LVDT Positioning

What is an LVDT?

• A Linear Variable Differential Transformer (LVDT) is an electromechanical sensor designed to measure linear displacement (straight-line position or movement).

• It's a highly accurate and reliable device commonly used in various industries.

Construction

• Three Coils: An LVDT has a central primary coil and two identical secondary coils placed symmetrically on either side of the primary, all wound around a hollow tube or cylinder.

• Movable Core: A ferromagnetic cylindrical core slides freely along the axis of the tube. The position of this core determines the output of the LVDT.

How It Works

1. AC Excitation: The primary coil is energized with an alternating current (AC).

2. Electromagnetic Coupling: This AC excitation induces voltages in the secondary coils through electromagnetic coupling.

3. Differential Output: The secondary coils are connected in series opposition, meaning their voltages are subtracted. When the core is perfectly centered, the induced voltages in the secondary coils cancel out, resulting in a zero output.

4. Position Measurement: As the core moves from the center position, the differential output voltage changes proportionally to the core's displacement and its direction.

Applications

• Industrial Automation: Measuring positions of valves, actuators, robotic components

• Aerospace: Monitoring flight controls, landing gear, instrumentation

• Power Generation: Tracking positions in turbines and generators

• Medical Equipment: Patient positioning systems, surgical equipment

• Materials Testing: Measuring strain and displacement in test specimens

• Automotive: Suspension systems, throttle position sensors

Advantages

• High Accuracy and Resolution: LVDTs provide exceptionally accurate and precise linear displacement measurements.

• Robustness: With no physical contact between the core and coils, there's minimal wear making them durable for harsh environments.

• Infinite Resolution: Theoretically, LVDTs have infinite resolution, allowing detection of the smallest changes in position.

• Null Position: The zero output at the center position provides a reference point.

• Rugged Construction: Suitable for demanding industrial environments.

Disadvantages

• Cost: LVDTs can be more expensive than other position sensors.

• Signal Conditioning: They usually require external signal conditioning electronics to convert the output signal into a usable format.

• Stray Magnetic Fields: Their operation can be affected by external magnetic fields.

• Limited Range: The measurable displacement is limited by the length of the LVDT.

Demodulation

LVDTs by themselves output an AC voltage that changes in amplitude and phase based on the core's position. This raw AC signal isn't directly usable in most applications, requiring demodulation circuits to convert it into a more meaningful and usable form, typically a DC voltage proportional to the core's displacement.

Here's a breakdown of LVDT demodulation circuits:

Why Demodulation is Needed:

• LVDT output is an AC waveform, but most applications require a DC voltage representing the core's position.

• The AC output contains both positive and negative voltage swings, making it difficult to directly interpret the information about core position.

Demodulation Techniques:

Two main types of demodulation circuits are commonly used:

• Rectifier-Type Demodulators:

  • Simpler and cheaper

  • Use diodes to rectify (convert) the AC signal to a pulsating DC voltage.

  • May suffer from sensitivity to temperature and non-linearities, leading to inaccuracies.

  • Typically used in less critical applications.

• Synchronous Demodulators:

  • More complex and costlier

  • Use a reference signal synchronized with the LVDT's excitation frequency.

  • Multiply the LVDT output with the reference signal, resulting in a DC voltage directly proportional to the core's displacement.

  • Offer superior accuracy, better noise rejection, and wider temperature range compared to rectifier-type.

  • Preferred for high-precision and critical applications.

Additional Considerations:

• Filtering: Demodulated signals often require filtering to remove unwanted noise and improve accuracy.

• Calibration: LVDT demodulator circuits may require calibration to ensure the output accurately reflects the core's displacement across its entire range.