Linear Variable Differential Transformer (LVDT) Lecture Notes

Introduction to LVDT

• Definition: LVDT stands for Linear Variable Differential Transformer.
• Principle: Works on the principle of variation of mutual inductance.
• Application: Popular device for measuring linear displacement.
• Displacement Range: Commercial LVDTs have a range of ±0.25 mm to ±250 mm.

Construction of LVDT

• Components:
  • Cylindrical Former: The structure housing the components.
  • Primary Winding:
    • Located at the center, excited by an AC voltage source (1 to 10 volts).
  • Secondary Windings:
    • Two coils on both sides of the primary coil (Secondary Coil 1 and Secondary Coil 2).
    • Equal number of turns connected in series.
  • Movable Core: A ferromagnetic core that moves freely within the former.

Working Principle of LVDT

• Positions of Core:
  • Null Position: Core at the center.
  • Left Side of Null Position: Core shifts left.
  • Right Side of Null Position: Core shifts right.

Case 1: Core at Null Position

• Output: Voltage induced in both secondary coils is equal, resulting in zero net output.
• Direction Sensitivity: A phase-sensitive detector is necessary to determine the direction of movement.

Case 2: Core Moves Left

• Output: EMF voltage induced in Secondary Coil 1 is greater than in Secondary Coil 2.
• Result: Gives a negative displacement result.

Case 3: Core Moves Right

• Output: EMF voltage induced in Secondary Coil 2 is greater than in Secondary Coil 1.
• Result: Gives a positive displacement result.

Advantages of LVDT

1. Wide Measurement Range: Can measure displacements from 0.25 mm to 250 mm.
2. No Frictional Losses: Core moves inside a hollow former, ensuring accuracy.
3. High Sensitivity: Output is high and doesn’t require amplification (typically 40 volts/mm).
4. Low Hysteresis: Excellent repeatability under varying conditions.
5. Direct Electrical Signal Conversion: Converts linear displacement to electrical voltage easily.
6. Secondary Transducer Capability: Can be used with primary transducers (e.g., a pressure transducer) to measure fluid pressure.

Disadvantages of LVDT

1. Sensitivity to Stray Magnetic Fields: Requires protection from stray magnetic fields.
2. Effects of Vibrations and Temperature: Performance can be impacted by external vibrations and temperature changes.

Conclusion

• LVDTs are versatile and accurate devices for measuring linear displacement but come with certain limitations regarding environmental sensitivity.