accoustic chemical & optical transducers

8/8/2019 Accoustic Chemical & Optical Transducers

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Acoustic, Chemical &

Optical TranducersTheir Purpose & Applications

Basic concept and purpose of a Transducer has been

explained along with emphasis on Acoustic, Chemical

and Optical Transducer. Their characteristics, need and

various applications have been discussed in detail.

Arvind Kumar Sharma

August 20, 2010


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ACOUSTIC, CHEMICAL & OPTICAL TRANSDUCER

Arvind Kumar Sharma

B.Tech (IV Year)Pulp & Paper TechnologyIIT Roorkee Saharanpur Campus,Saharanpur (U.P)India 247001

INTRODUCTION

Transducers are electric or electronic devices that transform energy fromone manifestation into another. Most people, when they think of transducers,think specifically of devices that perform this transformation in order togather or transfer information, but really, anything that converts energy can

be considered a transducer.

Transducers that detect or transmit information include common items suchas microphones, Geiger meters, potentiometers, pressure sensors,thermometers, and antennae. A microphone, for example, converts soundwaves that strike its diaphragm into an analogous electrical signal that can

be transmitted over wires. A pressure sensor turns the physical force beingexerted on the sensing apparatus into an analog reading that can be easilyrepresented. While many people think of transducers as being some sort oftechnical device, once you start looking for them, you will find transducers

everywhere in your everyday life.

Most transducers have an inverse that allows for the energy to be returned toits original form. Audio cassettes, for example, are created by using atransducer to turn the electrical signal from the microphone pick-up whichin turn went through a transducer to convert the sound waves into electrical

signal into magnetic fluctuations on the tape head. These magneticfluctuations are then read and converted by another transducer in this casea stereo system to be turned back into an electrical signal, which is thenfed by wire to speakers, which act as yet another transducer to turn the

electrical signal back into audio waves.

Other transducers turn one type of energy into another form, not for thepurpose of measuring something in the external environment or to
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communicate information, but rather to make use of that energy in a more

productive manner. A light bulb, for example, one of the many transducersaround us in our day-to-day lives, converts electrical energy into visible light.

Electric motors are another common form of electromechanical transducer,converting electrical energy into kinetic energy to perform a mechanical task.The inverse of an electric motor a generator is also a transducer, turningkinetic energy into electrical energy that can then be used by other devices.

As in all energy conversions, some energy is lost when transducers operate.The efficiency of a transducer is found by comparing the total energy put intoit to the total energy coming out of the system. Some transducers are veryefficient, while others are extraordinarily inefficient. A radio antenna, forexample, acts as a transducer to turn radio frequency power into anelectromagnetic field; when operating well, this process is upwards of 80%efficient. Most electrical motors, by contrast, are well under 50% efficient,and a common light bulb, because of the amount of energy lost as heat, isless than 10% efficient.

OPTICAL TRANSDUCERS

Introduction

NEW REVOLUTION OF OPTICAL FIBER SENSORS IT IS A SPIN-OFF FROM OTHER OPTICAL TECHNOLOGIES SEEING THE POTENTIAL IN SENSING APPLICATIONS DEVELOPED

AS ITS OWN FIELD

Why Optical Transducers?

ELECTROMAGNETIC IMMUNITY ELECTRICAL ISOLATION COMPACT AND LIGHT

BOTH POINT AND DISTRIBUTED CONFIGURATION WIDE DYNAMIC RANGE AMENABLE TO MULTIPLEXING

Techniques by which the measurements are made can be broadly grouped inthree categories depending on

(a) how the sensing is accomplished,
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(b) the physical extent of the sensing, and

(c) the role of the optical fiber in the sensing process.

Means of sensing

In this category, sensors are generally based either on measuring an

intensity change in one or more light beams or on looking at phase changesin the light beams by causing them to interact or interfere with one another.Thus sensors in this category are termed either intensity sensors orinterferometric sensors. Techniques used in the case of intensity sensorsinclude light scattering (both Rayleigh and Raman), spectral transmissionchanges (i.e., simple attenuation of transmitted light due to absorption),microbending or radiative losses, reflectance changes, and changes in the

modal properties of the fiber. Interferometric sensors have beendemonstrated based upon the magneto-optic, the laser-Doppler, or theSagnac effects, to name a few

Extent of sensing

This category is based on whether sensors operate only at a single point orover a distribution of points. Thus, sensors in this category are termed either

point sensors or distributed sensors. In the case of a point sensor, thetransducer may be at the end of a fiber the sole purpose of which is to bringa light beam to and from the transducer. Examples of this sensor type areinterferometers bonded to the ends of fibers to measure temperature and

pressure. In the case of a distributed sensor, as the name implies, sensing isperformed all along the fiber length. Examples of this sensor type are fiberBragg gratings distributed along a fiber length to measure strain or

temperature.

