LVDT Displacement Sensor

What an LVDT Is

A linear variable differential transformer is an electromechanical transducer that produces an AC voltage proportional to the linear displacement of a movable magnetic core. Mechanically it is a transformer: one primary winding sits at the center of a cylindrical bobbin, and two secondary windings of equal turns are placed symmetrically on either side. The two secondaries are connected in series opposition, meaning their induced voltages subtract rather than add. A high-permeability soft-iron core slides freely along the axis of the coils and is mechanically coupled, usually through a non-magnetic pushrod, to the object being measured.

The device works on mutual induction. When the primary is energized with AC excitation, magnetic flux links into both secondaries through the core. At the electrical null, the core is centered, equal flux reaches each secondary, the two opposed voltages cancel, and the differential output approaches zero. Move the core toward one secondary and that winding couples more flux while the opposite winding couples less, so the differential output rises in proportion to displacement. The output phase, measured against the excitation, is one sense on one side of null and inverts on the other, which is how the demodulator recovers direction as well as magnitude.

Two characteristics flow directly from this construction and explain why LVDTs persist where cheaper sensors exist. First, the core makes no electrical contact with the windings, so there is no wiper, no sliding resistance track, and no contact to oxidize or wear. That gives the LVDT theoretically infinite resolution, since the only limit on the smallest detectable step is the electronic noise floor of the conditioner, not any mechanical quantization. Second, the same frictionless coupling yields outstanding null repeatability and long-term stability: a well-made LVDT returns to the same null reading after millions of cycles, which is why it dominates fatigue-test and flight-control feedback.

The principle dates to the 1930s, when the differential-transformer arrangement was developed for position sensing, and it matured through aerospace and instrumentation use across the following decades. The core idea has barely changed because it does not need to: the physics of a balanced differential transformer is hard to improve on for a robust, contactless, absolute-within-stroke linear measurement. What has evolved is the electronics, from discrete demodulators to integrated DC-LVDT conditioners and digital signal processing that linearizes and temperature-compensates the raw differential signal.

It helps to separate the LVDT from its relatives at the outset. A potentiometric position sensor uses a sliding wiper and wears; an LVDT does not. A magnetostrictive or laser sensor measures longer strokes but costs more and is more sensitive to environment over short ranges. The LVDT occupies the band where short to medium stroke, high repeatability, ruggedness, and tolerance of heat, pressure, and radiation matter more than absolute span. Its close cousin the RVDT applies the identical opposed-secondary principle to rotary motion within a bounded angle.

AC and DC Architectures

The single most consequential split in the LVDT catalog is between AC-LVDTs and DC-LVDTs. The transducer element is the same balanced differential transformer in both, but the location of the excitation oscillator and the demodulator differs, and that placement decides wiring, temperature range, accuracy, and how you read the signal. Choosing the wrong architecture is the most common LVDT selection error, so it deserves a chapter of its own. The table below frames the trade-off before the discussion.

An AC-LVDT ships as a passive coil set. You supply primary excitation from an external oscillator, commonly 3 to 15 V RMS depending on the series, at a carrier frequency between 1 and 10 kHz, and you recover displacement with an external synchronous demodulator. Because no semiconductors live inside the sensor body, an AC-LVDT tolerates the widest environment: high-temperature variants run to 150 degrees Celsius and beyond, survive radiation and strong vibration, and place all heat-sensitive electronics in a remote conditioner where they can be cooled and shielded. The penalty is system complexity. You must provide a matched conditioner, route a shielded multi-conductor cable, and respect the excitation frequency at which the quoted sensitivity in millivolts per volt per millimetre applies.

A DC-LVDT integrates the oscillator, synchronous demodulator, and output amplifier inside the housing. It runs from a plain DC rail, commonly 6 to 30 VDC, and presents a ready-to-use DC output proportional to core position, readable on a multimeter or a standard PLC analog input. That simplicity is its whole appeal: three or four wires, no separate conditioner, no carrier to match. The cost is temperature ceiling and ultimate accuracy. The onboard electronics typically cap continuous operation near 120 degrees Celsius and add their own thermal drift, so reference-grade and high-temperature work still favors AC units with remote conditioning. A DC-LVDT is, in effect, an AC-LVDT plus a built-in conditioner, trading environmental range for plug-and-play wiring.

