Lecture 2 Sensor and Actuation Systems for Mechatronics

08-Jan-21 By Amanuel Diriba 1
Course title: Introduction to mechatronics
Course No : MEng 5271
Jimma University
Jimma Institute of Technology
Faculty of Mechanical Engineering
Sensor and Actuation Systems for Mechatronics
LECTURE – 2

Transducers:
 Transducer is a device which convert signal from one form to another.
 Transduces used to convert measured value to electric signal.
.
Sensor and Actuation Systems for Mechatronics
2.1 Sensors

What is the difference between sensors and transducers ?
Sensors Vs. Transducers
Sensors
1. detect the state of the system
2. convert signal from one form
to another
3. Some sensors does not convert
one signal to another
4. All sensors are not Transducers
Transducers
1. detect the state of the system
2. convert signal from one form
to another
3. All transducers convert one
signal into another signal
4.All transducers can be a sensors

Range and Span:
 The range of a transducer defines the limits between which the input can
vary.
 The difference between the limits (maximum value - minimum value) is known
as span
For example: -
A load cell is used to measure force. An input force can vary from 20 to 100N.
Then The Range of load cell is 20 to 100 N. And The Span of load cell is 80 N
(i.e.100- 20)
Error:
The algebraic difference between the indicated value and the true value of
the measured parameter is termed as the error of the device.
 Error = Indicated value — true value
For example:
If the transducer gives a temperature reading of 30°C when the actual
temperature is 29°C, then the error is + 1°C.
PERFORMANCE TERMINOLOGY:

Accuracy:
 Accuracy is defined as the ability of the instrument to respond to the true
value of the measure variable under the reference conditions.
For example:
A thermocouple has an accuracy of ± 1° C. This means that reading given by
the thermocouple can be expected to lie within + 1°C (or) — 1°C of the true value
Sensitivity:
 The sensitivity is the relationship showing how much output we can get per
unit input.
 sensitivity = Output / Input
Precision:
 It is defined as the degree of exactness for which the instrument is intended
to perform.

Repeatability:
 The repeatability and reproducibility of a transducer are its ability to give the same
output for repeated applications of the same input value.
Reliability:
 The reliability of a system is defined as the possibility that it will perform its
assigned functions for a specific period of time under given conditions.
Stability:
 The stability of a transducer is its ability to give the same output when used to
measure a constant input over a period of time.
Drift:
 The term drift is the change in output that occurs over time
Dead band:
 There will be no output for certain range of input values. This is known as dead
band.
 There will be no output until the input has reached a particular value.
.

Resolution:
 Resolution is defined as the smallest increment in the measured value that can
be detected.
 The resolution is the smallest change in the input value which will produce an
observable change in the input.
Backlash:
 Backlash is defined as the maximum distance (or) angle through which any
part of a mechanical system can be moved in one direction without causing
any motion of the attached part.
Dead time:
 It is the time required by a transducer to begin to respond to a change in
input value

1. Position sensors/ transducers
• A position sensor is a sensor that facilitates measurement of
mechanical position.
o Used to determine the position of object w.r.t reference point.
Classification of sensors/transducers

a.PHOTO ELECTRIC
o Used to detect the position of object by breaking or reflecting a beam of
light.
o Photo detector receives a beam of light produced by the LED.
o An object breaks or reflects the beam of light, then voltages decrease
and then the position is detected.
Common types of position sensors include:

Example: Automatic gate

• The Hall effect is the production of voltage difference (the hall
voltage) across a current carrying conductor (in the presence
of magnetic field), perpendicular to both current and the
magnetic field.
Hall effect sensors work on the principle that when a beam of
charge particles passes through a magnetic field, forces act on
the particles and the current beam is deflected from its straight
line path.
Thus one side of the disc will become negatively charged and
the other side will be of positive charge.
This charge separation generates a potential difference which
is the measure of distance of magnetic field from the disc
carrying current.
b. Hall effect

• When a current carrying conductors is placed into a magnetic field,
a voltage will be generated perpendicular to both current and the field.
• The generated voltage is called Hall voltage.

