Lecture Notes on LVDT (Linear Variable Differential Transformer)

Introduction to LVDT

• LVDT stands for Linear Variable Differential Transformer.
• Operates on the principle of variation of mutual inductance.
• Widely used for measuring linear displacement.
• Commercial ranges: ±0.25 mm to ±250 mm.

Construction of LVDT

• Cylindrical Former: Central component of the LVDT.
• Primary Coil:
  • Located at the center.
  • Excited by an AC voltage source (1 to 10 volts).
• Secondary Coils:
  • Two coils on both sides of the primary coil (Secondary Coil 1 and Secondary Coil 2).
  • Equal number of turns, connected in series.
• Movable Ferromagnetic Core:
  • Freely moves through the former.

Working Principle of LVDT

1. Core at Null Position:

   • Both secondary coils have equal voltage induced (EMF).
   • Net output = 0.
   • Requires a phase-sensitive detector for direction measurement.

2. Core at Left Side of Null Position:

   • External force moves core left.
   • EMF in Secondary Coil 1 > EMF in Secondary Coil 2.
   • Net output indicates negative displacement.

3. Core at Right Side of Null Position:

   • External force moves core right.
   • EMF in Secondary Coil 2 > EMF in Secondary Coil 1.
   • Net output indicates positive displacement.

Advantages of LVDT

1. Wide Measurement Range:
   • Effective displacement measurement (0.25 mm to 250 mm).
2. No Frictional Losses:
   • Core movement in a hollow former prevents friction losses, enhancing accuracy.
3. High Sensitivity:
   • Output is typically 40 volts per mm, often requiring no amplification.
4. Low Hysteresis:
   • Excellent repeatability under various conditions.
5. Direct Conversion to Electrical Signals:
   • Converts linear displacement to easily measurable electrical voltage.
6. Secondary Transducer Capability:
   • Can act with primary transducers, e.g., converting pressure into electrical signals.

Disadvantages of LVDT

1. Sensitivity to Stray Magnetic Fields:
   • Requires setup to protect from interference.
2. Susceptibility to Vibrations and Temperature Changes:
   • Performance may be affected by environmental conditions.

Conclusion

• Recap of LVDT’s construction, working, advantages, and disadvantages.
• Invitation to watch the next video on calibration of LVDT.