Mechatronics - Unit - II

8/13/2019 Mechatronics - Unit - II

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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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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.



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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.

Mechatronics - Unit - II

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