mechatronics - unit - ii
TRANSCRIPT
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UNIT12
Sensors and Transducers
Sensors: It is used for an element which produces signal relating to the quantity
being measured
Example: An electrical resistance temperature element, the quantity being measured is
temperature and the sensor transforms an input of temperature into a change in resistance
Transducer: These are defined as elements that when subject to some physical changeexperience a related change.
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Performance Terminology
Range and span:
The range of a transducer defines the limits between which the input can vary
The span is the maximum value of the input minus the minimum value.
Span = Max. value of inputMin. value
Ex: A load cell for the measurement of forces might have a range of 0 to 50 KN.
Span = 500 = 50 KN.
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Error:
It is the difference between the result of the measurement and
the true value of the quantity being measured.
Error = Measured valueTrue value
Error = 2524 = 1c
Error = 2526 = -1c
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Accuracy:
It is the extent to which the value indicated by a measurement systemmight be wrong.
Ex: A measuring system having an accuracy of 2c implies that the
reading given by the instrument can be expected to lie within + or - 2c of
the true (actual) value
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Sensitivity:
It is the relationship indicating how much output you get per unit input
i.e., output / input
Ex: A resistance thermometer may have a sensitivity of 0.5/c
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Hysterisis Error:
Transducers can give different outputs from the same value of quantity
being measured according to whether that value has been reached by a
continuously increasing change or a continuously decreasing change. This
effect is called Hysterisis.
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Repeatability / Reproducibility:
The transducer that are used to describe its ability to give the same output
for repeated applications of the same input value.
Repeatability = ((Max.Min. values given)/Full range)* 100
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Stability:
Its ability to give the same output when used to measure a constant input
over a period of time.
Resolution:
It is the smallest change in the input value that will produce an observable
change in the output.
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Displacement sensors:
These are concerned with the measurement of the amount by which some
object has been moved.
Position sensors:
These are concerned with the determination of the position of some object
with reference to some reference point.
Proximity sensors:
These are a form of position sensors and are used to determine when an
object has moved to within some particular critical distance of the sensor.
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Consideration for selecting a displacement, position and proximity sensor
1. The size of the displacement: for a proximity sensor, how close
should the object be before it is detected.
2. Whether the displacement is linear or angular. Linear displacement
sensors might be used to monitor the thickness or other dimensions of
sheet materials.
Angular displacement methods might be used to monitor the angular
displacement of shafts.
3. The resolution and accuracy required.
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Continued.
4. What material the measured object is made i.e., some sensors
will only work with ferromagnetic materials some with only metals,
some with only insulators.
5. The cost.
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Inductive proximity switch
Number of forms of switch which can be activated by the presence of
an object in order to give a proximity sensor with an output which is
either ON or OFF.
The micro switch is a small electrical switch which requires physicalcontact and a small operating force to close the contact.
Ex: In the case of determining the presence of an item on a conveyor belt,
this might be actuated by the weight of the item on the belt depressing the
belt and hence a springloaded platform under it, with the movement of
this platform then closing the switch.
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Pneumatic proximity sensor
Pneumatic sensor involve the use of compressed air displacement or
the proximity of an object being transformed into a change in air
pressure.
From the fig.(a) low pressure air is allowed to escape through a port in
the front of the sensor, this escaping air in the absence of any close
by object, escapes and in doing so also reduces the pressure in the
nearby sensor output port.
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From fig. (b) if these is a closeby object, the air cannot so readily escape and
the result is that the pressure increases in the sensor output port. The output
pressure from the sensor thus depends on the proximity of the object.
Such sensors are used for the measurement of displacements of fractions of mm
in ranges which typically are about 3 to 12 mm
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Incremental Encoder
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Circular disk contains alternative evenly spaced opaque and transparent
segment over a circle.
Light source located on one side of the disk .
Photo detecting device (Photo detector) is placed on the other side of the disk.
LED is used as the light source that provides continuous signal.
The line of action of light source, the circular pattern and the photo detector
must match with each other.