Excitation choices inside the AC family also matter. Raising the carrier frequency increases sensitivity and frequency response, which helps in dynamic servo loops, but it can excite eddy currents in nearby conductive structures and add phase-shift error, so most general-purpose units settle around 2.5 to 5 kHz. Driving an AC-LVDT below its rated excitation voltage reduces output and degrades signal-to-noise; driving it above causes self-heating and zero drift. The datasheet sensitivity figure is only valid at the stated excitation voltage and frequency, so the conditioner must reproduce both. For DC-LVDTs the manufacturer fixes the internal carrier, removing this degree of freedom and the matching errors that come with it.

One more distinction concerns ratiometric versus fixed output. Many precision conditioners compute the ratio of the differential secondary voltage to the sum of the two secondary voltages, which cancels excitation-amplitude drift and improves long-term stability. This ratiometric scheme is most easily exploited with AC-LVDTs feeding a dedicated conditioner, and it is a quiet reason that the most demanding metrology channels stay AC even when DC units would wire up faster.

Sensing Variants and Gaging Probes

Beyond the AC/DC split, LVDT-family sensors come in mechanical and electrical variants tuned to a duty. Three groupings cover most of the market: free-core (unguided) LVDTs, guided-core and spring-loaded gaging probes, and the inductive cousins, half-bridge probes and LVITs, that share the conditioner ecosystem but not the exact transformer principle. The table compares them on the parameters that drive selection.

Free-core LVDTs have an unconstrained core attached by a non-magnetic rod to the moving member. The core floats inside the bore without a bearing, so there is zero added friction and zero side load on the measured motion, which is ideal for servo-valve spools, hydraulic-cylinder rods, and fatigue specimens where any drag would corrupt the result. The penalty is that the user must guide the core within the bore; misalignment shortens the linear range and, in the extreme, lets the core rub. Free-core units span the widest stroke envelope, from sub-millimetre precision elements up to half-metre long-stroke designs.

Guided-core and spring-loaded gaging probes add a precision linear bearing and a return spring so the shaft tip can be pressed against a workpiece and follow its surface. These dimensional gaging probes are the metrology backbone of automotive and bearing production lines: typical gaging ranges run from about plus or minus 0.5 mm to plus or minus 50 mm with sub-micron resolution, and the spring delivers a controlled measuring force. The bearing trades a touch of friction for guaranteed alignment and a usable mechanical interface, which is exactly the right deal for a benchtop or in-line gage station.

Half-bridge probes and LVITs share the LVDT signal-conditioning world but are not strict three-coil differential transformers. A half-bridge probe uses two coils whose impedances change oppositely with core position, forming one arm of an inductive bridge. It is simpler and cheaper than a full LVDT but generally less linear and less standardized, and it requires a half-bridge-compatible conditioner rather than an LVDT one. An LVIT, linear variable inductive transducer, drives a single coil whose inductance varies as a conductive target moves, with electronics onboard; it suits rugged factory-automation and OEM duties where cost and packaging matter more than the last tenth of a percent of linearity. The practical warning is that LVDT, half-bridge, and LVIT outputs are not interchangeable on the same input, so the conditioner must match the probe family.

The RVDT rounds out the family by applying the opposed-secondary principle to rotation. A cam-shaped core on a shaft makes the differential output proportional to angle. Linear-range RVDTs work within roughly plus or minus 30 to plus or minus 40 degrees with sensitivity on the order of a few millivolts per degree, and accuracy falls off quickly past about plus or minus 35 degrees. RVDTs are widely deployed as aircraft flight-control surface and actuator feedback sensors precisely because, like LVDTs, they are contactless, rugged, and stable for decades. Choose an RVDT when the motion is rotary and bounded, and an LVDT when it is straight-line, since converting linear to rotary through a linkage introduces backlash error.

Mounting, Environment, and Standards

An LVDT lives or dies on its installation. The transducer can be inherently accurate to a fraction of a percent, yet a careless mounting will throw away that accuracy through misalignment, side load, thermal expansion, or electromagnetic interference. This chapter covers the mechanical and environmental discipline that protects the measurement, and the calibration standard that proves it.

