What is an LVDT displacement sensor?
An LVDT (linear variable differential transformer) is an electromagnetic sensor that converts linear position into an electrical signal with no physical contact between its moving and stationary parts. Because the core glides freely inside the coil assembly, an LVDT does not wear, has effectively unlimited mechanical life and resolves very small movements.
The broader family of displacement sensors also includes non-contact inductive proximity probes, magnetostrictive rod sensors and gauging probes. Engineers use them to measure stroke, deflection, expansion, valve position and part dimensions wherever precision and durability outweigh the lowest possible cost.
How LVDT technology works
An LVDT has a primary winding between two identical secondary windings, all around a hollow bore. A ferromagnetic core, attached to the object being measured, moves inside the bore.
- The primary is excited with an AC signal, typically in the kilohertz range.
- The core couples magnetic flux into both secondaries.
- At the null position, the secondary voltages are equal and opposite, and the output is near zero.
- As the core moves, coupling shifts toward one secondary. The differential voltage grows linearly with displacement, and its phase relative to the excitation indicates direction.
A signal conditioner, either external or built into a DC LVDT, demodulates this AC signal into a DC voltage or current. Since nothing touches, output is smooth and continuous, and the sensor tolerates shock, vibration and contamination that would destroy a contacting sensor.
| Sensor type | Typical strength | Best fit |
|---|---|---|
| AC LVDT | High temperature, radiation and small size | Test rigs, turbines, research reactors |
| DC LVDT | Simple wiring with direct voltage or 4–20 mA output | Industrial position feedback, PLC inputs |
| Gauging probe | Sub-micron repeatability | Dimensional inspection fixtures |
| Submersible LVDT | Sealed for fluids or seawater | Hydraulics, subsea actuators |
| Inductive proximity sensor | Non-contact gap measurement | Shaft position, thermal expansion |
| Magnetostrictive | Long stroke in a cylinder | Hydraulic cylinder feedback |
Common applications
- Aerospace and defense test: structural deflection, actuator feedback and cryogenic component tests at labs around Huntsville’s NASA and Army test community.
- Power generation: turbine valve position, casing expansion and monitoring in nuclear and fossil plants across the Southeast.
- Automotive test: suspension travel, fatigue rigs and component durability testing for the region’s assembly plants and suppliers.
- Subsea, marine and offshore: ROV and actuator position, wellhead equipment and long-term seawater immersion.
- Manufacturing quality: in-process gauging and automated inspection fixtures.
- Civil and research: crack, settlement and bearing movement monitoring, plus university materials testing.
Choosing the right LVDT
Most LVDT problems are mechanical, not electrical. A core rubbing the bore, a misaligned mounting bracket or a range sized too tight will undermine a precise sensor. Start by defining travel, mounting and how the core connects to the moving part, then work out the electrical interface.
Environment usually narrows the choice quickly. Temperature, radiation, pressure and immersion may rule out internal electronics, which points to an AC LVDT with a remote conditioner. For a PLC input in a clean enclosure, a DC LVDT with 4–20 mA output saves panel space and wiring. When stated accuracy matters, buy and calibrate the sensor and conditioner as a matched pair.
Manufacturers we represent for LVDTs and displacement sensors
RDP Electrosense is the North American office of RDP Electronics, founded in Wolverhampton, UK, in 1966 and one of the early LVDT manufacturers. RDP was among the first to build signal conditioning into the LVDT body. Its current range runs from GT gauging probes and D5/D6 industrial LVDTs to DC LVDTs, submersible and seawater sensors, and LIN and PY inductive sensors for extreme temperature and radiation. RDP also supplies matching amplifiers, indicators and the Modular 600 multi-channel system, and it builds special versions when catalog sensors fall short.
Get local application support
Vaughn Associates’ Huntsville-based engineers work with aerospace, defense and industrial test teams on sensor selection, mounting and signal conditioning. Visit engineering support, request a quote, or find your area manager on the territories page.
