SENSORS AND TRANSDUSERS

Nikhilesh Vaidya,

T.Y.

Instrumentation Engg.,

S.G.G.S.I.E.&T., Nanded.

SENSORS

A sensor is a device that, as its name signifies, measures a physical quantity and then converts it into signals that can be read by the user or by any other instrument.

It detects events or changes in quantities and provides a corresponding output,generally as an electrical or optical signal; for example,a thermocouple converts temperature to an output voltage. But a mercury-in-glass thermometer is also a sensor; it converts the measured temperature intoexpansion and contraction of a liquid which can be read on a calibrated glasstube.

Light,motion,temperature,magneticfields, gravity, humidity, moisture, vibration, pressure, electrical fields, sound, and other physical aspects of the externalenvironment

TRANSDUSERS

A transducer is a device that converts one form of energy to another form of energy. Energy types include electrical, mechanical, electromagnetic(including light), chemical, acoustic, and thermal energy. Usually a transducer converts a signal in one form of energy to a signal in another[1] (for example, a loudspeaker converts an electric signal to sound).

Actuator , microphones , Resistance temperature detector, Geophone & combination transducers have both functions; they both detect and create action, example is an antenna, a transducer of radio waves.

Evangelista Torricelli invented the barometer in 1643. Torricelli’s tube

1] Barometer

The pressure of the air molecules pushed the mercury up into the glass tube.

Torricelli’s barometer used a glass column suspended in a bowl of mercury.

Barometer

• He concluded that the weight of the mercury in the tube was equal to the weight of the air pressing down on the mercury in the dish.

• Water is 13.6 times less dense than mercury.

So, if we were to use water in a barometer, it would be 13.6 times taller than a mercury barometer.So it would be 10.3 meters tall!

Barometer

Airplanes use a special type of barometer, called an altimeter, to measure altitude

Higher atmospheric pressure is usually associated with fair weather and clearing skies.

Tornadoes and hurricanes can produce very low barometric readings.

Barometers

Mount Everest

Sea level On top of Mount EverestSea level

fraction

of 1 atm

average altitude

(m) (ft)

1 0 0

1/2 5,486 18,000

1/3 8,376 27,480

1/10 16,132 52,926

1/100 30,901 101,381

1/1000 48,467 159,013

1/10000 69,464 227,899

1/100000 96,282 283,076

Bimetallic strip

A bimetallic strip is used to convert a temperature change into mechanical displacement.

change in L = * Lo * (T To)

Lower temperature Higher temperature

Fire Alarm If the fire breaksout, the heat from the fire will causethe bimetallic stripsto bend upwardsand complete thecircuit. The alarmbell then ring.

Bimetallic strip & its application

Smoke Sensors/detectors

A smoke detector is a device that senses smoke, typically as an indicator of fire. Commercial and residential security devices issue a signal to a fire alarm control panel as part of a fire alarm system, while household detectors, known as smoke alarms, generally issue a local audible or visual alarm from the detector itself.

TYPES:

Optical

Ionization

Optical

Uses incandescent bulb or Light-emitting diode,a lens, and a photoelectric receiver (typically a photodiode)

Two types

Scattered light

Blocked light

Scattered Light Method

Under normal operation, the light passes the photodetector

Smoke scatters the light

Photodetector catches scattered light

http://www.smokealarmdetectors.com/images/photoelectric%20smoke%20detector.jpg

Blocked Light Method

A tight beam of light shines from source to detector

Smoke blocks path, triggering alarm

This type can span large rooms

http://www.sas.org/tcs/weeklyIssues/2004-07-16/feature1/index.html

Pros

Less Prone to false alarms

Less sensitive to steam and cooking fumes

Better at detecting smoke from smoldering fires

Cons

Slower to detect smoke than ionization method

Image courtesy of Wikimedia Commons

LED Photodetector

Ionization

A radioactive source ionizes the air between two electrodes.

An ionization smoke detector uses a radioisotope such as americium-241 to produce ionization in air; a difference due to smoke is detected and an alarm is generated. Ionization detectors are more sensitive to the flaming stage of fires than optical detectors, while optical detectors are more sensitive to fires in the early smouldering stage.

