Object Recognition For Pick And Place Tasks.
i
“I / We admit that I/we have read this literature work through my/our observation
which has fulfilled the scope and quality in order to be qualified for the conferment
of Bachelor Degree in Electrical Engineering (Industry Power).”
Signature
: ………………………………..
Supervisor’s name I : ………………………………..
Date
: ………………………………..
Signature
: ………………………………..
Supervisor’s name II : ………………………………..
Date
: …………………………………
ii
OBJECT RECOGNITION FOR PICK AND PLACE TASKS
LIM HUEY KIAN
This Report Is Submitted In Partial Fulfillment Of Requirements For The Degree Of
Bachelor In Electrical Engineering (Industry Power)
Faculty Electrical Engineering
Kolej Universiti Teknikal Kebangsaan Malaysia
March 2005
iii
“I admit that this is done by myself except the conclusion and extracts taken from
other sources that I explained each in detail.”
Signature
: ……………………
Author’s Name : LIM HUEY KIAN
Date
: 9TH MARCH 2005
iv
This literature piece is dedicated to my beloved mother and father
v
Appreciation
In this section, I would like to take an opportunity to thank my supervisor, Prof. Dr
Marizan Sulaiman, who assist and guide me a lot in executing my project. I would
also like to thank Mr. Mohd Ariff Mat Hanafiah, Mr. Mohd Syakrani Akhbar, all my
family members and course mates for giving moral supports.
vi
Abstract
The object recognition is a process of making sequences of decisions to gives
signal to pick and place system to separate the object. This system are commonly
use in industrial field such as manufacturing, automation, fabrication and etc. The
conveyor belt is one of the most efficient devices available for moving goods over
short distance. The sensor emits, receives, and converts the light energy into an
electrical signal. The use of the recognition algorithm as part of the evaluation
function for sensing segmentation gives rise to significant improvement of the
system performance by automatic generation of recognition strategies. The conveyor
system will roll the object and the sensor detects the presence of object and sends the
signal to the controller. The controller will distinguish the different objects and
instruct the robot arm to pick and place the object to bin 1 (for object A), bin2 (for
object B), bin3 (for object C) or bin4 (for object D).
vii
Abstrak
Pengesanan objek adalah proses yang melibatkan satu keputusan untuk
memberi isyarat kepada sistem “ambil dan letak” untuk mengasingkan objek. Sistem
ini biasa digunakan dalam bidang industri seperti pembuatan, automasi,
pembentukan dan sebagainya. Alat pengangkut adalah peranti yang paling cekap
dan mudah diperoleh untuk menggerakkan barang-barang dalam jarak yang agak
jauh. Fungsi penderia (alat pengesan) melibatkan proses memancar, menerima dan
menukar tenaga cahaya kepada bentuk isyarat elektrik. Algoritma pengesan
digunakan sebagai sebahagian daripada fungsi penilaian bagi pembahagian pengesan
untuk memberi peningkatan kecekapan yang ketara dalam sesebuah sistem dengan
mengaplikasikan strategi-strategi penjanaan pengesanan secara automatik. Sistem
pengangkut akan membawa objek dan penderia pula akan mengesan objek dan
menghantar isyarat kepada pengawal. Alat pengawal akan menilai perbezaan objek
dan mengarahkan lengan robot untuk mengambil dan meletakkan objek ke dalam bin
1(bagi objek A), bin 2(bagi objek B), bin 3(bagi objek C) atau bin 4(bagi objek D).