 The typical application of Hall effect sensor is the measurement of fluid
level in a container.
 The container comprises of a float with a permanent magnet attached at its
top. An electric circuit with a current carrying disc is mounted in the
casing.
 When the fluid level increases, the magnet will come close to the disc and a
potential difference generates.
 This voltage triggers a switch to stop the fluid to come inside the
container.

o An optical encoder is a sensor which converts a mechanical rotary
motion (angular position or speed) into a usable electronic signal.
o It is used to measure position, velocity, acceleration and direction of
movement of rotors.
C. Digital optical encoder

I. Incremental encoder
o It consists three components light source, coded disk and photo
detector
o The wheel is between light and photo detector.
o When a beam of light passes through slots in a disc, it is sensed by the
light sensor opposite to the light source
o When the disk is rotated, a pulsed output is produced by sensor with
number of pulses being proportional to the position of the disc
o The photo detector receives the light signal alternatively which is
converted into electrical signal.

 The principle of operation is that they provide a unique output corresponds to each
rotational position of the shaft.
 The output is in the form of binary numbers representing the angular position.
 The disc has four concentric slots and four photo detectors to detect the light pulse.
 The slots are arranged in such way that they give a binary number.
 The number of bits in binary number will be equal to the number of tracks.
II. Absolute encoder

For example: - 8 tracks, 4LEDS and 4Light transducer.
2
8
= 256 position
360/256 = 1.406° angular displacement

o Used to find closeness of object and presence or Absence of object at
certain place.
o Detection occurs when object approaches within detection range.
o Common types of Proximity sensors include:
A. Eddy current proximity sensors
B. Inductive proximity sensors
C. Capacitive proximity sensor
D. Pneumatic proximity sensors
E. Proximity switches
2. Proximity sensors / transducers

 Eddy current proximity sensors can detect the position of
conductive objects with high resolution.
A. Eddy Current proximity sensors

Working principle of eddy current proximity sensors
 Eddy current proximity sensors has two identical coils, One
reference coil & another sensing coil which senses the
magnetic current in the object.
 When a coil is supplied with alternating current, A coil at the face
of the sensor is used to generate an alternating magnetic field.
 As a conductive object approaches the magnetic field, eddy
currents are induced in the target object (Eddy current start to flow
due to AC(conducting object) close to sensor).
 Eddy current always try to oppose that motion due to which it
is produced by creating opposite magnetic field08-Jan-21 By Amanuel Diriba 23

 An inductive sensor is a device that uses the principle of
electromagnetic induction to detect or measure objects.
 Inductive sensors use currents induced by magnetic fields to
detect nearby metal objects.
 The inductive sensor uses a coil (an inductor) to generate a
high frequency magnetic field as shown in figure below.
 An alternating current is supplied to the coil which generates a
magnetic field.
 When, a metal object comes closer to the end of the coil,
inductance of the coil changes.
B. Inductive proximity sensors

• This resulting current flow sets up a new magnetic field that opposes the original
magnetic field.
• The net effect is that it changes the inductance of the coil in the inductive sensor

 Capacitance proximity sensors are similar to inductive proximity
sensors except that they sense the presence of almost any material.
C . Capacitive proximity sensors
Example : smart phone

 Pneumatic sensors are transducers, generating an electrical signal in
proportion to the pressure they measure.
 This allows pressure to be monitored by a range of electronic devices.
D . Pneumatic Sensors

Working principle of pneumatic sensors
 It contains of three ports.
 Low pressure air is allowed to escape through port A. In the absence of
any obstacle / object, this low pressure air escapes and in doing so,
reduces the pressure in the port B.
 However when an object obstructs the low pressure air (Port A), there is
rise in pressure in output port B.

