LDC1312: Inductive Sensing Working

But why does inductance varies with opposing magnetic field? if it is like transformer action then opposing magnetic field won't change the inductance. It will just change reflected resistance value. Is it something to do with permeability of magnetic circuit?

Hello Adesh,

This scenario is different than a transformer where a transformer essentially transfers energy from the primary coil inductor to a secondary coil inductor and these inductors can be different in value or amount of windings. Also note that when you add the secondary inductor and close it, the inductance of the primary will change.

Following the same analogy, then for the LDC devices we are generating a magnetic field in an inductive loop as the primary but the conductive target that is brought into this field is not an inductor. Note, that an inductor needs at least 2 terminals and a closed loop for current to flow, therefore the surface of the conductor does not inherently form inductance. Instead, the conductive object represents a medium for the magnetic field to induce eddy currents on its surface. These currents form a closed loop path which in turn generates a magnetic field that is reflected back and interacts with the primary coil as mutual inductance. Therefore depending on the size and shape of the inductor that is generating the field, the conductive surface will essentially form a mirror image of that coil that is approximately twice the distance. Parameters like conductivity, material thickness, and skin depth will affect how strong the interaction is. For more information please refer to research papers on physics of eddy currents from electromagnetic induction. Also, Wikipedia is a good place to start if you search for Eddy Currents, Skin Effect and Inductance.

Regards,
Luke LaPointe

Hello Luke,
Are you saying that inductance is changing because the Magnetic circuit is changing?
See according to my understanding, the inductance will not depend on the magnitude of eddy current in that case, it will totally the function of Magnetic permeability. To change the primary inductance of transformer the only way is changing its magnetic path.