Working principle of I/P converter

The purpose of any “I/P” transducer is to convert an electrical signal into a corresponding pneumatic signal. In most cases, this means an input of 4-20 mA DC and an output of 3-15 PSI, but alternative ranges do exist.

An example of an I/P transducer manufactured by Foxboro is the model E69, shown here:

Two pressure gauges indicate supply and output pressure, respectively. Wires convey the 4-20
mA electrical signal into the coil unit inside the transducer.

A view with the cover removed shows the balancing mechanism used to generate a pneumatic
pressure signal from the electric current input. The baffle/nozzle may be seen at the left of the
mechanism, the nozzle located at the end of a bent tube, facing the flat baffle on the surface of the
circular coil unit:

As electric current passes through the coil, it produces a magnetic field which reacts against a permanent magnet’s field to generate a torque. This torque causes the coil to rotate counterclockwise (as viewed in the picture), with the baffle connected to the rotating assembly. Thus, the baffle moves like the needle of an analog electric meter movement in response to current: the more current through the coil, the more the coil assembly moves (and the baffle moves with it). The nozzle faces this baffle, so when the baffle begins to move toward the nozzle, backpressure within the nozzle rises. This rising pressure is amplified by the relay, with the output pressure applied to a bellows. As the bellows expands, it draws the nozzle away from the advancing baffle, achieving balance by matching one motion (the baffle’s) with another motion (the nozzle’s). In other words, the nozzle “backs away” as the baffle “advances toward:” the motion of one is matched by the motion of the other, making this a motion-balance instrument.

A closer view shows the baffle and nozzle in detail:

Increased current through the wire coil causes the baffle to move toward the right (as pictured) toward the nozzle. The nozzle in response backs away (also to the right) to hold the baffle/nozzle gap constant.

Interestingly the model E69 transducer employs the same pneumatic amplifying relay used in
virtually every Foxboro pneumatic instrument:

This amplifying relay makes the system more responsive than it would be otherwise, increasing sensitivity and precision.

As in all instruments, the zero adjustment works by adding or subtracting a quantity, while the span adjustment works by multiplying or dividing a quantity. In the Foxboro model E69 transducer, the quantity in question is motion, since this is a motion-balance mechanism. The zero adjustment adds or subtracts motion by offsetting the position of the nozzle closer to or farther away from the baffle. A close-up photograph of the zero adjustment screw shows it pressing against a tab to rotate the mounting baseplate upon which the coil unit is fixed. Rotating this baseplate add or subtracts angular displacement to/from the baffle’s motion:

The span adjustment consists of changing the position of the nozzle relative to the baffle’s center of rotation, so that a given amount of rotation equates to a different amount of balancing motion required of the nozzle. This adjustment consists of a pair of nuts locking the base of the bellows unit at a fixed distance from the baffle’s center of rotation. Changing this distance alters the effective radius of the baffle as it swings around its center, therefore altering the gain (or span) of the motionbalance system: