Inductive Transducers

Inductive transducers work on the principle of inductance change due to any appreciable change in the quantity to be measured i.e. measured. For example, LVDT, a kind of inductive transducers, measures displacement in terms of voltage difference between its two secondary voltages. Secondary voltages are nothing but the result of induction due to the flux change in the secondary coil with the displacement of the iron bar. Anyway, LVDT is discussed here briefly to explain the principle of inductive transducer. LVDT will be explained in another article in more detail. For the time being let’s focus on the basic introduction of inductive transducers.

Inductive Transducers
An example of an Inductive Transducer (LVDT)

Now first our motive is to find how the inductive transducers can be made to work. This can be done by changing the flux with the help of measured and this changing flux obviously changes the inductance and this inductance change can be calibrated in terms of measured. Hence inductive transducers use one of the following principles for its working.

  1. Change of self inductance
  2. Change of mutual inductance
  3. Production of eddy current

Let’s discuss each principle one by one.

Change of Self Inductance of Inductive Transducer

We know very well that self inductance of a coil is given by

Where,
N = number of turns.
R = reluctance of the magnetic circuit.

Also we know that reluctance R is given by

Where, μ = effective permeability of the medium in and around the coil.

Where,
G = A/l and called the geometric form factor.
A = area of cross-section of the coil.
l = length of the coil.

So, we can vary self inductance by

  • Change in number of turns, N,
  • Changing geometric configuration, G,
  • Changing permeability

For the sake of understanding we can say that if the displacement is to be measured by the inductive transducers, it should change any of the above parameter for causing in the change in self inductance.

Change of Mutual Inductance of Inductive Transducer

Here transducers, which work on change of mutual inductance principle, use multiple coils. We use here two coils for the sake of understanding. Both coils have their self-inductance as well. So let’s denote their self-inductance by L1 and L2.

Mutual inductance between these two coils is given by

Thus mutual inductance can be changed by varying self inductance or by varying coefficient of coupling, K. The methods of changing self inductance we already discussed. Now coefficient of coupling depends on the distance and orientation between two coils. Thus for the measurement of displacement we can fix one coil and make other movable which moves with the source whose displacement is to be measured. With the change in distance in displacement coefficient of coupling changes and it causes the change in mutual inductance. This change in mutual inductance can be calibrated with the displacement and measurement can be done.

Production of Eddy Current of Inductive Transducer

We know that when a conducting plate is placed near a coil carrying alternating current, a circulating current is induced in the plate called “EDDY CURRENT”. This principle is used in such type of inductive transducers. Actually what happens? When a coil is placed near to coil carrying alternating current, a circulating current is induced in it which in turn produces its own flux which try to reduce the flux of the coil carrying the current and hence inductance of the coil changes. Nearer the plate is to the coil, higher will be eddy current and higher is the reduction in inductance and vice versa. Thus inductance of coil varied with the variation of distance between coil and plate. Thus the movement of the plate can be calibrated in terms of inductance change to measure the quantity like displacement.

Real Life Application of Inductive Transducer

Inductive transducers find application in proximity sensors which are used for position measurement, dynamic motion measurement, touch pads etc. Particularly inductive transducer is used for the detection of type of metal, finding missing parts or counting the number of objects.

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