Chapter 4 Photo Sensor

8/3/2019 Chapter 4 Photo Sensor

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INDUSTRIAL AUTOMATION SECTIONSensor Technology (FAB 10402)

Page 1Sensor Technology

Compiled by: Miss Juniza Md Saad & Mdm. Hasrine Abdullah

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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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.


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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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.


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.



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


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


Absence of object output is ON


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


T

2

1

R~

~

T

2

1

R~

~

T

2

1

R~

~


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Absence of object output is ON


7.2.2 Thru beam or Reflex sensors

Object presence output is ON


Absence of object output is

OFF


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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Chapter 4 Photo Sensor