LVDT Sensors: How Linear Variable Differential Transformers Measure Position

LVDT sensors measure linear position using a transformer with a movable core. An AC signal drives a primary coil, and two secondary coils pick it up. The core's position determines which secondary receives more of the signal. Because the core never touches the coils, nothing wears out, and resolution is limited only by the readout electronics.

LVDT stands for linear variable differential transformer. The design dates to the 1940s and remains the default choice in jet engine actuators, turbine valves, hydraulic cylinders, and metrology gauge heads. These are places where a position reading has to survive heat, vibration, and millions of cycles without drifting.

How does an LVDT work?

An LVDT works by mutual induction. An AC excitation voltage on the primary coil induces voltages in two secondary coils wired in series opposition. With the core centered, those voltages cancel. Move the core and one secondary couples more strongly than the other, so output amplitude tracks distance and output phase shows direction.

Here is the sequence:

1. Excitation. A sine wave drives the primary coil, typically a few volts rms at 1 to 10 kHz.
2. Coupling. A ferromagnetic core on a nonmagnetic push rod channels flux from the primary into the two secondaries. The core rides in the bore without touching it.
3. Null. With the core dead center, both secondaries see equal flux. Wired in opposition, their voltages subtract to nearly zero. The small leftover is the null voltage.
4. Displacement. Move the core toward secondary A and its voltage rises while B's falls. The difference grows in proportion to displacement across the rated stroke.
5. Direction. Crossing null flips the output phase by 180 degrees relative to the excitation. Amplitude alone can't tell left from right, but phase can.

Think of a balance scale. The secondaries are the pans and the core is a weight sliding along the beam. Centered, the scale reads zero. Slide the weight either way and the scale tips in proportion to the displacement, with the lower pan showing direction.

Within the linear range, the output reduces to a simple relationship, where k is sensitivity and x is core displacement from null:

V_out = V_A − V_B ≈ k × x

Datasheets usually quote k in mV per volt of excitation per millimeter (mV/V/mm), so you can predict output at any excitation level.

AC vs DC LVDT sensors: which output do you need?

An AC LVDT is just the coils and core, so it needs an external signal conditioner to supply excitation and demodulate the output. A DC LVDT puts the electronics inside the housing: apply DC power and read a DC voltage or a 4-20 mA current. Choose AC for extreme environments and DC for fast integration.

Either way, the signal chain does the same three jobs:

• Phase-sensitive demodulation. A synchronous demodulator compares the output with the excitation, producing a DC value whose sign carries direction.
• Ratiometric measurement. Five-wire LVDTs bring out both secondaries separately. Computing (V_A − V_B) / (V_A + V_B) cancels drift in excitation amplitude and much of the temperature effect.
• Filtering. A low-pass filter removes the carrier. Keep usable bandwidth at roughly a tenth of the excitation frequency, so a 5 kHz carrier supports about 500 Hz. This matters for fast actuators.

What specs matter on an LVDT sensor datasheet?

Four specs decide fit: stroke, linearity, sensitivity, and operating temperature. Stroke tells you whether the sensor covers your travel. Linearity sets the accuracy you can claim. Sensitivity sets signal level at your excitation. The temperature rating tells you whether it survives the install. Core style and output type come next.

• Stroke (full-scale range). Quoted as ± from null, for example ±25 mm. Keep normal travel well inside it, since linearity degrades at the ends.
• Linearity error. Maximum deviation from a best-fit straight line, as a percentage of full scale. Many general-purpose units land in the tenths of a percent.
• Sensitivity. Output per volt of excitation per unit of travel (mV/V/mm). Higher sensitivity means more signal above the noise floor.
• Excitation. The recommended voltage and frequency range for AC units, or the supply range for DC units. Running outside it shifts sensitivity and phase.
• Temperature range and coefficient. The survivable range, plus zero and sensitivity drift per degree.
• Null voltage. Residual output at center. Lower is better for measurements that hover near zero.
• Core style. Free cores ride loose on your own linkage. Guided cores run on a captive bearing and tolerate side loads. Spring-return gauge heads push against the part like a dial indicator.
• Environmental sealing. IP rating, pressure rating, and submersion capability for hydraulic or outdoor installs.
• Output (DC units). Common options are 0 to 10 V, ±10 V, and 4 to 20 mA. Match the input card you already have.

LVDT advantages and limitations

The main advantage of LVDT sensors is contactless sensing: no wiper, no brush, no wear, so accuracy holds over years of cycling. The main limitations are the need for AC excitation and demodulation, a housing longer than the stroke, and a higher price than simple potentiometers. For harsh or high-cycle duty, the trade usually favors the LVDT.

