Electrical and Electronics Measurements and InstrumentationProf. Yousef B. Mahdy -2019-2020, Assuit...

Prof. Yousef B. Mahdy -2019-2020, Assuit University, Egypt

Electrical and Electronics

Measurementsand Instrumentation

Prof. Yousef B. Mahdy

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Introduction

Electrical measurement quantities are used for theanalysis and to check the performance of the electricaldevices. These instruments are available in analog ordigital form.

Mostly, analog meters are used in the electricallaboratory for practical studies and digital meters areused in commercial, industrial and various otherpurposes.

Electrical measuring instruments can be classified intodifferent types.


METHODS OF MEASUREMENT

The measurement methods can be classified as

–Direct comparison methods

–Indirect comparison methods


Direct measurement methods

In direct measurement methods, the unknown quantityis measured directly. Direct methods of measurementare of two types, namely, deflection methods andcomparison methods.

In deflection methods, the value of the unknownquantity is measured by the help of a measuringinstrument having a calibrated scale indicating thequantity under measurement directly, such asmeasurement of current by an ammeter.

In comparison methods, the value of the unknownquantity is determined by direct comparison with astandard of the given quantity, such as measurement ofemf by comparison with the emf of a standard cell.

Comparison methods can be classified as null methods,differential methods, etc.


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In null methods of measurement, the action of theunknown quantity upon the instrument is reduced tozero by the counter action of a known quantity of thesame kind, such as measurement of weight by abalance, measurement of resistance, capacitance, andinductance by bridge circuits.


Indirect measurement methods

In indirect measurement methods, the comparison isdone with a standard through the use of a calibratedsystem.

These methods for measurement are used in thosecases where the desired parameter to be measured isdifficult to be measured directly, but the parameter hasgot some relation with some other related parameterwhich can be easily measured.

For instance, the elimination of bacteria from some fluidis directly dependent upon its temperature. Thus, thebacteria elimination can be measured indirectly bymeasuring the temperature of the fluid.

In indirect methods of measurement, it is generalpractice to establish an empirical relation between theactual measured quantity and the desired parameter.


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The different methods of measurement are summarizedwith the help of a tree diagram in the following Figure.


MEASUREMENT SYSTEM AND ITS ELEMENTS

A measurement system may be defined as a systematicarrangement for the measurement or determination ofan unknown quantity and analysis of instrumentation.

The generalized measurement system and its differentcomponents/elements are shown in in the next slide.

The operation of a measurement system can beexplained in terms of functional elements of thesystem.

Every instrument and measurement system iscomposed of one or more of these functional elementsand each functional element is made of distinctcomponents or groups of components which performsrequired and definite steps in measurement.

The various elements are the following:


Generalised measurement system


Primary Sensing Elements

It is an element that is sensitive to the measuredvariable. The physical quantity under measurement,called the measurand ( a quantity intended to bemeasured), makes its first contact with the primarysensing element of a measurement system.

Primary sensing elements may have a non-electricalinput and output such as a spring, manometer or mayhave an electrical input and output such as a rectifier.

In case the primary sensing element has a non-electrical input and output, then it is converted into anelectrical signal by means of a transducer.

The transducer is defined as a device, which whenactuated by one form of energy, is capable ofconverting it into another form of energy.


Many a times, certain operations are to be performedon the signal before its further transmission so thatinterfering sources are removed in order that the signalmay not get distorted. The process may be linear suchas amplification, attenuation, integration,differentiation, addition and subtraction or nonlinearsuch as modulation, detection, sampling, filtering,chopping and clipping, etc. The process is called signalconditioning. So a signal conditioner follows the primarysensing element or transducer, as the case may be.

The sensing element senses the condition, state orvalue of the process variable by extracting a small partof energy from the measurand, and then produces anoutput which reflects this condition, state or value ofthe measurand.


Variable Conversion Elements

After passing through the primary sensing element, theoutput is in the form of an electrical signal, may bevoltage, current, frequency, which may or may not beaccepted to the system.

For performing the desired operation, it may benecessary to convert this output to some other suitableform while retaining the information content of theoriginal signal.

For example, if the output is in analog form and thenext step of the system accepts only in digital formthen an analog-to-digital converter will be employed.

Many instruments do not require any variableconversion unit, while some others require more thanone element.


Manipulation Elements

Sometimes it is necessary to change the signal levelwithout changing the information contained in it for theacceptance of the instrument.

The function of the variable manipulation unit is tomanipulate the signal presented to it while preservingthe original nature of the signal.

For example, an electronic amplifier converts a smalllow voltage input signal into a high voltage outputsignal.

