A linear differential transformer (LVDT) is an inductive position sensor that converts straight-line mechanical movement into a proportional alternating current (AC) voltage signal. In a real closed-loop control circuit, swapping a resistive slide potentiometer for an LVDT fundamentally changes the system by eliminating physical wiper friction, removing dead spots, and providing infinite resolution for position feedback. Makers and junior technicians commonly confuse the LVDT with a standard AC step-down power transformer or a linear variable reluctance transducer; however, an LVDT relies specifically on a movable ferromagnetic core altering mutual inductance between isolated coils, rather than just varying an air gap or stepping down mains voltage.
How a Linear Differential Transformer Actually Works
The architecture of an LVDT consists of three coils wound around a hollow, non-magnetic bobbin. A primary coil sits in the exact center, flanked by two identical secondary coils wound in opposite directions (series opposition). A separate, rod-shaped ferromagnetic core is attached to the moving object you want to measure and slides freely through the hollow center of the bobbin.
When the core is dead center (the electrical null), the magnetic coupling to both secondary coils is perfectly equal. The induced AC voltages cancel out, yielding a net output of zero. As the core moves left, coupling increases in Secondary 1 and decreases in Secondary 2, resulting in a net positive AC voltage. Move it right, and the opposite happens. Crucially, the amplitude of the output voltage tells you the distance moved, while the phase of the AC signal (0° or 180° relative to the primary excitation) tells you the direction.
Worked Example: Calculating LVDT Output Voltage
To size an LVDT for a precision actuator, you need to calculate the expected output voltage at maximum stroke to ensure your analog-to-digital converter (ADC) or signal conditioner won't clip the signal. Let us use the specifications of a standard industrial-grade sensor, similar to the TE Connectivity GHSD series.
- Total Stroke: ±25 mm (50 mm total travel)
- Sensitivity: 2.5 mV/V/mm
- Excitation Voltage ($V_{exc}$): 5.0 V RMS at 3 kHz
- Target Displacement: +14 mm from the null position
The formula for LVDT output voltage is:
V_out = Sensitivity × V_exc × Displacement
At +14 mm of displacement, the secondary coils will output 175 mV RMS. Because the core moved in the positive direction, this signal will be in-phase (0°) with the 3 kHz excitation. If the core moves to -14 mm, the output remains 175 mV RMS, but the phase flips to 180° out-of-phase. Note that in the real world, at the exact 0 mm null position, you will still read a small 'residual null voltage' (often 1% to 2% of full-scale output) due to minor winding asymmetries and harmonic distortion.
Where You Meet This In Practice
You will rarely find a raw LVDT on a hobbyist workbench; they are the backbone of heavy-duty, high-reliability industrial and aerospace position feedback systems. Because they have no physical electrical contact between the moving core and the stationary coils, they are virtually immortal in high-vibration environments.
- CNC Machine Tool Axes: Used to verify exact ball-screw positioning, bypassing the backlash inherent in rotary encoders mounted on the motor.
- Hydraulic Cylinder Positioning: Integrated directly into the piston rod of heavy machinery. Models like the Temposonics R-Series are hermetically sealed to survive 350 bar (5,000 psi) of hydraulic fluid pressure.
- Aerospace Flight Controls: Redundant LVDTs measure the deflection of ailerons and elevators, chosen because they do not suffer from the wear-out mechanisms of resistive wipers.
From a procurement and integration standpoint, a basic 10mm stroke analog LVDT (like the Honeywell 060-1000 series) runs about $150 to $250. Industrial IP67 hydraulic versions push $800 to $1,200. You cannot wire a raw LVDT directly to an Arduino or a standard PLC analog input. You must use a synchronous demodulator IC—such as the Analog Devices AD698 LVDT signal conditioner—which converts the AC amplitude and phase into a clean, bipolar DC voltage (e.g., ±10V) or a 4-20mA current loop that a PLC can easily read.
LVDT vs. Resistive Potentiometer Comparison
While both devices measure linear displacement, their underlying physics dictate entirely different use cases. Refer to the Omega Engineering LVDT guide for deeper application notes on sensor selection.
| Criteria | Linear Differential Transformer (LVDT) | Resistive Slide Potentiometer |
|---|---|---|
| Resolution | Infinite (limited only by ADC/signal conditioner noise) | Finite (limited by wire winding pitch or carbon track granularity) |
| Friction & Wear | Zero friction; infinite mechanical life | High friction; wiper wears out over 100k-1M cycles |
| Signal Type | AC Voltage (requires demodulation) | DC Voltage (simple voltage divider) |
| Environmental Sealing | Excellent (core can be fully encapsulated in resin) | Poor (wiper slot allows dust and moisture ingress) |
| Typical Cost | $150 - $1,200+ | $5 - $50 |
Linear Differential Transformer FAQ
How do you wire a linear differential transformer to a PLC?
You do not wire the raw LVDT directly to the PLC. The LVDT outputs a high-frequency AC signal with phase-shifting properties that a standard DC analog input card cannot interpret. Instead, you wire the LVDT's primary and secondary leads to a dedicated LVDT-to-DC signal conditioner module (like a 4-20mA transmitter). The conditioner handles the AC excitation, synchronous demodulation, and phase detection, outputting a standard 4-20mA or 0-10V DC signal to the PLC. At the null position, the output is typically 12mA or 5V; full positive stroke is 20mA/10V, and full negative stroke is 4mA/0V.
What is the difference between a linear differential transformer and a potentiometer?
A potentiometer relies on physical contact: a metal wiper slides across a resistive carbon or wire-wound track, acting as a variable voltage divider. This contact causes friction, generates electrical noise (wiper bounce), and eventually wears out. A linear differential transformer uses electromagnetic induction. The ferromagnetic core slides through the air bore of the coils without touching them, resulting in zero friction, zero wiper noise, and a theoretically infinite mechanical lifespan, though it requires more complex AC drive circuitry.
Why does my linear differential transformer output read zero at the center position?
This is by design and is known as the 'electrical null.' The two secondary coils are wound in opposite directions and wired in series opposition. When the core is perfectly centered, it couples an identical amount of magnetic flux into both secondary coils. Because they are wired in opposition, the induced electromotive force (EMF) in Secondary 1 is exactly equal and opposite to the EMF in Secondary 2. They cancel each other out, resulting in a net zero voltage output, providing a highly accurate, repeatable physical reference point for your machine.
