Magnetic induction is the underlying principle for Linear Variable Differential Transformers. It is similar to the working principle of many electronic devices. Historically, LVDTs were commonly used in manufacturing and military applications. Some of the desirable characteristics of LVDTs include high resolution, robustness, and simple design. Compared with other types of position sensors, a Linear Variable Differential Transformer produces highly accurate readings which are also highly linear and repeatable.
Other terms used to describe an LVDT include a passive inductive transducer or electronic transformer used in measuring the linear movement of an object.
Conception and history
The original concept of an LVDT was introduced in 1936 by George B. Hoadly. Since then, predecessors to the unique design have had structural differences, but the fundamental nature and working principle have remained the same. It was in 1946 that Herman Shaevitz published a paper about Linear Variable Differential Transformers which describes the technology we know today.
What sets modern LVDTs apart from earlier models are the types of materials used in fabrication as well as the success in miniaturising the device which allowed for smaller sensors with higher reading accuracy. The military was the first sector that could afford the widespread use of LVDTs, but when the costs became lower, various industries began to see the value of using LVDTs and made it a preferred position sensor technology for many applications.
Composition and construction
The device consists of seven essential components, excluding the external circuitry.
- Ferromagnetic core
- Primary coil
- Two secondary coils
The moving element in the sensor is the ferromagnetic core within the cylinder. The three coils are inductors wound around the shaft. The secondary coils, which are in opposite directions need to have the same length and make the same amount of turns to maintain linearity and work in relation to the sensor’s null position.
The shield is cylindrical and protects the windings and internal components from wear and tear as well as enclosing the magnetic field used in sensing movement. The object whose movement the device measures is connected to the ferromagnetic core using a handle. Depending on the requirements of the applications, some LVDT assemblies may include a guide system.
LVDT signal conditioning
Another essential component of an LVDT is signal conditioning. It plays a crucial role in the sensor’s measurement accuracy and linearity. The first step in the process is an oscillator exciting the primary coil which is connected to the two secondary windings. Each of these windings has a varying degree of dependence on the ferromagnetic core position. If the core is located in the centre, the secondary coils will have a parallel connection to the primary coil. Displacement of the ferromagnetic coil will cause one of the secondary coils to have a stronger coupling, thus producing a higher degree of signal excitation.
The secondary coil wound in a different direction will produce an opposite phase signal which becomes the basis for the differential voltage corresponding to the object’s displacement. The output generated by each of the secondary windings is summed to get the total voltage with linear variation equivalent to the maximum positive and negative displacement with respect to the null position.