 These are small electrical switches which require physical contact and a
small operating force to close the contacts.
 They are basically employed on conveyor systems to detect the presence of an
item on the conveyor belt.
 Also Used in , robotics, NC machines, material handling systems and assembly
lines to detect the presence of object.
E. Proximity switches

Q.1 convert Automatic door system into mechatronics block diagram
A. Real life application of position sensors (PHOTO ELECTRIC )

Q. 2 Convert this system into mechatronics through block diagram
B. Real life application of proximity sensors (Inductive)

Q. 3 Convert hand sanitizing system into mechatronics block diagram.
C. Real life application of proximity sensors

Q. 4.Using a radar to measure distance to autonomously maintain desired distance
Between vehicles Quiz 1

 Used to monitor linear and angular velocity.
A.Incremental encoder
 Number of pulse is directly proportional to velocity.
 When number of pulse increase , velocity also increase.
3. Velocity sensor / transducers

• The word Tachogenerator comes from Greek word ‘TACHO’ mean
speed. Generator means an instrument which generate power.
• Tachnogenerator (Tachometer generators )are electromechanical device which
is used for measuring the speed of engines and motors ,and converting into
a voltage so that it can be measured.
• In other words, it converts angular velocity into voltage
• They are used to measure the speed of motors, conveyor belts,
engines, machine tools, mixers, Fans, and other rotational
devices.
B. Tachogenerator

• It works on the principle of relative motion between the magnetic field and shaft
of the coupled device. The relative motion induces the EMF in the coil which is
placed between the constant magnetic field of the permanent magnet.
• It consists of an assembly of a toothed wheel and a magnetic circuit.
• Toothed wheel is mounted on the shaft or the element of which angular motion is
to be measured.
• Magnetic circuit comprising of a coil wound on a ferromagnetic material core. As
the wheel rotates, the air gap between wheel tooth and magnetic core changes
which results in cyclic change in flux linked with the coil.
• The alternating emf generated is the measure of angular motion.
• Air gap , when velocity (flux in the coil change & emf produced)
Working Principle of Tachogenerators

Types of Tachogenerators
There are Two types of Tachogenerators
A. A.C. Tachogenerators (A.C. Tachometer Generators)
B. D.C. Tachogenerators (D.C. Tachometer Generators)

Example1: measure the speed of motors
Example 2: measure the speed of engines

4. Motion sensors / transducers
 These sensors work on the principle of pyro electricity, which states that a
crystal material such as Lithium tantalite generates charge in response to
heat flow.
 In presence of an electric field, when such a crystal material heats up, its
electrical dipoles line up. This is called as polarization.
 On cooling, the material retains its polarization. In absence of electric field,
when this polarized material is subjected to infra red irradiation, its
polarization reduces.
 This phenomenon is the measure of detection of movement of an object.

Pyro electric sensors
o Pyro electric material generate charge in response to heat flow.
I. Lithium tantalite
 When object interrupt lithium tantalite it reduces heat flow which in turn reduces
charge (charge α heat flow)
 When temperature fall, it become unpolarized and voltage produced.
Figure of Lithium tantalite

II. Infrared
o Object interrupt infrared reduce heat flow which in turn reduced charge (charge α heat
flow)
o When object interrupt infrared polarized ion change & voltage change.
Figure of infrared sensors/ transducers

6. Force sensors/ transducers
a) Strain gauge
o The electrical resistance is changed due to applied force which cause change in
length (increases) and cross sectional area (decreases) of the strain gauge.
o This change in resistance is then usually converted into voltage by connecting one,
two or four similar gauges as an arm of a Wheatstone bridge (known as Strain
Gauge Bridge)
o The bridge output voltage is then a measure of strain, sensed by each strain gauge
Figure of Strain gauge08-Jan-21 By Amanuel Diriba 43

 It contains piezoelectric ionic crystal materials such as Quartz.
 On application of force or pressure these materials get stretched or compressed. During
this process, the charge over the material changes and redistributes.
 One face of the material becomes positively charged and the other negatively charged.
 The net charge q on the surface is proportional to the amount x by which the charges have
been displaced. The displacement is proportion to force.
b) Piezoelectric force Sensors

