ELECTRICAL INSTRUMENTS First lecture USU

ELECTRICAL INSTRUMENTS &MEASUREMENTS

RAHMAN SONOWIJOYO

130421036

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DEPARTEMEN TEKNIK MESINFAKULTAS TEKNIK

UNIVERSITAS SUMATRA UTARAMEDAN

2013/2014

Book

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Book

Lesson Plan

Week - 1INTRODUCTION TO MEASUREMENTMeasurement units, Measurement system applications, Elements of ameasurement system, Choosing appropriate measuring instruments

Week - 1INSTRUMENT TYPES AND PERFORMANCECHARACTERISTICSReview of instrument types, Active and passive instruments, Null-typeand deflection-type instruments, Analogue and digital instruments,Indicating instruments and instruments with a signal output, Smart andnon-smart instruments, Static characteristics of instruments, Accuracyand inaccuracy(measurement uncertainty), Precision / repeatability /reproducibility , ToleranceRange or span, Linearity, Sensitivity of measurement, Threshold,Resolution, Sensitivity to disturbance, Hysteresis effects, Dead space,Dynamic characteristics of instruments, Zero order instrument, First orderinstrument, Second order instrument, Necessity for calibration

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AGENDA CHAPTER – 1 & 2• INTRODUCTION TO MEASUREMENT

– Measurement units

– Measurement system applications

– Elements of a measurement system

– Choosing appropriate measuring instruments

• INSTRUMENT TYPES AND PERFORMANCE

• Review of instrument types

– Active and passive instruments

– Null-type and deflection-type instruments

– Analogue and digital instruments

– Indicating instruments & instruments with a signal output

– Smart and non-smart instruments 5

AGENDA

• Static characteristics of instruments– Accuracy and inaccuracy (measurement uncertainty)– Precision/repeatability/reproducibility– Tolerance– Range or span– Linearity– Sensitivity of measurement– Threshold– Resolution– Sensitivity to disturbance– Hysteresis effects– Dead space

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AGENDA

• Dynamic characteristics of instruments

– Zero order instrument

– First order instrument

– Second order instrument

– Lecture Summary

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Introduction to Measurement

• Measurement techniques have been of immense importance eversince the start of human civilization, when measurements were firstneeded to regulate the transfer of goods in barter trade to ensurethat exchanges were fair.

• The massive growth in the application of computers to industrialprocess control and monitoring tasks has spawned a parallel growthin the requirement for instruments to measure, record and controlprocess variables.

• As modern production techniques require high accuracy limits sothe requirement for instruments to be both accurate and cheapbecomes ever harder to satisfy.

• In the past few years, the most cost-effective means of improvinginstrument accuracy has been found in many cases to be theinclusion of digital computing power within instrumentsthemselves.

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Measurement Units

• The first improved measurement unit was a unit of length (the metre)defined as 1/10000000 times the polar quadrant of the earth.

• A platinum bar made to this length was established as a standard of lengthin the early part of the nineteenth century.

• This was superseded by a superior quality standard bar in 1889,manufactured from a platinum–iridium alloy.

• Since that time, technological research has enabled further improvementsto be made in the standard used for defining length.

• Firstly, in 1960, a standard metre was redefined in terms of 1.65076373 x106 wavelengths of the radiation from krypton-86 in vacuum.

• More recently, in 1983, the metre was redefined yet again as the length ofpath travelled by light in an interval of 1/299 792 458 seconds.

• In a similar fashion, standard units for the measurement of other physicalquantities have been defined and progressively improved over the years.

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Measurement Units

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Fundamental Units

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Supplementary Fundamentals

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Derived Units

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Derived Units

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Derived Units

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Elements of Measurement System

• A measuring system exists to provide information about the physical valueof some variable being measured.

• In simple cases, the system can consist of only a single unit that gives anoutput reading/ signal according to the magnitude of the unknownvariable applied to it.

• However, in more complex measurement situations, a measuring systemconsists of several separate elements.

• The term measuring instrument is commonly used to describe ameasurement system, whether it contains only one or many elements.

• A measuring system normally consists of following elements:– Sensor– Variable conversion element– Signal processing unit– Signal presentation or recording/Signal Utilization unit

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Block diagram of Measurement System

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Functional Elements of an Instrument

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Typical Measurement System Architecture

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The Principles of Basic Electronic Instrument

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Instrumentation and Measurement Applications

Measurement System Applications• Applications of measuring instruments can be classified

into three major areas.• The first of these is their use in regulating trade, applying

instruments that measure physical quantities such aslength, volume and mass in terms of standard units.

