Principle And Composition Of An I/P Transducer

Pneumatic instrumentation feels unfamiliar in a highly integrated electronic era, and people assume there is no way to analyse it. In fact the principle is the same as an electronic circuit. Pneumatic signal control is regulating a high-power compressed air flow, and the electro pneumatic transducer is developed from the classic pneumatic instrument principle.

The two core components

The core components of the instrument are the nozzle baffle mechanism and the pneumatic power amplifier. Everything else in the housing exists to support those two: to hold them in position, to adjust them and to keep dirt out of them.

Electromagnetic change becomes spatial change

The method of electrical conversion is to turn an electrical signal into a physical displacement of a mechanical part through electromagnetic conversion. That mechanical part is the baffle in the nozzle baffle mechanism.

What the baffle actually is

The baffle is a solid thin metal disc in the middle. The other side of the disc is designed as a thin metal coil structure, convex outside and hollow inside. The metal coil passes through a strong magnetic permanent magnet cylinder. The current in the coil is the 4-20 mA control signal from the control room. As the current value changes, the attraction of the strong magnet changes, and that causes a physical displacement of the disc baffle which is integrated with the metal coil.

From displacement to usable pressure

Displacing the baffle restricts or opens the air flow through the nozzle, which changes the nozzle back pressure. That back pressure is small and cannot drive anything on its own, which is what the pneumatic power amplifier is for: it turns a pilot pressure into an output with enough flow behind it to move a final control element.

Why this design keeps being used

It has very few moving parts, no electronics in the air path, and it can be adjusted with two screws on the front face. On the T1000 and T1500 patterns a thermistor circuit in series with the coil adds temperature compensation, which removes the main remaining source of drift.