7) FLUID PRESSURE SENSORS/ transducers
a) Diaphragm Type:
o In the diaphragm type sensor, when there is a difference in pressure between
the two sides then the Centre of the diaphragm becomes displaced.
o Corrugations in the diaphragm result in a greater sensitivity.
o This movement can be monitored by some form of displacement sensor, e.g: a
strain gauge.

b) Capsule and Bellow Types:
o Capsules are two corrugated diaphragms combined to give greater accuracy
o Capsules and bellows are made up of stainless steel, phosphor bronze, and nickel with
rubber and nylon
o Pressure range 103to 108 Pa
Figure of Capsule and Bellow Types

c) Tube Pressure Sensor:
o A different form of deformation is obtained using a tube with an elliptical cross
section
o Increase in pressure in tube causes it tend to circular cross – section
o C – Shaped tube is generally known as a Bourdon tube.
o C opens when pressure in the tube increases
o A helical form gives more sensitivity
o Tubes are made up of stainless steel, phosphor bronze, and nickel with rubber & nylon
o Pressure range 103to 108 Pa.

d) Tactile Sensor:
• It is used on fingertips of robot hands and for touch display screen.
• It has two PVDF(polyvinylidene fluoride) layers separated by a soft film which transmits
the vibrations.
• An alternating current is applied to lower PVDF layer which generates vibrations due to
reverse piezoelectric effect. These vibrations are transmitted to the upper PVDF layer via
soft film. These vibrations cause alternating voltage across the upper PVDF layer.
• when some pressure is applied on the upper PVDF layer the vibrations gets affected and
the output voltage changes.

e) Piezoelectric Sensors:
o Used to measure pressure and force.
o Piezoelectric materials when stretched or compressed generate electric charges with
one face of the managerial becoming positively charged and the opposite face
negatively charged.
o As a result a voltage is produced. The net charge q on a surface is proportional to the
amount x by which the charges have been displaced, and since the displacement is
proportional to the applied force F.
o q = kx= SF Where k is a constant and S a constant termed the charge sensitivity

8. Temperature sensors/ transducers
o used to monitor temperature changes such as
a) Expansion or contraction of solids, liquid or gas.
b) Change in resistance and emf.
A. Bimetallic Strips:
o A Bimetallic thermostat consists of two different metal strips bounded together and they
cannot move relative to each other.
o These metals have different coefficients of expansion and when the temperature changes
the composite strips bends into a curved strip, with the higher coefficient metal on the
outside of the curve.

Figure of Bimetallic Strips
o The basic principle in this is all metals try to change their physical dimensions
at different rates when subjected to same change in temperature.
o This deformation may be used as a temperature- controlled switch, as in the
simple thermostat.

b) Resistance Temperature Detectors (RTDs):
o The materials used for RTDs are Nickel, Iron, Platinum, Copper, Lead, Tungsten,
Mercury, Silver, etc.
o The resistance of most metals increases over a limited temperature range and the
relationship between Resistance and Temperature is shown below.
o Resistance increase with increase in temperature.
o The Resistance temperature detectors are simple and resistive elements in the form of
coils of wire.

C) Thermistors:
o Thermistor is a semiconductor device that has a negative temperature coefficient
of resistance in contrast to positive coefficient displayed by most metals.
o Thermistors are small pieces of material made from mixtures of metal oxides, such as
Iron, cobalt, chromium, Nickel, and Manganese.
o The thermistor is an extremely sensitive device because its resistance changes
rapidly with temperature.
o The resistance of conventional metal-oxide thermistors decreases in a very non-
linear manner with an increase in temperature.

Figure of Thermistors
o The resistance-temperature relationship for a thermistor can be described by an equation
of the form
Where Rt is the resistance at temperature t, with K and β being constant

d) Thermocouples:
o Thermocouple works on the fact that when a junction of dissimilar metals heated, it
produces an electric potential related to temperature.
o The thermocouple consist of one hot junction and one cold junction
o Hot junction is inserted where temperature is measured
o Cold junction is maintained at a constant reference temperature.