• The second application area of measuring instruments is inmonitoring functions. These provide information thatenables human beings to take some prescribed actionaccordingly.

• The third application area of measurement systems isautomatic feedback control systems.

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Instrumentation and Measurement Applications

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Application Areas

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Choosing Appropriate Measuring Instruments

• The starting point in choosing the most suitable instrument touse for measurement of a particular quantity in amanufacturing plant or other system is the specification of theinstrument characteristics required, especially parameters likethe desired measurement accuracy, resolution, sensitivity anddynamic performance.

• It is also essential to know the conditions that the instrumentwill be subjected to, as some conditions will immediatelyeither eliminate the possibility of using certain types ofinstrument or else will create a requirement for expensiveprotection of the instrument.

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• It should also be noted that protection reduces theperformance of some instruments, especially in terms of theirdynamic characteristics

• A skilled instrument engineer, having knowledge of all theinstruments that are available for measuring the quantity inquestion, will be able to evaluate the possible list ofinstruments in terms of their accuracy, cost and suitability forthe environmental conditions and thus choose the mostappropriate instrument

• The instrument characteristics are the features that form thetechnical basis for a comparison between the relative meritsof different instruments.

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• Generally, the better the characteristics, the higher the cost.However, in comparing the cost and relative suitability ofdifferent instruments for a particular measurement situation,considerations of durability, maintainability and constancy ofperformance a re also very important because the instrumentchosen will often have to be capable of operating for longperiods without performance degradation and a requirementfor costly maintenance.

• In consequence of this, the initial cost of an instrument oftenhas a low weighting in the evaluation exercise.

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• So instrument choice is a compromise between performancecharacteristics, ruggedness and durability, maintenancerequirements and purchase cost.

• To carry out such an evaluation properly, the instrumentengineer must have a wide knowledge of the range ofinstruments available for measuring particular physicalquantities and must also have a deep understanding of howinstrument characteristics are affected by particularmeasurement situations and operating conditions

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INSTRUMENT TYPES AND PERFORMANCE

Review of Instrument Types

• Active and passive instruments

• Null-type and deflection-type instruments

• Analogue and digital instruments

• Indicating instruments and instruments with asignal output

• Smart and non-smart instruments

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Active and Passive Instruments

• Instruments are divided into active or passiveones according to whether the instrument outputis entirely produced by the quantity beingmeasured or whether the quantity beingmeasured simply modulates the magnitude ofsome external power source.

• In active instruments, the external power sourceis usually in electrical form, but in some cases, itcan be other forms of energy such as a pneumaticor hydraulic one.

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• One very important difference between activeand passive instruments is the level ofmeasurement resolution that can be obtained.

• In terms of cost, passive instruments are normallyof a more simple construction than active onesand are therefore cheaper to manufacture.

• Therefore, choice between active and passiveinstruments for a particular application involvescarefully balancing the measurement resolutionrequirements against cost.

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Null-Type and Deflection-Type Instruments

• The pressure gauge is a good example of adeflection type of instrument, where the value ofthe quantity being measured is displayed in termsof the amount of movement of a pointer.

• An alternative type of pressure gauge is thedeadweight gauge which is a null-typeinstrument. Here, weights are put on top of thepiston until the downward force balances thefluid pressure.

• Weights are added until the piston reaches adatum level, known as the null point.

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• Pressure measurement is made in terms ofthe value of the weights needed to reach thisnull position.

• The accuracy of these two instrumentsdepends on different things.

• For the first one it depends on the linearityand calibration of the spring, whilst for thesecond it relies on the calibration of theweights.

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• As calibration of weights is much easier thancareful choice and calibration of a linearcharacteristic spring, this means that the secondtype of instrument will normally be the moreaccurate.

• This is in accordance with the general rule thatnull-type instruments are more accurate thandeflection types.

• In terms of usage, the deflection type instrumentis clearly more convenient.

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Analogue and Digital Instruments

• An analogue instrument gives an output thatvaries continuously as the quantity beingmeasured changes. The output can have aninfinite number of values within the rangethat the instrument is designed to measure.

• A digital instrument has an output that variesin discrete steps and so can only have a finitenumber of values.

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Analogue and Digital Instruments

• The distinction between analogue and digitalinstruments has become particularly importantwith the rapid growth in the application ofmicrocomputers to automatic control systems.