viii
CONTENTS
CHAPTER
1
2
T IT L E
PAGE
TITLE
ACKNOWLEDGEMENT
DEDICATION
APPRECIATION
ABSTRACT
ABSTRAK
CONTENTS
LIST OF TABLES
LIST OF FIGURE
LIST OF APPENDIXES
i
ii
iii
iv
v
vi
vii
ix
x
x ii
INTRODUCTION
LITERATURE REVIEW
2.1 Type Of Sensors
2.1.1 Inductive Proximity Switch
2.1.1.1 Standard Barrel Types
2.1.1.2 Operation
2.1.1.3 Advantages Of Inductive
Proximity Switch
2.1.2 Capacitive Proximity Sensor
2.1.3 Photoelectric Proximity Switches
2.2 Controller
2.2.1 Definitions Of PLC
2.2.2 Procedures Of PLC
2.2.3 Advantages Of Using PLC In
Automation
1
3
4
6
6
7
8
10
12
14
15
ix
3
4
5
6
2.2.4 Basic Instructions
2.2.4.1 Load
2.2.4.2 Load Bar
2.2.4.3 Output
2.2.4.4 Output Bar
2.2.5 Simple Examples For PLC Instruction
PROJECT DEVELOPMENT
3.1 Objective
3.2 Project Development
PROBLEM STATEMENT AND PROJECT
METHODOLOGY
4.1 Problem Statement
4.2 Project Methodology
4.2.1 Input (Sensor System)
4.2.2 Controller (PLC)
4.2.3 Output
FUNCTIONS OF PICK AND PLACE TASKS
5.1 Flowchart
5.2 PLC Instruction
DISCUSSION & CONCLUSION
6.1 Application In Automation Field
6.2 Conclusion
16
16
17
17
17
18
REFERENCES
APPENDIXES
56
57
21
22
23
24
24
26
26
28
30
34
51
52
55
x
LIST OF TABLE
No
2.1
T IT L E
Logic state definitions for load, load bar, output, and output
PAGE
17
bar instruction
5.1
Input data instruction
34
5.2
Output data instruction
35
5.3
Keep address for different data instruction
35
5.4
Keep address for output valve instruction
36
5.5
Counter address instruction
37
5.6
Timer address instruction
37
xi
LIST OF FIGURE
T IT L E
No
PAGE
2.1
Inductive Proximity Switches
5
2.2
Housings of standard inductive
6
2.3
Block diagram of the operation principle of inductive
7
proximity sensor
2.4
Capacitive Proximity Switches
9
2.5
Block diagram of the operation principle of capacitive
9
proximity sensor
2.6
Photoelectric Proximity Switches
11
2.7
Typical PLC (Programmable Logic Controller)
12
2.8
Block Diagram of PLC operation
14
2.9
A simple series connection circuit of two switches,
18
a coil and a battery.
2.10
The equivalent ladder diagram for Figure 2.9
18
2.11
The ladder diagram for latch instruction
19
2.12
The ladder diagram for timer instruction
20
4.1
Block Diagram for the system object recognition for
24
pick and place task
5.1
Flowchart for object recognition for pick and place tasks
30
x ii
5.2
Flowchart for status A refer to Figure 5.1
32
5.3
Flowchart for status B refer to Figure 5.1
32
5.4
Flowchart for status C refer to Figure 5.1
33
5.5
Flowchart for status D refer to Figure 5.1
33
5.6
Ladder diagram of object recognition for pick and place
38
6.1
Counting objects
52
6.2
Level control
53
6.3
Checking the missing component
53
6.4
Detecting the inverted caps
53
6.5
Detecting of height for sheet / files / papers
54
6.6
Bottle sensing
54
6.7
Die attachment
54
xiii
LIST OF APPENDIXES
No
T IT L E
PAGE
A
CX-Programmer
57
B
Photoelectric Sensors
60
C
Inductive Proximity Switch
64
D
Capacitive Proximity Switch
79
E
Programmable Logic Controller (PLC) CQM1H
87
Chapter 1
INTRODUCTION
In the field of industrial robotics, many different calibration methods exist to help
reduce error in the robot system. One of the most common calibration methods, locating
the manipulator home position, requires that the robot be positioned with all joint angles
specified to have a value of either zero or 90 degrees [1].
To improve the calibration of the home position, first wanted to simplify the
method of calibration. The home position can be found using one of the following three
separate methods of calibration [2].
i.
Relative calibration - Expensive process that requires each component in the
robotic structure to be defined relative to the previous component. Accuracy
from relative calibrations can vary based on the accuracy of the robot
components.
ii.
Optimal calibration - This process uses a measurement system combined with
kinematics models of the robot to measure many positions of the robot and
correct any errors present in structure. Accuracy from optimal calibrations can
vary based on the robot positions and kinematics model.
iii.
Leveling based calibration - Process uses simple electronic levels known as
inclinometers to easily orient each component of the robot structure with
respect to the angle read by the inclinometer.
To improve the initial device, the prototype was reduced to a single right angle
plate with the two sensors mounted perpendicular to each other within the structure.
Operation of the calibration procedure was also improved with automated software to
adjust the position of the robot joints to match the values required by the sensor values
[ 3] .
