Like other inductive transducers, this transducer is also used for converting a linear motion into an electrical signal. The basic construction of an LVDT is explained and shown in the figure below.

Construction

The device consists of a primary winding (P) and two secondary windings named S1 and S2. Both of them are wound on one cylindrical former, side by side, and they have equal number of turns. Their arrangement is such that they maintain symmetry with either side of the primary winding (P). A movable soft iron core is placed parallel to the axis of the cylindrical former. An arm is connected to the other end of the soft iron core and it moves according to the displacement produced.

Working

As shown in the figure above, an ac voltage with a frequency between (50-400) Hz is supplied to the primary winding. Thus, two voltages VS1 and VS2 are obtained at the two secondary windings S1 and S2 respectively. The output voltage will be the difference between the two voltages (VS1-VS2) as they are combined in series. Let us consider three different positions of the soft iron core inside the former.

• Null Position – This is also called the central position as the soft iron core will remain in the exact center of the former. Thus the linking magnetic flux produced in the two secondary windings will be equal. The voltage induced because of them will also be equal. Thus the resulting voltage VS1-VS2 = 0.

• Right of Null Position – In this position, the linking flux at the winding S2 has a value more than the linking flux at the winding S1. Thus, the resulting voltage VS1-VS2 will be in phase with VS2.

• Left of Null Position – In this position, the linking flux at the winding S2 has a value less than the linking flux at the winding S1. Thus, the resulting voltage VS1-VS2 will be in phase with VS1.

From the working it is clear that the difference in voltage, VS1-VS2 will depend on the right or left shift of the core from the null position. Also, the resulting voltage is in phase with the primary winding voltage for the change of the arm in one direction, and is 180 degrees out of phase for the change of the arm position in the other direction.

The magnitude and displacement can be easily calculated or plotted by calculating the magnitude and phase of the resulting voltage.

The graph above shows the plot between the resulting voltage or voltage difference and displacement. The graph clearly shows that a linear function is obtained between the output voltage and core movement from the null position within a limited range of 4 millimeter.

The displacement can be calculated from the magnitude of the output voltage. The output voltage is also displayed on a CRO or stored in a recorder.

Advantages

1. Maintains a linear relationship between the voltage difference output and displacement from each position of the core for a displacement of about 4 millimeter.

2. Produces a high resolution of more than 10 millimeter.

3.Produces a high sensitivity of more than 40 volts/millimeter.

4. Small in size and weighs less. It is rugged in design and can also be assigned easily.

5. Produces low hysteresis and thus has easy repeatability.

Disadvantages

1. The whole circuit is to be shielded as the accuracy can be affetced by external magnetic field.

2. The displacement may produce vibrations which may affect the performance of the device.

3. Produces output with less power.

4. The efficiency of the device is easily affected by temperature. An increase in temperature causes a phase shift. This can be decreased to a certain extent by placing a capacitor across either one of the secondary windings.

5. A demodulator will be needed to obtain a d.c output.

This type of transducer is used for finding the linear displacement in terms of voltage or other digital parameters. Another transducer for finding the linear displacement is the Linear Motion Variable Inductance Transducer.

There are mainly two types of proximity inductance transducers. They are

Mutual Inductance Type Proximity Transducer

This transducer consists of a primary and secondary coil. The constructional diagram of the transducer is shown below.

Working

An ac source is given to the primary. This ac source excites the primary and a flux is produced. This flux is linked to the secondary coil and thus a voltage ‘V’ is induced. If the mutual inductance between the primary and secondary windings is represented by ‘M’ (Hertz) and the frequency of ac excitation is represented by ‘w’, then the voltage ‘V’ developed in secondary coil can be written as V = MwI_p.
I_p – The current due to excitation in primary (Amperes).

As shown in the figure, a ferromagnetic displacement plate is placed very near to the windings. The current in the primary coil produces a magnetic flux that links with the secondary coil through the displacement plate. Thus, the movement of the ferromagnetic plate to the right causes a greater value of flux linkage between the two terminals. This in turn causes an increase in the resulting output voltage across the secondary terminal with a value of (T1-T2). This output is given to the input of the CRO or a recorder and the amount of displacement can be known in terms of voltage. A

Advantages

1. The device is small when compared to other transducers.

2. Wear and tear is minimized as there is no physical contact between the target and coil configuration.

3. The output value will be accurate for small displacements as there is a linear relation between the output voltage and linear displacement.

4. There will not be any external effects by contamination.

5. The device works accurately even at higher temperatures up to 400 degree Celsius.

Disadvantages

1. The accuracy decreases when it comes to the measure of large displacement. This is because the linear relation between voltage and displacement is less at higher ranges.

2. The external magnetic field may cause harm to the ferromagnetic material.

Variable Reluctance Type Proximity Transducers

This device can be set up in two ways. Both the diagrams are shown below.

Working

The device consists of a coil that is wound on a core made up of ferromagnetic material. The displacement is given to the core through a target that makes an upward and downward movement according to the displacement produced. It does not touch the core of the coil and a smaller air gap is made between them.
When the target moves closer to the coil due to the displacement, the air gap becomes less causing the reluctance of the magnetic field to reduce and thus the coil inductance to increase. The value of inductance keeps on varying according to the variation in target movement. A CRO or a recorder takes these values and displays it to the user.

In the right side figure shown above, an E-type core is used for finding the displacement. The target is also pivoted at the central limb of the core. Thus, a single coil is divided into two turns and the end of each coil works as the arms of an inductance bridge.

As the displacement value changes, an output signal is produced. This is given to a CRO after amplification.

The biggest advantage of this device is that it shows a linear relationship between the output and the displacement.