What are LVDT Sensors and How They Work?

An LVDT (Linear Variable Differential Transformer) is basically an electrochemical transducer or sensor. It is also known as the linear variable displacement transformer and linear variable displacement transducer. It is basically used to measure linear displacement, as it can convert rectilinear motion of the object to which it is coupled mechanically into an electrical signal.

LVDT linear position sensors can measure both movement and position of objects. Extremely small movements such as a few millionth of an inch, as well as movements of several inches can be measured by using LVDT. LVDT sensors can also measure positions up to + 30 inches to -30 inches.

The basic LVDT sensor includes three coils - a primary coil and two secondary coils spaced symmetrically on each side of the primary coil. An LVDT assigns a specific value for a particular position of the core and thus, measures the displacement of an object. This occurs through the coupling of electromagnetic field generated by an AC excitation of the primary coil to the core and then to the secondary coils.

The core of an LDVT is made of magnetically permeable or ferromagnetic material, which links the electromagnetic field generated on the primary coil to the secondary coils. The core is the moving part of the sensor that can move along the axis of the tube within the coil’s hollow bore. It is that part of the sensor which is mechanically coupled to the object to be displaced. The core does not come in contact with the coil while moving, which is the reason why an LVDT has a hugely extended life, if used properly.

An alternating current (AC) is used to energize the primary coil (the inner coil) – a process known as primary excitation. This primary excitation produces magnetic flux in the primary coil, which in turn, induces a voltage in each of the secondary coils. The magnetic flux generated by the primary coil is coupled to the secondary coils by the core.

When the core stands at the centre between the secondary coils and the primary coil, voltages generated in both the secondary coils are equal but opposite in direction. Therefore, they cancel each other and the output voltage of the LVDT becomes zero.

But, when the core moves from the centre, the secondary coil closer to it gets more strongly coupled to the primary coil. As a result, the voltage produced in this coil is greater than the voltage produced in the other secondary coil.  So, there is an output voltage of the LVDT that can be in phase or out of phase with the primary voltage, depending on the direction in which the core moves.

LVDTs are known for their robustness and low power consumption, for which they are preferred over other displacement transducers. LVDT sensors are primarily used to get position feedback in hydraulic applications. They can monitor the performance accuracy of actuators and cylinders, which can help enhance operational efficiencies.

Due to their reliability and the ability to withstand high temperatures, LVDT sensors are also used to monitor the position of steam control valves in the rehabilitation of power generation plants. These sensors have found applications in petroleum extraction as well. They are used in downhole drilling equipment for position feedback control and also in drilling cutters to obtain a perfectly cut or drilled hole.

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