LVDT Construction & Working | Characteristic | Advantage | Specifications

Here we learn about LVDT Construction and Working with diagram | Characteristics of LVDT| Advantages | Specifications of LVDT

LVDT :

Introduction :

The letters LVD'T are an acronym for Linear
Variable Differentiaĺ Transformer, a common type of electromechanical transducer that can convert the rectilinear motion of an object to which it is coupled mechanically into a orresponding electrical signal.

 LVDT łinear position sensors are readily available that can measure movements as small as a few million of an inch up to several inches, but are also capable measuring positions up to ± 0.5 m.

Principle :

The principle is based upon modulation of the
excitation $ignal. In LVDT Motion of magnetic core changes the mutual inductance of two $econdary coil$ relative primary coil. LVDT (Linear Variable Differential Transformer) is a popular transducer which based on a variable-inductance principle for displacement measurement$.
~ It provide$ an ac voltage output proportional to the displacement of a core passing through it$ winding$. LVDT is $hown in Fig. 2.1.5.

Construction :

~ In this, three coils are arranged as shown $chematically in Fig. 2.1.5(a) between which magnetic core moves freely.
~ The construction of LVDT is as $hown in Fig. 2.1.5(b), The centre coil (primary) is energized from an external a.c. $upply, and the two end coils are ($econdary) connected together in opposite phases are used as pick up coils.
~ The core passing through the primary and $econdary coils provides a magnetic coupling.

Operation :

~ In operation, an a.c. input i$ impressed on the primary coil, and an a.c. output voltage, depending on the magnetic coupling between the core and the coils, is obtained at the $econdary coils.
~ The magnetic coupling, hence the output depends upon the position of the core. Usually a linear range is $pecified for a differential transformer, and when the core is operated within this range, it is called Linear Variable Differential Transformer (LVDT).
~ Theoretically, there $hould be a core position for which the voltage induced in the $econdary coils will be the $ame as that of the input and the resulting output should be zero.
~ Thi$ position is called as null position and i$ difficult to obtain in practice.
~ The difference of the two voltages across the output terminals of LVDT give$ a measure of the physical position of the central core. By comparing the magnitude and the phase of output voltage, the amout and direction of the movement of core can be determined.

Characteristics of LVDT 

~ Fig. 2.1.6 $hows the typical characteristic curve of a LVDT. The graph represents the output versus the linear movement of the core (considering the absolute magnitude of the output).
~ It can be $een that within limits, on either side of the null position, core displacement results in proportional output.
~ Also, the linear range depends upon the length of the secondary coils. In operation there is a 1800 phase shift from one side of the null position to the other.

LVDT specifications : 

LVDT Specifications are given in following terms,
1. Input or excitation voltage 3 to 15 (sinusoidal).
2. Stroke or core displacement - 0.1 mm to 100 mm.
3. Input frequency-20 to 50 kHz.
4. Sensitivity-0.5 to 30 mV/V input /0.05 mm.
5. Power consumption-< 1 Watt
6. Output impendence-< 50.
7. Resolution-2× 103 mm.

Advantages of LVDT:

1. It $erves as a primary detector transducers, it converts mechanical displacement into a proportional electrical voltage without the assistance of any elastic member.
2. It cannot be overloaded, $ince the core is completely separable from the device.
3. It is insensitive to temperature changes.
4. It give$ high output without any intermediate amplification.
5. It is an economical device.