Chapter 2
LITERATURE REVIEW
2.1 TYPE OF SENSORS
There are many type of sensor used in new technology system. This is because
their ability beyond convention switching devices. Sensors are divided into many
categories based on their input parameters such as inductive proximity sensor, capacitive
proximity sensor, photo sensor, magnetic sensor and etc. In my project, four types of
sensor are used, such as the inductive proximity sensor, capacitive proximity sensor,
photo sensor and magnetic sensor. The functions and more details of these sensors will
be further explained according to subtitle below.
2.1.1 Inductive Proximity Switches
A proximity switch is a device (refer to Figure 2.1), which causes a switching
action without physical contact. Inductive proximity switches only react to metal
material up to an operating distance of approximately 50 mm. Proximity switches
respond to targets that come within the active range of their generated sensing fields.
These units are completely self-contained, and house a field generator, amplifier, and
other necessary circuitry to accomplish electronic switching.
The units are all solid state and have no moving parts that can wear out. The
electronic switches are not susceptible to contact contamination, contact erosion, or
material transfers are mechanical switches. Their service life, within their specified
ratings, is virtually unlimited. The switching is insensitive to vibration, and is positive
(full step function) without chatter, regardless of how slowly the target approaches or
recedes from the sensor.
The oscillator resonant circuit, located in the proximity switch, uses an open core
coil to help produce a concentrated high frequency electro-magnetic (RF) field, which
emerges from the active surface of the sensor. If a metal target (e.g. metal) enters this
field, eddy currents are induced. The floating induced eddy current draws energy from
the LC circuit (L : coil, C : capacitor). The load on the oscillator circuit evokes a
decrease in the oscillating amplitude. The oscillator is attenuated.
The decrease of the oscillating amplitude is converted into an electrical signal by
the electronic circuit, which leads to a change of switching state of the proximity switch.
When the electric conductive material is removed from the inductive field, the pulse
amplitude increases and via the electronic circuit the original switching position is
recreated [6].
Figure 2.1: Inductive Proximity Switches
2.1.1.1 Standard Barrel Types
Housings of standard inductive ranges are made of Nickel coated brass barrels
(refer Figure 2.2). They are widely used in non-corrosive atmospheres.
Figure 2.2: Housings of standard inductive
2.1.1.2 Operation
The oscillator creates high frequency electromagnetic field, which radiates from
sensing face of the sensor. When damped with metallic object, eddy currents are induced
in metal causing change in amplitude of oscillations. This signal is conditioned to change
Schmitt trigger output and state of output amplifier. Analogue DC output voltage and
current are available in addition to standard PNP/NPN switching outputs. Figure 2.3
shows the principle of inductive proximity switch operation [6].
Oscillator
circuit
Rectifier
circuit
Switching
circuit
Output
Detecting Coil
Power
Supply
Power
Supply
Figure 2.3: Block diagram of the operation principle of inductive proximity sensor
2.1.1.3 Advantages of inductive proximity switches
The advantages of inductive proximity switches are:
i.
High reliability with frequent or rare switching processes, there are no contact
bouncing or contact wear
ii. Contact less function; therefore no influence of the material surface
iii. High switching speeds up to 5000 Hz
iv. Large contamination insensitivity in relation to non-metallic contamination, e.g.
dust, oil or aqueous/organic solvents
v.
Small current consumption
vi. Cheap
2.1.2 Capacitive Proximity Sensors
Capacitive switches (refer Figure 2.4) can also detect non-metallic materials, but
only over relatively short operating distances.
Capacitive proximity switches are
contactlessly working position switches, which react to the presence of objects of nearly
any materials within the supervised range. Capacitive proximity switches operate using
an RC resonant circuit (resistor-capacitor), where the capacity is affected. To achieve
this, the electrodes of the capacitor are separated. One electrode is located in the
proximity switch on the active surface. The second electrode is either the target with
earth or ground as return line, or ground itself, whereby the target causes a change in the
dielectric medium.
When this medium approaches the active surface and thus the capacitor electrode
in the sensor, capacitance increases to the extent where, with the resistor, the value for
tripping the resonant circuit is reached and the oscillator starts oscillating.
