LINEAR VARIABLE DIFFERENTIAL TRANSFORMER (LVDT)

Principle of LVDT:

LVDT works under the principle of mutual induction, and the displacement which is a non-electrical energy is converted into electrical energy. And the way how the energy is getting converted is described in the working of LVDT in a detailed manner.

The most widely used variable-inductance displacement transducer in Industry is a 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.

Specification of LVDT :

1 Measurement Range 0-50 mm
2 Accuracy ± 1% of the FSR
3 Linearity ±2% of the total range
4 Operating Temperature -20 to 1200C
5 Supply Voltage 5 V
6 Sensitivity 27mV/V

Characteristics of LVDT & its significance

Significance:-

As the core is moved in one direction from the null position, the differential voltage i.e. the difference of the two secondary voltages will increase while maintaining an in-phase relationship with the voltage from the input source. In the other direction from the null position, the differential voltage will also increase, but will be 1800 out of phase with the voltage from the source
The output voltage of an LVDT is a linear function of core displacement within a limited range of motion says about 5mm from the null position. Fig shows the variation of output voltage against displacement for various positions of the core. The curve is practically linear for small displacements. Beyond this range of displacement, the curve starts to deviate from a straight line.

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.

Some Questions and Answers :

Explain with neat sketch working principle of LVDT.

The LVDT transformer consists of a single primary winding P1 and two secondary windings S1 and S2, wound on a cylindrical former. The secondary windings have an equal number of turns and are identically placed on either side of the primary winding. The primary winding is connected to an alternating current source.

A movable soft iron core is placed inside the former. The displacement to be measured is applied to an arm attached to the soft iron core. In practice, the core is made of Ni-Fe alloy which is slotted longitudinally to reduce eddy current losses. When the core is in its normal (null) position, equal voltages are induced in the two secondary windings. Accordingly, output voltage ES1 of the secondary winding S1 is more than ES2, the output voltage of secondary winding S2. The magnitude of voltage is thus ES1- ES2 and the output voltage is in phase with ES1, the output voltage of secondary winding S1. Similarly, if a core is moved to the of null position, then the flux linking with winding S2 becomes larger than that with winding S1. This results in ES2 becoming larger than Es1. The output voltage in this case is E0 = ES2- ES1 and is in phase with ES2; i.e., the output voltage of secondary winding S2.

The amount of voltage change in either of secondary windings is proportional to the amount of movement of the core. Hence, we have an indication of the amount of linear motion. By nothing which voltage output is increasing or decreasing, we can determine the direction of motion. In other words, any physical displacement of the core causes the voltage of one secondary winding to increase while simultaneously reducing the voltage in the other secondary winding. The difference of two voltages appears across the two output terminals of the transducer and gives a measure of the physical position of the core and hence, the displacement. As the core is moved in one direction from the null position, the differential voltage i.e., the difference of two secondary voltages, will increase while maintaining an in-phase relationship with the voltage from the input source.

In the other direction from the null position, the differential voltage will also increase, but will be 1800 out of phase with the voltage from the source. By comparing the magnitudes and phase of the output (differential) voltage with that of the source, the amount and direction of the movement of the core and hence, of displacement, may be determined.