A Linear variable differential transformer, or LVDT, is based on the principles of magnetic induction, the same principle of a “wall-wart” voltage transformer utilized in many electronic device designs. LVDTs have historically been a popular sensor for military and manufacturing applications (Nyce2004), partially due to their extremely high resolution but also due to their simplicity and robustness. Since LVDTs are non-contact, motion of the measured object is not constrained due to frictional forces. They also have a high linearity, repeatability and accuracy (Wilson2005).
LVDTs have been described as “a passive inductive transducer” (Webster1999), “an accurate and reliable method for measuring distance” (Wilson2005) and “a type of electrical transformer used for measuring linear displacement” (Wikipedia).
The original concept of a LVDT was proposed in a patent by George B. Hoadly titled “Telemetric System”, intended as ”… a system for the electrical transmission of intelligence at a distance” (Hoadly1936). The original design has some structural and operational differences to its successors, but the fundamental technology of the sensor has not changed significantly since its invention (Nyce2004). In 1946, Herman Schaevitz published “The Linear Variable Differential Transformer”, a paper describing an LVDT design which is nearly identical to current commercially available LVDTs (Wilson2005). The primary differences between modern versions of the sensor and its predecessors lay mainly in the areas of miniaturization and materials, allowing smaller and more accurate sensors to be made (Nyce2004). The use of the LVDT was historically dominated by the military until lowering costs lead to widespread use in a variety of industries (Nyce2004). LVDTs require few components and can be hand-constructed (Powell2009), although it is very difficult to match the quality of commercial units.
An LVDT is composed of seven components, not including the conditioning circuitry:
- Primary coil
- Secondary coil 1
- Secondary coil 2
- Ferromagnetic core
The ferromagnetic core is the moving component whose position within the shaft is sensed. Around the shaft are wound three inductors, the primary winding in the middle and the secondary windings (wound in opposite direction) on either side of the primary. Each of the secondaries should have the same number of turns and be of the same length, otherwise the null position and linearity will be affected. A cylindrical shield protects the windings from damage and also serves to contain the magnetic field used for sensing. The physical component which will be measured using the LVDT is mechanically coupled to the ferromagnetic core using a threaded (and non-ferromagnetic) handle. For some applications, a guide system and even a spring-return (which is known as a “gagehead” configuration) may be included in the assembly.
Communication between the core and coils of the LVDT is achieved by means of inductive coupling.
An oscillator excites the primary coil which is coupled with the secondary coils. The amount of coupling between the primary and secondary coils is dependent upon the position of the ferromagnetic core. If the core is in the center position, then the secondary coils will be equally coupled to the primary. If the core is displaced from its center position, then one of the secondary coils will be more strongly coupled and will echo the excitation signal to a greater degree.
One of the secondary coils, being wound in opposite direction to the other two inductors, will output the excitation signal in opposite phase. A differential voltage indicates the direction of the displacement: the outputs of the secondary coils are rectified and summed to produce a voltage which varies linearly between the +/- the maximum displacement with the center position being at zero volts. The phase of the secondary voltage indicates if the core is above or below the center position, while the amplitude indicates by how far the core is displaced from the center position.
Reading the RMS voltage level across the two secondary coils will provide magnitude but not phase. Both the direction and distance of the core can be measured after demodulating the signal in preparation for a usable DC voltage output.
The conditioning circuit for an LVDT plays an important role in the linearity and precision of the device (Nyce2004). A complete signal conditioning circuit is characterized by three sections: a signal generator, a demodulator, and a DC voltmeter.
A signal generator is an oscillator that excites the primary windings. Sine wave oscillations are most often used to drive the circuit. Square-wave oscillations are suitable for some circuits, like FET synchronous demodulators (Nyce2004). Response time of the sensor increases with an increasing oscillation frequency, however, the output signal level dissipates and temperature sensitivity increases above 10 kHz. Therefore, signal generator oscillation frequencies typically range from 250 Hz to 10 kHz.
The demodulator converts the AC output of the LVDT into a DC signal. In a simple case, this can be achieved with a diode demodulator. A differential amplifier allows one to scale the output voltage to a suitable range for an Analog-to-Digital Converter.
LVDT signal conditioning circuits can be bought as complete solutions, which contain the signal generator, demodulator, and DC voltmeter in one chip, or as separate elements. Some LVDTs come with the signal conditioning circuits built into the device, which are usually called dc LVDTs, since the input and output signal is a DC voltage. Design notes from companies like Analog Devices and Linear Technologies have helped to ease the customization and construction process of LVDT signal conditioning circuits (Pei2005) & (AnalogDevices2014).
LVDTs are mainly used for industrial, military, and aerospace applications due to their nearly infinite resolution, excellent linearity, and mechanical durability. Aircraft wing flaps usually have LVDTs to measure the displacement of flap actuators. Spring-loaded LVDTs are often used to measure the off-axis rotational movement of wheels.
Depending on the application, an appropriate displacement range can be determined. Measurement range depends on the size of the LVDT. The most compact LVDTs can measure a range of 1 mm, while some of the largest sensors cover a range of 100 mm. LVDTs are a strong choice of sensor for applications that require high precision linear displacement measurements.
- (AnalogDevices2014) Analog Devices, “Universal LVDT Signal Conditioning Circuit”. Circuit Note, CN-0301, 2014. Circuit Note CN-0301
- (Bartelt2008) Terry Bartelt, The LVDT DC Conversion Circuit
- (Herceg1976) Edward E. Herceg, “Handbook of Measurement and Control”. Schaevitz Engineering, Pennsauken, pp. 3-4, 1976.
- (Hoadley1936) George B Hoadley, “Telemetric System”. US Patent Application #2196809, March 17th 1936 (Patented on April 9th 1940).
- (Khazan1994), Alexander D. Khazan, “Transducers and Their Elements”. Prentice Hall, 1994.
- (Nyce2004) David S Nyce, “Linear Position Sensors”. John Wiley & Sons, Hoboken, New Jersey, 2004.
- (Pei2005a), Cheng-Wei Pei, “Low-Distortion Sine Wave Oscillator with Precise Low-Amplitude Stability”. Linear Technology Magazine, 2005.Low-Distortion Sine Wave Oscillator LVDT
- (Pei2005b), Cheng-Wei Pei, “Precision LVDT Signal Conditioning Using Direct RMS to DC Conversion”. Linear Technology Design Notes, 2005. LT Design Note 362
- (Powell2009) Mike Powell, Mike’s Flight Deck
- (Webster1999) John G Webster, “The Measurement, Instrumentation, and Sensors Handbook”. CRC Press with IEEE press, Boca Raton, Florida, 1999.
- (Wilson2005) John S Wilson, “Sensor Technology Handbook”. Elsevier, Burlington, Massachusetts, 2005.
- RDP Group, 2002, LVDT principle of operation.