When the actuating target is removed from the active surface, the opposite
occurs, and the oscillator stops oscillating. Commencement and ceasing of oscillation,
evaluated by the connected electronic circuitry, produces a change in the switching state
of the proximity switch. A built-in potentiometer permits fine adjustment of the actuating
distance within the field. The sensor responds to all solid and liquid media, such as
water, glass, wood, paper, metal, plastic, foodstuff, etc. Figure 2.5 shows the principle of
capacitive proximity switch operation [6].
Figure 2.4: Capacitive Proximity Switches
Oscillator
circuit
Rectifier
circuit
Switching
circuit
Output
Base Electrode
Power
Supply
Power
Supply
Figure 2.5: Block diagram of the operation principle of capacitive proximity sensor
2.1.3 Photoelectric Proximity Switches
The IFO photoelectric switches are non-contact switches, which are suitable for
use as a diffuse-reflective sensor (without reflector) or as a retro-reflective sensor (with
reflector). Light emitter, receiver, electronic evaluation circuitry and amplifier for AC or
DC are all in one common housing (self-contained type). No additional power supply,
switching units or amplifiers are necessary. The operating principle is based on
modulated light, which is emitted through the front lens of the switch directly to the
object or reflector, which is to be detected. The reflected light reaches the receiver
through a second lens, and is processed electronically, causing a change in the output
condition of the switch. Removal of the object from the detection zone causes the switch
to return to its original switching position. Due to the synchronization of emitter and
receiver circuitry, the photoelectric switch is insensitive against interference and
external light.
Photoelectric switches (refer Figure 2.6) can only detect objects, which reflect
sufficient light. Therefore, the operating distance depends a lot on the surface condition
(reflectivity) of the object to be detected. A smooth white surface allows for a much
larger operating distance compared to a dull black surface finish. With some models, the
optimum operating distance for each application can be set, using the built-in
potentiometer. In this way, undesired background reflections are eliminated. The
provided LED is also helpful when setting the sensing distance, as it shows switching
condition and is used as a function indicator. It is further possible to choose between
light operation (ON with reflection, corresponding to NO contact) and dark operation
(OFF with reflection, corresponding to NC contact) by repositioning a small jumper at
the terminal screws [6].
Figure 2.6: Photoelectric Proximity Switches
“I / We admit that I/we have read this literature work through my/our observation
which has fulfilled the scope and quality in order to be qualified for the conferment
of Bachelor Degree in Electrical Engineering (Industry Power).”
Signature
: ………………………………..
Supervisor’s name I : ………………………………..
Date
: ………………………………..
Signature
: ………………………………..
Supervisor’s name II : ………………………………..
Date
: …………………………………
ii
OBJECT RECOGNITION FOR PICK AND PLACE TASKS
LIM HUEY KIAN
This Report Is Submitted In Partial Fulfillment Of Requirements For The Degree Of
Bachelor In Electrical Engineering (Industry Power)
Faculty Electrical Engineering
Kolej Universiti Teknikal Kebangsaan Malaysia
March 2005
iii
“I admit that this is done by myself except the conclusion and extracts taken from
other sources that I explained each in detail.”
Signature
: ……………………
Author’s Name : LIM HUEY KIAN
Date
: 9TH MARCH 2005
iv
This literature piece is dedicated to my beloved mother and father
v
Appreciation
In this section, I would like to take an opportunity to thank my supervisor, Prof. Dr
Marizan Sulaiman, who assist and guide me a lot in executing my project. I would
also like to thank Mr. Mohd Ariff Mat Hanafiah, Mr. Mohd Syakrani Akhbar, all my
family members and course mates for giving moral supports.
vi
Abstract
The object recognition is a process of making sequences of decisions to gives
signal to pick and place system to separate the object. This system are commonly
use in industrial field such as manufacturing, automation, fabrication and etc. The
conveyor belt is one of the most efficient devices available for moving goods over
short distance. The sensor emits, receives, and converts the light energy into an
electrical signal. The use of the recognition algorithm as part of the evaluation
function for sensing segmentation gives rise to significant improvement of the
system performance by automatic generation of recognition strategies. The conveyor
system will roll the object and the sensor detects the presence of object and sends the
signal to the controller. The controller will distinguish the different objects and
instruct the robot arm to pick and place the object to bin 1 (for object A), bin2 (for
object B), bin3 (for object C) or bin4 (for object D).
