A linear variable differential transformer (LVDT) is an electromechanical transducer that converts straight-line physical displacement into a proportional alternating current (AC) voltage output. Unlike resistive sensors that rely on physical wiper contact, an LVDT uses magnetic coupling between a movable ferromagnetic core and stationary wire coils, yielding theoretically infinite resolution, zero friction, and exceptional lifespan in harsh environments.
How the Magnetic Coupling Actually Works
Inside the stainless steel or Inconel housing of an LVDT, you will find three distinct wire coils wound around a hollow, non-magnetic bobbin. The center coil is the primary winding, and the two outer coils are the secondary windings (S1 and S2). A separate, rod-shaped ferromagnetic core slides freely through the center of the bobbin, attached to the object you are measuring.
The primary coil is driven by an AC excitation signal, typically ranging from 1 kHz to 10 kHz. This alternating magnetic field induces a voltage in both secondary coils. The secondaries are wired in a series-opposing configuration, meaning their voltages subtract from one another.
The Water Valve Analogy: Think of the LVDT like a balanced water valve with one pressurized inlet (the primary coil) and two outlet pipes (the secondary coils). When the physical plug (the core) is dead center, equal water flows out both sides, and the net pressure difference between the two outlets is zero. Push the plug to the left, and the left outlet gets more flow while the right starves, creating a measurable pressure differential.
When the core is exactly in the center (the null position), the magnetic coupling to S1 and S2 is identical. The induced voltages are equal in magnitude but opposite in phase due to the series-opposing wiring, resulting in a net output of 0V. As the core moves away from null, the coupling to one secondary increases while the other decreases. The resulting differential AC voltage is directly proportional to the distance moved. Crucially, the phase of the output voltage relative to the excitation signal flips by 180 degrees when the core crosses the null point, which tells your control system which direction the core moved.
Worked Numeric Example: Sizing and Signal Output
To understand how this translates to real bench measurements, let us calculate the expected output of a standard industrial LVDT (similar to an Omega LD620 or TE Connectivity GHSD series) used for valve positioning.
- Excitation Voltage ($V_{exc}$): 5V RMS at 3 kHz
- Sensor Sensitivity: 2.4 mV / V / mm
- Full Scale Range: ±25 mm
- Measured Displacement: +12 mm from null
The formula for the RMS output voltage is:
V_out = V_exc × Sensitivity × Displacement
Plugging in our real values:
V_out = 5V × 2.4 mV/V/mm × 12 mm
V_out = 144 mV RMS
At +12 mm displacement, your multimeter (set to AC millivolts) will read exactly 144 mV RMS, and the phase will be in-phase (0°) with the 3 kHz excitation.
If the valve shifts to -12 mm (the opposite direction), the magnitude remains 144 mV RMS, but the phase shifts to 180° out-of-phase. A standard AC multimeter cannot read phase, which is why LVDTs require dedicated signal conditioning hardware to convert this AC differential signal into a usable DC voltage (e.g., 0-10V or 4-20mA) for a PLC or microcontroller.
Where You Meet LVDTs in Practice
You will rarely find an LVDT in consumer electronics or hobbyist 3D printers; they are strictly heavy-duty industrial, aerospace, and civil engineering components. According to Omega Engineering's sensor guides, their frictionless design makes them mandatory in applications where a sensor must survive millions of cycles without degrading.
| Feature | LVDT | Linear Potentiometer | Magnetostrictive Sensor |
|---|---|---|---|
| Wear Mechanism | None (magnetic coupling) | High (carbon track wear) | None (time-of-flight pulse) |
| Resolution | Infinite (limited by electronics) | Limited by wiper granularity | Very high (micron level) |
| Typical Cost (2026) | $150 - $800+ | $10 - $50 | $400 - $1,500+ |
| Best Application | Hydraulic cylinders, CNC axes | Joysticks, consumer pedals | Long-stroke extrusion presses |
In civil engineering, LVDTs are epoxied across bridge expansion joints to monitor structural creep over decades. In aerospace, redundant LVDTs are physically bolted to flight control surface actuators to provide closed-loop feedback to the fly-by-wire computers.
What People Commonly Confuse It With (And What It Changes in Your Circuit)
The most common mistake makers and junior engineers make is confusing an LVDT with an RVDT (Rotary Variable Differential Transformer). An RVDT uses the exact same AC differential principle but measures angular rotation (usually limited to ±40 degrees) via a shaped rotor rather than a linear sliding core.
They are also frequently confused with string potentiometers (which are just rotary pots with a spring-loaded pull-cord) or linear Hall effect sensors (which measure magnetic field density and suffer from severe temperature drift and non-linearity over long distances).
What this changes in your circuit: If you are replacing a 10kΩ linear potentiometer with an LVDT in an installation, you cannot simply wire it to a DC voltage divider. You are ripping out a passive DC circuit and installing an active AC system. You must add an AC oscillator to drive the primary, and a synchronous demodulator IC (like the Analog Devices AD598) to rectify the AC output, track the phase for directionality, and output a clean DC voltage. If you skip the demodulator, your microcontroller will just read noise.
Frequently Asked Questions
What is the difference between an LVDT and an RVDT?
An LVDT measures linear (straight-line) displacement using a cylindrical core that slides in and out of a coil bobbin. An RVDT measures rotary (angular) displacement using a butterfly-shaped ferromagnetic rotor that spins inside the stator coils. Both output AC differential voltages and require the same type of signal conditioning, but their mechanical form factors and measurement axes are entirely different.
Can I wire an LVDT directly to an Arduino or ESP32 ADC?
No. An Arduino or ESP32 analog-to-digital converter (ADC) reads DC voltages between 0V and 3.3V (or 5V). An LVDT outputs an AC voltage (e.g., 144 mV RMS at 3 kHz) that swings positive and negative. Feeding an AC signal into a microcontroller ADC will result in garbage data and could damage the pin if the negative swing exceeds the IC's absolute maximum ratings. You must use an LVDT signal conditioner board to convert the AC RMS voltage into a 0-3.3V DC signal first.
Why does my LVDT output read a residual voltage at the null position?
This is known as null voltage or residual voltage. Even when the core is perfectly centered, you might read a few millivolts on your oscilloscope. This happens due to slight asymmetries in the coil winding, stray capacitance between the windings, and harmonics from the excitation source. High-quality signal conditioners use a low-pass filter and a null-offset trim pot to cancel this residual error out of the final DC signal.
How much does a commercial LVDT cost in 2026?
Pricing depends heavily on the stroke length, housing material, and environmental sealing. A basic, unamplified AC-operated LVDT with a 10mm stroke and an aluminum body typically costs between $150 and $250. Heavy-duty models with Inconel housings, welded hermetic seals for subsea or high-pressure hydraulic use, and stroke lengths over 100mm easily range from $500 to $850+ from manufacturers like TE Connectivity, Sensata, or Macro Sensors.
