chapter 4 photo sensor
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CHAPTER 4: PHOTO-ELECTRIC SENSORS
1. Introduction
A photoelectric sensor is another type of position sensing device. Photoelectric sensors, similar to
the ones shown below, use a modulated light beam that is either broken or reflected by the target.
The control consists of an emitter (light source), a receiver to detect the emitted light, and
associated electronics that evaluate and amplify the detected signal causing the photoelectric outputswitch to change state.
2. Function
It detects all type of materials: liquid, solid, powder, cream, opaque, transparent, reflective.
There are different forms of photo-electric detector:
i) Miniature Small size
Small objects
Amplifier incorporated
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ii) Compact
Size irrelevant
Normal objects
Amplifier incorporated
iii) Fiber optic head (with separate amplifier)
Very small size
Small objects
Remote amplifier
iv) Fiber optic head (with separate amplifier)
Minimum size
Very small objects Remote amplifier
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3. Operating principles
A photo-electric detector uses light beam (emission) for its functioning. When an object interrupt
the light beam, it modify the quantity of light receive by the receiver and provoke a
transformation of the state of the output.
There are two methods of detection:
4. Composition
The basic components of a photo-electric detector are:
Transmitter : Transmit a light beam
Receiver : Receive the light beam
When the object enters the light beam, it modifies the quantity of the light received by the
receiver and provokes a transformation of the output.
Object to be detected blockthe light from the transmitter.
Object to be detected reflectsthe light from the transmitter
Control PartOperative Part
Receiver
TransmitterOutputstage
Outputdriver
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5. Symbol
6. Types & Applications
There are 5 types of photo-electric detectors. There are classified according to the
method of detection.
Thru-beam Reflex Polarised Reflex
Object Blocks Light
Diffuse Diffuse
w/bg. supp.
Object Reflects Light
PHOTO-ELECTRIC
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6.1 Thru beam
6.1.1 Description
The receiver and transmitter are placed in two separate casings.
The detection is made by the interruption of light beam when the object
passes between the transmitter and receiver.
The light beam is transmitted using infra red.
6.1.2 Characteristics
Advantages Disadvantages Long range distance ( up to 30m)
Suitable for polluted environment
(smoke, dust.)
Suitable for opaque or reflective
object
Position of object to be detected is
very precise.
Rigid alignment between the
receiver and transmitter.
Unable to detect transparent
material.
Vibration can cause
alignment problem
Object
ReceiverTransmitter
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6.1.3 Application Example
Detect the wheel passing.
6.2. REFLEX or RETRORELECTIVE SCAN
6.2.1 Description
The receiver and transmitter are placed in the same casing.
When there is no object in front of the detector, the light beam transmitted by
the transmitter is reflected towards the receiver by the reflector.
The detection is made by the interruption of light beam when the object
passes between the two cases.
The reflector consists of multiples trihedron trirectangle which reflects the light
beam.
Reflecto
Object
Receiver
Transmitter &
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This type of sensor may not be able to detect shiny object. Shiny object reflect
light back to the sensor. The sensor is unable to differentiate between light
reflected from the shiny object and light reflected from the reflector.
6.2.2 Characteristic
Advantages Disadvantages
Cost less because only one casing and
one cable (compared to thru-beam).
Can be used for polluted environment
(smoke, dust.)
The alignment between transmitter and
the reflector can be less accurate than
thru-beam system.
Limited range (up to 10m). Unable to detect reflective
(shiny) materials.
6.2.3 Precaution during installation
The reflector must be smaller than the object to detect.
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Position the transmitter/receiver and the reflector for maximum reflection
of the light beam towards the receiver (normally depends on the position
of the reflector).
Incorrectly oriented Incorrectly centered Correctly positioned
Install the casing and the reflector in a such way to avoid reflection from
the object to detect
Reflective object approaching the sensorThe object reflects the light beam back to the
receiver (acting as the reflector of the sensor)
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Install the detector and its
reflector so as to avoid direct
reflection from the object
6.2.4 Application example
To control the passage of packed mineral water.
Detection of people entering a shop.