vii
Abstrak
Pengesanan objek adalah proses yang melibatkan satu keputusan untuk
memberi isyarat kepada sistem “ambil dan letak” untuk mengasingkan objek. Sistem
ini biasa digunakan dalam bidang industri seperti pembuatan, automasi,
pembentukan dan sebagainya. Alat pengangkut adalah peranti yang paling cekap
dan mudah diperoleh untuk menggerakkan barang-barang dalam jarak yang agak
jauh. Fungsi penderia (alat pengesan) melibatkan proses memancar, menerima dan
menukar tenaga cahaya kepada bentuk isyarat elektrik. Algoritma pengesan
digunakan sebagai sebahagian daripada fungsi penilaian bagi pembahagian pengesan
untuk memberi peningkatan kecekapan yang ketara dalam sesebuah sistem dengan
mengaplikasikan strategi-strategi penjanaan pengesanan secara automatik. Sistem
pengangkut akan membawa objek dan penderia pula akan mengesan objek dan
menghantar isyarat kepada pengawal. Alat pengawal akan menilai perbezaan objek
dan mengarahkan lengan robot untuk mengambil dan meletakkan objek ke dalam bin
1(bagi objek A), bin 2(bagi objek B), bin 3(bagi objek C) atau bin 4(bagi objek D).
viii
CONTENTS
CHAPTER
1
2
T IT L E
PAGE
TITLE
ACKNOWLEDGEMENT
DEDICATION
APPRECIATION
ABSTRACT
ABSTRAK
CONTENTS
LIST OF TABLES
LIST OF FIGURE
LIST OF APPENDIXES
i
ii
iii
iv
v
vi
vii
ix
x
x ii
INTRODUCTION
LITERATURE REVIEW
2.1 Type Of Sensors
2.1.1 Inductive Proximity Switch
2.1.1.1 Standard Barrel Types
2.1.1.2 Operation
2.1.1.3 Advantages Of Inductive
Proximity Switch
2.1.2 Capacitive Proximity Sensor
2.1.3 Photoelectric Proximity Switches
2.2 Controller
2.2.1 Definitions Of PLC
2.2.2 Procedures Of PLC
2.2.3 Advantages Of Using PLC In
Automation
1
3
4
6
6
7
8
10
12
14
15
ix
3
4
5
6
2.2.4 Basic Instructions
2.2.4.1 Load
2.2.4.2 Load Bar
2.2.4.3 Output
2.2.4.4 Output Bar
2.2.5 Simple Examples For PLC Instruction
PROJECT DEVELOPMENT
3.1 Objective
3.2 Project Development
PROBLEM STATEMENT AND PROJECT
METHODOLOGY
4.1 Problem Statement
4.2 Project Methodology
4.2.1 Input (Sensor System)
4.2.2 Controller (PLC)
4.2.3 Output
FUNCTIONS OF PICK AND PLACE TASKS
5.1 Flowchart
5.2 PLC Instruction
DISCUSSION & CONCLUSION
6.1 Application In Automation Field
6.2 Conclusion
16
16
17
17
17
18
REFERENCES
APPENDIXES
56
57
21
22
23
24
24
26
26
28
30
34
51
52
55
x
LIST OF TABLE
No
2.1
T IT L E
Logic state definitions for load, load bar, output, and output
PAGE
17
bar instruction
5.1
Input data instruction
34
5.2
Output data instruction
35
5.3
Keep address for different data instruction
35
5.4
Keep address for output valve instruction
36
5.5
Counter address instruction
37
5.6
Timer address instruction
37
xi
LIST OF FIGURE
T IT L E
No
PAGE
2.1
Inductive Proximity Switches
5
2.2
Housings of standard inductive
6
2.3
Block diagram of the operation principle of inductive
7
proximity sensor
2.4
Capacitive Proximity Switches
9
2.5
Block diagram of the operation principle of capacitive
9
proximity sensor
2.6
Photoelectric Proximity Switches
11
2.7
Typical PLC (Programmable Logic Controller)
12
2.8
Block Diagram of PLC operation
14
2.9
A simple series connection circuit of two switches,
18
a coil and a battery.