Light signals can be received if the transparent segment of the pattern is
in between the light source and the photo detector.
The detector receives no light signal if opaque segment of the pattern is
moved.
Disk is made to rotate, the photo detector will receive pulsed light signal for
every time it sees the light source i.e., LED The disk is rigidly fixed with rotor or shaft of rotating element. The evenly
spaced transparent radial lines on its surface, rotates part the light source.
The output is taken from the photo detector. The number of pulses determines
the position of the disk and the number of pulses / sec. measures the velocity
of the disk.
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The pulsed light signal generated by utilizing the two patterns are
usually called channelA signal and channelB signal.
In fig. (b) an equivalent relative angular placements of two are shown.
The relative angular placements of transparent segments of these two
patterns are such that the outputs of two photo detectors are shown in
fig . The phase difference between the two signals is 90 deg. such
signals together are referred to as Quadrature Square waves means
that two squarewaves that are phaseshifted w.r.t. each other by 90deg.
If channelA leads channelB, then the direction of movement is
clockwise direction and vice versa.
The number of transparent segments on the disk determines the
resolution of the encoder. More the number of transparent segments
more accurate position (angular) information can be obtained.
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Absolute Encoders
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Absolute encoder for the measurement of angular displacement.
It gives an output in the form of Binary number of several digits, each
such number representing a particular angular position.
The rotating disk has four concentric circles of slots and four sensors
to detect the light pulses. The slots are arranged in such a way that the
sequential output from the sensors is a number in the binary code.
Typical encoders tend to have upto 10 or 12 tracks. The number of bits
in the binary number will be equal to the number of tracks. Thus with
10 tracks there will be 10 bits and so the number of position that can be
detected is 210
i.e., 1024 , a resolution of (360/1024) = 0.35 deg.
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Normal form of binary code is generally not used because changing
from one binary number to the next can cause result in more than one
bit changing and if, through some misalignment, one of the bits
changes fractionally before the others then an intermediate binary
number is momentarily indicated and so can lead to false counting.
To over come this gray code is generally used with this code only one
bit changes in moving from one number to the next.
Edd t i it S
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Eddy current proximity Sensors
If a coil is supplied with an alternating current, an alternating magnetic field isproduced.
If a metal object is in close proximity to this alternating magnetic field, then
eddy currents are induced in it.
The eddy current themselves produce a magnetic field. This distorts the
magnetic field responsible for their production.
As a result, the amplitude of the alternating current changes. At some preset
level this change can be used to trigger a switch.
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It is used for the detection of nonmagnetic but conductive materials. They
have the advantages of being relatively inexpensive, small in size with high
reliability and can have high sensitivity to small displacements.
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Hall effect sensors
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Beam of charged particles passes through a magnetic field, forces act
on the particles and the beam is deflected from its straight line path.
A current flowing in a conductor is like a beam of moving charges and
thus can be deflected by a magnetic field. This effect is called Hall
effect.
Electrons moving in a conductive plate with a magnetic field applied at
right angles to the plane of the plate.
As a consequence of the magnetic field, the moving electron is
deflected to one side of the plate and thus that side become negatively
charged while opposite side becomes positively charged since the
electrons are directed away from it.
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Used to determine the level of fuel in automobile fuel tank
Magnet is attached to a float and as the level fuel changes so the float
distance from the Hall sensor changes. The result is a Hall voltage
output which is a measure of distance of the float from the sensor and
hence the level of fuel in the tank.
A li i f ll ff
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Application of Hall effect sensor
Hall effect sensors commutate 3- brushless DC motor.
The simple cylindrical 3- motor Figure aboveis commutated by a Hall effect
device for each of the three stator phases. The changing position of the
permanent magnet rotor is sensed by the Hall device as the polarity of the
passing rotor pole changes. This Hall signal is amplified so that the stator coils
are driven with the proper current. The Hall signals may be processed by
combinatorial logic for more efficient drive waveforms.