9) light sensors/ transducers
o Used to monitor light intensity.
o Light sensors are used in cameras, infrared detectors, and ambient lighting
applications
o Sensor is composed of photoconductor such as a photo resistor, photodiode, or
phototransistor.
A. Photo diode
o Photodiode is a solid-state device which converts incident light into an electric
current.

b) Photo transistor
o When light fall on base, current is produced this is proportional to the light
intensity.
Figure of Photo transistor

10. LIQUID FLOW SENSORS / transducers
A. Turbine Flow Meter
 It has a multi blade rotor mounted centrally in the pipe along which the flow is to be
measured.
 The fluid flow rotates the rotor. Accordingly the magnetic pick up coil counts the number
of magnetic pulses generated due to the distortion of magnetic field by the rotor blades.
 The angular velocity is proportional to the number of pulses and fluid flow is proportional
to angular velocity.

b) Orifice Plate
 It is a simple disc with a central hole and it is placed in the tube through which the
fluid flows.
 The pressure difference measured between a point equal to the diameter of
the tube upstream and half the diameter of downstream.
 The accuracy of this instrument is ±1.5%

11. liquid level sensors/transducers
A. Differential Pressure Sensor:
 This sensor is used to measure pressure less than the atmospheric pressure at a given
location.
 This sensor measures the difference between two or more pressures introduced as inputs
to the sensing unit.
 For example, measuring the pressure drop across an oil filter.

b. Float System:
 In this method the level of liquid is measured by movement of a float.
 The movement of float rotates the arm and slider will move across a potentiometer.
 The output result is related to the height of the liquid.

o Machine vision – camera lighting technique.
o A machine vision system uses a camera to view an image,
o Machine vision uses sensors (cameras), processing hardware and software
algorithms to automate complex visual inspection tasks and precisely guide
handling equipment during product assembly.
o Applications include Positioning, Identification, Verification, Measurement,
and Flaw Detection.
o Vision systems are capable of measuring parts, verifying parts are in the
correct position, and recognizing the shape of parts.
o Also, vision systems can measure and sort parts at high speeds.
o Computer software processes images captured during the process you are
trying to assess to capture data.
10. Machine vision

Machine vision Steps
• Image formation
Light source
Cameras
• Image processing
– Sampling
– Quantization
Image storing
– Frame grabber
– Frame buffer
• Image analysis
– Direct imaging technique
– Equivalent ellipse technique
– Region growth
• Image Interepretation (comparing with stds)

1. Image formation
• Its image capturing process using camera and lights.
• An image is formed because light emanates from an object in a variety of
directions.

Typical machine vision system consists of
cameras,
image processor,
a response system controller.08-Jan-21 By Amanuel Diriba 66

2. Image processing
67
• Image is a signal (Analog signal ).
• To generate digital image from sensed data, image processing consists of two major
process
a) Sampling
b) Quantization
A) Sampling
• is the process of converting a signal (e.g., a function of continuous time or space) into a
numeric sequence (a function of discrete time or space).
• The process is also called analog-to-digital conversion, or simply digitizing".
08-Jan-21 By Amanuel Diriba

Quantization
• The quantization level determines the number of grey levels in the digitized
image.

3. Image storing
• Frame grabber - an electronic device that captures individual,
digital still frames from an analog video signal or a digital
video stream.
• Frame buffer - Store the information and data.

4. Image analysis
72
• Image analysis is the extraction of meaningful information
from images

5. Image interpretation
73
• Image interpretation is the process of examining an image and identifying the
features in that image.
Template matching-
• It involves the identification of an object based on recognition of its image
various conclusions are drawn by comparing the result of analysis with
prestored set of standard criteria