2.10
The equivalent ladder diagram for Figure 2.9
18
2.11
The ladder diagram for latch instruction
19
2.12
The ladder diagram for timer instruction
20
4.1
Block Diagram for the system object recognition for
24
pick and place task
5.1
Flowchart for object recognition for pick and place tasks
30
x ii
5.2
Flowchart for status A refer to Figure 5.1
32
5.3
Flowchart for status B refer to Figure 5.1
32
5.4
Flowchart for status C refer to Figure 5.1
33
5.5
Flowchart for status D refer to Figure 5.1
33
5.6
Ladder diagram of object recognition for pick and place
38
6.1
Counting objects
52
6.2
Level control
53
6.3
Checking the missing component
53
6.4
Detecting the inverted caps
53
6.5
Detecting of height for sheet / files / papers
54
6.6
Bottle sensing
54
6.7
Die attachment
54
xiii
LIST OF APPENDIXES
No
T IT L E
PAGE
A
CX-Programmer
57
B
Photoelectric Sensors
60
C
Inductive Proximity Switch
64
D
Capacitive Proximity Switch
79
E
Programmable Logic Controller (PLC) CQM1H
87
Chapter 1
INTRODUCTION
In the field of industrial robotics, many different calibration methods exist to help
reduce error in the robot system. One of the most common calibration methods, locating
the manipulator home position, requires that the robot be positioned with all joint angles
specified to have a value of either zero or 90 degrees [1].
To improve the calibration of the home position, first wanted to simplify the
method of calibration. The home position can be found using one of the following three
separate methods of calibration [2].
i.
Relative calibration - Expensive process that requires each component in the
robotic structure to be defined relative to the previous component. Accuracy
from relative calibrations can vary based on the accuracy of the robot
components.
ii.
Optimal calibration - This process uses a measurement system combined with
kinematics models of the robot to measure many positions of the robot and
correct any errors present in structure. Accuracy from optimal calibrations can
vary based on the robot positions and kinematics model.
iii.
Leveling based calibration - Process uses simple electronic levels known as
inclinometers to easily orient each component of the robot structure with
respect to the angle read by the inclinometer.
To improve the initial device, the prototype was reduced to a single right angle
plate with the two sensors mounted perpendicular to each other within the structure.
Operation of the calibration procedure was also improved with automated software to
adjust the position of the robot joints to match the values required by the sensor values
[ 3] .
Chapter 2
LITERATURE REVIEW
2.1 TYPE OF SENSORS
There are many type of sensor used in new technology system. This is because
their ability beyond convention switching devices. Sensors are divided into many
categories based on their input parameters such as inductive proximity sensor, capacitive
proximity sensor, photo sensor, magnetic sensor and etc. In my project, four types of
sensor are used, such as the inductive proximity sensor, capacitive proximity sensor,
photo sensor and magnetic sensor. The functions and more details of these sensors will
be further explained according to subtitle below.
2.1.1 Inductive Proximity Switches
A proximity switch is a device (refer to Figure 2.1), which causes a switching
action without physical contact. Inductive proximity switches only react to metal
material up to an operating distance of approximately 50 mm. Proximity switches
respond to targets that come within the active range of their generated sensing fields.
These units are completely self-contained, and house a field generator, amplifier, and
other necessary circuitry to accomplish electronic switching.
The units are all solid state and have no moving parts that can wear out. The
electronic switches are not susceptible to contact contamination, contact erosion, or
material transfers are mechanical switches. Their service life, within their specified
ratings, is virtually unlimited. The switching is insensitive to vibration, and is positive
(full step function) without chatter, regardless of how slowly the target approaches or
recedes from the sensor.
The oscillator resonant circuit, located in the proximity switch, uses an open core
coil to help produce a concentrated high frequency electro-magnetic (RF) field, which
emerges from the active surface of the sensor. If a metal target (e.g. metal) enters this
field, eddy currents are induced. The floating induced eddy current draws energy from
the LC circuit (L : coil, C : capacitor). The load on the oscillator circuit evokes a
decrease in the oscillating amplitude. The oscillator is attenuated.
The decrease of the oscillating amplitude is converted into an electrical signal by
the electronic circuit, which leads to a change of switching state of the proximity switch.
When the electric conductive material is removed from the inductive field, the pulse
amplitude increases and via the electronic circuit the original switching position is
recreated [6].
Figure 2.1: Inductive Proximity Switches
2.1.1.1 Standard Barrel Types
Housings of standard inductive ranges are made of Nickel coated brass barrels
(refer Figure 2.2). They are widely used in non-corrosive atmospheres.
