A linear voltage differential transformer (LVDT) is a frictionless electromagnetic sensor that converts precise physical linear movement into a proportional alternating current (AC) voltage output. In real-world installations, it changes the game by eliminating the mechanical wiper wear that inevitably destroys standard resistive potentiometers in high-cycle machinery, replacing physical contact with magnetic coupling. People commonly confuse LVDTs with basic inductive proximity sensors or RVDTs; a proximity sensor gives you a binary 'target present' switch, an RVDT measures rotational angle, while an LVDT gives you continuous, micron-level proportional linear position data.
The Core Mechanism: How an LVDT Actually Works
Think of an LVDT as a magnetic voltage divider. Inside the stainless steel or Inconel housing, there are three coils of wire wrapped around a hollow, non-magnetic bobbin. The center coil is the primary, and the two outer coils are the secondaries (Secondary 1 and Secondary 2). The secondaries are wound in series opposition, meaning their voltages subtract from one another.
A separate, cylindrical ferromagnetic core is attached to the object you are measuring. As the core slides in and out of the bobbin, it changes the magnetic coupling between the primary and the secondaries.
- At the Null Position (Center): The core couples equal magnetic flux to both secondaries. The induced voltages are equal and opposite, canceling each other out. The net output is theoretically zero volts.
- Moving toward Secondary 1: Flux increases in S1 and decreases in S2. The output voltage rises, and its AC phase aligns (0°) with the primary excitation.
- Moving toward Secondary 2: Flux increases in S2 and decreases in S1. The output voltage rises again, but the AC phase flips 180° relative to the primary.
This phase shift is critical: the amplitude of the AC output tells you how far the core moved, while the phase tells you which direction it moved.
Worked Numeric Example: Calculating Displacement Output
Let's run the math on a standard industrial AC-operated LVDT to see how the signal translates to physical movement. You will need the excitation voltage and the sensor's sensitivity rating, usually provided in millivolts per Volt per millimeter (mV/V/mm) or inch (mV/V/in).
Primary Excitation ($V_{exc}$): 3.0V RMS @ 3.0 kHz
Sensor Sensitivity ($S$): 4.2 mV/V/mm
Core Displacement ($d$): +5.0 mm from null
The formula for the secondary output voltage ($V_{out}$) is:
V_{out} = V_{exc} imes S imes d
Step 1: Calculate the RMS Output Voltage
V_{out} = 3.0V imes 4.2 mV/V/mm imes 5.0 mm
V_{out} = 63.0 mV RMS
Step 2: Determine the Phase
Because the displacement is positive (+5.0 mm, moving toward S1), the output signal is in-phase (0°) with the 3.0 kHz primary excitation. If the core had moved -5.0 mm, the amplitude would still be 63.0 mV RMS, but the phase would be 180° out-of-phase.
Where You Meet LVDTs in Practice
You will rarely find an LVDT in consumer electronics; they are heavy, require AC signal conditioning, and are relatively expensive (typically $250 to $800+ per unit). Instead, you meet them in harsh, high-stakes industrial and aerospace environments:
- Hydraulic Press Brakes & CNC Machinery: Mounted on the side cylinders to provide closed-loop feedback for bending angles. The LVDT survives the heavy vibration and oil mist that would blind an optical encoder.
- Automatic Gauge Control (AGC) in Steel Mills: Measuring the exact gap between massive rolling mill stands to control sheet metal thickness down to microns, despite extreme ambient heat and scale dust.
- Aerospace Flight Controls: Used inside hydraulic actuators to measure the position of ailerons and rudders. They are qualified to standards like MIL-PRF-23411 to survive extreme temperature cycling and radiation.
- Valve Position Feedback: Monitoring the stroke of main steam valves in power plants, where high ambient temperatures destroy standard plastic-housed potentiometers.
Decision Tree: LVDT vs. Potentiometer vs. Magnetostrictive
Choosing a linear position sensor comes down to balancing stroke length, environmental harshness, cycle life, and budget. Use this decision matrix to terminate your selection process.
| Condition / Requirement | Potentiometer (Resistive) | Magnetostrictive (Time-of-Flight) | Linear Voltage Differential Transformer (LVDT) |
|---|---|---|---|
| Cycle Life (>10 million cycles) | Fails (wiper wears out) | Passes (non-contact) | Passes (frictionless core) |
| Stroke Length (> 36 inches) | Available but fragile | Best Choice (up to 300+ inches) | Impractical (coil gets too long/heavy) |
| Harsh Environment (Oil, Coolant, Dust) | Fails (contaminates tracks) | Passes (sealed tube) | Best Choice (hermetically sealed, no electronics in probe) |
| Resolution & Linearity | Poor (~0.1%) | Good (~0.02%) | Best Choice (Infinite resolution, <0.1% linearity) |
| Budget | Lowest ($20 - $100) | High ($800 - $1,500+) | Medium/High ($300 - $800) |
Wiring and Signal Conditioning Pitfalls
If you are wiring a bare AC LVDT to a PLC or external signal conditioner, the physical wiring topology dictates your accuracy. The most common bench and jobsite mistake is ignoring cable length voltage drop.
4-Wire vs. 6-Wire Configurations
A basic LVDT has three coils, which theoretically requires 6 wires (2 for primary, 4 for secondaries). However, manufacturers often tie the secondary commons together internally, bringing out 4 wires.
- 4-Wire Hookup: Fine for short runs (under 10 feet). Over long cable runs, the resistance of the copper wire causes a voltage drop on the primary excitation lines. If the sensor only receives 2.8V instead of the expected 3.0V, your calculated displacement will be skewed by nearly 7%.
- 6-Wire Hookup: Uses two extra 'sense' wires connected directly to the primary coil terminals inside the sensor. The signal conditioner uses these sense wires to measure the actual voltage at the sensor and actively adjusts the drive current to maintain exactly 3.0V RMS at the primary, regardless of cable length or temperature-induced resistance changes. Always use 6-wire for runs over 15 feet.
Frequently Asked Questions
Q: What exactly is an LVDT in one sentence?
A: It is an electromagnetic transducer that uses a moving ferromagnetic core to vary the mutual inductance between a primary and two secondary coils, translating linear physical displacement into a proportional AC voltage signal.
Q: What does an LVDT change in a real circuit or installation?
A: It changes the feedback architecture from a simple DC voltage divider (which suffers from wiper noise, dead zones, and mechanical wear) to a ratiometric AC magnetic system that requires a carrier oscillator and demodulator but offers infinite mechanical life and infinite resolution.
Q: What do people commonly confuse an LVDT with?
A: They confuse it with inductive proximity sensors (which are binary on/off switches, not proportional) and RVDTs (Rotary Variable Differential Transformers, which measure angular rotation up to ±60° rather than linear stroke). They also confuse it with strain gauges, which measure material deformation (force/load) rather than gross physical displacement.
When specifying your next position feedback loop, default to the integrated DC-EC series for ease of use, or a bare 6-wire AC LVDT for maximum precision in custom test rigs. Do not compromise on signal conditioning; the sensor is only as accurate as the demodulator reading its phase.