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Potentiometer Sensor
Consists of resistance element with a sliding contact which can be moved over thelength of the element. Such elements can be used for linear or Rotary displacements,
the displacement being converted into a potential difference.
The rotary potentiometer consists of a circular wire wound track or a film of
conductive plastic over which a rotatable sliding contact can be rotated. The track may
be single turn or helical.
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Pyroelectric Sensors
Pyroelectric materials example Lithium tanlate are crystalline materials
which generate charge in response to heat flow.
When it is heated to a temperature just below curie temperature about
610 deg. and the material cooled while remaining in the field, electric
dipoles within the material line up and it becomes polarized.
The effect is like magnetizing a piece of iron by exposing it to a
magnetic field.
When the pyro electric material is exposed to infrared radiations, its
temperature rises and this reduces the amount of polarization in the
material, the dipoles being shaken up more and losing their alignment
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Pyroelectric Sensors
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Pyroelectric Sensors
It consists of a polarized pyroelectric crystal with thin metal film
electrodes on opposite faces.
Because the crystal is polarized with charged surfaces, ions are drawn
from the surrounding air and electrons from any measurement circuit
connected to the senor to balance the surface charge as shown in fig. (a)
If infrared radiation is incident on the crystal and changes it temperature
the polarization in the crystal is reduced and consequently that is a
reduction in charge at the surfaces of the crystal as shown in fig. (b).
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The charge leaks away through the measurement circuit until the
charge on the crystal once again is balanced by the charge on the
electrodes.
Thus it behaves as charge generator which generates charge when
there is a change in temperature as a result of incidence of infrared
radiations.
Pyroelectric Sensors
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An IR emitting body moving across the front of a sensor will expose first
one, then both and then the other sensor element.
The output signal waveform from an analog sensor shows that for motion in
one direction, first a positive, then zero and then a negative transition results.
Motion in the other direction will produce first a negative, then zero and then
a positive transition
Pyro electric Sensors
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PVDF T til
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PVDF Tactile sensors
It uses Piezo electric Polyvindylidene Fluoride (PVDF) film.
Two layers of the film are used and are separated by a soft film whichtransmits vibrations.
The lower PVDF film has an alternating voltage applied to it and this
results in mechanical oscillations of the film.
The intermediate film transmits these vibrations to the upper PVDF
film.
As a consequence of the piezo electric effect, these vibrations cause an
alternating voltage to be produced across the upper film when pressure
is applied to the upper PVDF film its vibrations are affected and the
output alternating is changed.
Light Sensors
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Light Sensors
Sensors which senses the presence of light are called light sensors or
photo sensors. These sensors are also called photo electric transducers
because when light falls on these sensors, there exist a change in their
electrical property. i.e., light signals induce change in electrical
properties of conductance, resistance, inductance etc., of the material.
This change in electrical properties of the material is used to measure a
wide range of radiations including intensity of light.
The phenomenon is observed in terms of three effects known as
photoemmissive, photoconductive and photovoltaic transducers
Photo emissive Transducer
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Photo emissive Transducer
It is a vacuum chamber containing photo emissive cathode, an anode
and photomultiplier consists of many dynodes.
The light sensitive cathode emits electrons by virtue of photoelectric
effect.
Electrons thus ejected from a photosensitive cathode are attracted
toward and strike the 1stpositive electrode (Dynode-1) of the
multiplier, liberating secondary electrons.
The photoelectrons thus generated from the first dynode are again
accelerated towards second dynode since it is kept at a higher potentialcompared to first dynode.
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The secondary electrons bombard the second dynode generating more
secondary electrons and so on, until a desirable current signal is produced
through several stages.
Finally a shower of electrons is produced and collected at the anode
constituting anode current, which is proportional to the intensity of light.
Photoconductive Transducers (Photo resistors)
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Photoconductive Transducers (Photo resistors)
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Photo conductive or light dependent resistorLDR material can be used for
the construction of photo conductive transducers.
