LVDT Working Principle

LVDT (Linear Variable Differential Transformer) Working Principle:

The differential transformer is an electromechanical transducer which produces an electrical output proportional to the displacement of a movable core. These are essentially variable reluctance transducers. They have an advantage over the synchro, in that there is no need for brushes and slip rings. So the friction is reduced to a minimum. The disadvantage is that it can be used over only a limited range of input position. The basic principle involved is shown in the schematic diagram below. These are called Linear Variable Differential Transformers (LVDT). The LVDT is shown in Figure.
The primary is supplied form an AC source. The two identical secondary coils are connected in phase opposition. The voltage induced in the two secondary coils is dependent upon their respective induction with the primary winding.

 This is governed by the position of the core. There exists a null position of the movable core when both secondary coils induce the same voltage i.e., e1 = e2. Then e0 the output voltage is equal to zero. A slight movement of the core upwards from this position will produce variation in the coupling and e1 will be greater than e2. Consequently e0 being equal to el - e2 will be non zero.

It is in fact proportional to the displacement x1. If the displacement takes place in the downward direction from the null position, e2 will be greater than e1. Thus there will be an output with 180°, out of phase with the voltage produced by upward motion. The differential transformers are also known as pick off. The excitation of such devices normally ranges from 0 V R.M.S. to 15 V R.M.S. The frequency range is from 60 Hz to 20 kHz,

(a) Advantages of LVDT:

1. Good linearity [linear up to 5 mm, practically 0.005% linear]
2. The resolution is infinite
3. Presents high output
4. Good sensitivity [of the order of 40 V/mm]
5. Power consumption is low
6. Rugged in construction
7. Low hysteresis         
8. Offers no friction

(b) Disadvantages:

1. To obtain fair differential output displacement required is large
2. As they are sensitive to stray magnetic fields shielding required
3. The receiving instrument must work on AC.
4. Demodulator is necessary if the receiver has to work on DC.
5. It is affected by temperature
6. It has limited dynamic response by the mass of the core and applied voltage.

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