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The Rotational Variable Differential Transformer (RVDT) is used to measure rotational angles and operates under the same principles as the LVDT sensor. Whereas the LVDT uses a cylindrical iron core, the RVDT uses a rotary ferromagnetic core. A schematic is shown below.

Typical RVDT Sensor
Common Specifications

Common specifications for commercially available RVDT'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.
  Angle: Capable of continuous rotational measurement. However, most RVDTs have effective angle limits of up to ±60°.
  Nonlinearity: Higher accuracy in the smaller angle range: 0.25% @ ±30°, 0.50% @ ±40°, 1.50% @ ±60°.
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 very long service life.
  - High signal to noise ratio and low output impedance.
  - Negligible hysteresis.
  - Infinitesimal theoretical resolution. In reality, angle resolution is limited by the resolution of the amplifiers and voltage meters used to process the output signal.
  - No permanent damage to the RVDT if measurements exceed the designed range.
  • Cons:
  - The core must be in contact (directly or indirectly) with the measured surface which is not always possible or desirable.