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Inductive Sensor

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Principle

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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

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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.

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

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Structure of LVDT

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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.

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Applications

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