An LVDT (Linear Variable Differential Transformer) is an electromagnetic sensor that converts precise linear mechanical displacement into a proportional alternating current (AC) electrical signal without any physical electrical contact between its moving core and the coil assembly. When you drop an LVDT into a control system, it fundamentally changes your circuit architecture: you can no longer just read a simple DC voltage with a microcontroller's ADC. Instead, you must provide a stable AC excitation signal (typically a 1 kHz to 10 kHz sine wave) and use a synchronous demodulator to decode both the amplitude (distance) and phase (direction) of the returning differential signal.
Core Specifications and Selection Data
Before wiring up a sensor, you need to understand the datasheet parameters. LVDT performance varies wildly between standard industrial automation models (like the TE Connectivity GH-Series) and high-temperature aerospace variants. Below is a data-dense comparison of what you will actually see on a spec sheet when sourcing these components.
| Parameter | Standard Industrial (e.g., TE GH-Series) | High-Precision / Aerospace (e.g., Sensata/Macome) | Unit / Notes |
|---|---|---|---|
| Linearity Error | ±0.25% of Full Scale | ±0.05% of Full Scale | Lower is better; dictates absolute accuracy |
| Static Sensitivity | 2.5 mV/V/mm | 1.8 mV/V/mm | Output per volt of excitation per mm of travel |
| Excitation Voltage | 1.0 to 5.0 Vrms | 3.0 to 10.0 Vrms | Must be a pure sine wave to avoid harmonic distortion |
| Excitation Frequency | 1 kHz to 5 kHz | 400 Hz to 10 kHz | Higher frequencies allow faster dynamic response |
| Operating Temperature | -55°C to +150°C | -55°C to +260°C | Coil insulation limits the upper bound |
| Core Pull Force | 0.5 to 2.0 grams | < 0.1 grams (air bearing) | Magnetic attraction between core and stator |
Notice the Static Sensitivity row. This is the most critical number for your signal conditioning design. It tells you exactly how many millivolts you get out for every volt you put in, per millimeter of travel. If your excitation voltage drifts by 5%, your position reading will drift by 5% unless you use a ratiometric measurement technique.
How an LVDT Works: The Magnetic Circuit
Think of an LVDT as a center-tapped transformer with a movable core. Inside the stainless steel housing, there are three coils wound around a hollow, non-magnetic bobbin. The primary coil sits exactly in the center, flanked by two identical secondary coils (Secondary 1 and Secondary 2). The two secondaries are wired in series-opposition, meaning their voltages subtract from one another.
A separate ferromagnetic rod (the core) slides freely through the hollow center. When you apply an AC sine wave to the primary coil, it generates an alternating magnetic field. This field couples to the secondary coils via the movable core.
- Null Position (Dead Center): The core is perfectly centered. The magnetic flux couples equally to both secondary coils. Because they are wired in opposition, the voltages cancel out perfectly. Output = 0 Vrms.
- Positive Displacement: The core moves toward Secondary 1. More flux couples to S1 than S2. The differential output voltage rises, and the phase matches the primary excitation (0° phase shift).
- Negative Displacement: The core moves toward Secondary 2. More flux couples to S2. The output voltage rises again, but the phase flips 180° relative to the primary.
Worked Numeric Example: Calculating LVDT Output
Given: An LVDT with a sensitivity of 2.4 mV/V/mm. You drive the primary coil with a 3.0 Vrms sine wave at 3 kHz. The core is pushed +6.0 mm from the null position.
Formula: V_out = Excitation × Sensitivity × Displacement
Calculation: V_out = 3.0 V × 2.4 mV/V/mm × 6.0 mm = 43.2 mVrms
Result: Your demodulator will read a 43.2 mVrms signal at a 0° phase angle. If the core had moved -6.0 mm instead, the magnitude would still be 43.2 mVrms, but the phase angle would be 180°, telling the controller the core moved left instead of right.