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6.3. POLARISED REFLEX SYSTEM
6.3.1 - Description
This type of detector transmits a visible red light.
It has two opposites polarising filters:
A filter located at the transmitter which allows only the transmission of
vertical component of the light.
A filter located at the receiver which allows only the transmission of
horizontal component of the light.
Without object in front of the sensor
NO OBJECT Reflector
Transmitter
Receiver
Reflected light beam depolarised bythe reflector. Regeneration of horizontalplane rays. Receiver filter blocked thevertical rays and horizontal rays are
received by the receiver.
Light beam transmitted is
polarised vertically
NO OBJECT Reflector
Transmitter
Receiver
Reflected light beam depolarised bythe reflector. Regeneration of
horizontal plane rays. Receiver filterblocked the vertical rays and
horizontal rays are received by thereceiver.
Light beam transmitted isolarised verticall
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With object in front of the sensor
OBJECT PRESENCEReflector
Reflectiveobject
Non depolarised lightbeam reflected by the objectis blocked by the receiver
filter. Only rays in thevertical plane will be
accepted.
Light beam transmitted ispolarised vertically
Transmitter
Receiver
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6.3.2 Characteristic
Advantages Disadvantages
Detect reflective (shiny) object
Not sensitive to parasite reflection.
Operation could be disturbed by external
light.
Cant be used to detect materials which
can depolarise the light beam.
6.3.3 Application Example
Detection at the entrance to a car park (shining metallic car).
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6.4 DIFFUSE
6.4.1 - Description
The transmitter and the receiver are grouped in the same casing.
When there is no object passes in front of the detector, no light beam is reflected
back towards the receiver.
When there is an object passes in front of the detector, a part of the light beam is
reflected towards the receiver by the object.
Transmitter &Receiver
NO OBJECT
Object
Transmitter &Receiver
OBJECT PRESENCE
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6.4.2 Characteristic
Advantages Disadvantages
Adjustable sensitivity
Suitable to detect transparent
materials such as bottles.
Operation range limited.
Not suitable for polluted environment.
Detection could be disturbed by a
back ground.
6.4.3 Application Example
Detection of can in food processing industry.
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6.5. DIFFUSE SYSTEM with BACK GROUND SUPPRESSION
6.5.1 Function
This sensor is used to detect object up to a certain distance. Detectors with this
system are equipped with sensing distance adjustment potentiometer. This
potentiometer allows focusing a specific detection zone whilst ignoring parasite
reflections.
It can detect objects which have different colours and different reflectivity at the
same distance.
When there is no object passes in front of the detector, no light beam is reflected
back towards the receiver.
When there is an object passes in front of the detector, a part of the light beam is
reflected towards the receiver by the object.
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Background suppression is accomplished with a position sensor detector (PSD).
Reflected light from the target hits the PSD at different angles, depending on the
distance of the target. The greater the distance the narrower the angle of the
reflected light.
6.5.2 Characteristic
Advantages Disadvantages
Detection of an object whilst ignoring
background.
Detection of objects up to a given
distance (adjustment potentiometer)
Detection of objects irrespective of
their colour.
Operation range limited.
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6.5.3 - Adjusting Sensing Distance
Sensor is detecting the back ground.
Adjust the sensing distance so as not to include the background.
Check that the detector switches when an object is present.
The background remains out of range. i.e. not detected.
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7. Switching (Operating Modes)
Instead of NO or NC contact, photoelectric sensors provide light switching or dark
switching as one criteria for selection.
71. Light Switching
The output is on when the light beam is received by the receiver.
7.1.1 Diffuse sensors
Absence of object output is OFF
The equivalent contact diagram:
Object presence output is ON
The equivalent contact diagram:
T
2
1
R ~
~
T
2
1
R~
~
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7.1.2 Thru beam or Reflex sensors
Object presence output is OFF
The equivalent contact diagram:
Absence of object output is ON
The equivalent contact diagram:
7.2. Dark Switching
The output is on when the light beam is not received by the receiver.