Thus, the voltage amplifier acts as a variablemanipulation unit. Some of the instruments mayrequire this function or some of the instruments maynot.


Data Transmission Elements

The data transmission elements are required totransmit the data containing the information of thesignal from one system to another.

For example, satellites are physically separated fromthe earth where the control stations guiding theirmovement are located.


Data Presentation Elements

The function of the data presentation elements is toprovide an indication or recording in a form that can beevaluated by an unaided human sense or by acontroller.

The information regarding measurand (quantity to bemeasured) is to be conveyed to the personnel handlingthe instrument or the system for monitoring, controllingor analysis purpose. Such a device may be in the formof analog or digital format.

The simplest form of a display device is the commonpanel meter with some kind of calibrated scale andpointer.

In case the data is to be recorded, recorders likemagnetic tapes or magnetic discs may be used. Forcontrol and analysis purpose, computers may be used.


Steps of a typical measurement system

The stages of a typical measurement system aresummarized below with the help of a flow diagram inFigure shown in the next slide.


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Basic classification of measuring instruments

Mechanical instruments: They are very reliable forstatic and stable conditions. The disadvantage is theyare unable to respond rapidly to measurement ofdynamic and transient conditions.

Electrical instruments: Electrical methods of indicatingthe output of detectors are more rapid than mechanicalmethods. The electrical system normally depends upona mechanical meter movement as indicating device.

Electronic instruments: These instruments have veryfast response. For example a cathode ray oscilloscope(CRO) is capable to follow dynamic and transientchanges of the order of few nano seconds (10-9 sec).


CLASSIFICATION OF INSTRUMENTS

The classification of measuring instruments is shown inFigure shown in the next slide.


Classification of measuring instruments


Absolute instruments or Primary Instruments

These instruments gives the magnitude of quantityunder measurement in terms of physical constants ofthe instrument e.g. Tangent Galvanometer. Theseinstruments do not require comparison with any otherstandard instrument

These instruments give the value of the electricalquantity in terms of absolute quantities (or someconstants) of the instruments and their deflections.

In this type of instruments no calibration or comparisonwith other instruments is necessary.

They are generally not used in laboratories and areseldom used in practice by electricians and engineers.They are mostly used as means of standardmeasurements and are maintained lay nationallaboratories and similar institutions.


Some of the examples of absolute instruments are:

–Tangent galvanometer

–Raleigh current balance

–Absolute electrometer


Secondary instruments

These instruments are so constructed that the quantitybeing measured can only be determined by the outputindicated by the instrument. These instruments arecalibrated by comparison with an absolute instrumentor another secondary instrument, which has alreadybeen calibrated against an absolute instrument.

Working with absolute instruments for routine work istime consuming since every time a measurement ismade, it takes a lot of time to compute the magnitudeof quantity under measurement. Therefore secondaryinstruments are most commonly used:

They are direct reading instruments. The quantity to bemeasured by these instruments can be determinedfrom the deflection of the instruments.

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They are often calibrated by comparing them witheither some absolute instruments or with those whichhave already been calibrated.

The deflections obtained with secondary instrumentswill be meaningless while it is not calibrated.

These instruments are used in general for all laboratorypurposes.

Some of the very widely used secondary instrumentsare: ammeters, voltmeter, wattmeter, energy meter(watt-hour meter), ampere-hour meters etc.

Secondary instruments are further classified as

–Indicating instruments

–Integrating instruments

–Recording instruments


Indicating Instruments

Indicating instruments are those which indicate themagnitude of an electrical quantity at the time when itis being measured.

The indications are given by a pointer moving over acalibrated (pre graduated) scale.

Ordinary ammeters, voltmeters, wattmeters, frequencymeters, power factor meters, etc., fall into thiscategory.


Integrating Instruments

Integrating instruments are those which measure thetotal amount of either quantity of electricity (ampere-hours) or electrical energy supplied over a period oftime.

The summation, given by such an instrument, is theproduct of time and an electrical quantity undermeasurement.

The ampere-hour meters and energy meters fall in thisclass.


Recording Instruments

Recording instruments are those which keep acontinuous record of the variation of the magnitude ofan electrical quantity to be observed over a definiteperiod of time.

In such instruments, the moving system carries aninked pen which touches lightly a sheet of paperwrapped over a drum moving with uniform slow motionin a direction perpendicular to that of the direction ofthe pointer.

Thus, a curve is traced which shows the variations inthe magnitude of the electrical quantity underobservation over a definite period of time.

Such instruments are generally used in powerhouseswhere the current, voltage, power, etc., are to bemaintained within certain acceptable limit.


Analog and Digital Instruments

Analog signals are those which vary in a continuousfashion and take on infinity of values in any givenrange. The devices which produce these signals arecalled analog devices.