Core alignment is the first rule. The core must travel coaxially with the coil bore; any lateral offset or angular tilt of the pushrod adds non-linearity and can let the core scrape the bore on long strokes. Free-core designs leave alignment to the user, so the application must provide a guide, while guided-core probes solve it internally with a precision bearing at the cost of a little friction. Always couple the core through a non-magnetic, low-friction rod and avoid placing magnetic or large conductive masses close to the coil, since they distort the flux path and shift the calibration.

Thermal management matters because the secondaries, the core permeability, and the conditioner all carry temperature coefficients. AC-LVDTs with remote conditioners separate the heat-sensitive electronics from the hot zone, which is why high-temperature and cryogenic work uses them; specialised AC units operate well above 150 degrees Celsius. DC-LVDTs cap out near 120 degrees Celsius because their onboard electronics set the limit. For wide-temperature channels, request the manufacturer thermal-drift curve for both zero and sensitivity, and where possible use a ratiometric conditioner to cancel excitation-amplitude drift.

Electrical environment determines wiring practice. The low-level AC secondary signal of an AC-LVDT is vulnerable to pickup, so use shielded, twisted multi-conductor cable, ground the shield at one end only, and keep the run away from variable-frequency drives and contactors. DC-LVDTs are more forgiving because the demodulated DC output is higher level, but they still benefit from clean power and short analog runs. Sealing class also belongs here: washdown, submerged, and pressurized-bore variants exist for hydraulic and subsea duty, and the housing protection rating should match the splash, dust, and immersion the unit will see.

On the standards side, the governing calibration practice for displacement channels on material testing machines is ASTM E2309 / E2309M, Standard Practices for Verification of Displacement Measuring Systems and Devices Used in Material Testing Machines. It verifies the whole measuring system in place against a calibrated reference such as a gauge-block stack or laser interferometer, requires a minimum of two runs of readings, and sorts each channel into accuracy classes defined by a Relative Error in percent of reading and a Fixed Error static floor, with results traceable to NIST and re-verification at least annually. The table summarises the standards and verification touchpoints an LVDT channel commonly references.

The recurring theme across these references is that an LVDT specification is only meaningful with its conditions attached. A linearity of plus or minus 0.25 percent is bound to a stated range, and a sensitivity in millivolts per volt per millimetre is bound to a stated excitation voltage and frequency. Verification under ASTM E2309 makes the channel, sensor plus conditioner plus cabling, traceable as a system, which is the only number that matters on the test floor.

Key Specification Parameters

Reading an LVDT datasheet is a skill in itself, because manufacturers quote each figure under specific conditions that are easy to miss. Across most spec sheets, eight parameters actually drive selection: nominal linear range, linearity, sensitivity, resolution, null voltage, frequency response, operating temperature, and output type. Each is explained below.

Nominal linear range (stroke) is the total core travel, usually expressed as a bipolar figure about the null such as plus or minus 2.5 mm, plus or minus 25 mm, or plus or minus 250 mm, over which the quoted linearity holds. LVDTs are always operated inside this band because non-linearity rises sharply near the stroke ends. Watch for a longer over-travel range listed alongside the linear range; linearity does not apply there. As a rule, place the working motion well within the nominal range so wear, thermal growth, and installation tolerance never push the core past the linear band.

Linearity (non-linearity) is the maximum deviation of the output from a best-fit straight line, expressed as plus or minus a percent of full range. General-purpose units are typically plus or minus 0.25 percent FS, while precision and gaging units reach plus or minus 0.1 percent or better over a reduced stroke. Because the error concentrates at the ends, choosing a longer-stroke unit and using only its center can buy back linearity. Always confirm whether the number applies to the nominal range or a longer range, since the two are not comparable.

Sensitivity (scale factor) is the output per unit displacement, for AC-LVDTs given in millivolts per volt of excitation per millimetre, and for DC-LVDTs in volts per millimetre of the conditioned output. Sensitivity varies with excitation frequency and voltage, so the datasheet value is only valid at the stated carrier; a conditioner running a different excitation will read a different scale factor. Higher sensitivity eases the conditioner design but can come with a shorter stroke, so it trades against range.

