LVDT construction, working and applications

The LVDT is a passive inductive transducer that converts linear displacement into an ac voltage whose amplitude is proportional to the displacement and whose phase gives the direction.

Construction

• One primary winding at the centre and two identical secondary windings $S_1$, $S_2$ wound symmetrically on either side of it, on a hollow non-magnetic former.
• A movable ferromagnetic core (soft iron or nickel-iron) slides inside the former and is attached to the object whose displacement is measured.
• The primary is excited by a 1-10 V, 50 Hz-20 kHz ac supply. The two secondaries are connected in series opposition.

                ~ E (ac excitation)
                     |
      +---------+--------+---------+
      |   S1    | Primary|   S2    |
      |  (((    |  (((   |   (((   |
      +----[=== core ===]----------+
                 <-- x -->
   S1 and S2 in series opposition: e_o = e1 - e2

Working

The primary field induces emfs $e_1$ and $e_2$ in the secondaries, and the output is $e_o = e_1 - e_2$.

• Core at centre (null position): equal flux links both secondaries, $e_1 = e_2$, so $e_o = 0$.
• Core moved toward $S_1$: $S_1$ gets more flux, so $e_1 > e_2$ and $e_o$ is in phase with the primary voltage.
• Core moved toward $S_2$: $e_2 > e_1$ and $e_o$ is $180^\circ$ out of phase with the primary.

The magnitude of $e_o$ is proportional to the displacement over the linear range. A phase-sensitive demodulator is used to obtain a dc signal whose sign gives the direction.

 |e_o|
    \               /
      \           /     <- linear range on each side
        \       /
          \   /
 -----------+----------- x
           null
  (180 deg out of phase) | (in phase)

Advantages

• Frictionless, infinite resolution, no contact wear, long life.
• High sensitivity (up to about 2 mV/V/mm); good linearity (about 0.25%).
• Rugged and tolerant of shock, vibration, dirt and the environment.
• Small null error, and the core moves freely without touching the coils.

Limitations

• Needs ac excitation and a demodulator.
• Stray magnetic fields and temperature affect it; the frequency response is limited by the carrier (the carrier must be at least about 10 times the signal frequency).
• Limited range, and a small residual voltage at null.

Applications

Displacement and position, thickness, force and pressure (with an elastic element such as a diaphragm or bourdon tube), acceleration, liquid level and load cells.