Role of optical fiber

Further distinction is often made in the case of fiber sensors as to whethermeasurands act externally or internally to the fiber. Where the transducersare external to the fiber and the fiber merely registers and transmits thesensed quantity, the sensors are termed extrinsic sensors. Where the

sensors are embedded in or are part of the fiber -- and for this type there isoften some modification to the fiber itself -- the sensors are termed internalor intrinsic sensors. Examples of extrinsic sensors are moving gratings tosense strain, fiber-to-fiber couplers to sense displacement, and absorptioncells to sense chemistry effects. Examples of intrinsic sensors are those thatuse microbending losses in the fiber to sense strain, modified fiber claddingsto make spectroscopic measurements, and counter-propagating beams

within a fiber coil to measure rotation.


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Working Principle

LIGHT BEAM CHANGES BY THE PHENOMENA THAT IS BEINGMEASURED.

LIGHT MAY CHANGE IN ITS FIVE OPTICAL PROPERTIES i.eINTENSITY, PHASE, POLARIZATION, WAVELENGTH ANDSPECTRAL DISTRIBUTION.

Sensing Details

General Equation of light wave:

Y= EP(t)cos[t+(t)]

INTENSITY BASED SENSORS EP (t) FREQUENCY VARYING SENSORS - P(t) PHASE MODULATING SENSING- (t) POLARIZATION MODULATING FIBER SENSING


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CLASSIFICATION

Extrinsic Sensor

WHERE THE LIGHT LEAVES THE FEED OR TRANSMITTING FIBER TO BE CHANGED

BEFORE IT CONTINUES TO THE DETECTOR BY MEANS OF THE RETURN OR

RECEIVING FIBER.


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Intrinsic Sensor

INTRINSIC SENSORS ARE DIFFERENT IN THE SENSE THAT LIGHT BEAM DOES NOT

LEAVE THE OPTICAL FIBER BUT IS CHANGED WHILE STILL CONTAINED WITHIN IT.

Advantages and Disadvantages of Optical Sensors

R&D in the optical sensor field is motivated by the expectation that opticalsensors have significant advantages compared to conventional sensor types,

in terms of their properties.

Taking advantage of the capacity of optical fibers to send and receive opticalsignals over long distances, a current trend is to create networks of sensors,or sensor arrays. This avoids having to convert between electronics and

photonics separately at each sensing site, thereby reducing costs andincreasing flexibility.

A difficulty of all sensors, both optical and non optical, is interference frommultiple effects. A sensor intended to measure strain or pressure may bevery temperature-sensitive. Intense R&D over the last five years to providemeans for distinguishing between various effects has been conducted for

optical sensors. Considerable progress has been made, as will be discussedbelow.


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APPLICATIONS

MILITARY AND LAW ENFORCEMENT

THIS SENSOR ENABLES LOW LIGHT IMAGING AT TV FRAME

RATES AND ABOVE WITHOUT THE LIMITATIONS OF VACCUM TUBE

BASED SYSTEMS.

COMPRISES OF :

AMPLIFIED CCD SENSOR ANTI BLOOMING TECHNOLOGY CRYSTAL POLYMER SHUTTER

ADVANTAGES:

EXCEPTIONAL DAY LIGHT RESOLN. IMMUNE TO OVER EXPOSURE VERY HIGH CONTARAST LEVELS NO HALOING OR SCINTILLATIONS

BIOMETRICSYOUR FACE, FINGERS AND EYES IN A WHOLE NEW LIGHT


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IMAGE CAPTURE IMAGE PROCESSING FEATURE EXTRACTION FEATURE COMPARISON

CHEMICAL TRANSDUCERS

Devices used to monitor, measure, test, analyse data as generated due tochanges in a measured norm (usually concentration for chemical sensors).

