LVDT: Linear Variable Differential Transformer

Working Principle

A Linear Variable Differential Transformer (LVDT) is a precision non-contact displacement sensor that uses electromagnetic coupling to measure linear position. It consists of one primary coil and two secondary coils wound around a hollow tube, with a ferromagnetic core (the “armature”) that slides freely through the centre.

An AC excitation signal drives the primary coil. The core’s position modulates the magnetic coupling to each secondary coil differently. When centered, both secondaries produce equal voltages that cancel out (null output). As the core moves, one secondary couples more strongly, producing a differential voltage proportional to displacement.

Key advantage: The core does not touch the coils — there is zero friction, zero wear, and infinite resolution. LVDTs can last billions of cycles with no degradation.

Electrical Characteristics

Parameter	Typical Value
Measurement Range	±0.5 mm to ±500 mm
Linearity	±0.1–0.5% of full range
Resolution	Infinite (analog, limited by electronics)
Repeatability	<0.01% of full range
Excitation	AC: 1–10 kHz, 1–10 V RMS
Output	AC voltage (mV/mm), or DC with signal conditioner
Operating Temperature	−55 to +150 °C (specialized: +200 °C)
Lifetime	Essentially unlimited (no contact wear)

Interfacing with an MCU

LVDTs require a signal conditioner (excitation + demodulation) to convert the AC output to a DC voltage proportional to position. Common solutions:

• Dedicated LVDT conditioner IC (e.g., AD698, LTC1966): Provides excitation, demodulation, and DC output in one chip
• External module: Calex, Measurement Specialties, Micro-Epsilon offer DIN-rail modules with ±10 V or 4–20 mA output
• DIY with op-amp: Synchronous demodulation circuit using the excitation signal as reference

Calibration

• Null adjustment: Position the core at mechanical zero; trim the conditioner offset to read exactly 0 V
• Span calibration: Move to a known displacement (gauge block or micrometer); adjust gain to read correct value
• Phase check: Verify the demodulator phase matches the excitation; incorrect phase inverts the polarity
• 5-point linearity: Measure at 0%, 25%, 50%, 75%, 100% stroke; linearity should be within ±0.5%

Code Example

/*
 * LVDT with DC-output conditioner module — Arduino
 * Conditioner outputs 0-5V for ±25mm range
 * Wiring: Conditioner Vout → A0
 */
#define LVDT_PIN A0
#define RANGE_MM 50.0    /* Total range (±25mm = 50mm span) */
#define ZERO_V   2.5     /* Voltage at center (0 mm) */
#define SPAN_V   5.0     /* Full scale voltage */

void setup() {
    Serial.begin(9600);
    Serial.println("LVDT Position Sensor Ready");
}

void loop() {
    /* Average 16 samples for stability */
    long sum = 0;
    for (int i = 0; i < 16; i++) {
        sum += analogRead(LVDT_PIN);
        delayMicroseconds(500);
    }
    float voltage = (sum / 16.0) * (5.0 / 1024.0);

    /* Convert voltage to displacement */
    float displacement = (voltage - ZERO_V) / SPAN_V * RANGE_MM;

    Serial.print("Position: ");
    if (displacement >= 0) Serial.print("+");
    Serial.print(displacement, 2);
    Serial.print(" mm  (");
    Serial.print(voltage, 3);
    Serial.println(" V)");

    delay(200);
}

Real-World Applications

Limitations

• Signal conditioning: Requires excitation, demodulation, and possibly amplification circuitry — not plug-and-play.
• Cost: Industrial LVDTs cost $50–$500+; much more expensive than potentiometers.
• Core alignment: The core must move coaxially; lateral misalignment introduces errors.
• AC susceptibility: Sensitive to external magnetic fields; shielded versions available at extra cost.
• Physical size: The coil assembly can be bulky for small displacement ranges.