The document discusses Linear Variable Displacement Transducers (LVDTs) and how they work. LVDTs use mutual induction to convert displacement into an electrical signal. They consist of a primary coil surrounded by two secondary coils, and an iron core that can move within the coils. As the core moves, it changes the voltages induced in the secondary coils, producing an output voltage proportional to the core's position. The document also discusses how LVDTs are constructed and their operating principles, advantages, disadvantages, and applications.

1. What is LVDT
Pedro Gaspar C
Linear Variable Displacement Transducer (LVDT):
A very basic transducer which is always useful in the
field of instrumentation. Now let me explain about the
LVDT with its Principle of Operation and I will explain
how it is constructed for its well known operation and
you can understand the working of LVDT.
https://instrumentationtools.com/what-is-lvdt/

2. What is LVDT
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Principle of LVDT:
LVDT works under the principle of mutual
induction, and the displacement which is a non-
electrical energy is converted into an electrical
energy. And the way how the energy is getting
converted is described in working of LVDT in a
detailed manner.

3. What is LVDT
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4. What is LVDT
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Construction of LVDT:
LVDT consists of a cylindrical former where it is
surrounded by one primary winding in the centre of
the former and the two secondary windings at the
sides. The number of turns in both the secondary
windings are equal, but they are opposite to each
other, i.e., if the left secondary windings is in the
clockwise direction, the right secondary windings
will be in the anti-clockwise direction, hence the net
output voltages will be the difference in voltages
between the two secondary coil.

5. What is LVDT
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The two secondary coil is represented as S1 and S2. Esteem iron core is
placed in the centre of the cylindrical former which can move in to and
fro motion as shown in the figure. The AC excitation voltage is 5 to 12V
and the operating frequency is given by 50 to 400 HZ.
Working of LVDT:
Let’s study the working of LVDT by splitting the cases into 3 based on
the iron core position inside the insulated former.
Case 1:
On applying an external force which is the displacement, if the core
reminds in the null position itself without providing any movement then
the voltage induced in both the secondary windings are equal which
results in net output is equal to zero
i.e., Esec1-Esec2=0

6. What is LVDT
Pedro Gaspar C
Case 2:
When an external force is appilied and if the steel iron core
tends to move in the left hand side direction then the emf
voltage induced in the secondary coil is greater when
compared to the emf induced in the secondary coil 2.
Therefore the net output will be Esec1-Esec2
Case 3:
When an external force is applied and if the steel iron core
moves in the right hand side direction then the emf induced
in the secondary coil 2 is greater when compared to the emf
voltage induced in the secondary coil 1. therefore the net
output voltage will be Esec2-Esec1

7. What is LVDT
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Advantages of LVDT:
• Infinite resolution is present in LVDT
• High output
• LVDT gives High sensitivity
• Very good linearity
• Ruggedness
• LVDT Provides Less friction
• Low hysteresis
• LVDT gives Low power consumption.

8. What is LVDT
Pedro Gaspar C
Disadvantages of LVDT:
Very high displacement is required for generating high
voltages.
Shielding is required since it is sensitive to magnetic field.
The performance of the transducer gets affected by
vibrations
Its is greatly affected by temperature changes.
Applications of LVDT:
LVDT is used to measure displacement ranging from
fraction millimeter to centimeter.
Acting as a secondary transducer, LVDT can be used as a
device to measure force, weight and pressure, etc..

9. Flapper Nozzle System
A very important signal conversion is from pressure to
mechanical motion and vice versa. This conversion can
be provided by a flapper/nozzle system (sometimes
called a baffle /nozzle system). A diagram of this device
is shown in Fig.I below.
What is Flapper Nozzle System
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10. What is Flapper Nozzle System
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11. What is Flapper Nozzle System
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A regulated supply of pressure, usually 20 psig, provides a
source of air through the restriction. The nozzle is open at
the end where the gap exists between the nozzle and
flapper, and air escapes in this region. If the flapper
moves down and closes off the nozzle opening so that no
air leaks, the signal pressure will rise to the supply
pressure.
As the flapper moves away, the signal pressure will drop
because of leaking of the leaking air. Finally, when the
flapper is far away, the pressure will stabilize at some
value determined by the maximum leak through the
nozzle.

