LVDT fundamentals — In a linear variable differential transformer (LVDT), which component is the moving element that couples mechanically to the measured displacement?

Introduction:
An LVDT is an electromechanical sensor that converts linear motion into an electrical signal using transformer action. Understanding which part moves helps with mounting, mechanical coupling, and interpreting sensor specifications such as range, linearity, and sensitivity.

Given Data / Assumptions:

• An LVDT contains a primary coil and two secondary coils wound on a stationary former.
• A high-permeability ferromagnetic core slides within the bore.
• Excitation is applied to the primary; the differential secondary output indicates core position.

Concept / Approach:

The movable core alters magnetic coupling to the two secondaries. At the electrical null (center), induced voltages in the secondaries are equal and opposite, giving zero differential output. Displacement from center unbalances the coupling, producing a differential voltage proportional to position and indicating direction via phase.

Step-by-Step Solution:

Verification / Alternative check:

Manufacturer datasheets depict a free-moving core (often attached to a pushrod), confirming that the core is the moving member while windings remain stationary.

Why Other Options Are Wrong:

• Primary / Secondary: These windings are fixed; they do not move.
• Diaphragm: Pertains to pressure sensors, not LVDTs.
• Frame: Structural, non-moving support.

Common Pitfalls:

• Rigidly fixing the core to the coils; it must slide freely to avoid friction errors.
• Ignoring end-effects beyond the linear range, which degrade linearity.

Final Answer:

Core