Introduction
The Linear Variable Differential Transformer (LVDT) is an electromechanical transducer used to measure linear displacement. It is also known as the Linear Variable Displacement Transducer or simply Differential Transformer. LVDTs are highly robust and can operate in extreme environments, including cryogenic temperatures and high heat up to 650°C (1200°F), thanks to their non-electronic AC-operated design.
The LVDT converts mechanical displacement into a proportional electrical signal that contains both amplitude (distance) and phase (direction) information. Its operation is based on electromagnetic coupling, ensuring no physical contact between the moving core and the coil assembly. This provides extremely long life, high resolution, and excellent reliability.
Construction of LVDT
An LVDT consists of:
- A primary winding (P) placed at the center of a cylindrical hollow former
- Two identical secondary windings (S1 and S2) placed symmetrically on either side of the primary
- A movable soft iron core placed inside the former
The primary winding is energized with an AC supply, creating an alternating magnetic flux that couples with the secondary windings. The secondary coils have an equal number of turns and are connected so that their output represents the difference in induced voltages. The soft iron core, linked to the displacement being measured, moves freely inside the hollow former and ensures high sensitivity. The entire assembly is enclosed in a stainless-steel housing to protect against electromagnetic interference.
Working Principle
Three solenoid coils are arranged end-to-end: the primary coil at the center and two secondary coils on either side. A cylindrical magnetic core slides along the axis and is mechanically connected to the object whose displacement is being measured.
When an AC signal excites the primary coil, voltages are induced in the secondary coils. The output voltage is the difference between the two secondary voltages, making the device a “differential transformer.”
Operating Conditions
- Null Position: When the core is centered, equal voltages are induced in S1 and S2, resulting in zero output (ideal null).
- Core Moves Upward: Voltage in S1 increases while S2 decreases, producing an in-phase output.
- Core Moves Downward: Voltage in S2 increases while S1 decreases, creating an out-of-phase output.
The amplitude of the output voltage indicates the magnitude of displacement, while the phase determines the direction. Because the sliding core does not physically touch the coil structure, friction is eliminated, providing very high repeatability and long operational life.
Quadrature Error and Modern Improvements
Small non-zero voltages at the null position (quadrature error) arise due to coupling imperfections.
Modern LVDT systems use precision rectifiers, digital processing, or microcontrollers/FPGA-based circuits to remove such errors and enhance accuracy.
Advanced systems often perform ratiometric processing by dividing the differential voltage by the sum of secondary voltages, making measurements independent of excitation voltage variations.
Advantages of LVDT
- High Range: Can measure displacements from 1.25 mm to 250 mm.
- No Friction: Core moves without contact, ensuring excellent accuracy.
- High Sensitivity: Typically around 40 V/mm; no external amplification required.
- Low Hysteresis: Ensures high repeatability.
- Low Power Consumption: Typically around 1 watt.
- High Reliability: Can be fully sealed against the environment.
Disadvantages of LVDT
- Highly sensitive to stray magnetic fields; proper shielding required.
- Performance may be affected by extreme vibrations or temperature variations.
Applications of LVDT
- Measurement of small displacements from millimeters to several centimeters
- Weight, force, and pressure measurement systems
- Backup or secondary displacement transducers
- ATM machines for measuring currency thickness
- Soil moisture testing instruments
- Tablet and pill manufacturing machines
- Robotic vacuum cleaners
- Medical devices (e.g., neurological probes)
- Load and pressure measurement systems
- Industrial automation and servomechanisms
- Power turbines, hydraulic systems, aircraft, and satellites