Thursday, September 13, 2018

Pneumatic Signal Transmission

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.
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:
Current to Pressure transducer

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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