Displacement Sensors
L.V.D.T
by
Dr. Sotiris Omirou
AMEM 211
LVDT
What is an LVDT?
An LVDT is a Linear Position Sensor
With a Proportional Analog Output
An LVDT has 2 Elements, a Moving
Core and a Stationary Coil Assembly
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LVDT
What Do the Letters LVDT Stand For?
LVDT
Linear Variable Differential Transformer
Transformer: AC Input / AC Output
Differential: Natural Null Point in Middle
Variable: Movable Core, Fixed Coil
Linear: Measures Linear Position
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LVDT
LVDT linear position sensors are readily
available that can measure movements as
small as 0.001 mm up to 75 mm.
The structure of an LVDT
secondary coil 1
Primary coil
Vout
Vin
Secondary coil 2
Movable core
3
The structure of an LVDT
The structure of an LVDT
4
How an LVDT operates
A linear variable displacement transformer (LVDT) is
a 33-coil transducer.
– The secondary voltages are
determined by:
+
S1
+
vin
P
–
–
+
S2
–
vout
vS1 = k1vin ,
Secondary windings are
connected in series opposition
–
ferrite rod
vS2 = k 2 vin
∴ vout = vS1 − vS 2
–
When the rod is centred, equal
secondary voltages are induced
and the output voltage is zero
How an LVDT operates
+
S1
+
–
As the rod moves from the
centre, as shown, k1
increases, while k2
decreases
–
vout is linearly related to the
change in position of the
rod
vout
–
vin
–
P
+
S2
–
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How an LVDT operates
k1 and k2 depend on the amount of
coupling between the primary and
the secondary coils, which is
proportional to the position of the
coil
When the coil is in the central position,
k1=k2 ⇒ VOUT=V1=V1-V2=0
When the coil is is displaced x units,
k1≠
k1≠k2 ⇒ VOUT=(k1=(k1-k2)VIN
How an LVDT operates
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Why Use An LVDT?
LVDTs have certain significant
features and benefits, most of
which derive from fundamental
physical principles of operation
or from materials and techniques
used in their construction.
Why Use An LVDT?
Friction-Free Operation
One of the most important features of an LVDT is
its frictionfriction-free operation. In normal use, there is
not any mechanical contact between the LVDT's
core and its coil assembly. There is no rubbing,
dragging, or other source of friction. This feature
is particularly useful in materials testing, vibration
displacement measurements, and highhigh-resolution
dimensional gaging systems.
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Why Use An LVDT?
High Resolution
Since an LVDT operates by using electromagnetic
coupling principles in a frictionfriction-free structure, it
can measure very small changes in core position.
These same factors also give an LVDT its
outstanding repeatability.
Why Use An LVDT?
Unlimited Mechanical Life
Because there is normally no contact between
an LVDT's core and coil structure, no parts can
rub together or wear out. This means that an
LVDT features unlimited mechanical life. This
factor is especially important in highhigh-reliability
applications such as aircraft, satellites and
space vehicles, and nuclear installations. It is
also highly desirable in many industrial process
control and factory automation systems.
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Why Use An LVDT?
Over travel Damage Resistant
The internal bore of most LVDTs is open at both
ends. In the event of unanticipated overtravel,
overtravel,
the core is able to pass completely through the
sensor’s coil assembly without causing damage.
This invulnerability to position input overload
makes an LVDT the ideal sensor for applications
like extensometers that are attached to tensile
test samples in destructive materials testing.
Why Use An LVDT?
Single Axis Sensitivity
An LVDT responds to motion of the core along
the coil's axis, but is generally insensitive to
crosscross-axis motion of the core or to its radial
position. Thus, an LVDT can usually function
without adverse effect in applications involving
misaligned moving members, or in cases where
the core doesn't always travel in a precisely
straight line.
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Why Use An LVDT?
Environmentally Robust
The materials and construction techniques used to
assemble an LVDT result in a rugged and durable
sensor robust to a wide variety of environmental
conditions. Bonding of the coil windings is followed
by epoxy encapsulation into the case, resulting in
superior humidity resistance, as well as the
capability to take substantial shock loads and high
vibration levels in all axes. An internal highhighpermeability magnetic shield minimizes effects of
external AC fields on LVDT operation.