• Analogue instruments must be interfaced to themicrocomputer by an analogue-to-digital (A/D)converter, which converts the analogue outputsignal from the instrument into an equivalentdigital quantity that can be read into thecomputer.

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Indicating Instruments & Instruments with a Signal Output

• Instruments can be divided is between those thatmerely give an audio or visual indication of themagnitude of the physical quantity measured andthose that give an output in the form of ameasurement signal whose magnitude isproportional to the measured quantity.

• The class of indicating instruments normallyincludes all null-type instruments and mostpassive ones.

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Indicating Instruments & Instruments with a Signal Output

• Indicators can also be further divided into thosethat have an analogue output and those thathave a digital display.

• Instruments that have a signal-type output arecommonly used as part of automatic controlsystems.

• In other circumstances, they can also be found inmeasurement systems where the outputmeasurement signal is recorded in some way forlater use.

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Smart and Non-Smart Instruments

• The advent of the microprocessor has createda new division in instruments between thosethat do incorporate a microprocessor (smart)and those that don’t.

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Performance Characteristics of Instruments

• Static Characteristics

• Dynamic Characteristics

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Static Characteristics of Instruments

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Accuracy and Inaccuracy (Measurement Uncertainty)

• The accuracy of an instrument is a measure ofhow close the output reading of the instrument isto the correct value.

• In practice, it is more usual to quote theinaccuracy figure rather than the accuracy figurefor an instrument.

• Inaccuracy is the extent to which a reading mightbe wrong, and is often quoted as a percentage ofthe full-scale (f.s.) reading of an instrument.

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• It is an important system design rule that instrumentsare chosen such that their range is appropriate to thespread of values being measured, in order that the bestpossible accuracy is maintained in instrument readings.

• The term measurement uncertainty is frequently used in place of inaccuracy.

• Example:• A pressure gauge of range 0–10 bar has a quoted

inaccuracy of 1.0% f.s then the maximum error to beexpected in any reading is 0.1 bar. This means thatwhen the instrument is reading 1.0 bar, the possibleerror is 10% of this value.

Accuracy and Inaccuracy (Measurement Uncertainty)

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Precision/Repeatability/Reproducibility

• Precision is a term that describes aninstrument’s degree of freedom from randomerrors.

• If a large number of readings are taken of thesame quantity by a high precision instrument,then the spread of readings will be very small.

• High precision does not imply anything aboutmeasurement accuracy. A high precisioninstrument may have a low accuracy.

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• Low accuracy measurements from a high precisioninstrument are normally caused by a bias in themeasurements, which is removable by recalibration.

• Repeatability describes the closeness of output readingswhen the same input is applied repetitively over a shortperiod of time, with the same measurement conditions,same instrument and observer, same location and sameconditions of use maintained throughout.

• Reproducibility describes the closeness of output readingsfor the same input when there are changes in the methodof measurement, observer, measuring instrument, location,conditions of use and time of measurement.


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• Both terms thus describe the spread of outputreadings for the same input.

• This spread is referred to as repeatability if themeasurement conditions are constant and asreproducibility if the measurement conditionsvary.

• The degree of repeatability or reproducibilityin measurements from an instrument is analternative way of expressing its precision.


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Precision/Repeatability/Reproducibility Example

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Tolerance

• Tolerance is a term that is closely related to accuracy anddefines the maximum error that is to be expected in somevalue.

• Tolerance describes the maximum deviation of amanufactured component from some specified value. –Example: One resistor chosen at random from a batchhaving a nominal value 1000W and tolerance 5% mighthave an actual value anywhere between 950W and 1050 W.

• Range or Span• The range or span of an instrument defines the minimum

and maximum values of a quantity that the instrument isdesigned to measure.

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Linearity

• It is normally desirable that the outputreading of an instrument is linearlyproportional to the quantity being measured.

• The non-linearity is then defined as themaximum deviation of any of the outputreadings from linear behavior.

• Non-linearity is usually expressed as apercentage of full-scale reading

• Example:?????????

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Sensitivity of Measurement

• The sensitivity of measurement is a measureof the change in instrument output thatoccurs when the quantity being measuredchanges by a given amount.

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• The sensitivity of measurement is thereforethe slope of the straight line drawn oninstrument response/output characteristics.

Example: A pressure of 2 bar produces adeflection of 10 degrees in a pressuretransducer, the sensitivity of the instrument is5 degrees/bar (assuming that the deflection iszero with zero pressure applied).