Changes in the current, due to particulate matter, trigger an alarm

http://www.smokealarmdetectors.com/images/ionization%20smoke%20detector.jpg

Pros

Detects particles too small to be visible

Cheaper than optical detector

Better at detecting smoke from hot fires

Cons

Most prone to false alarms

Ionization Chamber

Image courtesy of Wikimedia Commons

Gas detector

A gas detector is a device that detects the presence of gases in an area, often as part of a safety

system. Gas detectors can be used to detect

combustible, flammable and toxic gases, and oxygen depletion.

Gas leak detection is the process of identifying potentially hazardous gas leaks by sensors.

Common sensors include infrared point sensors, ultrasonic sensors, electrochemical gas sensors, and semiconductor sensors.

Applications of Gas Sensor:

Process control industries

Environmental monitoring

Boiler control

Fire detection

Alcohol breath tests

Detection of harmful gases in mines

Home safety

Grading of agro-products like coffee and spices

Rain sensor

A rain sensor or rain switch is a switching device activated by rainfall.

There are two main applications for rain sensors.

1. The first is a waterconservation device connected to an automatic irrigation system that causes the system to shut down in the event of rainfall. 2. The second is adevice used to protect the interior of an automobile from rain and tosupport the automatic mode of windscreen wipers

The sensor projects infrared light into the windshield at a 45degreeangle. If the glass is dry, most of this light isreflected back into the sensor by the front of the windshield. If water droplets are on the glass, they reflect the light indifferent directions thewetter the glass, the less light makes it back into the sensor.

Physics of Optical Sensor

Optical sensor works on concept of reflection and refraction.

At critical angle of incidence only reflection occurs.

Air

Glass

Air

Glass

(a) Reflection and Refraction (b) Total Internal Reflection

θr

θi θi

θi>θcritReflection

Refraction

Total Internal Reflection

Physics of Optical Sensor

Critical angle of incidence for some material combinations

Material Critical angle

Windshield Glass / Air 41.15 degree

Windshield Glass / Water 61.28 degree

Windshield Glass / Ice 59.52 degree

Windshield Glass / PVC 76.83 degree

PVC / Air 42.52 degree

Physics of Optical Sensor

Angle of incidence is kept slightly more than critical angle for Windshield material/air combination.

Presence of rain changes critical angle value.

1

2

3

Prisms

Air

IR LED IR Phototransistor

Measure area

Plastic

Ceilometer

The Cloud Height Sensor (the ceilometer), determines cloud height and levels in the atmosphere. The ceilometer uses invisible laser radiation to detect cloud levels.

The ceilometer works by transmitting a pulse of laser light into the atmosphere and sensing the light return as it is reflected back toward the ceilometer by objects in its path. By timing the interval between the transmission and reception, the height of particles (such as water droplets or ice crystals in clouds) above the ceilometer is calculated and reported to the Data Collection Package.

Ceilometer

One important use of the ceilometer is to determine cloud ceilings at airports.

Height =h = ct/2 where:

h = Height of Cloud c = Speed of Light (3 x 10^8 m/second) t = Time from Transmission to Reception

Radar gun

A radar speed gun (also radar gun and speed gun) is a device used to measure the speed of moving objects.

For things such as the measurement of bowling speeds in cricket, speed of pitched baseballs, athletes and tennis serves.

Radar gun

Mode of operation

Radar guns that operate using the X band (8 to 12 GHz) frequency range are becoming less common because they produce a strong and easily detectable beam.

V=(∆f*c)/(f*2)

The velocity of moving object is given by,

31

Doppler effect

It is most common means of measuring remote moving objects.

The police radar trap is a well known example of this technique.

The Doppler effect is a very effective and accurate means of

measuring velocity. If a narrow radio beam or ultrasonic beam

is aimed at an object the beam will be reflected back to the

source. However, if the object is moving the frequency of the

received signal differs from that of the transmitted signal.

The difference between the two frequencies being a measure of

the velocity of the moving object. The received frequency will

be higher than the transmitted frequency if the moving object is

travelling towards the receiver and lower if the object is

travelling away.

Metal detector

A Metal Detector is a device which is used to detect a metal object that is not visible to our naked eye .

