Linear Variable Differential Transformer (LVDT)

linear variable differential transformer (LVDT) is one of the most popular electromechanical devices used to convert small mechanical displacements (of the order of a few millimeters or fractions of a millimeter) into amplified electrical signals.

An LVDT provides an alternating current (AC) voltage output proportional to the relative displacement of a transformer core with respect to a pair of electrical windings. It provides a high degree of amplification and is very popular because of its ease of use.

Moreover, it is a non-contact-type device, where there is no physical contact between the plunger and the sensing element. As a consequence, friction is avoided, resulting in better accuracy and long life for the comparator. It can be conveniently packaged in a small cartridge.

Cutaway view of an LVDT. Current is driven through the primary coil at A, causing an induction current to be generated through the secondary coils at B. [1]

An LVDT produces an output proportional to the displacement of a movable core within the field of several coils. As the core moves from its “null” position, the voltage induced by the coils change, producing an output representing the difference in induced voltage. It works on the mutual inductance principle. A primary coil and two secondary coils, identical to each other, are wound on an insulating form.

An external AC power source is applied to the primary coil and the two secondary coils are connected together in phase opposition. In order to protect the device from humidity, dust, and magnetic influences, a shield of ferromagnetic material is spun over the metallic end washers. The magnetic core is made of an alloy of nickel and iron.

The motion of the core varies the mutual inductance of secondary coils. This change in inductance determines the electrical voltage induced from the primary coil to the secondary coil. Since the secondary coils are in series, a net differential output results for any given position of the core.

Sensitivity of an LVDT is stated in terms of millivolts output per volt input per 1 mm core displacement. The per-volt input voltage refers to the exciting voltage that is applied to the circuit. Sensitivity varies from 0.1 to 1.5 mV for a range varying from 0.01 to 10 mm of core displacement.

Sensitivity is directly proportional to excitation voltage, frequency of input power, and number of turns on the coils. An LVDT enjoys several distinct advantages compared to other comparators.

Advantages of LVDTs

  1. It directly converts mechanical displacement into a proportional electrical voltage. This isunlike an electrical strain gauge, which requires the assistance of some form of elastic member.
  2. It cannot be overloaded mechanically. This is because the core is completely separated fromthe remainder of the device.
  3. It is highly sensitive and provides good magnification.
  4. It is relatively insensitive to temperature changes.
  5. It is reusable and economical to use.

The only disadvantage of an LVDT is that it is not suited for dynamic measurement. Its corehas appreciable mass compared, for example, to strain gauges. The resulting inertial effectsmay lead to wrong measurements.

References

  1. Image based on: https://commons.wikimedia.org/wiki/File:LVDT.png
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