Often such a signal is a measured response to changesin physical phenomena, such as temperature, pressure,sound or position and is achieved using a transducer.

An analog signal is one where at each point in time thevalue of the signal is significant.

For example, in sound recording, fluctuations in airpressure (that is to say, sound) strike the diaphragm ofa microphone which induces corresponding fluctuationsin the current produced by a coil in an electromagneticmicrophone, or the voltage produced by a condensermicrophone.


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Digital Signal: In contrast the analog signals whichvary in a continuous fashion and take on infinity ofvalues in any given range, the digital signals vary indiscrete steps and thus take up only finite differentvalues in a given range. The devices that produce suchsignals are called digital devices.

Analog and Digital signals are presented in the Figure inthe next slides.

In an analog system, the function varies continuously.On the other hand, the digital values are discrete andvary in equal steps.

The figure below illustrates how both an analog voltageand a digital voltage vary with time.


Analog signal


Digital signal


Analog Versus Digital Modes

An analog signal is one where at each point in time thevalue of the signal is significant, where as a digitalsignal is one where at each point in time, the value ofthe signal must be above or below some discretethreshold.

The display of the quantity to be measured in analoginstruments is in terms of deflection of a pointer, whereas digital instruments indicate the value to bemeasured in terms of decimal number.

The main advantage of the analog signal is its finedefinition which has the potential for an infinite amountof signal resolution. Compared to digital signals, analogsignals are of higher density.


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Another advantage with analog signals is that theirprocessing may be achieved more simply than with thedigital equivalent.

An analog signal may be processed directly by analogcomponents, though some processes aren't availableexcept in digital form.

The analog instruments are less costly and simple indesign as compared to their digital counter parts.

The primary disadvantage of analog signaling is thatany system may have noise, that is, random unwantedvariation. As the signal is copied and re-copied, ortransmitted over long distances, these apparentlyrandom variations become dominant.


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The effects of noise create signal loss and distortion.This is impossible to recover, since amplifying the signalto recover attenuated parts of the signal amplifies thenoise (distortion/interference) as well. Even if theresolution of an analog signal is higher than acomparable digital signal, the difference can beovershadowed by the noise in the signal.

The digital devices have high speed and they alsoeliminate the human error. With increasing use ofdigital computers for data handling and automaticprocess control, the importance of digitalinstrumentation is increasing.

It has become necessary to have both analog to digitalconverter at input to the computers and digital toanalog converters at the output of the computers.


Analog to digital conversion of signal In order to convert an analog quantity into a digital

number, the vertical displacements in the Fig. of the nextslide are divided into equal parts.

If we divide the vertical quantities into 10 equal parts witheach part having a length of 1 unit.

While dealing with digital numbers, a quantity between 0to 0.5 are considered as 0, while a quantity between 0.5to 1.5 is 1 and similarly a quantity between 1.5 to 2.5 is2.

It is apparent that if we adopt digital system, the errorswill be involved. But if we further divide each of the stepsinto 2 equal parts, we get 20 steps instead of 10. And ifthese 20 steps are further divided into 2 parts each, wewill have 40 steps. By doing this we can get much betteraccuracy in converting analogue quantities into digitalnumbers.


Analog to digital conversion of signal


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We can go on subdividing each step further and further,till the desired accuracy is achieved.

However, it should be remembered that a digitalnumber is still a sum of equal units. And in a digitalsystem, magnitudes lying within one of these steps losetheir identity and are all defined by the same number.

For example, if we have ten steps, all the numberslying between 2.5 to 3.5, that is, 2.6, 2.7, 2.8, 2.9, 3.0,3.1, 3.2, 3.3, 3.4, would all be read as 3.

From the above discussion we concluded that thedifference between analogue and digital information isthat the analogue output is a continuous function whilethe digital output is a discrete number of units. The lastdigit of any digital number is rounded to 0.5 of the lastdigit.


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It should also be marked that the magnitude of thedigital quantity is measured only at the instant thereading is taken. One reading persists till anotherreading is taken (unlike the analogue quantity which isa continuous function).


Analog Instruments

The signals of an analog unit vary in a continuousfashion and can take on infinite number of values in agiven range. Fuel gauge, ammeter and voltmeters,wrist watch, speedometer fall in this category.


Digital Instruments

Signals varying in discrete steps and taking on a finitenumber of different values in a given range are digitalsignals and the corresponding instruments are of digitaltype.

Digital instruments have some advantages over analogmeters, in that they have high accuracy and high speedof operation. It eliminates the human operationalerrors.

Digital instruments can store the result for futurepurposes. A digital multimeter is the example of adigital instrument.


