Interview questions & answers
Q1. Explain the working principle of an LVDT.
An LVDT consists of a primary coil energised with AC excitation (typically 1–10 kHz) and two secondary coils wound in opposite directions on either side — a movable ferromagnetic core couples the primary flux into the secondaries differentially. When the core is displaced from the null (centre) position, one secondary has more flux coupling than the other, producing a net differential output voltage proportional to displacement. Because coupling is entirely magnetic (no physical contact between core and coils), the LVDT has infinite mechanical life — Measurement Specialties'' GCA-121-500 LVDT used in aircraft control surface feedback has operated for over 100 million cycles without measurable wear.
Follow-up: What determines the linearity range of an LVDT?
Q2. What is the null position and null voltage in an LVDT?
The null position is the core location where the two secondary voltages are exactly equal — the differential output is theoretically zero. In practice, a small residual null voltage of 0.1–1% of full-scale output remains even at null due to imperfect secondary winding symmetry, stray capacitive coupling between primary and secondaries, and core irregularities. A Solartron AX/2000 LVDT has a residual null voltage below 5 mV at 5 V excitation — for precision hydraulic servo applications, this null offset is trimmed out electronically in the signal conditioner by adjusting a balance potentiometer.
Follow-up: What is the effect of excitation frequency on LVDT output and null voltage?
Q3. How does the LVDT determine the direction of core displacement?
The direction of displacement is determined by the phase of the output voltage relative to the excitation voltage — when the core moves to the left of null, the output is in phase with the excitation (0°); when it moves to the right, the output is 180° out of phase. A synchronous demodulator (phase-sensitive detector) in the signal conditioning circuit uses the excitation reference signal to convert the AC output into a signed DC voltage: positive for one direction, negative for the other. A Texas Instruments PGA970 LDC/LVDT signal conditioner IC integrates the synchronous demodulation, providing a single ±5 V output representing both magnitude and direction of displacement.
Follow-up: What is synchronous demodulation and why is it preferred over simple rectification for LVDT signal conditioning?
Q4. What are the advantages of LVDT over a potentiometric displacement sensor?
LVDT advantages over a potentiometer include: infinite resolution (no resistive wiper, no quantisation), no friction (frictionless coupling through the air gap gives near-zero starting force), unlimited mechanical life (no wiper wear), and better immunity to contamination (sealed housing with no sliding electrical contact to corrode). A wirewound potentiometer for hydraulic spool position feedback in a 100 kN press requires replacement every 3–6 months due to wiper wear, while the LVDT equivalent (like the Moog D633 series built-in LVDT) needs only annual calibration checks. The only LVDT disadvantage is the requirement for AC excitation and demodulation circuitry.
Follow-up: How does the LVDT compare to an optical linear encoder in terms of resolution and environmental robustness?
Q5. What is the sensitivity of an LVDT and how is it specified?
LVDT sensitivity is specified as output voltage per unit displacement per volt of excitation — in units of mV/mm/V or mV/V per mm. A Schaevitz LVDT with 5 mV/V/mm sensitivity at 5 V excitation produces 25 mV per mm of displacement. Higher excitation voltage increases output proportionally (hence the normalised sensitivity specification), and higher excitation frequency generally reduces null voltage but increases eddy current losses in the core — most LVDT datasheets specify optimal excitation frequency where sensitivity is maximised and phase shift is minimised.
Follow-up: How does core material affect LVDT sensitivity and frequency response?
Q6. What is a DC-LVDT (DC-operated LVDT) and how does it differ from a standard LVDT?
A DC-LVDT integrates an oscillator (to generate AC excitation from DC supply), the sensing coils, and a synchronous demodulator into a single package — it accepts ±15 V or 24 V DC supply and provides a DC output voltage proportional to displacement, eliminating all external signal conditioning. An Omega Engineering LDCR-025 DC-LVDT provides ±10 V output for ±25 mm range on a single 24 V DC supply, directly compatible with a PLC analog input card. The trade-off is that the internal oscillator adds noise and limits dynamic bandwidth compared to an externally driven standard LVDT with optimised signal conditioning.
