Linear Variable Differential Transformer: LVDT

LVDT- Introduction, Operation, Advantages:


In this post we will understand about the Linear variable Differential Transformer which is one of the most popular transducer.
LVDT is an induction type transducer.
It is widely used to translate the linear displacement or motion into electrical signal.
Before proceeding further, let us refresh about inductive transducers principle and it is recommended to read Electrical transducer Basics, Types and Requirements

Principle  of Induction Type transducer :

             "When a force is applied to the ferromagnetic armature core, the air gap between displacement and the core is changed. It leads to varying the reluctance of the magnetic circuit. Thus the applied force is measured by the change of the inductance in a single coil." 

The inductive type tranducers enable static and dynamic measurements. They have the drawback of limited frequency response.

  • The LVDT has a primary coil and two similar secondary coils.
  • The secondary winnings have equal number of turns and are identically placed on either side of the primary winding.
  • It also consists of a rod-shaped magnetic core positioned centrally inside the coil.
  • The displacement to be measured is applied to an arm attached to the soft iron core.
  • The core is usually made of nickel-iron alloy with length wise slots to reduce eddy current losses.
  • When AC is fed into primary coil, voltages V1, V2 are induced in the secondary coils.
  • As these coils are connected in series opposition, the output voltage V0 = V1-V2.
  • If the core is placed exactly at central position(null position, reference position), V1 = V2. 
  • So the output voltage V0=0.
  • When the core is moved from the central position, the induced voltage in the secondary coil towards which the core has moved increases while that in the other secondary coil decreases. This results in a differential voltage output from the transformer.
  • The output voltage produced by the displacement of the core is linear over a considerable range but flattens out at both ends.
  • The voltage phase changes by 180 degree as the core moves through the center position.

LVDT - Advantages:

  1. LVDT provides continuous resolution and shows low hysteresis loss. So repeatability is very good.
  2. As there is no sliding contacts, there is less friction and noise.
  3. Output voltage is linear for displacement upto 5mm.
  4. Infinite resolution. Even a displacement of 10-3 mm can be recorded
  5. High output hence amplification may not be required.
  6. High sensitivity. It is highly sensitive to even slight movements.
  7. Ruggedness. They take up shock and vibrations easily.
  8. Small and light in weight.
  9. Less friction and less noise.
  10. Low hysteresis and hence gives reliable readings when used repeatedly.
  11. Low power consumption. It is around 1 watt.

LVDT - Disadvantages:

  1. Relatively large displacements are required for appreciable differential voltage output.
  2. Sensitive to stray(output) magnetic fields; hence shielding is essential
  3. It is sensitive to vibrations and temperature. 
  4.  Sometimes readings are affected due to vibration
  5. Performance is affected due to temperature rise.
  6. Dynamic response is affected by the mass of the apparatus and the frequency of supply.

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