Linear Variable Differential Transducer | LVDT

NOTES OF LVDT

1. LINEAR VARIABLE DIFFERENTIAL TRANSFORMER (LVDT) BY SHIWANI RAJ

2. LINEAR VARIABLE DIFFERENTIAL TRANSFORMER (LVDT) LVDT is a inductive transducer. LVDT is used to translate the linear motion into electrical signal.

3. CONSTRUCTION OF LVDT Soft iron core Made of high permeability nickel iron alloy which is hydrogen annealed providing low harmonics, low null voltage & high sensitivity. Slotted longitudinally to reduce eddy current losses. FIG1– LINEAR VARIABLE DIFFERENTIAL TRANSFORMER The transformer consists of a single primary winding P1 and two secondary windings S1 and S2 wound on a cylindrical former. The secondary windings have 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 OF FREQUENCY RANGING FROM 50Hz to 20Khz. 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.

4. The output voltage of secondary, S 1 is E S1 and that of secondary, S 2 is E S2 . To convert the outputs from S 1 and S 2 into a single voltage signal, the two secondaries S 1 and S 2 are in series opposition. The output voltage of the transducer is the difference of the two voltages. Differential output voltage, E o = E S1 – E S2 FIG – CIRCUITS OF AN LVDT

5. WORKING OF LVDt Case I When core is at null position: The flux linking with both secondary windings is equal and hence equal emfs are induced in them, i.e., E S1 = E S2. The output voltage at null position is, E o = E S1 – E S2 = 0 . Case II When core is moved to left of the null position: More flux links with winding S1 and less with winding S2, E S1 > E S2 . The output voltage is, E o = E S1 – E S2 and is in phase with E1. Case III When core is moved to right of the null position: More flux is linked with winding S2 and less with winding S1, E S1 < E S2 . The output voltage is, E o = E S2 – E S1 and is in phase with E2. The amount of voltage change in either secondary winding is proportional to the amount of movement of the core.

6. As the core is moved in one direction from the null position, the differential voltage 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 180° out of phase with the voltage from the source. By comparing the magnitude and phase of the output voltage with that of the source, the amount and direction of the movement of the core and hence displacement may be determined. The output voltage of an LVDT is a linear function of core displacement within a limited range of motion, about 5mm from the null position Beyond this range of displacement, the curve starts to deviate from a straight line.

7. RESIDUAL VOLTAGE Ideally the output voltage at the null position should be equal to zero. In actual practice there exists a small voltage at the null position. This null voltage is due to presence of harmonics produced in the input and output voltages Due to an incomplete magnetic or electrical balance or both a finite output voltage is produced at the null position which is generally 1% of the maximum output voltage in the linear range. Other causes are stray magnetic fields and temperature effects. With improved technological methods and with the use of better a.c . sources, the residual voltage can be reduced to almost a negligible value. FIG – RESIDUAL VOLTAGE

8. Advantages of lvdt A linearity of 0.05% is available. It gives a high output. It has high sensitivity as 40 v/MM. These transducers can tolerate a high degree of shock and vibration without any adverse effects. Less friction and less noise Low hysteresis Low power consumption less than 1 w.

9. Disadvantages of lvdt Relatively large displacements are required for appreciable differential output. They are sensitive to stray magnetic fields. The transducer performance is affected by vibrations. Temperature affects the performance of the transducer. The dynamic response is limited mechanically by the mass of the core and electrically by the frequency of applied voltage.

10. Applications of lvdt The lvdt act as primary transducer to convert the displacement directly into an electrical output proportional to displacements. The lvdt acting as secondary transducer can be used as a device to measure force, weight and pressure etc.