An LVDT (linear variable differential transformer) measures linear displacement through the magnetic coupling between an AC-excited primary coil, two secondary coils and a free-moving ferromagnetic core. Because the core never touches the coils, an LVDT has no friction, no mechanical wear and effectively unlimited resolution. That makes it the default choice when durability, harsh environments or fine resolution matter, while other technologies win on cost, very long strokes or measuring targets you can’t touch.
How does an LVDT work?
An LVDT converts core position into a voltage whose amplitude is proportional to displacement and whose phase indicates direction. Inside the housing, a primary coil is driven by an AC excitation signal. Two secondary coils sit on either side of it and are connected in series opposition.
When the core is centered at the null position, it couples equally into both secondaries, and their voltages cancel. As the core moves toward one secondary, that coil’s voltage rises while the other falls. The differential output grows in proportion to displacement across the sensor’s rated linear range, and its phase relative to the excitation flips as the core crosses null.
A signal conditioner supplies the excitation, then uses phase-sensitive demodulation to turn the AC output into a DC signal with polarity that indicates direction. Resolution is limited by the conditioning electronics and noise, not by the sensor’s mechanics.
AC LVDTs vs. DC LVDTs
- AC LVDTs contain only coils. They need an external conditioner but tolerate high temperatures, radiation and small packaging better, because there are no electronics inside.
- DC LVDTs (internally amplified) build the oscillator and demodulator into the sensor body. They run from a DC supply and output a ready-to-use signal. RDP was an early developer of this approach, and its DCTH, LDC, DCV and DCC families provide outputs such as 0–10 V, ±5 V or two-wire 4–20 mA.
Why choose an LVDT over other displacement sensors?
Choose an LVDT when you need long life, high resolution and reliable performance in a demanding environment. The main advantages are:
- No contact, no wear. Millions of cycles don’t degrade the measurement.
- Effectively unlimited resolution and excellent null repeatability.
- Rugged construction. The coil assembly can be sealed against fluids, pressure and contamination.
- Absolute output. Position is known immediately at power-up.
- Environmental range. Coil-only designs survive conditions that destroy electronics.
The limitations are worth weighing too. An LVDT needs excitation and conditioning, the body is considerably longer than its measuring range, the core must be kept aligned, and cost is higher than a simple potentiometer.
LVDT vs. potentiometer vs. magnetostrictive and non-contact sensors
Each displacement technology has a natural home. This comparison is qualitative; always check the specific model’s datasheet.
| Technology | Contact? | Strengths | Watch-outs | Typical fit |
|---|---|---|---|---|
| AC LVDT | No | Unlimited resolution, long life, heat and radiation tolerance | Needs external conditioner; core alignment | Test rigs, turbines, nuclear, gauging |
| DC LVDT | No | Simple wiring, direct voltage or 4–20 mA output | Internal electronics limit temperature range | Industrial position feedback, OEM machines |
| Linear potentiometer | Yes (wiper) | Low cost, simple DC output | Wiper wear, especially under vibration or dither | Low-cycle, cost-driven applications |
| Magnetostrictive | No | Long strokes, absolute output, fits inside hydraulic cylinders | Head electronics; stroke-dependent update rate | Hydraulic cylinder feedback |
| Draw-wire (string pot) | Yes (cable) | Very long ranges, easy retrofit | Cable wear and sag; lower resolution | Large structures, vehicle travel |
| Eddy current / inductive proximity | No | Small gaps, fast response, measures a target surface | Target material and short range | Shaft motion, gap measurement |
| Laser triangulation | No | Measures without touching the part | Surface finish, dirt and optical path | Profiling, delicate or hot parts |
RDP Electrosense’s range covers several of these: industrial and gauging LVDTs, DC LVDTs, magnetostrictive (Micropulse) position sensors, and PY non-contact proximity sensors that work with ferritic targets at gaps of about 1.5–6 mm. See the RDP Electrosense line page for current families.
How do you select an LVDT?
Selecting an LVDT means matching range, output, mechanical style and environment to the job. Work through these decisions in order.
Measuring range and linearity
Pick a range that covers your full travel with some margin, because linearity is specified over the rated range. RDP’s D5 and D6 industrial LVDTs span roughly ±0.25 mm to ±75 mm with ±0.5% linearity standard and ±0.25% or ±0.1% options, while the DCTH family extends to ±470 mm.
Mechanical configuration
- Unguided core: no contact at all, but your mechanism must guide the core.
- Guided or captive: bearings keep the core aligned for easier installation.
- Spring-return gauging probes: the tip rests on the part. RDP’s GT gauging heads offer repeatability of about 0.15 µm, with air-push options for automated fixtures.
Environment
Temperature, pressure, fluids and radiation often decide the design. RDP’s D5W submersible LVDT is rated to 3.4 MPa, SSA and SSD seawater models are designed for long-term immersion beyond 2 km, and LIN and PY differential-inductance sensors operate from −220 °C in standard models up to 600 °C in high-temperature variants, with pressure ratings to 20 MPa and radiation tolerance up to about 1 GGy on some models.
Signal conditioning and output
Match the conditioner to the sensor and installation. RDP’s S7AC panel amplifiers and DR7 DIN-rail units condition AC LVDTs, the S7TW is loop-powered for 4–20 mA, the E309 indicator adds a local display, and the Modular 600 rack handles multi-channel test stands mixing LVDT and strain gauge channels. The same families condition load cells and force sensors, which simplifies mixed test rigs.
Where are LVDTs used?
LVDTs appear wherever position must be measured reliably for years or under abuse:
- Aerospace and defense test: actuator position feedback, structural test deflection and cryogenic rigs, common in Huntsville-area test labs.
- Power generation: turbine casing expansion and valve position in fossil and nuclear plants across the Southeast.
- Automotive and rail test: suspension travel and fatigue rig displacement.
- Subsea and marine: ROV and subsea actuator position, including equipment built for offshore work in the Gulf.
- Manufacturing quality: in-process dimensional gauging and inspection fixtures.
- Civil monitoring: crack, settlement and bearing movement.
What are the most common LVDT installation mistakes?
Most field problems come from the installation, not the sensor:
- Core misalignment. An unguided core rubbing the bore adds friction and noise.
- Magnetic mounting materials. Ferromagnetic brackets or core extension rods near the coils distort the output. Use non-magnetic materials.
- Operating past the rated range. Output becomes nonlinear beyond the linear stroke.
- Calibrating without the real cable. Cable capacitance can shift phase and gain on AC LVDTs.
- Poor shielding and grounding. Ground loops show up as noise, especially near drives and welders.
- Mismatched excitation. Conditioner frequency and amplitude should match the sensor’s calibration.
Getting LVDT application help in the Southeast
Vaughn Associates represents RDP Electrosense across our territories (North Carolina only in our North Carolina, Virginia, Maryland and D.C. territory). Our sales engineers can review stroke, environment, mounting and output requirements, recommend sensor and conditioner combinations, and coordinate quotes directly from RDP.
Explore our LVDT and displacement sensor solutions, find your area manager on the territories map, or request application help with your stroke, environment and output requirements.
