LVDT Trainer Kit: Working, Experiments, Applications, and How to Choose Right One

TL;DR

1. This guide is intended for engineering students, polytechnic students, and laboratory instructors in India who want to understand LVDT trainer kits, perform basic experiments, and evaluate the right equipment for their laboratories.
2. An LVDT (Linear Variable Differential Transformer) converts the linear displacement of a movable ferromagnetic core into an electrical signal. The amplitude of its differential AC output indicates the magnitude of displacement within the linear operating range, while its phase relative to the excitation signal indicates the direction of displacement.
3. An LVDT trainer kit typically includes an LVDT sensor, a micrometer mechanism, an AC excitation source, a signal-conditioning circuit with a demodulator, and a display. Test points may also be provided so students can observe the signal at different stages of the measurement process.
4. Core tests include displacement vs output voltage, phase inversion, sensitivity, linearity, null voltages and repeatability.
5. Check stroke range, micrometer accuracy level, location for testing purposes; display options available to students’ needs before selecting a LVDT kit.

A LVDT trainer kit is a bench top teaching unit in which the core of a linear variable differential transformer is moved with a micrometer while output voltage is read on a display or an oscilloscope. LVDT stands for Linear Variable Differential Transformer. It is a displacement transducer that measures position without any physical contact between moving parts and coils.

LVDT is among the most common sensors in measurement and instrumentation laboratories. Valve positions, structural movement, and servo feedback are all measured with it, which is why LVDT experiment appears in the practical syllabus of most electrical, electronics, and instrumentation courses.

This guide explains what an LVDT is, how LVDT measurement works inside a trainer, which experiments you can perform, how to solve an exam style numerical, and what to check when selecting an LVDT kit for your lab.

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What Is LVDT and Why Is It Used for Displacement Measurement?

A transducer converts a physical quantity, such as displacement, pressure, or temperature, into a corresponding signal. A displacement transducer converts mechanical movement into an electrical signal that can be measured by instruments or processed by controllers and computers.

This is done by LVDTs on similar principles to transformers. An LVDT operates on the principle of mutual induction. An AC supply energises the primary winding, producing an alternating magnetic field that induces voltages in the two secondary windings. Moving the ferromagnetic core changes the magnetic coupling between the primary and secondary windings, causing the two secondary voltages to change relative to each other.

An LVDT consists of three coils wound on a hollow cylindrical former. The central coil is the primary winding and is supplied with an AC excitation voltage, often in the kilohertz range, depending on the design. The two outer coils are the secondary windings and are connected in series opposition. A movable ferromagnetic core slides axially inside the hollow former, changing the magnetic coupling between the primary and the two secondary windings.

LVDTs offer several advantages. Because the movable core does not make physical contact with the windings, mechanical friction and wear are minimal. The primary and secondary windings are also electrically isolated, as in a transformer. Depending on the model, an LVDT can measure displacements ranging from fractions of a millimetre to several centimetres or more.

An LVDT is classified as a passive inductive transducer because it requires an external AC excitation source. The core’s movement changes the magnetic coupling between the windings, producing a differential electrical output.

How Does an LVDT Work?

The easiest way to understand LVDT is to look at three core positions.

What happens when the core is at the centre?

When the core is at the electrical null position, the two secondary windings have equal induced voltage magnitudes. Because they are connected in series opposition, their voltages cancel, ideally producing zero differential output. In a practical LVDT, a small residual voltage may remain even at null.

What happens when the core moves to one side?

If core moves toward secondary S1, more flux links S1 and less links S2. voltage in S1 rises and voltage in S2 falls. The difference is no longer zero. On one side of the electrical null, the differential output has a particular phase relationship with the excitation reference. When the core moves to the other side, that relationship reverses by approximately 180 degrees. The exact phase convention depends on winding connections and the chosen reference polarity. and its amplitude increases as the core moves farther from the electrical null position, within the linear range.

What happens when the core moves to the other side?

