Mutual-induction Transducers

Mutual-induction Transducers

The basic arrangement of a mutual-induction transducer constitutes two coils, the primary winding and the secondary winding.

One of the coils (primary winding) carries an alternating current (ac) excitation, which induces a steady ac voltage in the other coil (secondary winding).

The level (amplitude, rms value, etc.) of the induced voltage depends on the flux linkage between the coils. None of these transducers employ contact sliders or slip rings and brushes as do resistively coupled transducers (potentiometer).

Consequently, they will have an increased design life and low mechanical loading. In mutual-induction transducers, a change in the flux linkage is effected by one of two common techniques. One technique is to move an object made of ferromagnetic material within the flux path.

This changes the reluctance of the flux path, with an associated change of the flux linkage in the secondary coil. This is the operating principle of the linear-variable differential transformer

(LVDT), the rotatory-variable differential transformer (RVDT), and the mutual-induction proximity probe. All of these are, in fact, variable-reluctance transducers. The other common way to change the flux linkage is to move one coil with respect to the other.

This is the operating principle of the resolver, the synchro-transformer, and some types of ac tachometer. These are not variable-reluctance transducers, however.

 The motion can be measured by using the secondary signal in several ways. For example, the ac signal in the secondary coil may be “demodulated” by removing the carrier signal

(i.e., the signal component at the excitation frequency) and directly measuring the resulting signal, which represents the motion.

 This method is particularly suitable for measuring transient motions.

Alternatively, the amplitude or the rms (root-mean-square) value of the secondary (induced) voltage may be measured. Another method is to measure the change of inductance.