Sensitivity of Measurement

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Threshold

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Resolution

• When an instrument is showing a particular outputreading, there is a lower limit on the magnitude of thechange in the input measured quantity that producesan observable change in the instrument output.

• One of the major factors influencing the resolution ofan instrument is how finely its output scale is dividedinto subdivisions.

• A car speedometer has subdivisions of typically 20km/h. This means that when the needle is between thescale markings, we cannot estimate speed moreaccurately than to the nearest 5 km/h. This figure of 5km/h thus represents the resolution of the instrument.

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Sensitivity to Disturbance

• All calibrations and specifications of aninstrument are only valid under controlledconditions of temperature, pressure etc.

• These standard ambient conditions are usuallydefined in the instrument specification. Asvariations occur in the ambient temperature etc.,certain static instrument characteristics change.

• The sensitivity to disturbance is a measure of themagnitude of this change.

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• Such environmental changes affect instruments intwo main ways, known as:– Zero drift or Bias

– Sensitivity drift or scale factor drift

• Zero drift or bias describes the effect where thezero reading of an instrument is modified by achange in ambient conditions. This causes aconstant error that exists over the full range ofmeasurement of the instrument.

• Zero drift is normally removable by calibration.

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Example :

• Zero drift is also commonly found ininstruments like voltmeters that are affectedby ambient temperature changes. Typicalunits by which such zero drift is measured arevolts/°C. This is often called the zero driftcoefficient related to temperature changes.

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Hysteresis Effects

• Next Figure illustrates the output characteristic of aninstrument that exhibits hysteresis.

• If the input measured quantity to the instrument issteadily increased from a negative value, the outputreading varies in the manner shown in curve (a).

• If the input variable is then steadily decreased, theoutput varies in the manner shown in curve (b).

• The non-coincidence between these loading andunloading curves is known as hysteresis.

• Two quantities are defined, maximum input hysteresisand maximum output hysteresis.

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Hysteresis Effects

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Hysteresis Effects Examples

• Hysteresis is most commonly found ininstruments that contain springs, such as thepassive pressure gauge

• Hysteresis can also occur in instruments thatcontain electrical windings formed round aniron core, due to magnetic hysteresis in theiron.

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Dead Space

• Dead space is defined as the range of differentinput values over which there is no change inoutput value.

• Any instrument that exhibits hysteresis alsodisplays dead space. However someinstruments that do not suffer from anysignificant hysteresis can still exhibit a deadspace in their output characteristics.

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Dead Space

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Dynamic Characteristics of Instruments - Introduction

• The static X-tics of measuring instruments are concerned onlywith the steady state reading that the instrument settlesdown to, such as the accuracy of the reading etc.

• The dynamic characteristics of a measuring instrumentdescribe its behaviour between the time a measured quantitychanges value and the time when the instrument outputattains a steady value in response.

• As with static characteristics, any values for dynamiccharacteristics quoted in instrument data sheets only applywhen the instrument is used under specified environmentalconditions. Outside these calibration conditions, somevariation in the dynamic parameters can be expected.

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• The dynamic characteristics are necessary to find dynamicresponse of the instrument.

• Input varies from instant to instant, so it is dynamic.Dynamic input may be:– Periodic– Transient– Random– Periodic – varying cyclically with time or repeating at constant

interval– Transient – varying non cyclically and signal is of definite

duration, becomes zero after regular intervals– Random – varying with respect to time no definite period &

amplitude is continuous but not cyclical

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• To study dynamic characteristics it is necessary torepresent each instrument by mathematical model. Bymathematical model we get laws governing I/p ando/p.

• Dynamic characteristics are experimentally determinedby dynamic input.

• For theoretical analysis for any dynamic input , asolution of it s governing equation is obtained bymathematical models.

• Two main parameters of system response are:– Speed of response– Fidelity

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• Speed of response is defined as rapidity with which ameasurement system responds to change in measuredquantity.– Rise Time– Time Constant

• Fidelity is quantity of indication by which theinstrument as regards the change of input Or change inquantity without dynamic error

• Dynamic behavior of measurement systems is studiedin two domains:– Time domains Analysis– Frequency domain analysis

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Response/Transfer Characteristics of Instruments

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Response/Transfer Characteristics of Instruments

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Zero Order Instrument

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Zero Order Instrument

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First Order Instrument

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First Order Instrument

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Second Order Instrument

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LECTURE SUMMARY

QUESTIONS

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ELECTRICAL INSTRUMENTS First lecture USU

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