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The Linear Variable Differential Transformer (LVDT) is the most broadly used variable-inductance transducer in industry. It is an electro-mechanical device designed to produce an AC voltage output proportional to the relative displacement of the transformer and the armature, as illustrated in the figure below.

Typical LVDT

Common Specifications
Common specifications for commercially available translational LVDT's are listed below:

  Input: Power input is a 3 to 15 V (rms) sine wave with a frequency between 60 to 20,000 Hz (the two most common signals are 3 V, 2.5 kHz and 6.3 V, 60 Hz).
  Stroke: Full-range stroke ranges from ±125 µm to ±75 mm (±0.005 to ±3 in).
  Sensitivity: Sensitivity usually ranges from 0.6 to 30 mV per 25 µm (0.001 in) under normal excitation of 3 to 6 V. Generally, the higher the frequency the higher the sensitivity.
  Nonlinearity: Inherent nonlinearity of standard units is on the order of 0.5% of full scale.
Pros and Cons

  • Pros:
  - Relative low cost due to its popularity.
  - Solid and robust, capable of working in a wide variety of environments.
  - No friction resistance, since the iron core does not contact the transformer coils, resulting in an infinite (very long) service life.
  - High signal to noise ratio and low output impedance.
  - Negligible hysteresis.
  - Infinitesimal resolution (theoretically). In reality, displacement resolution is limited by the resolution of the amplifiers and voltage meters used to process the output signal.
  - Short response time, only limited by the inertia of the iron core and the rise time of the amplifiers.
  - No permanent damage to the LVDT if measurements exceed the designed range.
  • Cons:
  - The core must contact directly or indirectly with the measured surface which is not always possible or desirable. However, a non-contact thickness gage can be achieved by including a pneumatic servo to maintain the air gap between the nozzle and the work piece.
  - Dynamic measurements are limited to no more than 1/10 of the LVDT resonant frequency. In most cases, this results in a 2 kHz frequency cap.