Figure 2.2: Housings of standard inductive
2.1.1.2 Operation
The oscillator creates high frequency electromagnetic field, which radiates from
sensing face of the sensor. When damped with metallic object, eddy currents are induced
in metal causing change in amplitude of oscillations. This signal is conditioned to change
Schmitt trigger output and state of output amplifier. Analogue DC output voltage and
current are available in addition to standard PNP/NPN switching outputs. Figure 2.3
shows the principle of inductive proximity switch operation [6].
Oscillator
circuit
Rectifier
circuit
Switching
circuit
Output
Detecting Coil
Power
Supply
Power
Supply
Figure 2.3: Block diagram of the operation principle of inductive proximity sensor
2.1.1.3 Advantages of inductive proximity switches
The advantages of inductive proximity switches are:
i.
High reliability with frequent or rare switching processes, there are no contact
bouncing or contact wear
ii. Contact less function; therefore no influence of the material surface
iii. High switching speeds up to 5000 Hz
iv. Large contamination insensitivity in relation to non-metallic contamination, e.g.
dust, oil or aqueous/organic solvents
v.
Small current consumption
vi. Cheap
2.1.2 Capacitive Proximity Sensors
Capacitive switches (refer Figure 2.4) can also detect non-metallic materials, but
only over relatively short operating distances.
Capacitive proximity switches are
contactlessly working position switches, which react to the presence of objects of nearly
any materials within the supervised range. Capacitive proximity switches operate using
an RC resonant circuit (resistor-capacitor), where the capacity is affected. To achieve
this, the electrodes of the capacitor are separated. One electrode is located in the
proximity switch on the active surface. The second electrode is either the target with
earth or ground as return line, or ground itself, whereby the target causes a change in the
dielectric medium.
When this medium approaches the active surface and thus the capacitor electrode
in the sensor, capacitance increases to the extent where, with the resistor, the value for
tripping the resonant circuit is reached and the oscillator starts oscillating.
When the actuating target is removed from the active surface, the opposite
occurs, and the oscillator stops oscillating. Commencement and ceasing of oscillation,
evaluated by the connected electronic circuitry, produces a change in the switching state
of the proximity switch. A built-in potentiometer permits fine adjustment of the actuating
distance within the field. The sensor responds to all solid and liquid media, such as
water, glass, wood, paper, metal, plastic, foodstuff, etc. Figure 2.5 shows the principle of
capacitive proximity switch operation [6].
Figure 2.4: Capacitive Proximity Switches
Oscillator
circuit
Rectifier
circuit
Switching
circuit
Output
Base Electrode
Power
Supply
Power
Supply
Figure 2.5: Block diagram of the operation principle of capacitive proximity sensor
2.1.3 Photoelectric Proximity Switches
The IFO photoelectric switches are non-contact switches, which are suitable for
use as a diffuse-reflective sensor (without reflector) or as a retro-reflective sensor (with
reflector). Light emitter, receiver, electronic evaluation circuitry and amplifier for AC or
DC are all in one common housing (self-contained type). No additional power supply,
switching units or amplifiers are necessary. The operating principle is based on
modulated light, which is emitted through the front lens of the switch directly to the
object or reflector, which is to be detected. The reflected light reaches the receiver
through a second lens, and is processed electronically, causing a change in the output
condition of the switch. Removal of the object from the detection zone causes the switch
to return to its original switching position. Due to the synchronization of emitter and
receiver circuitry, the photoelectric switch is insensitive against interference and
external light.
Photoelectric switches (refer Figure 2.6) can only detect objects, which reflect
sufficient light. Therefore, the operating distance depends a lot on the surface condition
(reflectivity) of the object to be detected. A smooth white surface allows for a much
larger operating distance compared to a dull black surface finish. With some models, the
optimum operating distance for each application can be set, using the built-in
potentiometer. In this way, undesired background reflections are eliminated. The
provided LED is also helpful when setting the sensing distance, as it shows switching
condition and is used as a function indicator. It is further possible to choose between
light operation (ON with reflection, corresponding to NO contact) and dark operation
(OFF with reflection, corresponding to NC contact) by repositioning a small jumper at
the terminal screws [6].
Figure 2.6: Photoelectric Proximity Switches