Gas Sensors

Applications: Controlled combustion (automobile, industrial furnaces) Toxic and inflammable gas detection (leakages) Electronic noses for air-quality monitoring, food quality and medical

diagnosis

Sensing Principles

Electrochemical (solid electrolyte and amperometric) Catalytic combustion (hot-wire) Semiconductor (conduction)

Solid electrolyte gas sensors

Today's automobiles monitor combustion efficiency using a galvanic oxygensensor in the exhaust manifold. This sensor measures the oxygen pressure of


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the exhaust gas. The potential between two electrodes depends solely on the

ratio of the partial pressures of oxygen at each electrode, separated by anoxygen ion conductor; zirconia . The chemical reactions (electron transfer) at

each electrode are the same but in reverse of one another; at one electrodethe reduced form of the chemical particle is being oxidised (releasingelectrons) and at the other electrode the oxidised form is being reduced(accepting electrons). The voltage output of the sensor is sent in a feedbackloop to control the air/fuel mixture for optimised combustion.

TWC

Cars are equipped with a three-way catalytic converter, so-called as ithelps decrease carbon monoxide, hydrocarbon and NOx emissions using bothreduction and oxidation catalysts (such as platinum, rhodium and/orpalladium).

In order to reduce emissions, modern car engines carefully control theamount of fuel they burn. They try to keep the air-to-fuel ratio very close tothe stoichiometric point, which is the calculated ideal ratio of air to fuel,using a lambda sensor feedback. Theoretically, at this ratio, all of the fuel willbe burned using all of the oxygen in the air. For petrol engines it is about14.7:1. As engine and driving conditions change, this ratio changes as well.Sometimes it will run richer or leaner than the ideal 14.7:1.


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AMPEROMETRIC SENSORS

An oxygen cell can simply be considered as an enclosure which holds a flatPtFe tape coated with an active catalyst (Pt), the cathode, and a metalanode.

This enclosure is airtight apart from a small capillary at the top of the cellwhich allows oxygen access to the working electrode. The two electrodes areconnected, via current collectors, to the pins which protrude externally andallow the sensor to be electronically connected to an instrument.


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Universal Exhaust Gas Oxygen (UEGO) Sensor

This is the combination of an amperometric sensor and a potentiometric

sensor for use with lean-burnengines.

The potentiometric, lambda, sensor determines whether the burn is lean orrich, while the amperometric sensor determines the precise oxygen pressure.

Toxic Gas Sensors

The reactions that take place at the electrodes in a carbon monoxide sensorare:Sensing: CO + H2O CO2 + 2H

+ + 2e

Counter: O2 + 2H+ + 2e- H2O

Overall reaction is: CO + O2 CO2

Similar reactions take place for all other toxic gases that are capable of beingelectrochemically oxidised or reduced (H2S,Cl2).


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Interferences

An auxiliary electrode can assist in overcoming cross interference from othergases. Typically carbon monoxide sensors show a significant response tohydrogen which can make the accurate measurement of CO difficult whenhydrogen is present.However, using a sensor with an auxiliary electrode all of the CO and some ofthe H2 reacts on the sensing electrode leaving only H2 to react with theauxiliary electrode. Once the ratio of the responses on each electrode inknown, a H2compensated signal can be obtained by subtracting theauxiliary signal from the sensing electrode signal with an analogue circuit orusing a microprocessor with appropriate software.

Applications

Oxygen:

Typical applications include the measurement of oxygen deficiency inconfined spaces such as tunnels, mines or chemical plant or for the analysisof combustion gases in flues and chimney stacks. Amperometric oxygensensors are also used in patient monitoring.

CO:

Sensors are available for a wide range of applications, including residentialsafety, fire detection (smouldering fires), and industrial safety devices.

Toxic gases:

Personal and industrial safety. Flue gas emission monitoring.


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Catalytic combustion sensors

A pellistor element is simply a platinum wire coil, coated with a catalyticslurry of an inert base material (e.g. alumina) and a metal catalyst whichaccelerates the oxidation reaction. This type of element is known as the"sensitive" element. There are a number of catalyst materials available and

the precise type and mix is carefully chosen to optimise sensor performance.

