Short Answer:

An LVDT (Linear Variable Differential Transformer) measures linear displacement using the principle of electromagnetic induction. It consists of a movable magnetic core placed inside a tube with one primary coil and two secondary coils. When the core moves, it changes the magnetic coupling between the coils, which in turn changes the output voltage.

The position of the core determines the voltage difference between the two secondary coils. This voltage is used to calculate how much and in which direction the core has moved. LVDTs are known for their high accuracy, reliability, and non-contact operation.

Detailed Explanation:

LVDT displacement measurement

An LVDT (Linear Variable Differential Transformer) is an electromechanical sensor used to measure small to large ranges of linear displacement with high precision. It works based on the principle of mutual induction and is commonly used in industrial automation, aerospace, and instrumentation systems.

Construction of LVDT:

An LVDT consists of three main parts:

1. Primary Coil (P):
   Located in the center of the LVDT and connected to an AC excitation source.
2. Secondary Coils (S1 and S2):
   Placed symmetrically on either side of the primary coil and connected in series opposition (their outputs are subtracted).
3. Movable Soft Iron Core:
   Placed inside a hollow cylindrical structure and free to move along the axis. It does not touch the coils and moves with the object whose displacement is being measured.
4. AC Excitation Source and Output Measurement Circuit:
   An alternating current (AC), usually 5-10 kHz, is applied to the primary coil.

Working Principle:

The working of an LVDT is based on electromagnetic coupling between the primary and secondary coils through the movable core. Here is how it operates step by step:

1. Initial Condition (Null Position):
   When the core is exactly centered between the two secondary coils, both coils receive equal magnetic flux from the primary coil. Since they are connected in opposite directions, the voltages cancel each other. The output voltage is zero — this is called the null position.
2. Core Displaced to the Right (Positive Direction):
   When the core moves towards secondary coil S1, more flux links to S1 than S2. As a result, S1 produces a higher voltage than S2. The output is a positive voltage, indicating direction and magnitude of movement.
3. Core Displaced to the Left (Negative Direction):
   When the core moves towards secondary coil S2, more flux links to S2. The result is a negative voltage at the output, again showing both the direction and amount of displacement.

The magnitude of the output voltage is proportional to how far the core has moved, and the polarity (positive or negative) indicates the direction of movement.

Key Features:

• High Accuracy and Sensitivity:
  Ideal for precise position measurements.
• Infinite Resolution:
  Can detect very small changes in position.
• Frictionless Operation:
  Since the core does not touch the coils, there is no mechanical wear.
• Robust and Reliable:
  Works well in industrial and harsh environments.
• Linear Output:
  Within a specific range, the output voltage changes linearly with displacement.

Applications:

• Position feedback in control systems
• Valve and actuator monitoring
• Aerospace and military equipment
• Testing machines and laboratory instruments
• Measuring expansion, movement, or shift in mechanical parts

Conclusion:

An LVDT measures linear displacement by detecting the position of a movable core using electromagnetic induction. As the core moves, it changes the voltage induced in two secondary coils, and this voltage is used to calculate the direction and distance of movement. LVDTs are widely used because they are accurate, durable, and work without physical contact, making them perfect for precise and long-lasting measurements.