Why Use An LVDT?
Environmentally Robust…
Both the case and core are made of corrosion
resistant metals, with the case also acting as a
supplemental magnetic shield. And for those
applications where the sensor must withstand
exposure to flammable or corrosive vapors and
liquids, or operate in pressurized fluid, the case
and coil assembly can be hermetically sealed
using a variety of welding processes.
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Why Use An LVDT?
Null Point Repeatability
The location of an LVDT's null point is extremely
stable and repeatable, even over its very wide
operating temperature range.
Why Use An LVDT?
Absolute Output
An LVDT is an absolute output device, as opposed
to an incremental output device. This means that
in the event of loss of power, the linear position
information being sent from the LVDT will not be
lost. When the measuring system is restarted, the
LVDT's output value will be the same as it was
before the power failure occurred.
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Why Use An LVDT?
CONCLUSION
The LVDT represents an optimal solution
to many linear measurement problems. An
LVDT possesses a number of advantages
over other types of position sensors and
is cost competitive in most cases.
Displacement Sensors (cont…)
Optical Encoder
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Encoders are a form of digital optical sensor, which
include a coded disc with information contained in a
series of concentric tracks, which are read by
photo-sensors to give a measure of angular position
Optical encoders are used to measure
displacement and derive speed.
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Photo-sensors are four pairs of LED’s and phototransistors mounted on either side. The sequence of
‘highs’ and ‘lows’ at the outputs of the four phototransistors are indicated by four LED’s, one in each
line.
The accuracy of the measurement depends on the
number of tracks (or bits). For the four-bit encoder
the resolution is 22.5°, or 16 unique positions.
Increasing this to five bits would give a resolution of
11.25°and so on.
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Two commonly used forms of encoder have binary or
Gray coded scales. For the the binary encoder each
transition, from one position to the next, the binary
output signal increments by 1 thus giving a unique
positional address anywhere around the encoder scale.
The Gray scale is designed so that all positional
transitions only involve the change of one bit of
information at a time, an increment or decrement
depending on the direction of movement.
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The Gray Code
The Gray code’
code’s most
important characteristic is
that only one digit
changes as you increment
or decrement the count.
The Gray code is
commonly associated
with input/output devices
such as an optical
encoder of a shaft’
shaft’s
angular position.
Decimal
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
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Gray code
00000
00001
00011
00010
00110
00111
00101
00100
01100
01101
01111
01110
01010
01011
01001
01000
11000
Question 1:
What is the resolution of an eight-bit encoder?
Answer
The resolution of an encoder with n bits is given by
2n. Hence, for the four-bit and five-bit examples the
resolution would be 16 and 32, respectively. For the
eight-bit encoder, 28 gives a resolution of 256 or
1.41 °.
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Question 2:
How can the encoder be used to measure speed
of rotation?
Answer
Using one of the bits, usually the LSB (least
significant bit), the frequency can be used to give
an indication of speed. This is related to the
speed and the number of cycles. It is also
governed by whether it is a binary or Gray scale
encoded encoder.
For the Gray scale coded encoder:
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For the binary scale encoder:
Proximity Sensors
The Reed switch sensor
Reed switches consist of two small ferromagnetic reeds
hermetically closed in a glass tube. The reeds are thin
and flexible, and because they are ferromagnetic they
become magnetised in the presence of a magnetic field.
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The Reed Switch
(cont…
(cont…)
The Reed Switch
(cont…
(cont…)
When a magnet is brought close to the glass tube,
the reeds move together and make contact and the
switch is turned on. The reeds open again when the
magnet is removed
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The Reed Switch
(cont…
(cont…)
Applications
Reed switches are common in alarm systems, for
example, in door frames. When the door is closed
the magnet keeps the switch on. When the door is
opened the alarm system senses the broken contact
and goes off.
Switching in dangerous situations such as where
flammable materials are present and there is a risk
of fire – non–
non–spark.
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