Pellistor systems

The standard sensor consists of a matched pair of elements, typicallyreferred to as a detector and compensator (reference element). The detectorcomprises a platinum wire coil embedded within a bead of catalytic material.The compensator is similar except that the bead does not contain catalyticmaterial and as a consequence is inert.

Both elements are normally operated in a Wheatstone bridge circuit, that willproduce an output only if the resistance of the detector differs from that ofthe compensator.


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Pellistor Principles

The bridge is supplied with a constant dc voltage that heats the elements to

500-550C. A chemical reaction (oxidation) occurs when a combustible gasreaches the sensing element. This increases the temperature of the element.This T rise is transmitted to the platinum heater coil which causes anincrease in the resistance of the wire. The inert element is unaffected andthis results in an electrical imbalance in the bridge circuit and a detectableoutput signal is obtained. The output voltage level depends on the type ofthe detected gas, but shows an excellent linearity with the gas concentrationlevel.

Theoretically dV=(dRV)/4R

where :

dR = kamQ/C

dV Output voltage

R Resistance value of sensor in clean air

V Bridge supply voltage

dR Resistance value variation of the heater

k Constant

m Gas concentration

a Thermal coefficient of heater material

C Thermal capacity of sensor

Q Molecular heat of combustion of gas

Pellistor Applications

Catalytic gas sensors (pellistors) are an industry standard for the detection offlammable gas.

Catalytic sensors will oxidise most combustible vapours and as such offer atrue "explosimeter". Their sensitivity to different substances varies,depending on the combustibility of the substance.

The sensitivity of a catalytic sensor is defined as its relative sensitivity tomethane. It is thus important to identify which substances are most likely tobe present and to set the sensitivity of the finished detector in accordancewith the substance that has the lowest relative sensitivity.


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Semiconductor gas sensors

Most widely studied area of solid-state gas sensors is that based on

semiconducting oxides.

The discovery in 1953 that adsorption of a gas onto the surface of a metaloxide semiconductor produced a large change in its electrical resistancesignalled the advent of mixed metal oxide semiconductor sensor(MMOS) technology. The effect is commercially exploited for only a fewoxides due to the requirement for a unique combination of resistivity,magnitude of resistance change in gas (sensitivity) and humidity effects.Amongst the oxides which are used as MMOS sensors areZnO2, TiO2, Cr2TiO3, WO3 and SnO2.

MMOS

The resistance change is caused by a loss or a gain of surface electrons as aresult of adsorbed oxygen reacting with the target gas. If the oxide is an n-type, there is either a donation (reducing gas) or subtraction (oxidising gas)of electrons from the conduction band. The result is that n-type oxidesincrease their resistance when oxidising gases such as NO2, O3 are presentwhile reducing gases such as CO, CH4, C2H5OH lead to a reduction inresistance. The converse is true for p-type oxides, such as Cr2TiO3.MMOS sensors can be made quantitative, as the magnitude of change in

electrical resistance is a direct measure of the concentration of the target gaspresent.

Digital smells!

An example of the electronic nose is given below, where an array of 8sensors output different patterns for each gas. If the array is trainedproperly it can recognise the individual gases in mixtures (chemometrics).


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Digital Tastes-the electronic tongue

This is generally the solution analogue of the electronic nose. That is, sensors

that can monitor classes of chemicals in solution are placed in an array tooutput a pattern that is indicative of a event of interest.

What is an electronic tongue?

Best for matching complex samples with subjective endpoints such as odouror flavour.

For example, when has milk turned sour? Or, when is a batch of coffee beansoptimally roasted? When is a water sample toxic?

The array can be trained to match a set of sensor responses to a calibration

set produced by the human taste panel or olfactory panel routinely used infood science. Although these arrays are effective for pure chemicals,conventional methods are often more practical.

Areas of application

Identification of spilled chemicals. Air quality monitoring


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Quality of foods and drinks. Water and wastewater analysis. Detection and diagnosis of infections.

ACOUSTIC TRANSDUCERS

An acoustic transducer is an electrical device that coverts sound wavevibrations into mechanical or electrical energy. They have various practicalapplications, including sound recording and sound playback. A specializedmodel, called an ultrasonic acoustic transducer, can be used to measure

distance to, as well as the mass of, an object.

