Linear Variable Differential Transformer
1. Linear Variable Differential Transformer
It is a Passive Transducer.
1. Principle
LVDT works under the principle of mutual induction and the displacement which is a non-electrical energy is converted into an electrical energy.
2. Contraction
1. Primary Coil
2. Secondary Coil 1 S1
3. Secondary Coil 2 S2
4. Ferromagnetic Core
5. Shaft
6. Shield
1. The ferromagnetic core is the moving component whose position within the shaft is sensed.
2. It is surrounded by one primary winding in the center of the former.
3. The two secondary windings are equal and opposite direction.
4. If the left secondary windings in the clockwise direction, the right secondary windings will be in the anticlockwise direction.
5. The net output voltages will be the difference in the voltages between the two secondary coils.
6. A cylindrical shield protects the windings from damage.
7. The two secondary coils are represented as S1 and S2.
8. The induced voltages V1 and V2 in the two secondary are equal but anti phase.
1. Working
The working of LVDT by splitting the cases into 3 based on the iron core position insulated former.
1. Center or Null Position
2. Right of Null Position
3. Left of Null Position
CASE-1
® On applying an External force which is the Displacement.
® If the core reminds in the null position it without providing any movement.
® Then the voltages induced in both the secondary coils are equal.
® The core is in the centre V1 and V2 are equal and Opposite.
® Hence they cancel out and the Output voltage V0 is zero.
V0 = V1 – V2 = 0
CASE-2
® When an external force is applied and it the steel iron core tends to moves towards coil S2.
® V2 is increased but V1 is decreased in magnitude will each other.
® Therefore the net output voltage
V0 = V2 – V1
® This is in phase with V2.
CASE-3
® When an external force is applied and it the steel iron core tends to moves towards coil S1.
® V1 is increased but V2 is decreased in magnitude will each other.
® Therefore the net output voltage
V0 = V1 – V2
® This is in phase with V1.
v The magnitude of V0 is a function of the distance moved by the one core.
v Phase indicates as to in which direction it has moved.
If core is attached to a moving object, the magnitude of V0 gives the position of that object.
4. Advantages
Ø High output.
Ø High sensitivity (50mV to 300mV).
Ø Very good linearity.
Ø Produces high resolution (>10mm).
Ø Low power consumption.
Ø Low hysteresis.
Ø Small in size and weight less.
Ø It is rugged in design and can also be assigned easily.
Ø Maintains a liner relationship between the voltage difference output and displacement from each position of the core for a displacement of about 4mm.
Disadvantages
Ø Very high displacement is required for generating high voltage.
Ø Shielded is required since it is sensitive external magnetic field.
Ø The Performance of the transducer gets affected by vibrations.
Ø Produces output with less power.
Ø The efficiency of the device affected by temperature.
Ø A demodulator will be needed to obtain a d.c. output
Applications
Ø Acting as a secondary transducer.
Ø LVDT can be used to measure force, weight and pressure.
Ø Measurement of roll position.
Ø Used as an absolute position sensor even if power is switched off.
Ø Used to measure displacement ranging from fraction mm to cm.
Ø Measurement of material thickness in hot strip or slab steel mills.
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