Linear Variable Differential Transformer LVDT

Definition of LVDT

An LVDT, or Linear Variable Differential Transformer, is an inductive transducer that converts axial core displacement into an AC electrical signal. Signal-conditioning electronics use its amplitude and phase to produce a signed displacement reading.

The two secondary windings of this transformer are connected in series opposition, so the raw output is their voltage difference. Accuracy belongs to the complete calibrated system and depends on range, linearity, excitation, conditioning, mounting and temperature.

Construction of LVDT

Main Features of Construction

• The transformer has a primary winding P and two secondary windings S₁ and S₂ arranged along a hollow cylindrical former.
• The secondary windings are nominally identical and are placed symmetrically on either side of the primary winding.
• An AC source excites the primary winding. The resulting alternating magnetic flux induces voltages in both secondary windings.
• A movable ferromagnetic core travels inside the former and attaches mechanically to the point whose displacement is measured.
• The high-permeability core changes magnetic coupling as it moves. Excitation frequency, core material and winding symmetry affect sensitivity, linearity and harmonics.
• A housing protects the coils and can support shielding, but shielding performance depends on enclosure material, geometry, grounding and the interference source.
• The secondary windings connect in series opposition, making the output equal to one secondary voltage minus the other.

Principle of Operation and Working

AC excitation of the primary creates alternating current and magnetic flux that couples to both secondary windings. The secondary S₁ produces e₁, and secondary S₂ produces e₂. With the secondaries in opposition, the differential output is their phasor difference.

The equation states the raw AC principle of Operation of LVDT. A phase-sensitive demodulator normally converts that signal to a signed DC voltage or digital value.

The following three cases describe motion relative to the nominal null position. Polarity labels depend on winding connections and the selected reference direction.

• CASE I When the core is at null position (for no displacement)
  At the nominal null position, equal coupling makes the two secondary voltages cancel. The ideal output e_out is zero because e₁ and e₂ are equal in magnitude. A real system can retain a small residual null voltage from winding imbalance, harmonics, temperature or signal conditioning.
• CASE II When the core is moved to upward of null position (For displacement to the upward of reference point)
  Motion towards S₁ increases its coupling while reducing coupling to S₂. The magnitude of e₁ then exceeds e₂. Under the shown wiring and reference convention, the demodulated output e_out is positive.
• CASE III When the core is moved to downward of Null position (for displacement to the downward of the reference point). Motion towards S2 makes the magnitude of e₂ exceed e₁. The raw AC output reverses phase relative to the other side of null, and the chosen demodulation convention makes e_out negative.

Output V_S Core Displacement

Within its specified central range, an LVDT’s conditioned output is approximately linear with core displacement. Outside that range, coupling geometry increases nonlinearity.

Some important points about magnitude and sign of voltage induced in LVDT

• Within the calibrated linear range, output magnitude represents distance from null.
• Output phase relative to the excitation identifies the side of null; a demodulator converts that phase relationship into a positive or negative reading.
• Sensitivity and linearity apply only under the stated excitation, loading, temperature and displacement range.

Advantages of LVDT

• Multiple ranges – Commercial LVDTs cover short and long strokes, but each model has a specified full range and linear operating interval.
• No Frictional Losses – The electromagnetic sensing element needs no sliding electrical contact with the core. Bearings, seals, linkages or side loading in the installation can still introduce friction and error.
• High Input and High Sensitivity – Sensitivity depends on excitation and device geometry. The transducer normally needs a suitable carrier source, demodulator and amplifier or data-acquisition input matched to its rated output.
• Low Hysteresis – Contactless coupling can provide low hysteresis and good repeatability when alignment, mounting, temperature and excitation remain controlled.
• Low Power Consumption – Excitation voltage, frequency and power are model-specific. The source must meet the manufacturer’s limits to avoid heating and calibration error.
• Direct Conversion to Electrical Signals – The differential AC output can be conditioned for voltage, current or digital acquisition and remote processing.

Disadvantages of LVDT

• Stray magnetic fields, cable coupling and ground loops can disturb the small AC signal, so shielding, cable routing and grounding must suit the installation.
• Temperature, excitation drift, vibration, misalignment, side load and signal-conditioner error can change the reading.

An LVDT is a strong option when its range, bandwidth, environment, mechanical installation and calibrated uncertainty match the measurement task.

Applications of LVDT

1. An LVDT measures linear position, stroke, deflection or dimensional change within its rated range. As a primary transducer, the LVDT converts displacement directly into a differential electrical signal.
2. An LVDT can act as a secondary transducer when another element first converts a quantity into displacement. For example, a Bourdon tube converts pressure into motion, and a calibrated LVDT measurement chain converts that motion into a pressure reading.