Common types of acoustic transducers used in sound recording includemicrophones, earphones, and guitar pickups. These create electrical energywhen moving parts inside the transducer, such as electrical plates or ribbons,are exposed to sound vibrations. The electrical energy produced inside thetransducer is sent first to an amplifier.

The amplifier then sends this energy to its final destination, usually aloudspeaker or recording device. The loudspeaker reproduces the sound at alevel that the human ear can hear. A recording device will retain theelectrical signal information. The recorder will send the stored signal to aloudspeaker during playback.

An ultrasonic acoustic transducer can be used to measure distance or themass of an object. The most common type is the piezoelectric acoustictransducer. These include a piezoelectric ceramic element that creates anddistributes ultrasonic sound waves.

Sound waves travel to an object from a piezoelectric transducer throughmaterial called a couplant. The couplant is usually water. Sound waves
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bounce off the object and return to the transducer in the form of an echo.

The time it takes for these echoes to return to the transducer is used tocalculate the distance to the object.

Underwater sound navigation and ranging (SONAR) is a common use of anultrasonic acoustic transducer. SONAR uses directional beams of soundwaves. This enables the SONAR operator to determine the direction and

distance to an object.

SONAR systems can be active or passive. An active system sends out soundwaves and listens for echoes. A passive system listens for noises made by

ships, fish, and landmasses.

An electromagnetic acoustic transducer (EMAT) is another form of ultrasonic

transducer. Instead of a ceramic element, an electro magnet is the maincomponent of an EMAT. This is a type of non-contact, or non-destructivetransducer. Unlike piezoelectric transducers, EMATs do not need a couplantto carry sound waves. Instead, two electromagnetic fields are generated to

disburse ultrasonic waves.

An electromagnetic acoustic transducer (EMAT), is a non-contactinspection device that generates an ultrasonic pulse in the part or sampleinspected, instead of the transducer. The waves reflected by the sampleinduce a varying electric current in the receiver (which can be the sameEMAT used to generate the ultrasound, or a separate receiver). This currentsignal is interpreted by software to provide clues about the internal structure

of the sample.

Any faults or cracks in a sample constitute a boundary, which results inpartial reflection of the incident ultrasonic pulse. Knowing the speed ofultrasound in the sample means that the depth of each crack can becalculated. This is done by halving the time taken between the generation ofthe pulse and the reception of the reflected signal, and multiplying by thespeed of ultrasound in the sample. Thus, using an EMAT, it is possible tobuild up a profile of the interior of a sample without having to damage or

deform it in any way.

As well as cracks in the interior, ultrasound will be reflected off the exteriorboundaries of samples, meaning that the technique can also be used tocalculate the thickness of samples. This is particularly useful when calculating

the thickness of metal pipes, as the pipe does not have to be opened up oreven empty for it to be tested. This is especially useful when dealing withpipes that are operational 24 hours a day - blockages, corrosion and otherproblems can be tested for and located without stopping the flow.
http://www.wisegeek.com/what-is-sonar.htmhttp://en.wikipedia.org/wiki/Ultrasonic_inspectionhttp://en.wikipedia.org/wiki/Ultrasoundhttp://en.wikipedia.org/wiki/Transducerhttp://en.wikipedia.org/wiki/Electric_currenthttp://en.wikipedia.org/wiki/Reflection_%28physics%29http://en.wikipedia.org/wiki/Deformation_%28engineering%29http://en.wikipedia.org/wiki/Corrosionhttp://en.wikipedia.org/wiki/Corrosionhttp://en.wikipedia.org/wiki/Deformation_%28engineering%29http://en.wikipedia.org/wiki/Reflection_%28physics%29http://en.wikipedia.org/wiki/Electric_currenthttp://en.wikipedia.org/wiki/Transducerhttp://en.wikipedia.org/wiki/Ultrasoundhttp://en.wikipedia.org/wiki/Ultrasonic_inspectionhttp://www.wisegeek.com/what-is-sonar.htm


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References:

1. Norton, Harry Handbook of Transducers, Prentice hall, 1989.2. Allocca, John, Stuart Allen, Transducers: Theory & Applications,Reston, 19843. http://www.wtec.org/loyola/opto/c6_s4.htm4. http://www.wisegeek.com/what-are-transducers.htm

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