What are Sensors?

American National Standards Institute (ANSI) Definition A device which provides a usable output in response to

a specified measurand

A sensor acquires a physical parameter and converts it into a signal suitable for processing (e.g. optical, electrical, mechanical)

A transducer Microphone, Loud Speaker, Biological Senses (e.g.

touch, sight,…ect)

Detectable Phenomenon

Need for Sensors

Sensors are omnipresent. They embedded in our bodies, automobiles, airplanes, cellular telephones, radios, chemical plants, industrial plants and countless other applications.

Without the use of sensors, there would be no automation.

Choosing a Sensor

Types of Sensors

Bimetallic Switch Colour Sensors Ultrasonic Distance

Sensors Light Section

Sensors LVDT Limit Switches Photoelectric

Devices Proximity Sensors Scan Sensors Force Sensors Load Sensors

Bimetallic Switch Sensor

A bi-metallic strip is used to convert a temperature change into mechanical displacement and thus acts as a temperature sensor

It is in the form of Cantilever beam The strip consists of two strips of different

metals which expand at different rates as they are heated, usually steel and copper

The different expansions force the flat strip to bend one way if heated, and in the opposite direction if cooled below its normal temperature. The metal with the higher expansion is on the outer side of the curve when the strip is heated and on the inner side when cooled.

Selection of Bimetallic strips

High coefficient of expansion High ductility High modulus of elasticity High electrical conductivity

Applications of Bimetallic sensors

Mechanical clock mechanisms are sensitive to temperature changes which lead to errors in time keeping. A bimetallic strip is used to compensate for this in some mechanisms

In the regulation of heating and cooling, thermostats that operate over a wide range of temperatures the bi-metal strip is mechanically fixed and attached to an electrical power source while the other (moving) end carries an electrical contact.

A direct indicating dial thermometer uses a bi-metallic strip wrapped into a coil. One end of the coil is fixed to the housing of the device and the other drives an indicating needle.

Advantages

Cost is low Negligible maintenance expenses Stable operation over extended period of

time

Disadvantages

Not suitable for more than 4000c. Permanent deformation of the metallic

strip may occur.

Colour Sensors

These types of sensors are used to help the robots differentiate between colours and decide on the appropriate actions.

The image of the colour sensor is shown in the next slide. We make use of the photo sensor to differentiate between colours.

In the above pic we place 3 LEDs of green, blue & red and the photo resistor in between them.

Now each colour is switched on for a period of time one by one and the readings of the photoresistor are noted.

After getting the readings of the different colours we compare how different each reading is from the calibrated reading of every colour

This way whichever calibrated value is closest to the recorded values, indicate that the object in the front is of that colour.

This happens because we know that when alight of a particular colour is incident on the same coloured object is reflected with more intensity than when a different coloured light falls on a object. Based on this we enter the calibrated values of all the colours required into the system and when the robot comes across the recorded value it compares it with the closest value.

Ultrasonic Distance sensors Time of Flight The measured pulses typically come form

ultrasonic, RF and optical energy sources. D = v * t D = round-trip distance v = speed of wave propagation t = elapsed time

These are waves of frequency greater than 20KHz. They travel with velocity of sound. Because of shorter wave length they travel as

sharp beam. They undergo reflection.

Basic principle of operation: Emit a quick burst of ultrasound (50kHz), (human

hearing: 20Hz to 20kHz) Measure the elapsed time until the receiver indicates

that an echo is detected. Determine how far away the nearest object is from the

sensor

D = v * t

Bat, dolphin, …

D = round-trip distancev = speed of propagation(340 m/s)t = elapsed time

Applications

Obstacle avoidance Navigation Map building Underwater exploration Car Parking System Robotics & Automation

Advantages

• Can detect things that are hard to see Not affected by colour Less affected by target surfaces

• Measures discrete distances to objects

• Resistance to many disturbances : vibration infrared radiation ambient noise EMI radiation

Disadvantages Don’t work in vacuum. Poor sound travel on the Mars, the moon Time of Flight error

Time of Flight error

Light Section Sensors

Measures the level of light as a number between 0% (Total darkness) and 100% (Very bright)

Can differentiate light levels reflected from bright and dark surfaces

Inside the light sensor is a photo-transistor.

The photo-transistor acts like a valve for electricity.

The more light energy it senses, the more electricity flows.

Ohm’s Law: V = I x R

voltagemeasurement

light energy

electrical flow

voltagemeasurement

light energy

Applications

Navigation - follow a black line on a white surface (or vice versa).

FireflyBot - find a very bright object in a room or area (light bulb).

Color sorter - tell the difference between black Lego bricks and yellow Lego bricks.

Input Device - different colors or gray levels on a piece of paper.

