Pneumatic controllers signal transmission

Signal transmission limitations of pneumatic control systems made it necessary to limit the distance between the control house and the transmitter or control valve. As a result, early control houses were located within or at the periphery of the process unit. 

Transmission lines in process control systems rarely make any significant contribution to the overall loop characteristics. Where signals are transmitted electrically, there is no detectable signal attenuation for any frequencies characteristic of the process components, and even for pneumatic transmission lines as long as 200 feet there is little loss of signal. Transmission lines have distributed properties, but according to Bradner (B3) who has studied pneumatic transmission lines extensively, they can be approximated as second-order systems. 

Transmission lines used to carry the measurement signal from the sensor to the controller and the control signal from the controller to the final control element. These lines can be either pneumatic (compressed air or liquid) or electrical. 

PLC (programmable logic controller), 482 PLS, 424 pneumatic controller, 135 control valve, 157 instrument, 135 signal transmission, 162 poles, 92,326... 

Differential pressure transmitters (or DP cells) are widely used in conjunction with any sensor that produces a measurement in the form of a pressure differential (e.g. orifice plate, venturi meter, flow nozzle, etc.). This pressure differential is converted by the DP cell into a signal suitable for transmission to a local controller and/or to the control room. DP cells are often required to sense small differences between large pressures and to interface with difficult process fluids. Devices are available that provide pneumatic, electrical or mechanical outputs. 

TRANSDUCER - The means by which the controller converts the signal from the sensing device into the means necessary to have the appropriate effect on the controlled device. For example, a change in air pressure in the pneumatic transmission piping. 

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