Linear Variable Differential Transformer (LVDT)

linear variable differential transformer (LVDT) is one of the most popular electromechanical devices used to convert small mechanical displacements (mechanical vibrations or mechanical motion of the order of a few millimeters or fractions of a millimeter) into amplified electrical signals (variable electrical current, voltage or electrical signals, and vice versa). LVDTs are linear position sensors. Drive mechanisms mainly used for automatic control systems or as mechanical motion sensors in measurement technologies. The classification of electromechanical transducers includes conversion principles or types of output 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]

Inside the LVDT’s housing is the primary coil. On either side of the coil assembly is a pair of secondary coils. Except for their physical positions, all three primary windings are identical. However, they are wired in series with the opposition, so that if they are energized equally, their outputs will add up to zero.

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

  • 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.
  • It cannot be overloaded mechanically. This is because the core is completely separated fromthe remainder of the device.
  • It is highly sensitive and provides good magnification.
  • It is relatively insensitive to temperature changes.
  • 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.

Types of LVDT

In terms of their shaft/armature construction there are several basic varieties available today:

  • Free (Unguided) Armature LVDTs: the shaft or armature is free to slide back and forth in the LVDT body. The armature is connected to the object under test, which moves in parallel with the LVDT body; the result is a virtually frictionless arrangement, resulting in a very long life span for the sensor. With infinite resolution quality, the unguided armature mechanism has a wear-resistant design that does not limit the resolution of the measured data. This type of mechanism is attached to the sample to be measured, fits loosely into the tube and requires the LVDT body to be supported separately.
  • Captive (Guided) Armature LVDTs: the armature (guided by a low-friction bearing assembly that restrains it) is connected both to the body and the object under test; this allows the LVDT to handle longer measurement ranges (e.g., ~25 mm to 45 cm), as well as scenarios when the object under test is moving transversely to the LVDT body. In these cases, misalignment would occur if the armature were not guided. Captive (guided) armature LVDTs are useful in both static and dynamic measurement applications (are best for long work intervals). Closed armatures help prevent misalignment as they are guided and locked by low-friction assemblies.
  • Forced or Spring-extended Armature LVDTs: has the same low-friction bearing assembly of a captive (guided) LVDT, but in addition, it uses a mechanical return mechanism like a spring, pneumatics, or a motor to lightly push the armature out to its full deflection. This is for applications where it is desired that the LVDT armature maintain a steady connection with the object under test (best for static and slow-moving applications). Force-extended armatures are used in LVDTs for slow moving applications. These mechanisms do not require any connection between the sample and the armature.

LVDT measurement applications

  • Machine tools
  • Tensile test stands
  • Aerospace test – landing gear, actuators, control surface positioning, hydraulics
  • Automotive and train tests – movements of suspension systems
  • Power generation – turbine testing
  • Robotics – position feedback
  • Manufacturing – Automation, process controls
  • Pulp & paper – tensioning arms positioning

References

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

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