When the core moves toward secondary winding S2, the induced voltage in S2 becomes greater than that in S1. The differential output amplitude increases with displacement within the linear range, but its phase reverses by approximately 180 degrees relative to the excitation signal. This phase reversal indicates that the core has moved to the opposite side of the electrical null position.

A simple way to understand an LVDT is to imagine two opposing signals. At the electrical null, their induced secondary voltages are equal in magnitude and cancel because the windings are connected in series opposition. When the core moves away from null, the imbalance produces a differential output. When one side becomes stronger, the balance changes, representing the difference between the secondary voltages. The magnitude of the difference indicates displacement, while its sign indicates the direction of movement.

output voltage can be written as:

V_out = V_S1 − V_S2

Within linear range, V_out is proportional to core displacement x from null position, so V_out = S × x, where S is sensitivity of LVDT.

Why is output never exactly zero at null?

An ideal LVDT produces zero differential output at electrical null. In a practical device, a small residual voltage may remain because of imperfect magnetic symmetry, winding differences, harmonic distortion, and parasitic coupling. The measured residual depends on the sensor and its signal-conditioning circuitry. In practice, a small residual voltage may remain because of excitation waveform harmonics, imperfect magnetic symmetry, and stray capacitive coupling between the primary and secondary windings. Good-quality LVDTs are designed to minimise this residual voltage, but it may not disappear completely and can be measured during a laboratory experiment.

What Is Inside an LVDT Trainer Kit?

A typical LVDT trainer is built so that each stage of the measurement chain can be inspected separately. The exact layout varies between manufacturers, but blocks below are common.

A micrometer jig is a feature that makes a unit a trainer rather than just a sensor. Laboratory equipment specifications for LVDT trainers commonly describe a micrometer jig fitted on a base plate that gives controlled displacement to the core. This lets students set a known displacement and compare it with electrical output.

Stroke and sensitivity depend on the model. Published specifications for individual LVDT trainer models may list different displacement ranges, sensitivities, and display types. For example, verify the Scientech 2303’s stated display, sensitivity, and measurement range against its current official datasheet before quoting these values. When comparing models, distinguish total travel from displacement on either side of null. Before selecting a sensor, compare LVDT sensor specifications and measurement ranges to understand how performance varies between models. 

How Does LVDT Measurement Work on a Trainer?

LVDT measurement on a trainer follows a typical signal path. The oscillator supplies AC excitation to the primary winding. The secondary windings produce an AC differential output whose amplitude and phase depend on the core’s position relative to the electrical null. That AC signal is then converted to DC and displayed.

A simple rectifier is not enough for this conversion. A rectifier would give the same DC level for a core displacement of +3 mm and  3 mm, because it only responds to amplitude. The sign would be lost. To preserve the direction information, an LVDT trainer uses a phase-sensitive detector that compares the secondary signal with the excitation reference. It converts the AC differential signal into a signed DC output: the polarity changes when the core moves across the electrical null. When the signals have the phase relationship associated with one direction of displacement, the demodulated output has one polarity. When the core moves to the other side of null, the phase relationship reverses by approximately 180 degrees, and the output polarity reverses. Within the linear range, the DC output magnitude is approximately proportional to displacement.

Two practical points are worth remembering. The LVDT secondary and signal-conditioning output should be connected to an appropriate input impedance, as specified by the manufacturer, to minimise loading errors. When measuring the raw AC secondary output, use an instrument with suitable input impedance and bandwidth. Second, the excitation frequency is normally selected to be much higher than the frequencies of the mechanical displacement being measured. The appropriate ratio depends on the LVDT design, signal-conditioning circuit, bandwidth, and required response.

Which Experiments Can You Perform on an LVDT Trainer Kit?

Most university lab manuals follow a similar sequence. The aim is usually to measure displacements set by a micrometer and then determine performance characteristics of LVDT.

Experiment 1: Study blocks and waveforms

Switch on the trainer and use an oscilloscope at each test point. Record amplitude and frequency of oscillator output, AC secondary signal, and demodulated DC level. This builds the habit of tracing a signal through each stage.

