This article describes linear variable differential transformer or LVDT. It works on the principle of differential transformer that has one primary winding and two secondary windings. The voltage output from the LVDT is proportional to the movement of the core whose displacement is to be measured.
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What is LVDT?
Linear variable differential transformer is popularly known as LVDT. As the name suggests LVDT comprises of the differential transformer that provides the AC voltage output proportional to the displacement of the core passing through the windings. The LVDT comprises of the one primary winding, and two secondary windings connected to each other in series opposing manner as shown in the figure below. The body whose displacement is to be measured is connected to the iron core.
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Construction of the LVDT
The LVDT is basically a differential transformer, whose voltage output is proportional to the displacement of the object hence it is given the name linear variable differential transformer. The differential transformer has a hollow magnetic core on which three coils are wound as shown in the figure below. There is one primary coil in the middle of the core through which the input voltage is applied. There are two secondary coils at the two ends of the central core, which are connected to each other in the phase opposition manner and through which the output is obtained. Through the hollow another solid core is passed, which is connected to the body whose displacement is to be measured.
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Working of the LVDT
The input voltage is supplied to the primary coil from the external source of power and the output is obtained from the secondary coils. The amplitude and the phase of the output depend on the relative coupling between the two output coils and primary coil. The relative coupling is in turn dependent on the position of the solid core inside the hollow core.
The figure below shows the output characteristics of the typical differential transformer. As shown in the figure there is certain position of the core, called as the null position, for which the output voltage is zero, this is an ideal position and is very difficult to attain. Beyond the null position the core moves either to the left or to the right and there is certain output voltage obtained from the differential transformer.
Within certain limits on either side of the null position the output obtained from the differential transformer is proportional to the movement of the core. This means the output from the differential transformer is linear with respect to the motion of the core. The linear variable differential transformer or LVDT works within this range of motion of the core. Thus in LVDT the voltage output obtained is linear with respect to the motion of the core moving inside it.
In LVDT the linear range obtained through the device is dependent on the length of the secondary coils. The magnitude of the output voltage obtained across the sides of the null position is same but they are opposite in phase. Thus it is possible to distinguish the two outputs from LVDT by determining the phase difference between the output voltages.
The output obtained from the LVDT is calibrated against the input motion of the core. The body whose displacement is to be measured is connected to this core, thus any motion of the body gives direct output from the LVDT in the form of the displacement.
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Advantages of LVDT
1) The biggest advantage of the LVDT is that the output obtained from it is proportional to the displacement of the mechanical member whose displacement is being measured.
2) LVDT cannot be overloaded mechanically since the core is completely separated from the other parts of the device.
3) Another important advantage of LVDT is that the output obtained from it is fairly high and it can be measured easily without requiring the need of the intermediate amplification.
3) LVDT is insensitive to the temperature and the changes in the temperature.
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Reference and Images Courtesy
1) Book: Measurement and Instrumentation Principle by Alan S. Morris, third edition, BUTTERWORTH HEINEMANN
2) Book: Mechanical Measurement by Thomas G. Beckwith and N. Lewis Buck, Oxford and IBH Publishing Co. Pvt Ltd.