Mechanical, Electrical and Electronics Instruments

1. Mechanical Instruments:

Mechanical instruments are very reliable for static andstable conditions. They are unable to respond rapidly tothe measurement of dynamic and transient conditionsdue to the fact that they have moving parts that arerigid, heavy and bulky and consequently have a largemass. Mass presents inertia problems and hence theseinstruments cannot faithfully follow the rapid changeswhich are involved in dynamic instruments. Also, mostof the mechanical instruments causes noise pollution.


1 Advantages of Mechanical Instruments

–Relatively cheaper in cost

–More durable due to rugged construction

–Simple in design and easy to use

–No external power supply required for operation

–Reliable and accurate for measurement of stableand time invariant quantity

Disadvantages of Mechanical Instruments

–Poor frequency response to transient and dynamicmeasurements

–Large force required to overcome mechanicalfriction

–Incompatible when remote indication and controlneeded

–Cause noise pollution


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2. Electrical Instruments:

When the instrument pointer deflection is caused by theaction of some electrical methods then it is called anelectrical instrument.

The time of operation of an electrical instrument ismore rapid than that of a mechanical instrument.

Unfortunately, an electrical system normally dependsupon a mechanical measurement as an indicatingdevice. This mechanical movement has some inertiadue to which the frequency response of theseinstruments is poor.


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3. Electronic Instruments:

Electronic instruments use semiconductor devices. Mostof the scientific and industrial instrumentations requirevery fast responses. Such requirements cannot be metwith by mechanical and electrical instruments.

In electronic devices, since the only movement involvedis that of electrons, the response time is extremelysmall owing to very small inertia of the electrons. Withthe use of electronic devices, a very weak signal can bedetected by using pre-amplifiers and amplifiers.


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Advantages of Electrical/Electronic Instruments:

–Non-contact measurements are possible

–These instruments consume less power

–Compact in size and more reliable in operation

–Greater flexibility

–Good frequency and transient response

–Remote indication and recording possible

–Amplification produced greater than that producedin mechanical instruments


Manual and Automatic Instruments

In case of manual instruments, the service of anoperator is required. For example, measurement oftemperature by a resistance thermometer incorporatinga Wheatstone bridge in its circuit, an operator isrequired to indicate the temperature being measured.

In an automatic type of instrument, no operator isrequired all the time. For example,measurement oftemperature by mercury-in-glass thermometer.


Self-operated and Power-operated Instruments

A self operated instrument does not require anyexternal power source for its operation. In suchinstruments the output energy is supplied by the inputsignal e.g. a dial indicator or mercury-in-glass typethermometer.

In power operated instruments some auxiliary powersource is required for its operation. This external powersource could be electricity, compressed air etc. In suchcases the input signal supplies only the insignificantportion of the output power e.g. an electro-mechanicalmeasurement system.

Electromechanical instruments shown in the next slidefall in this category.


Electromechanical measurement system


Self Contained and Remote Indicating Instruments

A self contained instrument has all the physicalelements in one assembly e.g. an analog ammeter or amercury-in-glass thermometer etc. Whereas, in aremote indicating instrument has primary sensoryelement and the secondary indicating element arelocated at two different locations linked by transmittingelement. These locations could be long distance apart.In modern instrumentation technology such type ofarrangement is quite necessary and vogue.


Contact Type and Non-Contact Type Instruments

In contact type instruments the sensing element of theinstrument contacts the control medium for themeasurements, for example mercury-in- glassthermometer. Where as in non-contact typeinstruments the sensor does not contact the controlmedium. The non-contact type measurement includesoptical, radioactive or radiation measurements. Suchas, radiation or optical pyrometer, non-touchtachometer etc.


Deflection and Null Output Instruments In a deflection-type instrument, the deflection of the

instrument indicates the measurement of the unknownquantity. The measurand quantity produces some physicaleffect which deflects or produces a mechanicaldisplacement in the moving system of the instrument.

An opposite effect is built in the instrument which opposesthe deflection or the mechanical displacement of themoving system. The balance is achieved when opposingeffect equals the actuating cause producing the deflectionor the mechanical displacement.

The deflection or the mechanical displacement at thispoint gives the value of the unknown input quantity.These type of instruments are suited for measurementunder dynamic condition. Permanent Magnet Moving Coil(PMMC), Moving Iron (MI), etc., type instruments areexamples of this category.


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In null-type instruments, a zero or null indication leadsto determination of the magnitude of the measurandquantity. The null condition depends upon some otherknown conditions. These are more accurate and highlysensitive as compared to deflection-type instruments. Adc potentiometer is a null- type instrument.


Objective-type Questions


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