--- title: "LVDT construction, working and applications" subject: "Instrumentation and Measurement" chapter: 5 key: "semester_4/instrumentation_and_measurement:lvdt-construction-working-principle" source: https://www.bctengineer.com/q/semester_4/instrumentation_and_measurement/lvdt-construction-working-principle site: BCT Engineering --- # LVDT construction, working and applications Instrumentation and Measurement · Chapter 5: Sensors - Asked in: year not given, 8 marks - Video: [LVDT - Linear Variable Differential Transformer or Transducer Working](https://www.youtube.com/watch?v=E-kDsP0wq6w) — ADTW Study - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_4/instrumentation_and_measurement/lvdt-construction-working-principle - Whole chapter: https://www.bctengineer.com/md/semester_4/instrumentation_and_measurement/chapter-5 ## Question Explain the construction and working principle of a linear variable differential transformer (LVDT). Draw its output against core displacement, and state its advantages, limitations and applications. ## Short answer An LVDT has a primary coil and two series-opposition secondaries with a movable core. Core displacement unbalances fluxes, giving output voltage proportional to displacement with phase indicating direction. Advantages: frictionless, high resolution, robust. Limitations: needs AC excitation and demodulator. Applications: displacement, force, pressure sensing. ## Answer The LVDT is a passive inductive transducer that converts linear displacement into an ac voltage whose amplitude is proportional to the displacement and whose phase gives the direction. #### Construction - One **primary winding** at the centre and two identical **secondary windings** $S_1$, $S_2$ wound symmetrically on either side of it, on a hollow non-magnetic former. - A movable **ferromagnetic core** (soft iron or nickel-iron) slides inside the former and is attached to the object whose displacement is measured. - The primary is excited by a 1-10 V, 50 Hz-20 kHz ac supply. The two secondaries are connected in series opposition. ```text ~ E (ac excitation) | +---------+--------+---------+ | S1 | Primary| S2 | | ((( | ((( | ((( | +----[=== core ===]----------+ <-- x --> S1 and S2 in series opposition: e_o = e1 - e2 ``` #### Working The primary field induces emfs $e_1$ and $e_2$ in the secondaries, and the output is $e_o = e_1 - e_2$. - **Core at centre (null position):** equal flux links both secondaries, $e_1 = e_2$, so $e_o = 0$. - **Core moved toward $S_1$:** $S_1$ gets more flux, so $e_1 > e_2$ and $e_o$ is in phase with the primary voltage. - **Core moved toward $S_2$:** $e_2 > e_1$ and $e_o$ is $180^\circ$ out of phase with the primary. The magnitude of $e_o$ is proportional to the displacement over the linear range. A phase-sensitive demodulator is used to obtain a dc signal whose sign gives the direction. ```text |e_o| \ / \ / <- linear range on each side \ / \ / -----------+----------- x null (180 deg out of phase) | (in phase) ``` #### Advantages - Frictionless, infinite resolution, no contact wear, long life. - High sensitivity (up to about 2 mV/V/mm); good linearity (about 0.25%). - Rugged and tolerant of shock, vibration, dirt and the environment. - Small null error, and the core moves freely without touching the coils. #### Limitations - Needs ac excitation and a demodulator. - Stray magnetic fields and temperature affect it; the frequency response is limited by the carrier (the carrier must be at least about 10 times the signal frequency). - Limited range, and a small residual voltage at null. #### Applications Displacement and position, thickness, force and pressure (with an elastic element such as a diaphragm or bourdon tube), acceleration, liquid level and load cells. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How are the two secondary windings of an LVDT connected? A. In parallel B. In series opposition C. In series aiding D. Each to a separate supply Answer: B. The secondaries are in series opposition so the output is e1 - e2, which is zero when the core is centred. (Key point: Secondary connection) 2. Why is the output zero when the core is at the centre? A. The primary is not excited at the centre B. Equal flux links both secondaries so e1 = e2 and they cancel C. The core short-circuits the secondaries D. The core saturates and blocks the flux Answer: B. Symmetry gives equal induced emfs in the two secondaries, and with series opposition e_o = e1 - e2 = 0. (Key point: Null position) 3. How does the LVDT output indicate the direction of core movement? A. By a change in frequency B. By a change in the primary current C. By the output being in phase or 180° out of phase with the primary voltage D. By a dc offset on the output Answer: C. Moving toward S1 gives output in phase with the primary, toward S2 gives 180° out of phase; a phase-sensitive demodulator extracts the sign. (Key point: Direction from phase) 4. Which is a genuine advantage of the LVDT? A. It needs no ac excitation B. It has no residual voltage at null C. Frictionless core movement gives infinite resolution and long life D. It has an unlimited range Answer: C. The core does not touch the coils, so there is no friction or wear and resolution is essentially infinite. The other options are limitations. (Key point: Advantages of LVDT) ## Related in Instrumentation and Measurement - [Variable reluctance (inductance) transducers](https://www.bctengineer.com/question.md?key=semester_4%2Finstrumentation_and_measurement%3Avariable-reluctance-transducer-types) — Instrumentation and Measurement, chapter 5 - [Potentiometer transducer and loading error](https://www.bctengineer.com/question.md?key=semester_4%2Finstrumentation_and_measurement%3Apotentiometer-transducer-loading-error) — Instrumentation and Measurement, chapter 5 ## Related in other subjects - [Stepper motor construction, principle and applications](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Astepper-construction-applications) — Electric Machines II, chapter 3 - [CT working principle and applications](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Act-definition-principle-applications) — Switchgear and Protection, chapter 2 - [MOSFET construction and working principle](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Amosfet-construction-working) — Basic Electronics Engineering, chapter 3, asked 2× - [Exciting current waveform with non-linear B-H core](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Anonlinear-bh-exciting-current) — Electrical Machines I, chapter 1