Resolution is effectively infinite at the transducer because there is no mechanical step; the real limit is the conditioner noise floor and the analog-to-digital converter. Short-stroke precision setups resolve sub-micron and even nanometre steps in the laboratory. Design to the resolution your electronics can deliver, not to an idealized infinite figure, and remember that wider bandwidth admits more noise and lowers usable resolution.

Null voltage is the small residual output that remains at the mechanical null, arising from coil asymmetry and quadrature components. A low null voltage eases conditioner nulling and improves near-zero accuracy; high-quality units specify it tightly. Frequency response caps the dynamic measurement and is governed largely by the excitation frequency for AC units, with a common guideline that usable bandwidth is roughly one tenth of the carrier. Servo and vibration applications need a higher carrier and a fast conditioner.

Operating temperature bounds the environment: AC-LVDTs reach 150 degrees Celsius and beyond because their electronics sit remotely, while DC-LVDTs are typically limited near 120 degrees Celsius by onboard components. Output type closes the loop with the control system: a raw AC differential requiring an external demodulator, a conditioned DC voltage, a 4-20 mA current loop, or a digital interface on the conditioner. The output choice should match the receiving PLC, DAQ, or DCS card and the cable run length.

Selection Decision Factors

To turn the preceding chapters into a specific model, follow the decision sequence below. As with most instrumentation, the costly mistakes come not from a single wrong figure but from deciding range or architecture before the application is fully characterized. These eight steps double as an RFQ template.

1. Stroke and core type: First fix the required travel, then choose free-core for friction-free servo and cylinder feedback or a guided spring-loaded probe for surface gaging. Size the nominal range so the working motion stays well inside it, since linearity degrades at the ends.
2. AC or DC architecture: Pick AC with a remote conditioner for the widest temperature range, the highest accuracy, and radiation or high-vibration zones; pick DC for the simplest three or four wire installation read directly by a PLC or multimeter.
3. Accuracy class: Distinguish loop and actuator feedback (plus or minus 0.25 percent FS is ample) from precision gaging and reference work (plus or minus 0.1 percent FS or better over a reduced stroke). Confirm the linearity figure is tied to the nominal range, not an over-travel range.
4. Excitation and conditioner match: For AC units, fix the excitation voltage and frequency to the datasheet conditions and select a conditioner that reproduces them; prefer a ratiometric conditioner to cancel amplitude drift. Verify the conditioner matches LVDT, half-bridge, or LVIT, since they are not interchangeable.
5. Environment and sealing: Set the operating temperature ceiling (AC for above 120 degrees Celsius), the housing protection rating for washdown, dust, or immersion, and any pressure-bore requirement for hydraulic or subsea service.
6. Mechanical interface: Define core coupling (non-magnetic low-friction rod), mounting (threaded body, flange, or bracket), and the guide that keeps a free core coaxial. Reserve clearance for thermal growth and over-travel.
7. Output and integration: Choose raw AC plus external demodulator, conditioned DC, 4-20 mA, or a digital interface to match the receiving DAQ, PLC, or DCS card and the cable length, then plan shielded twisted cabling with single-point shield grounding.
8. Calibration and traceability: Decide the verification regime up front. For material-test and metrology channels, plan ASTM E2309 / E2309M verification of the complete system against NIST-traceable references, with annual re-verification, and budget the calibration labor.

One last commonly overlooked dimension is manufacturer serviceability: availability of a matched conditioner from the same vendor, documented thermal-drift curves, spare-core and connector inventory, and field-calibration support. TE Connectivity, Honeywell, Macro Sensors, and Trans-Tek supply well-characterised LVDT and RVDT families, while Solartron Metrology, Heidenhain, TESA, and Sylvac dominate dimensional gaging probes. Trans-Tek, for example, publishes families such as the Series 240 DC-DC units at 0.25 to 0.5 percent FS linearity and the Series 210-220 long-stroke AC-LVDTs covering strokes up to about 1.5 m at better than plus or minus 0.25 percent FS, which makes datasheet comparison straightforward across a project.