What is LVDT | Diagram , Advantages and Disadvantages and Application 


LINEAR VARIABLE DIFFERENTIAL TRANSFORMER (LVDT)

The most widely used variable-inductance displacement transducer in Industry is Linear Variable Differential Transformer (LVDT). It is a passive type sensor. It is an electro-mechanical device designed to produce an AC voltage output proportional to the relative displacement of the transformer and the ferromagnetic core. The physical construction of a typical LVDT consists of a movable core of magnetic material and three coils comprising the static transformer shown in Figure 1.26. One of the three coils is the primary coil or excitation coil and the other two are secondary coils or pick-up coils. An AC current (typically 1 kHz) is passed through the primary coil and an AC voltage is induced in secondary coils. The magnetic core inside the coil winding assembly provides the magnetic flux path linking the primary and secondary coils.

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Construction of lvdt sensor
Construction of LVDT sensor

When the magnetic core is at the centre position or null position, the output voltages are being equal and opposite in polarity and, therefore, the output voltage is zero. The Null Position of an LVDT is extremely stable and repeatable. When the magnetic core is displaced from the Null Position, a certain number of coil windings are affected by the proximity of the sliding core and thus, an electromagnetic
imbalance occurs. This imbalance generates a differential AC output voltage across the secondary coil which is linearly proportional to the direction and magnitude of the displacement.

working of LVDT
Working of LVDT

Applications of LVDT sensors

  • Measurement of spool position in a wide range of servo valve applications
  • To provide displacement feedback for hydraulic cylinders
  • To control weight and thickness of medicinal products viz. tablets or pills
  • For automatic inspection of final dimensions of products being packed for dispatch
  • To measure distance between the approaching metals during Friction welding process
  • To continuously monitor fluid level as part of leak detection system
  • To detect the number of currency bills dispensed by an ATM

Advantages of LVDT Sensor 

(i) It is relatively low cost due to its popularity
(ii) It is solid and robust capable of working in a wide variety of environments
(iii) There is no friction resistance since the iron core does not contact the transformer coils thereby resulting in an infinite (very long) service life
(iv) High signal to noise ratio and low output impedance can be obtained
(v) It has negligible hysteresis
(vi) It has short response time, only limited by the inertia of the iron core and the rise time of the amplifiers
(vii) There is no permanent damage to the LVDT if measurements exceed the designed range
(viii) It can operate over a temperature range of-265°C to 600°C
(ix) It is has high sensitivity up to 40 V/mm
(x) It has less power consumption (less than 1 IF)

Disadvantages of LVDT Sensor : 

(i) The performance of these sensors is likely affected by vibration etc
(ii) Relatively large displacements are required for appreciable output
(iii) It is not suitable for fast dynamic measurements because of mass of the core
(iv) It is inherently low in power output
(v) It is sensitive to stray magnetic fields but the shielding is not possible

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