12. What is Flapper Nozzle System
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13. What is Flapper Nozzle System
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Fig II shows the relationship between signal pressure and gap
distance. Note the great sensitivity in the central region. A
nozzle/flapper is designed to operate in the central region where
the slope of the line is greatest. In this region, the response will
be such that a very small motion of the flapper can change the
pressure by an order of magnitude.
Note :
By adding a Electro-Magnet & supportive springs, we use this
system as Current to Pressure Converter (I/P Converter).
By adding an LVDT assembly & supportive springs, we use this
system as Pressure to Current Converter. (P/I Converter).
We will discuss in detail about I/P Converter and P/I Converter in
coming articles.

14. Pedro Gaspar C
Current to Pressure (I/P) Converter Principle
The I/P Converter works on Flapper/Nozzle principle.

15. Pedro Gaspar C
Current to Pressure (I/P) Converter Principle
In the Current to Pressure converter, we usually give input
current signal as 4 – 20 mA . We also give a continuous
supply of 20 P.S.I to the Flapper Nozzle assembly. As we
give input current signal, Electromagnet gets activated. If
the current is more, then the power of magnet will get
increased. The Flapper of the Flapper-Nozzle instrument is
connected to Pivot so that it can move up and down and a
magnetic material was attached to other end of flapper and
it is kept near the electromagnet.

16. Pedro Gaspar C
Current to Pressure (I/P) Converter Principle
As the magnet gets activated. the flapper moves towards
the electromagnet and the nozzle gets closed to some
extent. So the some part of 20 P.S.I supplied will escape
through nozzle and remaining pressure will come as output.
If the current signal is high, then power of the magnet will
increase, then flapper will move closer to the nozzle, so less
pressure will escape through nozzle and output pressure
increases.
In this way the output pressure will be proportional to the
input current.
For the input current of 4 – 20 mA we can get the output
pressure of 3 – 15 P.S.I

17. We can construct a pressure to current converter using a
Flapper- Nozzle arrangement , Bellows and a Linear
Variable Differential Transformer (LVDT) circuit. Input
pressure is given to Flapper-Nozzle arrangement and
the output current will come through the LVDT.
Flapper Nozzle arrangement: It is type of pressure
controlling device. It controls the input pressure by the
moving the Flapper away or towards the nozzle, if we
need high pressure, then nozzle will be closed by the
Flapper and if we need less pressure, then nozzle will be
opened by moving flapper away.
Pressure to Current (P/I) Converter Principle
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18. Bellows:- Bellows are one type of pressure measuring
devices. They are the elements which will expand as per the
applied pressure. Based on the displacement we can measure
the pressure.
Linear Variable Differential Transformer(LVDT): It works on
the principle of Mutual Inductance. In the LVDT three coils
will be present. Primary coil is connected to a power source
and secondary coils are connected in series opposition
method. At initial position, the voltage induced will be zero.
If the core displaces, then due to inductance, voltage will be
induced into the coils, resulting in the current.
Pressure to Current (P/I) Converter Principle
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19. Pressure to Current (P/I) Converter Principle
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20. Pressure to Current (P/I) Converter Principle
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First Input pressure is supplied to the Flapper- Nozzle
arrangement. Then it will supplied through a pipe and that
pressure is given as input to the bellows. These bellows are
connected to the Core of LVDT. When pressure is applied to
bellows, they will expand thus core displaces and the
voltage is induced on the secondary coils of LVDT. As
voltage is induced, current will flow through the coil. That
current is proportional to the input pressure applied. Thus
Pressure is converted into equivalent current.
When compared with standard values we can covert 3-15
P.S.I of pressure can be converted to 4-20 mA of current