on December 16th 2,573

Linear Variable Differential Transformer (LVDT): Working Principle, Types, Advantages, and Applications

A Linear Variable Differential Transformer (LVDT) is a sensor you use to measure linear position or movement. In this article, you will learn what an LVDT is, how it is built, and how it converts motion into an electrical signal. You will also see the main types of LVDTs, their advantages, and where they are commonly used. A comparison with other position sensors helps you understand when an LVDT is the better choice.

Catalog

Figure 1. LVDT

What is a Linear Variable Differential Transformer?

A Linear Variable Differential Transformer (LVDT) is a sensor used to measure linear position or displacement. In the figure, the LVDT appears as a cylindrical metal sensor with a movable probe that slides in and out as position changes. This movement shifts an internal magnetic core inside the transformer coils. As the core moves, the sensor generates an electrical signal proportional to the displacement. The cable shown carries this signal to external measuring or control equipment. Due to its non-contact operation, the LVDT sensor provides accurate and reliable measurements in industrial applications.

Construction of Linear Variable Differential Transformer

Figure 2. Construction of LVDT

The construction of a Linear Variable Differential Transformer (LVDT) consists of a cylindrical coil assembly and a movable magnetic core arranged to measure linear displacement. As shown in the figure, the LVDT has one primary winding placed at the center and two identical secondary windings positioned symmetrically on either side. These secondary windings are connected in series opposition, which is important for producing a differential output. The movable ferromagnetic core, shown inside the coils, is mechanically connected to the object whose position is being measured. When the core moves due to displacement, it changes the magnetic coupling between the primary and secondary windings. The lead wires carry excitation and output signals, completing the physical structure of the LVDT sensor.

Working Principle of Linear Variable Differential Transformer

Figure 3. Working Principle of LVDT

The working principle of a Linear Variable Differential Transformer (LVDT) is based on mutual inductance, where linear motion is converted into a proportional electrical signal. In the figure, an AC voltage is applied to the primary winding (P), which creates an alternating magnetic field linking the two secondary windings S1 and S2. When the core is at the null position shown in the center diagram, equal voltages are induced in both secondary coils, so the output voltage is zero.

As shown in the left diagram, when the core moves toward S1, the voltage induced in S1 becomes higher than S2, producing an output voltage Eout=E1−E2. In the right diagram, when the core moves toward S2, the induced voltage in S2 becomes higher, reversing the output to Eout=E2−E1. This change in voltage magnitude and phase clearly indicates both the amount and direction of displacement, which is why the LVDT sensor is widely used for precise linear position measurement.

Main Types of Linear Variable Differential Transformers

AC LVDT (AC-Operated LVDT)

Figure 4. AC LVDT

An AC LVDT (AC-Operated Linear Variable Differential Transformer) is a type of LVDT sensor that uses an external AC excitation to measure linear displacement. As shown in Figure 4, an alternating voltage is applied to the primary coil, creating a magnetic field that links with the secondary windings. When the movable core shifts up or down, the magnetic coupling changes, causing an AC output voltage whose amplitude varies with the core position. The figure clearly illustrates how the core movement directly controls the output signal level. Because the output remains AC, external signal conditioning such as demodulation and amplification is required to convert it into a usable DC signal. Due to their simple structure, high accuracy, and stable performance, AC-operated LVDTs are widely used in industrial measurement, laboratory experiments, and research applications.

DC-Output LVDT (Conditioned LVDT)

Figure 5. DC-Output LVDT

A DC-Output LVDT, also known as a Conditioned LVDT, is a Linear Variable Differential Transformer that provides a direct DC output proportional to linear displacement. As shown in Figure 5, this type of LVDT appears similar to a standard probe-style sensor but includes built-in electronic circuitry inside the cylindrical housing. Internally, it still operates on the AC LVDT principle, where the movable core shifts inside the coils as the measured object moves. The internal electronics (such as an oscillator, demodulator, and filter) convert the AC signal into a stable DC voltage or current output. This built-in conditioning eliminates the need for external signal processing equipment. Because of their simple wiring and easy integration, DC-output LVDTs are widely used in industrial automation, PLC systems, and control applications.

Advantages of Linear Variable Differential Transformer

• High accuracy and excellent linearity over a wide measurement range

• Frictionless operation due to the absence of mechanical contact

• Long operational life with minimal wear and maintenance

• High repeatability and low hysteresis

• Reliable performance in harsh industrial environments

Applications of Linear Variable Differential Transformer

• Industrial automation and process control

• CNC machines and machine tool positioning

• Aerospace and aircraft control systems

• Hydraulic and pneumatic actuator feedback

• Material testing and quality inspection equipment

• Structural monitoring and vibration measurement

LVDT vs Potentiometer vs Optical Encoder vs Capacitive Sensor

Specification	LVDT (Linear Variable Differential Transformer)	Potentiometer	Optical Encoder	Capacitive Sensor
Sensor Type	Inductive displacement sensor	Resistive position sensor	Optical position sensor	Capacitive displacement sensor
Measurement Motion	Linear only	Linear or rotary	Linear or rotary	Linear only
Contact Operation	Non-contact	Contact-based	Non-contact	Non-contact
Typical Accuracy	Up to ±0.25 percent of full scale	About ±1 percent of full scale	Up to ±0.01 percent	Up to ±0.05 percent
Resolution	Infinite theoretical resolution	Limited by resistive track	Limited by counts per revolution	Very high, near infinite
Output Signal	AC or conditioned DC	Analog DC voltage	Digital pulses	Analog voltage
Supply Voltage	1 kHz to 10 kHz AC excitation	5 V to 10 V DC	5 V or 12 V DC	5 V to 24 V DC
Signal Conditioning	Required for AC type	Not required	Required for decoding	Required for stability
Wear and Lifespan	No mechanical wear, very long life	High wear, short lifespan	Very long life	Very long life
Linearity	Excellent, up to ±0.25 percent	Moderate	Excellent	Very good
Environmental Resistance	Excellent against vibration and dust	Poor in harsh environments	Moderate, sensitive to dust	Sensitive to moisture and dirt
Temperature Stability	Very high	Low to moderate	Moderate	Moderate
Typical Measurement Range	1 mm to 500 mm	Up to 300 degrees or 300 mm	Depends on encoder design	0.1 mm to 10 mm
Common Applications	Industrial automation and aerospace	Low-cost position sensing	Motion control and robotics	Precision laboratory measurement

Conclusion

An LVDT provides accurate and reliable linear displacement measurement using a non-contact operating principle. Its simple construction, high accuracy, and long lifespan make it suitable for industrial and control applications. Different LVDT types allow easy integration into various systems. Compared with other position sensors, LVDTs offer stable performance and strong resistance to harsh environments.