Materials used for construction of photo conductive transducers are
cadmium sulphide (Cds), Lead Selenide (PbSe), Lead Sulfide (Pbs), Lead
Telluride (PbTe) etc.,
When light falls on the LDR the resistance of the transducer material
changes. If the LDR is made as a part of an electronic circuit, the changes
will be reflected in terms of current or voltage. The change in current or
voltage can now become the output signal, which simply appears as a
measure of input light intensity.
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A constant current source is connected across the LDR, when light
falls upon it the conductivity (resistance) of the material from which
the LDR has been made changes.
Depending upon the type of material the conductivity may increase or
decrease, since the LDR is supplied with a constant current source, the
effect is developed in terms of a rise or drop in voltage across the
LDR, which is proportional to the intensity of light. The change in
voltage can be calibrated. If the supplied constant source becomes a
voltage source, the measured signal would be a current signal insteadof voltage. In this case the change in current can be calibrated to
quantify the light intensity.
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These type of transducers are used for the detection of intensity of the
light. The principle is also employed for the design of the color sensor.
Ultrasonic sensors
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Ultrasonic sensors
Ultrasonic measurement systems consists of an ultrasonic transmitter, the
transmission medium and an ultrasonic receiver. The commonly used
ultrasonic sensors are the piezo electric sensing elements.
Piezo electric effect is reversible, the ultrasonic transmitter uses the
inverse piezo electric effect, i.e., if a voltage is applied to the transmitter
the crystal will undergo corresponding deformation (vibration)indirect.
The vibration of the crystal is transmitted through the media from one
end to the other.
The particle displacement sets up an accompanying pressure which is
picked up by the receiver.
The receiver use the direct piezo electric effect and converts the force
into corresponding voltage.
Applications
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1) Level measurement
T = (2*I)/C
C = velocity of ultrasonic waves
It is to be noted that the crystal must be placed at the bottom and not at the
top. If placed at the top due to the presence of air no wave will be able to
propagate thus giving an erroneous measurement
Applications
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pp
Crack detection:
If we measure the time then we will no exactly where the crack is developed.
Here crack or gap acts as the second medium and thus helps us to detect where
the crack has taken place.
It can also be used for measurement of flow
Advantages
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It is easy to direct and focus a beam of ultrasound as diffraction of these
waves are small due to their short wavelength.
Ultrasonic waves can easily pass through metals. This helps in mounting
the measurement system outside the system and it will lead to the
development of non-invasive sensor.
These sensors have wide application in Biomedical instrumentation such as
measure of blood flow in the artery, determination of Blood Pressure and
Ultrasonography.
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When a magnetic force is generated parallel to the reed switch, the
reeds become flux carriers in the magnetic circuit. The overlapping
ends of the reeds become opposite magnetic poles, which attract each
other. If the magnetic force between the poles is strong enough to
overcome the restoring force of the reeds, the reeds will be drawn
together.
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What magnetic force is required to close a reed switch ?
Minimum force - expressed in ampere-turns, will cause the reeds toclose, and is called the just-operate force. Since the force between the
poles increases as the gap decreases, a force of approximately half the
just-operate force will maintain the operated state. Speed of operation
of the reed switch is determined by the excess of operating force over
the just-operate force.
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In mobile devices with doors to protect the keypads, the power to the display
or central processing unit (CPU) can conveniently be switched on or off, just
by opening or closing the doors.
A reed sensor can be used in these applications because it does not drain any
power from the battery, irrespective of its contact state. Furthermore, when
the sensor is actuated, the resistance across the contacts is so low in the order
of a few milliohms, that unnecessary battery drain is avoided.
Reed sensors are also used in notebook computers to trigger the CPU to sleep
when the door is shut. Suitable products: R5, R3, and R2SMD reed sensors.
http://www.reed-sensor.com/Reed_Sensors/Ultraminiature_Reed_Sensors.htmhttp://www.reed-sensor.com/Reed_Sensors/Miniature_Reed_Sensors.htmhttp://www.reed-sensor.com/Reed_Sensors/Standard_Reed_Sensors.htmhttp://www.reed-sensor.com/Reed_Sensors/Standard_Reed_Sensors.htmhttp://www.reed-sensor.com/Reed_Sensors/Miniature_Reed_Sensors.htmhttp://www.reed-sensor.com/Reed_Sensors/Ultraminiature_Reed_Sensors.htm -
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Commercial coffee, tea, and milk vending machines need to dispense
enough milk or water for exactly one cup.
