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, S1 IS
ES1 AND THAT OF SECONDARY, S2 IS ES2.
• TO CONVERT THE OUTPUTS FROM S1 AND S2
INTO A SINGLE VOLTAGE SIGNAL, THE TWO
SECONDARIES S1 AND S2 ARE IN SERIES
OPPOSITION.
• THE OUTPUT VOLTAGE OF THE
TRANSDUCER IS THE DIFFERENCE OF THE
TWO VOLTAGES.
DIFFERENTIAL OUTPUT VOLTAGE, EO = ES1
– ES2
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., ES1=ES2.
THE OUTPUT VOLTAGE AT NULL POSITION IS, EO = ES1 – ES2
= 0.
• CASE II WHEN CORE IS MOVED TO LEFT OF THE NULL
POSITION:
MORE FLUX LINKS WITH WINDING S1 AND LESS WITH
WINDING S2, ES1 > ES2 .
THE OUTPUT VOLTAGE IS, EO = ES1 – ES2 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, ES1 < ES2 .
THE OUTPUT VOLTAGE IS, EO = ES2 –ES1 AND IS IN PHASE
WITH E2.
The amount of voltage change in either secondary winding is proportional to the amount of movement o
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
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.