Advantages

Energy saving Easier installation No background effect on object detection

Disadvantages

Different sensing distances and sensitivity settings are required for different objects (surface, colour)

Higher technical expertise High installation cost

LVDT

Electromechanical transducer Coupled to any type of

object/structure Converts the rectilinear motion

of an object into a corresponding electrical signal

Measures Displacement!!!!!!!! Precision of LVDT

Movements as small as a few millionths of an inch

Usually measurements are taken on the order of ±12 inches

Some LVDT’s have capabilities to measure up to ±20 inches

Applications

Automation Machinery Civil/Structural Engineering Power Generation Manufacturing Metal Stamping/Forming OEM Pulp and Paper Industrial Valves R & D and Tests Automotive Racing

Civil/Structural Engineering Examples Displacement measurement of imbedded

concrete anchors tested for tensile, compression, bending strength and crack growth in concrete

Deformation and creep of concrete wall used for retaining wall in large gas pipe installation

Dynamic measurement of fatigue in large structural components used in suspension bridges

Down-hole application: measuring displacement (creep) of bedrock

Advantages

Infinite resolution is present in LVDT High output LVDT gives High sensitivity Very good linearity Ruggedness LVDT Provides Less friction Low hysteresis LVDT gives Low power consumption.

Disadvantages

Internally non contact but externally has to be connected where the measurement has to be made

Not feasible for very long range measurements Very high displacement is required for

generating high voltages. Shielding is required since it is sensitive to

magnetic field. The performance of the transducer gets

affected by vibrations. Its is greatly affected by temperature changes.

Limit Switches

A limit switch is an electromechanical device that consists of an actuator mechanically linked to a set of contacts.

When an object comes into contact with the actuator, the device operates the contacts to make or break an electrical connection.

It can determine the presence or absence of an object. It was first used to define the limit of travel of an object; hence the name "Limit Switch."

Mechanical Advantages of Limit Switches Ease of use Simple visible operation Durable housing Well sealed for reliable operation High resistance to different ambient

conditions found in industry High repeatability Positive opening operation of contacts

(some models)

Electrical Advantages of Limit Switches

Suitable for switching higher power loads than other sensor technologies (5A at 24V DC or 10A at 120V AC typical vs. less than 1A for proximities or photoelectrics)

Immunity to electrical noise interference Immunity to radio frequency interference

(walkie-talkies) No leakage current Minimal voltage drops Simple Normally Open and/or Normally

Closed operation

Disadvantages

Shorter contact life than solid-state technology

Moving mechanical parts wear out eventually

Not all applications can use contact sensing

Applications

Conveyor systems Transfer machines Automatic turret lathes Milling and boring machines Radial drills High speed production equipment

Photoelectric Devices

A photoelectric effect is any effect in which light energy is converted to electricity.

When light strikes certain light-sensitive materials:

It may cause them to give electrons. It may change their ability to conduct electricity. It may cause them to develop an electrical

potential, or voltage across two surfaces. ( These are called photoelectric devices.)

Applications

Photomultipliers Image sensors Gold-leaf electroscope Photoelectron spectroscopy Spacecraft Moon dust Night vision devices

Advantages

Adapatble Clean Convenient Cost effective Free generation Inexhaustable Transcends National Borders No or Low Maintanence Supplemental Power Source

Disadvantages

High initial cost Low output efficiency

Proximity Sensors

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.

Types of Proximity Sensors

Inductive proximity sensors Capacitive proximity sensors Magnetic proximity sensors Photoelectric proximity sensors

Advantages

No physical contact required with the target to be detected, therefore, no moving parts so no friction and wear out.

Fast switching characteristics Unlimited number of switching cycles

since there is no mechanical contact Can work in harsh conditions Any type of target material can be

detected.

Disadvantages

It can detect only metallic targets Operating range may be limited

Applications

Parktronic, car bumpers that sense distance to nearby cars for parking

Ground proximity warning system for aviation safety Vibration measurements of rotating shafts in machinery Top dead centre (TDC)/camshaft sensor in reciprocating

engines. Sheet break sensing in paper machine. Anti-aircraft warfare Mobile phones Roller Coasters Conveyor systems Touch screens on mobile devices that come in close

proximity with the face

Force Sensors Metal foil strain-gage based (load

cell) Good in low frequency response High load rating Resolution lower than piezoelectricity-

based Rugged, typically big size, heavy

weight

Piezoelectricity based (force sensor) lower cutoff frequency at 0.01 Hz

can NOT be used for static load measurement Good in high frequency High resolution Limited operating temperature (can not be

used for high temperature applications) Compact size, light

Applications

Strain gauge must be made to react to a force. The strain gauge is attached to the member in which strain is

sensed, usually by bonding. Cannot be re-used! Special bonding agents exist for different applications and types

of materials Usually supplied by the manufacturers of strain gauges or

specialized producers. Strain gauges are often used for bending strain,

twisting (torsional and shear strain) and longitudinal tensioning/deformation (axial strain) of structures (engine shafts, bridge loading, truck weighing and many many others),

Biomechanics Traffic Monitoring

Thank you