An LVDT, or linear variable differential transformer, is a sensor that measures how far something moves in a straight line. It converts tiny physical movements into an electrical signal with exceptional precision, and because its core floats in an air gap with no physical contact, it can do this for decades without wearing out. LVDTs show up everywhere precision matters: aircraft control surfaces, submarine rudders, manufacturing quality checks, and deep-sea equipment.
What’s Inside an LVDT
An LVDT is built from three wire coils arranged in a line on a hollow cylindrical form. The center coil is the primary winding, and the two outer coils are the secondary windings. Running through the middle of all three coils is a rod-shaped core made of a high-permeability magnetic material like iron or ferrite. This core is the only moving part in the entire device. It slides freely inside the coil assembly without touching the walls, floating in a small air gap. That frictionless design is what gives the LVDT its remarkable durability and sensitivity.
The core is typically attached to whatever object you want to track. As that object moves, the core slides along with it inside the coil housing. The two secondary coils are wired in opposition to each other, meaning their output voltages naturally work against one another. This “series opposing” arrangement is the key to how the sensor extracts position information.
How It Turns Movement Into a Signal
The primary coil in the center is powered by a constant AC voltage source, typically oscillating at several kilohertz. This creates a magnetic field that radiates outward through the core and into the two secondary coils. How much of that magnetic energy reaches each secondary coil depends entirely on where the core sits.
When the core is centered perfectly between the two secondaries, equal amounts of magnetic flux reach both coils. Because they’re wired in opposition, their induced voltages cancel out, and the output is essentially zero. This center position is called the null point.
Move the core even slightly toward one secondary coil, and the balance breaks. The coil closer to the core picks up more magnetic flux and produces a higher voltage, while the coil farther away picks up less. The difference between the two voltages is your output signal, and it grows proportionally as the core moves further from center. Move the core the other direction, and the same thing happens in reverse, with the opposite coil now producing the stronger signal.
The output voltage follows a simple linear relationship: voltage equals a constant multiplied by the core’s displacement from center. This linearity holds across a useful range of travel on either side of the null point. The direction of movement is encoded in the phase of the AC output signal. One direction produces a signal in phase with the primary excitation; the other direction produces a signal 180 degrees out of phase.
Why Engineers Call It “Infinite Resolution”
Unlike digital encoders that measure position in fixed steps, an LVDT produces a continuously variable analog voltage. There are no discrete increments, no gear teeth, no optical lines to count. In theory, any movement of the core, no matter how small, changes the output voltage by some amount. This is why LVDTs are often described as having infinite resolution.
In practice, the smallest detectable movement is limited by external factors: electrical noise in the circuit, the quality of the signal conditioning electronics, and the precision of the analog-to-digital converter reading the output. But the sensor itself imposes no resolution limit. Paired with high-quality electronics, LVDTs can detect displacements measured in fractions of a micrometer.
Turning Raw Output Into Usable Data
The raw output of an LVDT is an AC voltage whose amplitude and phase both carry information. To get a clean, usable signal that tells you “the core moved 3.2 mm to the left,” you need signal conditioning. The core technique is called synchronous demodulation (sometimes called phase-sensitive demodulation). A demodulator compares the output signal’s phase to the original excitation signal, which lets it determine both how far the core moved and in which direction. The result is a DC voltage that’s directly proportional to position, with positive values for one direction and negative values for the other.
This demodulated signal then passes through filters to remove noise before being digitized by an analog-to-digital converter. Modern signal conditioners can achieve very high precision using 24-bit converters, capturing extremely fine position changes.
AC vs. DC Models
LVDTs come in two main configurations, and choosing between them depends on your environment and how much complexity you want to manage.
- AC LVDTs are the bare sensor with no built-in electronics. They require an external signal conditioner to provide the excitation voltage and process the output. The upside is ruggedness. With no onboard electronics to damage, AC LVDTs handle higher temperatures and harsher conditions. They’re the standard choice for extreme environments, long cable runs, and applications demanding the highest precision.
- DC LVDTs have the signal conditioning electronics built right into the sensor housing. You feed them a DC power supply and get back a clean DC output, typically 0 to 10 volts or a 4 to 20 milliamp current signal. They’re simpler to install and integrate, but the internal electronics limit their temperature range and make them more susceptible to electrical noise. They work well for straightforward industrial applications where ease of use matters more than extreme environmental tolerance.
Why LVDTs Last So Long
The defining mechanical advantage of an LVDT is that nothing touches anything. The core slides through an air gap inside the coil assembly with no friction, no wear surfaces, and no contact points that degrade over time. Compare this to a potentiometer, where a wiper physically scrubs along a resistive element and eventually wears through it. An LVDT has no such failure mode.
This frictionless operation translates to extraordinary service life. Subsea LVDT sensors designed for underwater pipeline monitoring and valve control are rated for more than 20 years of continuous operation. They endure pressures up to 350 bar (roughly 5,000 psi) in corrosive saltwater, housed in specialized alloy casings made from materials like Inconel or Monel. The absence of moving contact also means the sensor introduces essentially zero mechanical resistance to whatever it’s measuring, which matters when you’re tracking very small or delicate movements.
Where LVDTs Are Used
LVDTs tend to appear in applications where failure isn’t an option and precision is non-negotiable. In aerospace, they provide position feedback for flight control surfaces, landing gear, and engine components. In submarines, they measure rudder angles and the deflection of diving planes, providing the position data needed for stable maneuvering and attitude control underwater.
Subsea applications go further. LVDTs are integrated into valve actuators on underwater production systems to give real-time feedback on valve positions, controlling the flow of oil and gas at the seabed. Remotely operated vehicles (ROVs) and autonomous underwater vehicles (AUVs) use them at robotic arm joints, measuring the extension and rotation of each segment to enable precise gripping, welding, and sampling operations. They also monitor structural health by tracking tiny deformations in underwater platforms, mooring systems, and pipeline supports over months or years, catching problems before they become catastrophic.
In renewable energy, LVDTs measure hydraulic cylinder displacement in wave and tidal energy converters, helping optimize energy capture under constantly shifting dynamic loads. In manufacturing, they’re fixtures in quality control stations, measuring part dimensions with enough precision to catch deviations that would be invisible to other sensor types.