LVDT
abbreviated as Linear Variable Differential Transformer. It is an electrical
transformer containing a separable non conducting type of core.
Operating
principle:
The LVDT works on the principle if mutual
inductance, when the primary winding of transformer is supplied by the AC
voltage source, the alternating magnetic field induces voltage in secondary
winding.
Construction:
A
LVDT consists of one primary and two secondary windings with equal number of
turns ratio which is wound on a cylindrical former. The two secondary windings
are in series opposition and placed on either side of primary winding for AC
excitation. A soft iron core which is movable is placed with in the cylindrical
former.
Working:
The output voltage Vo is obtained due to
the displacement applied to the arm of the core. From circuit the output
voltage of secondary winding S1 is Vs1 and output voltage of secondary
winding S2 is Vs2. The secondary
windings are in series opposition connection, the differential output voltage
is given as Vo = Vs1 - Vs2.
*When core
is placed symmetrically with respect to secondary winding an equal amount of
voltage will be induced hence null position (output voltage becomes zero) is obtained.
Vs1 =
Vs2 then Vo = 0
*If core
moved up from null position the more voltage is induced in S1 due to magnetic flux
links hence the output voltage is given as Vo = Vs1 - Vs2.
*If core
moved down from null position the voltage is induced in S2 is more due to
magnetic flux links hence the output voltage is given as Vo = Vs2 – Vs1 which
is 180° out of phase with primary voltage.
Advantages:
1.It has
good linearity in output voltages
2.Very high
range displacements are measured
3.It has
high sensitivity and low hysteresis
4.It
consumes less power about <1W
5.It does
not require amplification devices
6.The
construction is rugged and simple to with stand the high degree of shocks and
vibrations
7.No
friction problem due to no sliding contact
8.The
operating temperature range is as high as +600°C and as low as -265°C.
Disadvantages:
1.It has
limited dynamic response
2.It
requires large displacement to provide high differential output
3.Easily
effected by temperature variations
4.It is
sensitive to stray magnetic fields
5.It
requires demodulator circuit when the device operates on DC
Applications:
1.Force and
vibrations are measured by using LVDT
2.It is used
in ballisto cardiography.
3.It also monitors
fluid level in hydraulic systems
4.It also
detect money in ATM's
5.Used to
measure tension in a Cord
6.Used in
railways for automatic open and closed gates
7.Used for
measuring weights and pressure
8.Used to
measure and control the thickness of metal steel being rolled.
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