Experiment 2: Plot displacement versus output voltage

Set the micrometer to a null position. One lab manual, for example, assumes null at 7 mm. Move the micrometer to one extreme in equal steps and note LVDT reading at each step. Then move it to the opposite extreme and repeat. Plot readings against displacement from null.

The following illustrative readings assume a sensitivity of approximately 10 mV/mm and an electrical null at a micrometer reading of 7 mm. The output is treated as a signed, demodulated DC voltage, with the sign indicating the direction from null.

Plot the signed output voltage against displacement measured from the electrical null. Within the linear operating range, the graph should approximate a straight line through the origin after accounting for any null offset. The slope of this graph gives the LVDT sensitivity.

Experiment 3: Observe phase reversal

Use a dual channel oscilloscope. Display primary voltage on one channel and secondary output on another. Move the core from one side of the null to the other and watch the secondary waveform shrink to almost nothing and then reappear flipped. This is the clearest way to see why phase carries direction.

Experiment 4: Find sensitivity and linearity

Calculate sensitivity as the change in output voltage divided by the corresponding change in displacement, using the linear operating region. Compare measured outputs with a defined reference straight line and calculate the maximum deviation. Express the linearity error as a percentage of full-scale output, following the reference-line method specified in the experiment or manufacturer’s documentation.

Experiment 5: Measure null voltage

With core at mechanical null, read residual output with the most sensitive range available. Note its value and relate it to harmonics and stray coupling.

Experiment 6: Check hysteresis and repeatability

Take readings while moving the core in one direction, then repeat the measurements while approaching the same displacement points from the opposite direction. The difference between the two sets of readings at corresponding positions indicates hysteresis. Repeat the measurements under the same conditions to assess repeatability.

Extension: Pressure measurement with a Bourdon tube

Some institutions pair LVDT with a Bourdon tube and a foot pump. The tube moves as pressure changes, and LVDT measures that movement. It shows how a displacement sensor can be used to measure a different physical quantity.

How Do You Solve an LVDT Numerical in Exam Style?

Question. An LVDT trainer has a sensitivity of 10 mV/mm over a range of plus or minus 10 mm. The excitation voltage is 3 V rms. Find:

(a) output for a core displacement of 6.5 mm, (b) displacement if demodulated output reads +38 mV, (c) normalised sensitivity in mV/V/mm, (d) linearity error if measured output at +8 mm is 77 mV.

Solution.

(a) Output = S × x = 10 mV/mm × 6.5 mm = 65 mV

(b) Displacement = V_out / S = 38 mV / (10 mV/mm) = 3.8 mm. Positive sign shows core is on side that gives an in phase output.

(c) Normalised sensitivity = S / V_excitation = 10 mV/mm / 3 V = 3.33 mV/V/mm

(d) Full scale output at 10 mm = 10 mV/mm × 10 mm = 100 mV. Ideal output at 8 mm = 10 mV/mm × 8 mm = 80 mV. Deviation = 80 mV 77 mV = 3 mV. Linearity error = (3 mV / 100 mV) × 100 = 3% of full scale

Exam trap. Normalised sensitivity is quoted per volt of excitation, so always divide by excitation voltage and not by output. Also check units. Many errors in this question type come from mixing mV and V.

Where Is an LVDT Used in Real Applications?

Outside the lab, LVDT appears wherever a reliable position measurement is needed.

Process plants use it to report the position of valve stems and control rods. Displacer type level transmitters use it to read small movement of a displacer. In civil engineering, spring loaded LVDT displacement sensors record movement in structural members, such as bridge deflection, expansion joint movement, and concrete crack monitoring. Miniature versions fit into tight spaces, and radiation resistant versions are made for nuclear power plants. Servo systems use LVDT as a position feedback element.

