PROXIMITY

SENSOR

NAME: KOUSTUBH CHAKRABORTY

STREAM: ELECTRONICS &

COMMUNICATION ENGINEERING

ROLL NO: 18700312067

TECHNO INDIA COLLEGE OF TECHNOLOGY

CONTENTS:

i. Definition

ii. Types of proximity sensors

iii. Inductive Proximity Sensor

iv. Capacitive Proximity Sensors

v. Ultrasonic Proximity Sensors

vi. Optical Proximity Sensor

vii. Conclusion

viii. Reference

DEFINITION:

A proximity sensor is a sensor able to detect the presence of nearby

objects without any physical contact. A proximity sensor often emits an

electromagnetic field or a beam of electromagnetic radiation (infrared,

for instance), and looks for changes in the field or return signal. The

object being sensed is often referred to as the proximity sensor's target.

Different proximity sensor targets demand different sensors. For

example, a capacitive or photoelectric sensor might be suitable for a

plastic target; an inductive proximity sensor always requires a metal

target.

Types of Proximity Sensors:

There are various types of proximity sensors. Among them four types of

sensors have wide range of applications. They are:

1. Inductive Proximity Sensors

2. Capacitive Proximity Sensors

3. Ultrasonic Proximity Sensors

4. Optical Proximity Sensors

Among these Ultrasonic Proximity Sensors are of two types: Active &

Passive. Optical Proximity Sensors are of basically of two types:

Thru beam and Reflective, in which Reflective Proximity Sensors are

divided in three types:

a) Direct Reflection

b) Reflection with reflector

c) Polarized reflection with reflector

INDUCTIVE PROXIMITY SENSORS:

Inductive proximity sensors are used for non-contact detection of

metallic objects. Their operating principle is based on a coil and

oscillator that creates an electromagnetic field in the close surroundings

of the sensing surface. The presence of a metallic object in the operating

area causes a dampening of the oscillation amplitude. The rise or fall of

such oscillation is identified by a threshold circuit that changes the

output of the sensor. The operating distance of the sensor depends on

the actuator's shape and size and is strictly linked to the nature of the

material.

COMPONENTS AND WORKING PRINCIPLE:

The main components of Inductive Proximity Sensors are coil, oscillator,

detector and the output circuit. The operating distance of the sensor

depends on object’s shape and size and strictly linked to the nature of

the material.

WORKING:

The coil generates high frequency magnetic field in front of the face.

When the metallic target comes in this magnetic field it absorbs some of

the energy. When a piece of conductive metal enters the zone defined

by the boundaries of the electromagnetic field, some of the energy of

oscillation is transferred into the metal of the target. This transferred

energy appears as tiny circulating electrical currents called eddy

currents. This is why inductive proximity sensors are sometimes called

eddy current sensors. This creates a small amount of power loss in the

form of heat (just like a little electric heater) so the amplitude (the level

or intensity) of the sensor’s oscillation decreases. Eventually, the

oscillation diminishes to the point that another internal circuit called a

Schmitt Trigger detects that the level has fallen below a pre-determined

threshold. This threshold is the level where the presence of a metal

target is definitely confirmed. Upon detection of the target by the

Schmitt Trigger, the sensor’s output is switched on.

ADVANTAGES OF INDUCTIVE PROXIMITY SENSORS:

Inductive Proximity Sensors are very accurate compared to other

technologies. They have high switching rate, and they can be used in

harsh environmental conditions.

DISADVANTAGES OF INDUCTIVE PROXIMITY SENSORS:

The only disadvantage of Inductive Proximity Sensors is that it can only

detect metallic objects. Thus its industry level applications are limited.

COMMON APPLICATIONS:

Inductive Proximity Sensors are used in metal detectors, car washes etc.

Because these sensors does not require physical contact these are useful

where access presents challenge and dirt is prevalent. Sensing range is

rarely greater than 6 cm but it has no directionality.

CAPACITIVE PROXIMITY SENSORS:

Capacitive Proximity Sensors are used for detection of metallic objects as

well as non-metallic objects (liquid, plastic, wooden material etc). These

sensors use variation of capacitance between the sensors and the

object. Variation in distance down to 1 micro inch can be measured

accurately. All targets having dielectric constant more than air can be

detected.