7.2.1 Diffuse sensors
Object presence output is OFF
The equivalent contact diagram:
T
2
1
R~
~
T
2
1
R~
~
T
2
1
R~
~
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Absence of object output is ON
The equivalent contact diagram:
7.2.2 Thru beam or Reflex sensors
Object presence output is ON
The equivalent contact diagram:
Absence of object output is
OFF
The equivalent contact diagram:
T
2
1
R~
~
T
2
1
R~
~
T
2
1
R~
~
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8. Fiber Optic
8.1 - Function
These detectors are composed by:
Remote amplifier (contain light transmitter and
receiver)
Fiber optic cable (acting as the detection point)
The light is transported between the amplifier and the detection point by the fiber optic.
Due to their size (very small), they can be placed at a narrow space.
Fiber optic sensors are suitable to detect very small objects such as screw, o-ring,
washer, capsule.
Three types of fiber optic sensors:
Thru-beam
Reflex
Diffuse
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8.2 - Principles of optical detection
The fiber acts as a light conductor. Light rays entering the fiber at a certain angle are
conveyed to the required place, with minimum loss.
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For a transmission with minimum of loss, the angle of incidence inside the fiber optic
must be smaller than the value of critical angle.
There are two types of fiber optics:
Glass fiber
Plastic fiber
8.2.1 - Glass Fiber Optic Cable
The core of the fiber is in silica. For maximum flexibility, each fiber is made of
numerous individual fibers approximately 50m in diametre.
The fibers are used with amplifiers transmitting infra red light.
Minimum bend radius:
10 mm with plastic sheath
25 mm with stainless steel sheath
There are specially made for high temperature:
90C with plastic sheath
250C with stainless steel sheath
8.2.2 - Plastic Fiber
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The core of the fiber is flexible plastic (PMMA). There is usually a single fiber,
0.25 to 1mm in diameter, depending on the model.
The fibers are used with amplifiers transmitting visible red light.
Minimum bend radius:
10 mm for fiber with 0.25mm core.
25 mm for fiber with 1mm core.
Plastic fibers are easy to use. It can be cut to required length without special
tools. They are available in different diameter, straight or spiral, standard or
deformable end. Certain reflex type has reception core multi thread for proximity
detection.
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8.3 - Application Example
Screw detection
Screw Counting
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9. Wiring
Power Supply
There are three types of power supply :
ac
dc
ac/dc
Output Types
There are two type of output:
Relay Solid state
Relay output
High switching current
Ease of setting-up
Solid state output
Long service life
High operating rates
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Electrical Wiring
2 wires
We can use either alternative current or direct current for the power supply.
There is no polarity to be respected when using alternative current.
The wiring connection is the same as a limit switch or a normal contact.
The detector is connected in series with the load.
(Eg. Signal, lamp, bell, PLC input)
It is not advisable to connect them in parallel.
It has a long live and can be used at high rate.
Electrical wiring to the input of PLC TSX 17.
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9.5.2 3-wires
We can use only direct current for the power supply.
We have to choose NPN or PNP depending on the signal processor.
It is composed of 3 wires:
o 1 wire to transmit the output signal. [Usually black (BK)]
o 2 wires for power supply.[usually brown (BN) for +ve and blue (BU) for -ve]
It is possible to connect them in parallel.
They are protected against bad connection (reverse polarity), over load and
short circuit of the load.
It has a long live and can be used at high rate.
Electrical wiring of a PNP sensor to the input of PLC TSX 17.
-
+
PowerSupply
NPN
Load
PNP
PowerSupply
+
Load
-
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Electrical wiring of a NPN sensor to the input of PLC Siemens S7 200.
9.5.3 5-wires
We can use either alternative current or direct current for the power supply.
It is composed of 5 wires:
2 wire for the power supply.
3 wire to transmit the output signal.
The wires for the power supply are isolated from the signal wires.
PowerSupply
ac/dc
Load
ac/dc
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It can support high current up to 2A.
The commutation frequency is limited up to 5Hz.
The operating cycle is limited (less than 1 million)
Electrical wiring of a 5-wires sensor to the inputs of PLC TSX 17.
24V0V 321 654 987 121110 151413 21
24V DC Inputs0.25A
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