In this lecture, we are going to learn about the LVDT transducer. The construction, the working principle of LVDT, characteristics, applications advantages, and disadvantages of LVDT. So let’s start with the definition of LVDT.
LVDT
The most widely used inductive transducer to translate linear motion into electrical signals is the LVDT. The Full form of LVDT is “Linear Variable Differential Transformer”. It uses transformer action so, soft iron core (Ni-Fe alloy) i.e. soft magnetic material is used to avoid error due to hysteresis ad eddy current loss.
Construction of LVDT
The transformer consists of a single primary winding P and two secondary winding S1 and S2 wound on a cylindrical former. A movable soft iron core is placed inside the former. The displacement to be measured is applied to the arm attached to the soft iron core. The basic construction of LVDT and equivalent circuit are shown in the figure below:
Working of LVDT
The primary winding is excited by an alternating current source, it produces an alternating magnetic field which in turn induces alternating current and voltages in the two secondary windings. From the above figure, the output voltage of the transducer is the difference between the two voltages.
I.e. Differential output voltage (E0) is given as,
E_0=E_{s1} - E_{s2}
When the core of LVDT is at its null position, the flux linking in both parts of secondary windings is the same, it means Es1=Es2. therefore the differential output is zero. When the core is moved in either direction the flux linkage in one part is different from another part. So secondary winding voltage is different. Therefore the difference between both is non-zero. The magnitude of the differential output voltage is proportional to the displacement of a core of the non-zero position and its polarity indicated the direction of motion.
Characteristics of LVDT
The variation of output voltage against displacement for the various positions of the core is shown in the figure below.
The output voltage of an LVDT is a linear function of core displacement within a limited range of displacement from the null position. Ideally, the output voltage at the null position is zero. however, in actual practice, there exists a small voltage at the null position. This small voltage is the residual voltage due to harmonics in the input supply voltage and also to harmonics produced in the output voltage on account of used iron core and due to stray magnetic field and stray capacitance. Therefore core is made of high permeability nickel-iron which is hydrogen annealed, this gives low harmonics low null voltage, and high sensitivity.
Advantages of LVDT
The advantages of LVDT are as per the following:
- A wide range of displacement can be measured from 1.25mm to 250 mm.
- High input and high sensitivity nearly 40volt/mm
- No friction losses.
- Low power consumption.
- Low hysteresis losses
- Highly ruggedness
- immunity from external effects is high.
Disadvantages of LVDT
The disadvantages of LVDT are as per the following:
- Relatively large displacements are required for appreciable differential output.
- Output is affected by a stray magnetic field.
- Dynamic responses are limited mechanically by the mass of the core and electrically by the frequency of applied voltages.
- Its output s affected by vibrations
- Temperature affects the performance of LVDT.
Application of LVDT
- LVDT can be used as a primary transducer for measurement of displacement with a strain gauge which is acting as a secondary transducer.
- LVDT can be used as a secondary transducer for used as a device to measure force, weight, pressure, etc.
- LVDT is used for the measurement of tension in a cord.
Frequently Asked Questions on LVDT
What is an LVDT used for?
Answer: An LVDT is an electromechanical sensor used to transform mechanical motion into a variable electrical signal (current or voltage) and is used for the measurement of displacement. They function as actuators for automatic control systems or as mechanical motion sensors in measurement technologies.
What is stray capacitance in transmission lines?
Answer: The charges carrying current in conductors make capacitance between each other as well as other nearby objects. This effect is called stray capacitance.
What is the output of LVDT?
Answer: the LVDT is a position-to-electrical sensor whose output is proportional to the position of a movable magnetic core.
What is the null voltage of LVDT?
Answer: The zero, called the Null Position, is defined as the core position where the phase shift of the (Va-Vb) differential output is 90 degrees. LVDT Schematic. The differential output between the two secondary outputs (Va-Vb) when the core is at a null position is called the Null Voltage.
What is the sensitivity of LVDT?
Answer: Sensitivity is usually specified in terms of milliVolt output per thousandths of an inch core displacement per Volt of excitation (mV/mil/Volt).
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