COMPOMENTS AND WORKING PRINCIPLE:

The main components of the Capacitive Proximity sensor Are Plate,

Oscillator, Threshold Detector and the Output Circuit.

WORKING:

Inside the sensor is a circuit that uses the supplied DC power to

generate AC, to measure the current in the internal AC circuit, and to

switch the output circuit when the amount of AC current changes. Unlike

the inductive sensor, however, the AC does not drive a coil, but instead

tries to charge a capacitor. Remember that capacitors can hold a charge

because, when one plate is charged positively, negative charges are

attracted into the other plate, thus allowing even more positive charges

to be introduced into the first plate. Unless both plates are present and

close to each other, it is very difficult to cause either plate to take on

very much charge. Only one of the required two capacitor plates is

actually built into the capacitive sensor! The AC can move current into

and out of this plate only if there is another plate nearby that can hold

the opposite charge. The target being sensed acts as the other plate. If

this object is near enough to the face of the capacitive sensor to be

affected by the charge in the sensor's internal capacitor plate, it will

respond by becoming oppositely charged near the sensor, and the

sensor will then be able to move significant current into and out of its

internal plate.

ADVANTAGES OF CAPACITIVE PROXIMITY SENSORS:

The main advantage of Capacitive Proximity Sensors is that it can detect

metallic as well as non metallic objects such as wood, liquid, plastics etc.

Also detection speed is very high and it has good stability. It uses low

power and it is less costly.

DISADVANTAGES OF CAPACITIVE PROXIMITY SENSORS:

The main disadvantages of these sensors are, they are affected by

temperate and humidity. Also these sensors are less accurate than

Inductive Proximity Sensors. These sensors are difficult to design.

COMMON APPLICATIONS:

Capacitive touch sensors are used in laptop track pads, digital displays,

mobile phones, mobile displays and many more. More and more design

engineers are selecting Capacitive Sensors for their versatility, reliability,

robustness and cost effectiveness over mechanical switches.

ULTRASONIC PROXIMITY SENSOSORS:

Ultrasonic sensors are based on measuring the properties of sound

waves with frequency above the human audible range. Systems typically

use a transducer which generates sound waves in the ultrasonic range,

above 18 kHz, generally in the range of 40 kHz, by turning electrical

energy into sound, then upon receiving the echo turn the sound waves

into electrical energy which can be measured. The Ultrasonic Sensor can

measure distances in centimeters and inches. It provides good readings

in sensing large-sized objects with hard surfaces.

COMPONENTS AND WORKING PRINCIPLE:

Active ultrasound sensors emit sound waves from quartz-crystal

transducers. The waves strike objects within the field of detection and as

long as there are no movement the waves are not disrupted. However,

when movement occurs the sound wave is disrupted and is reflected

back to the system’s receiver.

ADVANTAGES OF ULTRASONIC PROXIMITY

SENSORS:

Ultrasonic proximity sensors are not affected by atmospheric dust,

snow, rain etc. These sensors can work in adverse conditions where

application of other sensors is not easy. Sensing distance of these

sensors is more compared to inductive or capacitive proximity sensors.

DISADVANTAGES OF ULTRASONIC PROXIMITY

SENSORS:

These sensors find difficulties in sensing reflections from small, curved,

thin and soft objects.

GENERAL APPLICATIONS OF ULTRASONIC

PROXIMITY SENSORS:

Ultrasonic Thru beam sensors are used for bottle counting in drink filling

machines. The bottles pass the sensor too quickly and the gaps between

the bottles are often too small. For this reason, ultrasonic thru-beam

sensors are particularly suitable for bottle counting. Even in areas

with strong steam generation, reliable detection of bottles is guaranteed

with ultrasonic thru-beam sensors. These sensors also find application in

vehicle detection in automatic barrier systems, where proximity sensor

is used to detect any vehicle under the barrier, if a vehicle is detected

under the barrier, it automatically opens the barrier.

OPTICAL PROXIMITY SENSORS:

An optical proximity sensor offers non-contact sensing of almost any

object up to a range of 10 meters. It includes a light source, (usually an

LED in either infrared or visible light spectrum) and a detector

(photodiode). The light source generates light of a frequency that the

light sensor is best able to detect, and that is not likely to be generated

by other nearby sources. Infra-red light is used in most optical sensors.

