RVDT (Rotary Variable Differential Transformer)

Main Ideas / Concepts

• Context for transducers: Physical quantities can be converted into electrical quantities to make them easier to store, analyze, and process later in industrial applications.
• Purpose of RVDT: A Rotary Variable Differential Transformer (RVDT) converts angular displacement (angle changes) into an electrical voltage.
• RVDT is a differential transformer: Like other differential transformer designs, it includes:
  • A primary coil supplied with an AC input voltage
  • Two secondary coils (commonly Coil A and Coil B) whose generated voltages oppose each other
• Key measurement principle: The output voltage depends on how rotor/shaft movement changes the magnetic flux linkage between the primary and each secondary coil.
• Neutral (null) position behavior:
  • At the neutral position (no angular displacement), magnetic flux links equally with Coil A and Coil B
  • Therefore: VA = VB
  • The differential output is zero: Vout = VA − VB = 0

• Direction affects polarity:

  • Anticlockwise rotation: More flux links with Coil B than Coil A → VB > VA → Vout becomes negative

  • Clockwise rotation: More flux links with Coil A than Coil B → VA > VB → Vout becomes positive

• Magnitude relates to angle amount: As the angle moves farther from neutral, the difference in flux linkage (and thus between VA and VB) increases, so the output voltage magnitude increases, enabling precise angle measurement.

• Applications: Used in many places where precise angle/angular displacement measurement is required.

Method / Process (Implied Operation Steps)

1. Apply an AC source to the primary coil of the RVDT.
2. Position the rotor/shaft at the neutral (null) angular position so flux linkage to Coil A and Coil B is equal.
   • Output should be approximately 0 (since VA ≈ VB).
3. Rotate the shaft/rotor and observe the differential output:
   • Clockwise rotation
     • Flux linkage increases toward Coil A
     • Flux linkage decreases toward Coil B
     • Output polarity: positive (Vout = VA − VB > 0)
   • Anticlockwise rotation
     • Flux linkage increases toward Coil B
     • Flux linkage decreases toward Coil A
     • Output polarity: negative (Vout = VA − VB < 0)
4. Infer:
   • Direction of rotation from the sign of Vout
   • Amount of rotation from the magnitude of Vout

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