Analytical Instrumentation Instrumentation

Electropneumatic converter – Construction, Working

Electro-pneumatic is a pneumatic control system where air pressure and direction of the valve are controlled by an electrical current.

The electro-pneumatic converter is a device which provides an interface between the electronic and pneumatic systems either to convert current into a proportional air pressure or to convert current into a valve setting by controlling the air supply to a diaphragm actuator

These devices are suitable as an intermediate element between electrical and pneumatic measuring devices or between electrical control devices and pneumatic control valves.

Construction of Electro Pneumatic converter

The system consists of a coil assembly mounted on cross-flexures allowing it to rotate relatively freely around its axis but avoiding any axial movement.

In a powerful magnetic field, the coil is suspended so that it is rotated by a current passing through the coil. In the case of the electro-pneumatic converter, a flapper/nozzle system senses this rotation.

The instrument’s pneumatic section consists of a feedback bellow and a bias spring applying a force to a lever pivoting at one end through a flexure and carrying a nozzle at the free end.

Working of the Electro Pneumatic converter:

The nozzle is supplied via a restrictor and its back pressure is applied to a pneumatic relay whose output is applied to the bellows of feedback and is also used as the unit’s pneumatic output.

An increase in the input current causes the coil to rotate, and in so doing moves the flapper towards the nozzle so that the back pressure is increased.

The relative position of the flapper and nozzle is adjusted with the current corresponding to the lower range value (e.g. 4 mA for a 4 to 20 mA converter) so that the pneumatic output pressure is equal to the required value (e.g. 20 kPa for a 20 to 100 kPa system).

The change is amplified by the relay and applied to the feedback bellows so that the lever moves the nozzle away from the flapper until a new balance position is achieved.

When the coil current reaches the upper-range value (e.g., 20 mA) the pneumatic output signal reaches its corresponding upper-range value (e.g., 100 kPa).

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