Where You Meet LVDTs in Practice
You won't typically find LVDTs in consumer electronics or basic hobby projects due to the cost of the sensor and the required signal conditioning. You will, however, find them bolted to heavy machinery where failure is not an option and infinite mechanical life is required.
Hydraulic Press Brakes and CNC Machines: LVDTs are the standard for closed-loop position feedback on hydraulic cylinders. Because the core is completely enclosed and can be separated from the coil body, the coil assembly can be mounted outside a hydraulic cylinder while the core is attached to the piston rod inside, separated by a non-magnetic stainless steel barrier. This allows the LVDT to measure position inside a high-pressure, oil-filled environment without any seals or dynamic O-rings wearing out.
Steel Mill Automatic Gauge Control (AGC): In rolling mills, the thickness of the steel sheet is controlled by hydraulic actuators that require sub-millimeter precision under massive vibration and heat. LVDTs survive the 150°C+ ambient temperatures and constant mechanical shock that would instantly destroy an optical encoder or a resistive potentiometer.
Signal Conditioning in the Panel: On the bench or in the control panel, the raw LVDT wires connect to a signal conditioner. Modern designs use dedicated ICs like the Texas Instruments PGA970 or standalone DIN-rail modules. These modules generate the 3 kHz sine wave, excite the primary, read the secondaries, perform the synchronous demodulation (using a phase-locked loop to detect the 0° or 180° shift), and output a clean 0-10V DC or 4-20mA signal to the PLC.
LVDT vs. Potentiometers and Hall Effect Sensors
Beginners often confuse LVDTs with string potentiometers (string pots) or basic linear Hall effect sensors. Here is why engineers pay the premium for LVDTs in critical applications.
| Feature | LVDT (Inductive) | String Potentiometer (Resistive) | Linear Hall Effect (Magnetic) |
|---|---|---|---|
| Friction / Wear | Zero (frictionless core) | High (physical wiper on carbon track) | Zero (non-contact) |
| Linearity over Long Stroke | Excellent (±0.1% to ±0.5%) | Good, but degrades with wiper wear | Poor (requires complex magnet shaping) |
| Resolution | Infinite (analog magnetic coupling) | Limited by wire winding or carbon grain | Limited by ADC and thermal noise |
| Environmental Sealing | Excellent (core can be isolated) | Poor (spring and string mechanism jams) | Good, but magnet must be very close |
| Circuit Complexity | High (needs AC excitation + demodulator) | Low (simple DC voltage divider) | Low (DC supply, analog voltage out) |
Common Confusion: The RVDT. Do not confuse an LVDT with an RVDT (Rotary Variable Differential Transformer). An RVDT uses the exact same electromagnetic principle and requires the same AC signal conditioning, but it is designed to measure angular rotation rather than linear stroke. RVDTs are typically limited to a ±40° rotational range and are heavily used in aerospace flight control surface feedback (ailerons and flaps).
Frequently Asked Questions
Can I drive an LVDT with a DC square wave from a microcontroller?
Technically, the transformer will couple a square wave, but you will introduce massive harmonic distortion and severe eddy current losses in the ferromagnetic core, which will cause the core to heat up and alter the sensor's calibration. Always use a pure, low-distortion AC sine wave for the excitation source.
What happens if the LVDT core is pulled completely out of the coil?
The sensor will not be damaged. However, the magnetic coupling will drop to near zero, and the output voltage will collapse to the null point (0 Vrms) or saturate depending on the specific demodulator's fault-detection circuitry. Most industrial signal conditioners will flag a 'core out of range' fault if the signal drops below a specific threshold.
Why do LVDTs have a 'residual voltage' at the null point?
In a perfect world, the null voltage is exactly 0.000 V. In reality, slight asymmetries in the coil winding, stray capacitance between the primary and secondary wires, and external magnetic interference result in a small residual voltage (typically < 1% of full-scale output) at the exact center. High-end signal conditioners include a 'null offset' trim pot or digital register to zero this out during commissioning.
