A transducer is a device for converting energy from one form into another. A microphone, for example, is a transducer that converts sound energy into electric energy. The need for a transducer in instrumentation arises when a measurement of some inaccessible quantity, or parameter, is required or when data is required from this parameter for further analysis or computation. Common transducer systems are the oil-pressure gauges and fuel gauges used in many motor vehicles—they provide purely visual output. In other cases, it is often convenient or even necessary to have the data in electric-signal form. Most present-day transducers provide a measurement of the parameter in the form of an electric signal whose value is proportional to the quantity measured.
The automobile fuel gauge is a good example of an electric transducer. Although in the early days mechanical means were devised for measuring the fuel level, it is much more convenient and efficient to transmit this data from the fuel tank to the gauge by electric means.
The transducer itself is mounted on the fuel tank and consists of two main elements: a float that moves up and down with the fuel level, so providing a mechanical displacement, and a potentiometer element linked to the float, which converts this displacement into an electric signal. This signal is then transmitted by wire to the gauge on the dashboard.
The salient point about this example is that it illustrates the common transducer design practice of having two stages of transducing, first from parameter to displacement and then from displacement to electric output. The displacement transducer is one of the basic types of transducers.
The name given to a particular transducer usually indicates its operation. The fuel gauge, for example, is a potentiometric transducer, measuring fuel level by displacement and then providing an electric output by means of potentiometer. Similarly, a piezoelectric pressure transducer may sense pressure by means of a diaphragm or capsule mechanism and produce its electric output from a piezoelectric crystal.
Inductive transducers
Although the potentiometer is widely used, it is frequently necessary to use other types of transducers to avoid certain inherent disadvantages of the potentiometer. These occur mainly because of the necessary use of a sliding contact in a potentiometer, and arise from intermittent contact between the slider and the resistive element due to vibration, friction, or the ingress of dirt.
To overcome these problems, a number of noncontact designs have been developed, among the most common being the inductive transducers. These transducers take many physical forms, but all work on the same basic principle of having a moving armature and a stator of magnetic material and a system of coils that are coupled magnetically, the coupling being altered by any displacement of the armature.
An inductive system must be polarized by an AC voltage or current, and this necessity is a complication compared with the potentiometer, which basically requires only a DC battery type of supply for polarization. In industry, however, it is convenient to use the local power supply (transformed down to a suitable voltage), and for aircraft, to use their 400 Hz supply.
Typical examples of this type of transducer are the synchro, the AC pick-off and the linear variable differential transformer (LVDT). The LVDT is a good example of an inductive transducer, and it works on the E and I core principle with zero displacement arranged to give zero voltage output.
The fixed element is an E-shaped magnetic core with a coil of wire wound on each of the three limbs of the E. The two outer coils are connected in series and to the polarizing AC supply. The middle coil produces the output voltage. The I-section armature, a single magnetic bar, is mounted near the face of the three poles of the E and may be arranged to either rotate or move linearly with displacement.
Under zero displacement conditions, the magnetic fluxes generated by the two outer coils couple through the I core and into the center limb of the E, canceling each other out so that there is no output from the center coil. When displacement of the I core takes place, the flux balance is upset and greater coupling takes place from one outer coil than from the other, and so cancellation no longer exists, and the resulting flux in the center limb creates a voltage output.
This output varies in magnitude with respect to displacement. There is also a phase relationship that depends on the direction of the displacement. Frictional and dirt problems are minimal, and apart from its use within transducers, this principle is widely used in industry to measure the movement of parts of machinery, often as part of a servo control system.
