An LVDT (Linear Variable Differential Transformer) is an inductive sensor that converts linear mechanical motion into a proportional alternating current (AC) electrical signal without physical contact between the moving core and the coil assembly. In a real control circuit, swapping a standard DC position sensor for an LVDT fundamentally changes your hardware requirements: you must introduce an AC oscillator for excitation and a synchronous demodulator (signal conditioner) to convert the phase-sensitive AC output into a usable DC voltage for your PLC or microcontroller ADC.
The Core Working Principle (No Magnets, Just Induction)
Unlike Hall effect sensors that rely on permanent magnets, an LVDT operates purely on electromagnetic induction, functioning essentially as a variable-ratio transformer. The assembly consists of three solenoid coils wound around a hollow, non-magnetic cylindrical form:
- Primary Coil (P): Located in the center, driven by an external AC excitation source (typically 1 kHz to 10 kHz).
- Secondary Coils (S1 and S2): Wound symmetrically on either side of the primary, connected in series-opposition (differential).
A separate, ferromagnetic rod (the core) slides freely through the hollow bore. When the core is dead center (the null position), the magnetic flux coupling to S1 and S2 is identical. Because the secondaries are wired in opposition, their induced voltages cancel out, resulting in a net output of zero.
As the core moves off-center, flux coupling increases in one secondary and decreases in the other, generating a differential AC voltage. The most critical detail for circuit designers is the phase relationship. When the core crosses the null point from left to right, the output AC waveform undergoes an exact 180-degree phase shift relative to the primary excitation. This phase shift is how the signal conditioner determines the direction of travel, while the voltage amplitude dictates the distance.
Worked Numeric Example: Sizing the Signal Conditioner
To interface an LVDT with a standard 0-10V PLC analog input card, you need a signal conditioner. Sizing this module requires calculating the maximum expected raw AC output based on the sensor's sensitivity rating.
Let's assume we are using a standard industrial LVDT with the following datasheet specifications (assuming 25°C ambient and standard copper wiring):
- Sensitivity: 2.45 mV/V/mm
- Excitation Voltage: 5.0 VRMS
- Full Stroke Range: ±15 mm (Total 30 mm)
The Calculation:
If the mechanical linkage pushes the core +6.0 mm from the null position, the raw AC output voltage is calculated as:
V_out = Sensitivity × V_excitation × Displacement
V_out = 2.45 mV/V/mm × 5.0 V_RMS × 6.0 mm
V_out = 73.5 mV_RMS
At the absolute maximum stroke (+15 mm), the output peaks at 183.75 mVRMS. Therefore, your signal conditioner must be configured to accept a ±200 mVRMS AC input, synchronously demodulate it (using the phase to assign positive/negative polarity), amplify it, and output a ±10V DC signal to the PLC.
| Parameter | Typical Industrial Spec | Impact on Circuit Design |
|---|---|---|
| Excitation Frequency | 3 kHz - 5 kHz | Conditioner oscillator must match sensor tuning; mismatched frequencies cause severe linearity errors. |
| Linearity Error | ±0.25% of Full Scale | Determines the maximum resolution you can reliably extract without software calibration tables. |
| Input Impedance | ~150 Ω to 400 Ω | Excitation source must be capable of driving low-impedance inductive loads without voltage sag. |
Where You Meet LVDTs in Practice
You will rarely find LVDTs in consumer electronics; they dominate heavy industrial, aerospace, and high-precision manufacturing environments where reliability is non-negotiable. Common applications include:
- Hydraulic Cylinder Positioning: Mill-type hydraulic presses use internal LVDTs (like the TE Connectivity GH-Series) to monitor ram position. Because the core is frictionless, they survive millions of cycles without the wiper degradation that destroys potentiometers.
- CNC Machine Tool Axes: Used for closed-loop feedback on slide positions where sub-micron repeatability is required.
- Steam Turbine Valve Actuation: Power plants use high-temperature LVDTs to monitor governor valve stems, as the inductive design survives ambient temperatures exceeding 150°C without the thermal drift seen in semiconductor-based sensors.
What People Commonly Confuse With an LVDT
When specifying linear displacement sensors, engineers frequently conflate LVDTs with other technologies. Here is how to tell them apart:
- String Potentiometers (String Pots): These use a spring-loaded cable wound around a standard resistive potentiometer. They output a simple DC voltage ratio and require no signal conditioner. However, the mechanical wiper wears out over time, and the cable can snap. LVDTs are frictionless and virtually immortal, but require complex AC drive circuitry.
- Magnetostrictive Sensors (e.g., MTS Temposonics): Often housed in the same stainless-steel cylindrical form factors as LVDTs, these use a moving magnet and measure the time-of-flight of a torsional strain pulse down a waveguide. They are absolute position sensors (no homing required) but are significantly more expensive and susceptible to external magnetic interference.
- RVDTs (Rotary Variable Differential Transformer): The exact same electrical principle, but designed for angular rotation (typically ±40°) rather than linear stroke, commonly used in aircraft flight control linkages.
Frequently Asked Questions About LVDT Sensors
What is the difference between an LVDT and a string potentiometer?
A string potentiometer relies on physical contact between a carbon wiper and a resistive track to output a DC voltage, making it prone to mechanical wear, contact noise, and cable snap-back. An LVDT uses electromagnetic induction with a freely sliding ferromagnetic core, offering infinite mechanical life, zero friction, and higher resolution, though it requires an external AC signal conditioner to operate.
Can I wire an LVDT directly to an Arduino or PLC analog input?
No. An Arduino or standard PLC analog input expects a 0-5V or 0-10V DC signal. An LVDT outputs a low-level (typically <200mV) AC sine wave that contains both magnitude and phase data. You must place an LVDT signal conditioner (demodulator) between the sensor and the controller to convert the AC waveform into a proportional, polarity-aware DC voltage.
Why does my LVDT output read zero at the center position?
This is by design. The center of the coil assembly is the "null point." Here, the magnetic flux couples equally to both secondary coils. Because the secondaries are wired in series-opposition, their induced voltages are exactly equal and opposite, canceling each other out to produce 0V. As the core moves away from this null point in either direction, the voltage increases proportionally.
How much does a typical industrial LVDT cost?
Current market rates for industrial-grade, IP67-rated LVDTs (such as those from Omega Engineering or TE Connectivity) range from $250 to $800 depending on stroke length and housing material. The required DC-powered signal conditioner modules typically add another $150 to $400 to the total bill of materials.
