Inductive Sensor

Principle

In 1831, Michael Faraday in England and Joseph Henry in the U.S.A. discovered one of the most fundamental effects of electromagnetism: an ability of a varying magnetic field to induce electric current in a wire. Faraday's law of induction says that the induced voltage, or electromotive force (e.m.f.), is equal to the rate at which the magnetic flux through the circuit changes. If varying magnetic flux is applied to a solenoid, e.m.f. appears in every turn and all these e.m.f. must be added. If a solenoid, or other coil, is wound in such a manner as each turn has the same cross-sectional area (Figure I1), the flux through each turn will be the same, then induced voltage is

where N is the number of turns, B is the amplitude of magnetic field, and A is the area of the circuit where is in the magnetic field. The minus sign is an indication of the direction of the induced e.m.f.

In figure I1, the area A=lx. Consequently, the induced voltage is

Structure of LVDT

LVDT (linear variable differential transformer) is based on electromagnetic induction. The basic arrangement of a multiinduction transducer contains two coils-primary and secondary. The primary carries ac excitation (Vref) that induces a steady ac voltage in the secondary coil (Fig. I2). The movement of an object made of ferromagnetic material within the flux path alters the coupling between the coils. Consequently, the magnetic flux coupling between two coils is converted into voltage.

Applications

LVDT can be used to measure the displacement, deflection, position and profile of a workpiece.