Some LVDTs use sealed construction to protect their internal components from dust, moisture, or other environmental conditions. Because the core does not make sliding electrical contact with the windings, an LVDT can be designed for demanding environments, subject to the model’s specified protection rating.

For related lab work, a strain gauge based load measurement trainer such as Scientech 2307 covers linearity and sensitivity measurement with a different sensing principle, which is useful for comparison. The inductive proximity sensor section in Scientech 2313 introduces another inductive sensing method.

How Does LVDT Compare With Other Displacement Sensors?

LVDT is sensitive to stray magnetic fields, and its performance can be affected by vibration and temperature. These limits are worth noting in the lab report, since many questions ask for both advantages and disadvantages.

What Mistakes Should You Avoid While Using an LVDT Trainer?

Most poor results in the lab come from a few repeated habits.

first is not locating null properly. If null is set by eye rather than by watching output fall to its minimum, every reading is shifted. The second is pushing the core beyond linear range. graph bends at extremes, and sensitivity calculated from those points is wrong.

The third is backlash in micrometers. Always approach each reading from the same direction. If you overshoot a mark, back off and approach it again. Fourth is loading output with a low impedance meter, which pulls reading down. The fifth is treating AC and DC test points as interchangeable. A DC voltmeter on AC secondary will show a misleading value.

Finally, allow a short warm up and keep it away from large transformers and motors, since stray magnetic fields can disturb reading.

How Do You Choose the Right LVDT Trainer Kit?

Basic LVDT trainers in Indian supplier listings reviewed for this post were priced between roughly ₹13,000 and ₹16,500, and prices vary by supplier, configuration, and tax. Treat any figure as a starting point and ask for a current quote.

The table below lists what to check.

For a department buying several units, test one before ordering a batch. Check how repeatable the null is, move micrometers through full stroke, and look at waveforms on your own oscilloscope. A trainer that looks good on a datasheet but has a loose micrometer will waste lab time.

How Is LVDT Tested in GATE, SSC JE, and RRB JE?

LVDT belongs to the transducers and measurement portion of several papers. GATE Instrumentation Engineering covers transducers in depth. GATE Electrical Engineering has electrical and electronic measurements. SSC JE and RRB JE papers for electrical and electronics streams also test basic transducer knowledge. Always check the current syllabus for your exam, since topic lists change between cycles.

Question patterns tend to fall into three groups. Conceptual questions ask what output is at null, what phase indicates, or which transducer type LVDT belongs to. Numerical questions give a sensitivity and ask for output or displacement, as in the worked example above. Comparison questions ask for advantages and disadvantages against a potentiometer or capacitive transducer.

Common traps include:

Calling LVDT a variable self inductance device, when it works on variable mutual inductance. Forgetting that output at null is ideally zero but not exactly zero in practice. Ignoring that phase, not amplitude, gives direction. Mixing up output of raw LVDT, which is AC, with output after demodulation, which is DC.

Students preparing for GATE 2027 and upcoming SSC JE or RRB JE examinations should be able to draw an LVDT diagram, explain the differential output equation Vout=VS1-VS2 using a consistent polarity convention, and describe phase reversal in a few sentences.

Where Should You Start With an LVDT Trainer Kit?

LVDT trainer kit is one of clearest ways to see how a mechanical movement becomes an electrical signal. Start by tracing signals from the oscillator through LVDT to display. Then plot displacement characteristics, calculate sensitivity, and check linearity. After that, repeat measurement in both directions to understand hysteresis.

Five points are worth carrying out of the lab. output amplitude is proportional to core displacement from null. phase, either in step with primary or 180 degrees apart, shows direction. A phase sensitive detector is needed to recover signs after demodulation. Sensitivity and linearity are two numbers that describe how good an LVDT is. When buying an LVDT kit, test micrometer jig and test points before reading the specification sheet.

If you are setting up or upgrading an instrumentation lab, compare at least two LVDT trainers on points above and run null, linearity, and phase checks yourself. Share your questions or lab experience in comments, and explore other measurement topics on ScienTech to continue your preparation.

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