Follow-up: What is the bandwidth limitation of a DC-LVDT compared to an externally excited LVDT?
Q7. What is the RVDT (Rotary Variable Differential Transformer) and how does it differ from an LVDT?
An RVDT measures angular displacement using the same differential transformer principle as the LVDT but with a disc-shaped ferromagnetic rotor that rotates to vary coupling between primary and secondaries. The RVDT has a linear range typically ±40° to ±60°, beyond which the output deviates from linearity as the disc geometry causes non-uniform coupling. In aircraft, Moog RVDTs are used in throttle lever position sensing and control surface feedback for angles up to ±30° with 0.1° resolution — the inherent reliability of the contactless design is critical for flight safety applications.
Follow-up: What are the limitations of RVDT compared to a resolver for angular position measurement?
Q8. How is an LVDT calibrated and what is its typical linearity error?
LVDT calibration involves physically displacing the core through its full range using a precision micrometer or laser interferometer reference and recording the output voltage at each position — the sensitivity (mV/mm) and the best-fit linearity deviation are determined. Typical LVDT linearity is ±0.1% to ±0.5% of full-scale range — a 10 mm range LVDT with ±0.2% linearity has maximum deviation of ±20 µm from the ideal straight line. Temperature calibration is done by operating the LVDT in a temperature chamber from -40°C to +125°C to determine the thermal sensitivity coefficient, typically 0.01–0.05%/°C for precision types.
Follow-up: Why is dynamic calibration of an LVDT important for high-frequency vibration measurements?
Q9. What is a half-bridge LVDT configuration and when is it used?
A half-bridge LVDT has only one secondary winding tapped at its centre — displacement moves the core toward or away from the tap, changing the inductance of each half and producing a differential output. Compared to a full differential LVDT (two separate secondaries), the half-bridge has lower sensitivity but simpler winding geometry, making it suitable for very short stroke sensors (1–5 mm). Some embedded LVDT designs in hydraulic spool valves use half-bridge configurations to achieve a shorter sensor body length — the disadvantage is greater temperature sensitivity since there is no differential cancellation of common-mode thermal effects.
Follow-up: How does the full differential LVDT connection reject temperature-induced drift compared to a half-bridge?
Q10. What are the typical applications of LVDT in industrial and aerospace environments?
In industrial automation, LVDTs are used for hydraulic cylinder position feedback (replacing wear-prone potentiometers), die position in press brakes (±0.01 mm accuracy over 300 mm stroke), and valve position indication in power plant control valves. In aerospace, LVDTs are used for flight control surface position (elevator, aileron trim tabs), landing gear compression sensing, and engine thrust reverser position — the Parker Hannifin 400HR series LVDTs are qualified to MIL-SPEC and operate from -55°C to +175°C for engine compartment sensing. The combination of high reliability, wide temperature range, and sealed construction makes LVDT the preferred displacement sensor for safety-critical applications.
Follow-up: What is the difference between a free core LVDT and a captive core LVDT, and which is preferred for hostile environments?
Common misconceptions
Misconception: LVDT output is a DC voltage directly proportional to displacement.
Correct: Raw LVDT output is an AC differential voltage whose amplitude is proportional to displacement magnitude and whose phase indicates direction — signal conditioning (synchronous demodulation) converts this to a signed DC voltage.
Misconception: Higher LVDT excitation frequency always gives better performance.
Correct: Higher excitation frequency reduces null voltage and improves dynamic response but increases eddy current losses in the core, reducing sensitivity and increasing phase shift — each LVDT has an optimal frequency specified by the manufacturer.
Misconception: The LVDT null voltage is exactly zero at the null position.
Correct: A residual null voltage (typically 0.1–1% of full scale) always exists due to winding asymmetry and capacitive coupling — it must be trimmed out in the signal conditioner for precision applications.
Misconception: An LVDT and an RVDT are the same device used for different ranges.
Correct: LVDT and RVDT are mechanically distinct — LVDT uses a sliding cylindrical core for linear displacement, while RVDT uses a rotating disc-shaped rotor for angular displacement, and their coil geometries are fundamentally different.