Advantages

• No friction or wear. The core never touches the coils, so the sensing element has no mechanical life limit.
• Effectively infinite resolution. Output changes continuously with core position. The practical limit is noise in the signal conditioner.
• Stable null. Coil geometry sets the null point, so it repeats well and gives a reliable zero reference.
• Electrical isolation. Primary and secondary windings are separate, so excitation and output circuits are isolated.
• Survives extreme environments. With no internal electronics, AC units can be built for high temperature, radiation, high pressure, and full submersion.

Limitations

• Needs signal conditioning. AC units require excitation and demodulation electronics, which adds cost and design effort.
• Length. The housing must be longer than the stroke, which can be a problem in tight envelopes.
• Magnetic interference. Strong nearby fields can couple into the coils unless the housing is shielded.
• Bandwidth. Response speed is capped by the carrier frequency, so very fast dynamic events may need a different sensor.
• Cost. An LVDT plus conditioner usually costs more than a potentiometer covering the same travel.

How do LVDT sensors compare with other linear position sensors?

LVDT sensors beat linear potentiometers on life and resolution, and they beat most electronic sensors on heat and radiation tolerance. Magnetostrictive sensors win on long strokes, optical encoders win on digital precision in clean conditions, and potentiometers win on price. The right pick depends on stroke, environment, and budget.

Use the table as a first filter. Datasheet numbers for your shortlisted parts should settle the final choice.

Where are LVDT sensors used?

LVDT sensors show up wherever position feedback must stay accurate through heat, vibration, contamination, or very long service lives. Typical homes include aircraft actuators, steam and gas turbine valves, hydraulic cylinders, materials testing machines, and precision gauging stations. Nuclear plants use radiation-tolerant versions for control rod position.

• Aerospace. Position feedback on flight-control actuators, servo valves, and engine fuel metering.
• Power generation. Valve position on steam and gas turbines, where heat and vibration rule out most alternatives.
• Industrial hydraulics. Cylinder position inside or alongside the rod for closed-loop motion control.
• Test and measurement. Extensometers and displacement transducers on tensile and fatigue test frames.
• Metrology and quality control. Spring-return gauge heads that measure part dimensions on inspection fixtures.
• Civil and structural monitoring. Crack, joint, and settlement movement in bridges, dams, and buildings.

How to choose the right LVDT sensor

1. Define the stroke. Measure total travel and add margin so normal operation stays in the most linear part of the range.
2. Set the accuracy target. Convert your tolerance to a percentage of full scale and compare it with the linearity spec.
3. Describe the environment. List temperature extremes, pressure, fluids, vibration, shock, and any radiation exposure.
4. Pick AC or DC. Go AC when the environment is too hot for electronics or the sensor must be tiny. Go DC when you want to wire straight into a PLC or DAQ.
5. Choose the core style. Free core for clean linkages, guided core for side loads, spring-return gauge head for contact measurement.
6. Check mounting and cabling. Confirm housing length, connector or pigtail, cable length, and shielding before you freeze the mechanical design.

Our take: if a position sensor has to run hot, wet, or for millions of cycles, start your shortlist with LVDT sensors and move to a potentiometer only when cost forces it. The higher upfront price often pays for itself the first time you avoid a field replacement.

Not sure which LVDT fits your application? Talk to a CDI sensor specialist.

Frequently asked questions

What is the difference between an LVDT and an RVDT?

An LVDT measures linear displacement, while an RVDT (rotary variable differential transformer) measures angular displacement. Both use the same principle: an excited primary coil, two opposing secondaries, and a moving ferromagnetic core. In an RVDT the core is a shaped rotor, and the linear range typically covers a limited angle rather than full rotation.

Why is LVDT resolution described as infinite?

Because the output changes continuously as the core moves. There are no steps, wire turns, or wiper contacts to quantize the signal. In practice, resolution is set by the noise and bit depth of the signal conditioner and data acquisition system, not the sensor.

What is LVDT null voltage?

Null voltage is the small output an LVDT still produces with the core exactly at center. It comes from slight imbalances between the secondary windings, stray capacitance, and harmonics in the excitation. A phase-sensitive demodulator rejects most of it, and datasheets list the maximum so you can judge accuracy near zero.

Can LVDT sensors work in harsh environments?

Yes. LVDT sensors are among the most rugged position sensors available because the sensing element has no electronics or moving contacts. AC versions can be built for high temperature, radiation, high pressure, and full submersion in hydraulic fluid or water. DC versions are limited by their internal electronics, so check the rated temperature range.

Do LVDT sensors need calibration?

An LVDT arrives with a factory sensitivity figure, but you should calibrate the whole measurement chain together. Excitation level, cable length, and the signal conditioner all shift the output slightly. A two-point check at null and near full stroke is usually enough, with periodic rechecks in critical or high-temperature applications.