LVDTs and Other Non-Contact Sensor Technologies

Inductive non-contact sensor technology makes position sensors manufactured by Positek suitable for industrial applications and other uses exposing the device to the harshest working conditions. Positek has a range of standard linear sensors, as well as custom-design option.

Measuring displacement is a vital step in most mechanical processes. Machines and various equipment used in industrial and commercial applications use displacement sensors specifically for this purpose. But sensor technology is a broad category pertaining to a variety of devices with different specifications and uses. In this article, we will focus on linear displacement measurement, and understanding the working principle of linear variable differential transformers as well as other non-contact position sensors.

What does linear displacement measurement mean?

To better understand the technology behind LVDTs, it is also essential to define linear displacement measurement. In simple terms, linear displacement refers to an object's unidirectional movement from one point to another, along an axis. The purpose of a linear sensor is to produce an output or signal corresponding to the object's travelled distance from its reference point to its destination. Furthermore, a position sensor also identifies the movement's direction.

Working principle of an LVDT

A linear variable differential transformer or LVDT is the most common sensor device used to measure linear displacement. The assembly consists of three coils and a core. The coils are usually mounted to an immobile form, while the core couples directly with the moving body. The coils wound around a hollow tube does not touch the permeable core. The innermost coil is the primary coil, and it functions as it is excited by an alternating current. The coil will then produce a magnetic flux which will move towards the two other coils, spreading the AC voltage in both coils. To measure displacement using this principle, there is a corresponding value to each specific signal at any given core position. The electromagnetic coupling from the AC voltage exciting the primary coil, into the core, and back towards the secondary windings produces the signal value association.

This working principle, in which there is no contact between the device mechanisms make LVDTs an attractive choice for many industrial applications. Non-contact sensor technology ensures robustness and durability. For example, if you subject an LVDT to a working condition exposed to friction and vibration, there is virtually no damage due to wear and tear.

Moreover, since an LVDT uses an electromagnetic coupling and a magnetic flux, the device can have infinite resolution. Hence, it is possible for an LVDT to measure even the tiniest fraction of movement through signal conditioning.

The difference between contact and non-contact sensor technology

While it is true than non-contact sensor technology is most suitable for robust applications, there are still some mechanical assemblies which require simpler technology. Contacting sensor technology means that the device is directly coupled to the moving body while creating the signal output to measure displacement.

A potentiometer is the most basic type of contacting sensor consisting of a sliding wiper moving along a restrictive track. Potentiometers are still widely used today because the device does not require auxiliary power sources or complicated electronic set-up. These devices are also quite cheap and not hard to find. Since potentiometers suffer from wear and tear, these sensors are best used for mild applications where there is no exposure to vibration and harsh working conditions.

Other examples of non-contacting position sensor technology

The LVDT is the most common example of a non-contact sensor. But in addition to this, there are other sensor technologies which also have the same characteristic.

• Capacitive sensor. A capacitive sensor or transducer works by using the capacitance produced between elements to measure the displacement of an object.
• Magnetorestrictive sensor. As the name suggests, this type of non-contact position sensor works by emitting pulses that create a magnetic field. The devices use the magnetic field to locate the magnet's position. As a corresponding pulse emits from the magnet, the electronic components of the sensor times this transmission to measure the object's displacement.
• Eddy-current sensor. This type of sensor is also referred to as an inductive sensor. The working principle is to use eddy-current to measure rectilinear movement.

With these technologies as a precursor to many innovations in producing sensors, manufacturers like Positek use a patented inductive sensor technology to produce non-contact sensors with the highest level of robustness. Similar to an eddy-current sensor, Positek's range of displacement sensors can operate in the most extreme working conditions while maintaining output accuracy and repeatability.