This can be accomplished by filling up a secondary reservoir fitted with a
float and a reed switch which comes on when the reservoir is filled up to 200
ml. This signal can then be used to trigger other solenoids or pumps which
stop the main reservoir output and open the drain valve from the secondary
reservoir to dispense the liquid into a cup.
Using a narrow and long secondary reservoir ensures that errors in
measurement do not exceed 5 ml.
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Statistics say almost 80% of automobiles use reed switch based crash
sensors for triggering airbags.
A ring magnet is mounted on a very precise tension spring and this
assembly is slid over a reed switch. Depending on the impact which is
required to trigger the reed switch, different tension springs are used.
In most crash sensors, two reed switches are used instead of one as a
failsafe mechanism. When a crash occurs, the mass of the magnet
compresses the spring and moves closer to the reed switch, closing it.
This signal is used to trigger the compressed cylinder which inflates the
airbags.
Piezoelectric effect
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e oe ect c e ect
The piezoelectric effect occurs when the charge balance within the
crystal lattice of a material is disturbed.
When there is no applied stress on the material, the positive and
negative charges are evenly distributed so there is no potential
difference.
When the lattice is changed slightly, the charge imbalance creates a
potential difference, often as high as several thousand volts. However,
the current is extremely small and only causes a small electric shock.
The converse piezoelectric effect occurs when the electrostatic field
created by an electrical current causes the atoms in the material to
move slightly.
Piezoelectric Sensors
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A piezoelectric sensoris a device that uses thepiezoelectric effectto
measurepressure, acceleration, strainor forceby converting them to
an electricalcharge.
Piezoelectric Principle
http://en.wikipedia.org/wiki/Piezoelectric_effecthttp://en.wikipedia.org/wiki/Pressurehttp://en.wikipedia.org/wiki/Accelerationhttp://en.wikipedia.org/wiki/Strain_(materials_science)http://en.wikipedia.org/wiki/Forcehttp://en.wikipedia.org/wiki/Electricityhttp://en.wikipedia.org/wiki/Electricityhttp://en.wikipedia.org/wiki/Forcehttp://en.wikipedia.org/wiki/Strain_(materials_science)http://en.wikipedia.org/wiki/Accelerationhttp://en.wikipedia.org/wiki/Pressurehttp://en.wikipedia.org/wiki/Piezoelectric_effect -
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The active element of an accelerometer is a piezoelectric material.
Figure 1 illustrates the piezoelectric effect with the help of a
compression disk. A compression disk looks like a capacitor with the
piezo ceramic material sandwiched between two electrodes. A force
applied perpendicular to the disk causes a charge production and a
voltage at the electrodes.
Piezoelectric effect
The sensing element of a piezoelectric accelerometer consists of two
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The sensing element of a piezoelectric accelerometer consists of two
major parts:
Piezo ceramic material
Seismic mass
One side of the piezoelectric material is connected to a rigid post at the
sensor base. The so -called seismic mass is attached to the other side.
When the accelerometer is subjected to vibration, a force is generated
which acts on the piezoelectric element. According to Newtons Law
this force is equal to the product of the acceleration and the seismic
mass. By the piezoelectric effect a charge output proportional to the
applied force is generated. Since the seismic mass is constant the
charge output signal is proportional to the acceleration of the mass.
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Over a wide frequency range both sensor base and seismic mass have
the same acceleration magnitude. Hence, the sensor measures theacceleration of the test object.
The piezoelectric element is connected to the sensor socket via a pair
of electrodes. Some accelerometers feature an integrated electronic
circuit which converts the high impedance charge output into a low
impedance voltage signal.