LINEAR VARIABLE DIFFERENTIAL TRANSFORMER (LVDT)
AIM: To study LVDT transducer characteristics.
APPARATUS: Experiment board with LVDT device with 50 Hz a.c. power supply, D.C.P.S.(12v) , a.c. output voltmeter.
THEORY: The most widely used inductive transducer to translate the linear motion into electrical signals is the linear variable differential transformer (LVDT). The basic construction of LVDT consists of a single primary winding P and two secondary windings S1 and S2 respectively, wound on a cylindrical former, which is driven by a motorized screw. The motor used is 12V d.c. motor whose shaft drives a screw which further shifts the core in and out of the secondary of the two windings. The secondary of the windings have equal number of turns and are identically placed on either side of the primary winding. The two secondaries are connected in the differential form. The primary winding is connected to a low voltage alternating current source. The induction of secondary voltage can be altered by a slight movement of the core, in and out of the secondary winding. A movable soft iron core is placed inside the former. The displacement to be measured is applied to the arm attached to the soft iron core. In practice, the core is made of high permeability, nickel iron which is hydrogen annealed. This gives low harmonics, low null voltage and a high sensitivity. This is slotted longitudinally to reduce eddy current losses. The assembly is placed in a stainless steel housing and the end lids provide electrostatic and electromagnetic shielding. The frequency of a.c. applied to primary windings may be between 50Hz to 20kHz.
Since the primary winding is excited by an alternating current source, it produces an alternating magnetic field which in turn induces alternating current voltages in the two secondary windings.
The output voltage of secondary, S1, is Es1 and that of secondary, S2, is Es2. In order to convert the outputs from S1 and S2 into a single voltage signal, the two secondaries S1 and S2 are connected in series opposition. Thus the output voltage of the transducer is the difference of the two voltages.
Differential output voltage, E0 = Es1 � Es2.
When the core is at its normal (NULL) position, more flux links with winding S1 and less with winding S2. Accordingly, output voltage Es1, of the secondary winding S1, is more than Es2, the output voltage of secondary winding S2. The magnitude of output voltage is, thus, E0 = Es1 � Es2 and the output voltage is in phase with say, the primary voltage. Similarly, if the core is moved to the right of the null position, the flux linking with winding S2 becomes larger than that linking with winding S1. This results in Es2 becoming larger than Es. The output voltage in this case is E0 = Es2 � Es1, and is 180� out of phase with the primary voltage. Therefore, the two differential voltages are 180� out of phase with each other.
The amount of voltage change in either secondary winding is proportional to the amount of movement of the core. Hence, we have an indication of amount of linear motion. By noting which output voltage4 is increasing or decreasing, we can determine the direction of motion. Thus, 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 the two voltages appears across the output terminals of the transducer and gives a measure of the physical position of 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 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 180� out of phase with the voltage from the source. By comparing the magnitude 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.
The variation of the output voltage against displacement for various positions of core is to be plotted. The curve is practically linear for a limited range of displacement from the null position.
Advantages of LVDT:
1) High range from about 1.25mm to 250mm.
2) No Friction and good electrical isolation.
3) Immunity from external effects.
4) High input and high sensitivity.
5) Ruggedness.
6) Low hysteresis.
7) Low power consumption.
Applications of LVDT:
LVDT can be used in all applications where displacements ranging from fraction of a mm to a few cm are to be measured. It can be used as a device to measure force, weight and pressure, etc. It is also used for measurement and control of thickness of a metal sheet being rolled. Also, it can be used for measurement of tension is a cord.
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