to the maximum 3 psi by the set zero adjustment when
the current through the coil is 3 mA. Moving the nozzle along the flapper sets
the system gain and span.
You can also read: How Digital Communication is used in Industrial Control
A linear pneumatic amplifier or booster can be used to increase the pressure from a low-level pressure signal to a high-pressure signal for the actuator control.
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