To make the light sensing system more foolproof, most optical proximity

sensor light sources pulse the infra-red light on and off at a fixed

frequency.

COMPONENTS AND WORKING PRINCIPLE:

Optical sensors are basically of two types: Thru Beam Optical Sensors

and Reflective type Optical Sensors. Reflective type Optical sensors can

be again classified in three types: Direct reflection, Reflection with

reflector and Polarized refection with reflector.

THRU BEAM OPTICAL SENSOR:

In Thru Beam Optical Sensors the emitter and the receiver are housed

separately. Optical signal, which is pulsated by the pulse generator is

sent by emitter and is received by the receiver. Whenever there is an

object between emitter and the receiver light beam cannot pass

through, thus disrupting the optical signal, which is detected by the

sensor and it gives proper output. These sensors allow the longest range

of detection.

REFLECTIVE METHOD:

Unlike Thru Beam Sensors, in Reflective type sensors emitter and

receiver are housed together. In this case reflection occurs either from

the target or a reflector, which reflects the beam emitted by emitter,

and this reflected signal is received by receiver.

These sensors are divided in three types:

DIRECT REFLECTION:

In Direct Reflection method the light beam emitted is reflected of the

object, which is sensed by the sensor. The sensing range depends upon

the surface type and colour of the object.

REFLECTION WITH REFLECTOR:

Here the emitter and receiver is housed together at one point, but a

separate reflector is needed in another point of the system, which

reflects the light beam emitted, this reflected light beam is received by

receiver. An object is detected when it interrupts the light beam

between the sensor and reflector. These photocells allow longer sensing

distances, as the rays emitted are almost totally reflected towards the

receiver.

POLARIZED REFLECTION WITH REFLECTOR:

Often shiny objects reflect the light beam emitted by emitter in

reflection with reflector methods. Thus shiny objects are not properly

sensed. This problem is handled by using polarized reflective sensors.

Polarized reflective sensors contain polarizing filters in front of the

emitter and receiver that orient light into a single plane. These filters are

perpendicular or 90° out of phase with each other. The light beam is

polarized as it passes through the filter. When polarized light is reflected

off an object, the reflected light remains polarized. When polarized light

is reflected off a depolarizing reflector, the reflected light is depolarized.

The receiver can only detect reflected light that has been depolarized.

Therefore, the receiver cannot see (receive) light from reflective objects

that did not depolarize the light. The sensor can “see” a reflection from a

reflector, and it cannot “see” a reflection from most shiny objects.

All standard reflectors depolarize light and are suitable for polarized

reflective sensing.

ADVANTAGES OF OPTICAL PROXIMITY SENSORS:

These sensors are effective in Dusty or noisy environments, uses

focused beam, higher sensing distance compared to Inductive and

capacitive type sensors and they are immune to visible light

interference.

DISADVANTAGES OF OPTICAL PROXIMITY

SENSORS:

The main disadvantage of Optical sensors is they are costly.

APPLICATIONS OF OPTICAL PROXIMITY SENSORS:

Optical proximity sensors finds applications in lift door mechanisms,

components positioning sensing in electronic industry, security and

safety applications, counting of bottles/containers in factories etc.

CONCLUSION:

The global market for these proximity sensors is expected to grow at a

steady rate. Major industries using proximity sensors are machine

tools, woodworking machines, packaging machines and other types

of machinery.

Further applications of proximity sensors are automatic door units

such as garage doors or doors inside buildings, elevator doors or

doors inside railway coaches.

The building and automotive sector are further industries using high

volumes of proximity sensors.

REFERENCES:

http://literature.rockwellautomation.com/idc/groups/literature/docu

ments/ca/c116-ca502_-en-p.pdf

http://en.wikipedia.org/wiki/Proximity_sensor

http://www.fargocontrols.com/sensors.html

http://www.autonics.co.in/products/products_2.php?big=01&mid=0

1/01

http://www.ia.omron.com/support/guide/41/overview.html

http://www.sensorcentral.com/photoelectric/proximity01.php

http://www.engineershandbook.com/Components/proximitysensors.

htm

http://www.ab.com/en/epub/catalogs/12772/6543185/12041221/12

041223/Retroreflective-and-Polarized-Retroreflective.html