Pneumatic proportional control action and its Practical Automatic and manual mode

instrumentation

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What is a pneumatic system?

Pneumatic technology deals with the study of behaviour and applications of compressed air in manufacturing automation in particular. Pneumatic systems use air as the medium which is abundantly available and can be exhausted into the atmosphere after completion of the assigned task.

Proportional control action in Pneumatic system:

Most of the Pneumatic controller uses force balance technique. The input signal is exerted at one side of a beam by acting through the diaphragm, which is then counter-acted by the force exerted on the same beam by an output air pressure acting through a diaphragm, bellows, or bourdon tube. The self-balancing mechanical system “tries” to keep the beam motionless through an exact balancing of forces, the beam’s position precisely detected by a nozzle/baffle mechanism, shown in above figure.

When the process variable increases the baffle blocks the nozzle and increases the output pressure. Consider if it is a valve action is actuating when the PV increases the valve tends to close as the output pressure increases. The action can be reversed, all we would need to do is swap the pneumatic signal connections between the input bellows, so that the PV pressure was applied to the upper bellows and the SP pressure to the lower bellows

Automatic and manual modes:

The transfer between automatic and manual modes is a very important feature for any loop controller because it allows human operators to change the mode of the controller without introducing an unnecessary disturbance to the process being controlled.

In this particular pneumatic controller, bumpless auto/manual transfer is accomplished by the operator paying attention to the balance indicator revealing any air pressure difference between the output bellows and the output adjust pressure regulator.

When in automatic mode, a switch to manual mode involves adjusting the output regulator until the balance indicator registers zero pressure difference, then switching the transfer valve to the “manual” position.The controller output is then at the direct command of the output adjust pressure regulator, and will not respond to changes in either PV or SP.

“Bumplessly” switching back to automatic mode requires that the setpoint pressure regulator be adjusted until the balance indicator once again registers zero pressure difference, then switching the transfer valve to the “auto” position. The controller output will once again respond to changes in PV and SP.