Electromechanical Pressure Transducers: Types & Operation
Electromechanical Pressure Elements
They use an elastic mechanical element combined with a transducer that generates an electrical signal. The pressure (P) produces a force on a surface, which causes deformation. This mechanical deformation is converted into an electrical quantity. The most used electrical characteristics are electrical resistance, electrical capacity, and electrical reluctance.
A) Electronic Transmitter Equilibrium Pressure Force
For measuring, the mechanical element exerts a force on a rigid transmitter bar. For each value of P, the bar takes a particular position and the displacement transducer is excited. An oscillator circuit feeds a magnetic unit, generating a force (q) that repositions the bar, achieving a balance of forces. This completes the feedback circuit; the output current varies in proportion to the pressure range of the process.
B) Magnetic Transducers
B1) Variable Inductance Transducer
The movement of a plunger within a coil increases the inductance of the coil almost proportionally to the metallic portion of the core contained within the coil winding. The coil is fed with an alternating current, and the inductance-generated electromotive force (EMF) opposes the supply EMF. Therefore, as the plunger moves within the coil, the current in the circuit is reduced by increasing the inductance EMF.
B2) Variable Reluctance Transducer
It is based on an electromagnet that creates a magnetic field within which an armature of magnetic material moves. It feeds on a constant magnetomotive force (MMF). Therefore, changing the position of the armature varies the reluctance and the magnetic flux. This variation gives rise to an induced current in the coil that is proportional to the degree of displacement of the mobile armature. They present high sensitivity to vibrations, average stability over time, and are sensitive to temperature.
C) Resistive Transducer
It consists of an elastic element (Bourdon tube or capsule) that varies the ohmic resistance of a potentiometer according to the pressure. The potentiometer may take the form of a single continuous thread or be rolled into a coil following a linear resistance value. The movement of the pressure element is transmitted to a mobile arm resting on an isolated precision potentiometer. This is connected to a Wheatstone bridge circuit.
D) Piezoelectric Transducers
Piezoelectric elements are crystalline materials that, when physically deformed by the action of pressure, generate an electrical signal. They are light elements, small in size, and of robust construction.
E) Capacitive Transducers
They are based on the variation in capacitance that occurs in a capacitor when one of its plates moves due to the application of pressure. The moving plate is diaphragm-shaped and is located between two fixed plates. Thus, there are two capacitors: one with a fixed reference capacity and the other with a variable capacity, which can be compared in oscillating circuits or in a Wheatstone bridge circuit supplied with alternating current.