Block diagram of machine Vision

Inspection
Gauging
Guidance
Identification
Application of Machine vision

Before - Difficult to Read
After - Easy to Read with NERLITE

There are certain features which have to be considered when we choose a sensor.
o Accuracy – should be high.
o Stability – maintain under abnormal condition.
o Environmental condition – usually has limits for temperature and
humidity.
o Range – measurement limit of sensor.
o Calibration – essential for most of the measuring devices as the readings
changes.
o Resolution – smallest increment detected by the sensor.
o Cost – should be low.
o Power consumptions – low.
o Repeatability – the reading that varies is repeatedly measured under the
same environment.
3. Selection of sensors

o Is built on the micrometer scale using semiconductor fabrication techniques.
o One step further in the direction of miniaturization.
o Examples : - micro tactile sensors
Micro sensors

 Muscle of mechatronics system.
 Responsible for moving or controlling a mechanism or
system.
 It is operated by a source of energy, typically electric current,
hydraulic fluid pressure, or pneumatic pressure
 Converts that energy into motion.
 Normally, the actuators are used in conjunction with the power
supply and a coupling mechanism
2.2 Actuators

 Actuation system have three actuating units:- power supply, actuator and
coupling mechanism.
 Actuator can be : - stepper motor
- Dc motor
- Ac servo motor
- SCR (silicon controlled rectifier)

 Power supply;- some form of electric energy, liquids, compressed air etc.
 Coupling mechanism
 Act as the interface between the actuator and physical system.
 Example: - rack and pinion, belt drive, gear drive, lead screw & nut, piston,
linkages, etc..

From below identify and write :-
a) what types of Actuator is used ?
b) what type of coupling mechanics is used ?

2.2 1 Actuators Classification
 Actuators can be classified based on the type of energy
1. Electrical Actuators ,
2. Electromechanical Actuators,
3. Electromagnetic Actuators
4. Pneumatic & hydraulic Actuators
 Actuators can be classified based on the type of energy New
generation: -
1. Smart mtl actuators,
2. micro actuators,
3. Nano actuators..

TYPES OF ACTUATORS
1.ELECTERICAL ACTUATION
 Electrical Actuation is a device that can convert electrical energy to mechanical
energy.
 They are Switching devices such as diodes, transistors, triacs, MOSFET, and relays
For example

2. Electromechanical Actuators
 Electromechanical actuator are motors that converts electrical energy to
mechanical motion.
 They used to produce motion or action, such as linear motion or
angular motions.
 Broadly they can be classified as DC motors, AC motors, and stepper
motors.
DC motors
AC motors stepper motors

3. Electromagnetic Actuators
 Electromagnetic actuators convert electrical and mechanical energy into
one another.
 The solenoid is the most common electromagnetic actuator.

4. PNEUMATIC AND HYDRAULIC ACTUATORS
• Pneumatic actuators use air power (compressed air) to produce rotary and
linear motion.

HYDRAULIC ACTUATORS
• HYDRAULIC actuators use hydraulic power to produce linear, rotary or oscillatory
motion.

o Hydraulic and pneumatic systems are similar except that while a hydraulic system uses an
incompressible fluid as the working medium, a pneumatic system uses air, which is
basically compressible.
o Advantages of using air as the working medium are that it is readily available and no
recycling is necessary. It is nonflammable so that leakage does not create a threat to
safety. It has negligible change in viscosity, which controls the system’s performance.
o The major advantage of a hydraulic system is the incompressibility of the fluid helps
in positive action or motion, and faster response, unlike pneumatic systems where there
are longer time delays.

Micro and Nano actuators
• Nano or Micro actuators are used in assembly of extremely small
functional part.
• Is built on the Nano or micrometer scale using semiconductor
fabrication techniques.
• One step further in the direction of miniaturization.
• Any device produced by assembling extremely small functional parts of
around 1–15 mm is called a micro machine.
- Some types of micro actuators are: - micro motors, micro valves, micro
pumps…
Micro motors
Micro pumpsMicro valves

o Power needs or requirement
o Coupling mechanism
o Range of motion(linear/rotary)
o Accuracy
o Heat dissipation
o Speed characteristics
o Number of load speed
Selection criteria of actuators

Lecture 2 Sensor and Actuation Systems for Mechatronics

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