Working principle of electric to pneumatic converter
Electro-pneumatic transmitters are also called converters and transducers. They form the link between electric measurements and pneumatic control system. They also convert electronic controller outputs into air pressure for the operation of pneumatic valves.
The figure above illustrates one of these converters and also with which it is commonly combined. The input is usually a DC current in the range of 1 to 5, 4 to 20, 10 to 50 milliamps. Typically, a 4-20 mA input is converted to 3-15psi output. An Alnico permanent magnet creates a field that passes through the steel body of the transmitter and across a small air gap to the pole piece. A multi-turn, flexure- mounted voice coil is suspended in the air gap. The input current flows through the coil creating an electromagnetic force that tends to repel the coil and thus converts the current signal into a mechanical force.
Since the total force obtained in a typical voice coil motor with such small current inputs is only on the order of some ounces, thus the use of reaction nozzle is employed to convert the force into a pneumatic pressure. In this circuit the air supply flows through a restriction and out the detector nozzle. The reaction of the air jet as it impinges against the nozzle seat supplies the counter balancing force to the voice coil motor. The nozzle back pressure is the transmitted output pressure.
In order to make the transmitter insensitive to vibration, the voice coil is integrally mounted to a float submerged in silicone oil. The float is sized so that its buoyant force equals the weight of assembly, leaving a zero net force.
Zero is adjusted by changing a leaf-spring force. Span is adjusted by turning the range-adjusting screw to change the gap between the screw and the magnet, thus shunting some of the magnet field away from the pole piece.