LVDT – Linear Variable Differential Transformer Application

LVDT is a Linear Variable Differential Transformer. But don’t mistake it with a transformer because LVDT is one kind of a transducer.

The working principle of LVDT involves mutual inductance, just like what we find at transformers. LVDT also has a secondary coil just like a transformer. This secondary voltage has the characteristic of differential value, thus it is called a Linear Variable Differential Transformer.

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What is a Linear Variable Differential Transformer?

Just like mentioned above, LVDT is a transducer involved with the principle of mutual inductance, primary and secondary windings, and has differential characteristics.

Linear Variable Differential Transformer is an electromechanical inductive transducer. LVDT is able to convert the rectilinear displacement transducer into electric signals.

Physical quantities like force, weight, tension, and pressure are first transformed to displacement by a primary transducer before being measured by an LVDT in terms of the appropriate electrical signal because LVDT is a secondary transducer.

There are no electronics within the LVDT because it is an AC-controlled device. Due to its high level of accuracy, it is the most used inductive sensor. It is called a differential transformer because the difference in secondary voltages results in the electrical output.

We will also learn about an RVDT.

The Construction of LVDT

If a transformer has one primary winding and one secondary winding, an LVDT has more than that. An LVDT is constructed like a BJT, sandwiched layers. A Linear Variable Differential Transformer has:

One primary winding (P),
Two secondary windings (S1 and S2), and
An armature with a moving soft iron core.

These three windings are wound on a cylindrical former (in nature it is hollow).

The AC voltage source is connected to the primary winding to produce:

Magnetic flux in the air gap.
Induced voltage to the secondary windings.

There are some points to remember:

The soft iron core can move because it is placed inside the former.
We measure the displacement that is connected to the iron core.
Our iron core should have high permeability, making it reduce harmonic disturbance and provide high sensitivity for our LVDT.
We place our LVDT inside a housing made from stainless steel to provide good electrostatic and electromagnetic shielding.
The secondary windings are connected in a way so its output results in the voltage difference between two windings.

Working Principle of LVDT

After learning the working principle, we have to learn the working principle of Linear Variable Differential Transformer. As stated above, LVDT is energized by AC voltage and current and its outputs also produce AC voltage and current.

Observe the illustration of the working principle of LVDT.

Like what we said earlier, LVDT shares the same configuration with a transformer in terms of primary and secondary windings. Of course the primary and secondary voltage will not be very different with a common transformer.

The secondary output for first secondary winding (S₁) is e₁ and for second secondary winding (S₂) is e₂. We can calculate the differential output by

Thus, our circuit will be

Calculation of LVDT

Since the iron core can move to the S₁ or S₂, we have three conditions when using an LVDT. Because the iron core has three different positions, we will have three conditions:

Condition 1. When the core is not moving (null position) or no displacement.

If the LVDT is supplied by an AC source but it doesn’t move, we can say that the flux linking to the both secondary windings (S₁ and S₂) is equal. Since the flux is equal, the induced voltage for both windings is equal (e₁=e₂). When there is no displacement, the voltage out output, e_out is zero.

Condition 2. When the core is moving to the first secondary winding (S1)

The iron core moves toward the S₁ from the null position. This happens because the flux linked to the secondary winding S₁ is greater than the secondary winding S₂. Since the flux of S₁ is higher than S₂, the voltage e₁ is higher than e₂. Thus, the voltage output, e_out is positive.

Condition 3. When the core is moving to the second secondary winding (S₂)

The iron core moves toward the S₂ from the null position. This happens because the flux linked to the secondary winding S₂ is greater than the secondary winding S₁. Since the flux of S₂ is higher than S₁, the voltage e₂ is higher than e₁. Thus, the voltage output, e_out is negative.

We can put all three conditions in a single graph as shown below.

The 0% indicates that the iron core fully moves toward the S₂ thus the output is negative. The flux is 100% to the S₂.

The 50% indicates that the iron core is at null position thus the output is zero. The value 50% means the flux is divided equally to S₁ and S₂.

The 100% indicates that the iron core fully moves toward the S₁ thus the output is positive. The flux is 100% to the S₁.

Keep in mind that:

The amount of positive and negative changes depend on how big the movement of the iron core is. In other words, the output is proportional to the linear motion.
Only by looking at the value of the output, we can determine its movement direction and how far it moves.
The output of our LVDT is the linear function of the iron core displacement.

Advantages and Disadvantages of LVDT

After understanding the construction and working principle of an LVDT, we may able to spot what are their advantages and disadvantages if compared to other components that share the same function.

Advantages of LVDT

Wide range – LVDT is known for its wide range of displacement measurement.
Minimum frictional losses. Because we use an iron core in an air gap, there is no mechanical friction when there is movement of the core.
Very accurate device. This is achieved by the minimum friction from the core.
High sensitivity.
Low hysteresis. Thus, LVDT is very good for repeated uses.
Low power consumption.
Excels at converting mechanical movement to electrical signals.
Fast operation and response.

Disadvantages of LVDT

Sensitive to magnetic field. Since it operates with magnetic flux, any external magnetic field will affect this device.
Affected by vibration. Since we need the movement accuracy of the core, any vibration will affect its movement thus decreasing its accuracy.
Affected by temperature. Increasing temperature may affect the iron core.

LVDT Applications

LVDT is mainly used for measurement of pressure, tension, weight, and other mechanical force.
LVDT is used where displacement is the main factor of censoring. This displacement will be converted into an electrical signal if used as a primary transducer.
LVDT can be used as a secondary transducer. For example, Bourbon tube as a primary transducer while the LVDT as a secondary transducer. The Bourbon tube converts the pressure into linear displacement, then the LVDT converts the displacement into an electrical signal. This way we can calibrate pressure fluid measurement.