It consists of an oscillator which produces an alternating current that passes through coil producing alternating magnetic field

COMPONENTS OF TYPICAL METAL DETECTOR

Stabilizer (optional) - used to keep the unit steady as you sweep it back and forth

Control box - contains the circuitry, controls , speaker, batteries and the microprocessor

Shaft - connects the control box and the coil; often adjustable so you can set it at a comfortable level for your height

Search coil - the part that actually senses the metal; also known as the "search head," "loop" or "antenna"

OPERATING PRINCIPLE

1. Transmit signal from the electronics causes transmit electrical current in transmit coil.2. Electrical current in the transmit coil causes a transmitted magnetic field.3. Transmitted magnetic field causes electrical currents to flow in metal targets (called eddy currents.)4. Eddy currents generate a magnetic field. This field is altered compared to the transmitted field.5. Receive coil detects the magnetic field generated by eddy currents as a very small voltage.6. Signal from receive coil is amplified by receive electronics, then processed to extract signal from the target, rather thansignals from other environment magnetic sources such as earth’s magnetic field.

• Principle of operation of metal detector:

The operation of a metal detector is based on the principle of electromagnetic induction.

Metal detectors contain one or more inductor coils. When metal is placed in a close proximity to a varying magnetic field (generated by the coil or coils), currents are induced in the metallic part .

These current are called eddy Currents. The eddy Currents, in turn, induce their own magnetic field (called eddy fields) .

These fields act in such a direction as to oppose that generated by the coils. The resultant field ( H applied – H eddy ) and using a specially designed electronic circuit can indicate the type of material being magnetized.

APPILICATIONS

The most common applications for metal detector include the following :

Helpful for soldiers to detect land mines.

Security in the places like airports , metro railways etc.,

Archaeology department like treasure hunting.

Construction industry.

1. A battery in the top of the metal detector activates the transmitter circuit (red) that passes

electricity down through a cable in the handle to the transmitter coil (red) at the bottom.

2. When electricity flows through the transmitter coil, it creates a magnetic field all around it.

3. If you sweep the detector above a metal object (such as this old gray spanner), the magnetic field

penetrates right through it. 4. The magnetic field makes an electric current flow inside the metal object. 5. This flowing electric current creates another magnetic field all around the object.

The magnetic field cuts through the receiver coil (blue) moving about up above it. The magnetic

field makes electricity flow around the receiver coil and up into the receiver circuit (blue) at the

top, making a loudspeaker buzz and alerting you you've found something Generally speaking, metal detectors work at a maximum depth of about 20–50cm

(8–20in).

Anemometer

An anemometer or windmeter is a device used for measuring wind speed, and is a common weather station instrument.

1) Windmill anemometers

2) Hot Wire / Film Anemometer

3) Laser Doppler anemometers

4) Sonic anemometers

Anemometer

Anemometer

Uses of Anemometers Weather stations

Wind speed helps indicate a change in weather patterns, such as an approaching storm, which is important for pilots, engineers, and climatologists.

Aerospace engineers and physicists often use anemometers. This type of anemometer is used in velocity experiments. Velocity is the measurement of the rate and direction of change in the position of an object. Laser anemometers calculate the wind speed around cars, airplanes, and spacecraft, for instance. Anemometers help engineers make these vehicles more aerodynamic.

THE ELECTROMAGNETIC SPECTRUM

Hygrometer

A hygrometer is an instrument used for measuring the moisture content in the atmosphere.

1. WORKING OF Hair tension hygrometers

Hygrometer

These devices use a human or animal hair under tension. The length of the hair changes with humidity and the length change may be magnified by a mechanism and/or indicated on a dial or scale. Whale bone may be used in place of hair.

The instrument can be made more sensitive by removing oils from the hair, such as by first soaking the hair in diethyl ether.

Hygrometer

APPLICATIONS:

Besides greenhouses and industrial spaces, hygrometers are also used in some incubators (egg), and museums. They are also used in the care of wooden musical instruments such as guitars and violins

Hygrometers are also used in the coating industry because the application of paint and other coatings may be very sensitive to humidity and dew point.

EXAMPLE:

1kg of wet wood, which contains 500g of water has a moisture content u of 50%

Reference:

http://electronics.howstuffworks.com/gadgets/other-gadgets/metal-detector1.htm

http://education.nationalgeographic.com/education/encyclopedia/anemometer/?ara=1

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

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