ELECTRONIC INSTRUMENTATION(EC010704)

Reference Books• Measurement System : Ernest O Doebelin• Electronic Instrumentation : H.S Kalsi

Electronic Instrumentation• Instrumentation is a branch of engineering

that deals with the measurement and control of different parameters.

• or• Instrumentation is defined as "the art

and science of measurement and control".

• Measuring is used to monitor a process or operation

UNIT 1

Objectives of

Engineering Measurement

Objectives • At the end of this Unit

Basic measuring systemPerformance characteristics of

instrumentsErrors in measurementUnits-Dimensions Standards. Instrument calibration.

Objectives Of Engineering Measurements

1. Measurements of system parameter

information.

2. Automatic control of a system.

3. Simulation.

4. Experimental design.

5. To perform various manipulation.

6. Testing of materials and quality control.

7. Verification of scientific theories.

Basic Measuring System• A measurement assigns a specific value to a

physical variable. The physical variable now becomes the measured variable.

• A measurement system is a tool used to measure the physical variable.

• Methods of measurement can be classified in to two

• Direct methods– Un known quantity is directly compared against a

standard– Result is expressed as a numerical number

• In direct methods– In engineering application measurement systems

uses this methods

Simple measuring systemSignal conditioning element

Data trans element

Data processing element

Primary sensing element

Transducer

Data display

Transducer

Data recording

Basic Measuring System• Four Parts of Measurement System

– Sensor-Transducer Stage– Signal Conditioning Stage– Output Stage

Sensor• The sensor is a physical element that uses some

natural phenomenon to sense the variable being measured.

• The transducer changes this sensed information into a detectable signal form (electrical, mechanical, optical, etc.)

• A Transducer is a device which converts one form of energy into some other form of energy

• It is also known as 'Pickup Element'.

Sensor• Mainly Transducers can be classified into two types on

the basis of power supply required• Active Transducers• Passive Transducers.• Active transducers are those which does not requires

external power supply for their operation. • For example: Photo Voltage Cell, Piezo Electric Crystal,

Generator etc.• Passive Transducers: Passive Transducers are those

transducers which requires external power supply for their operation.

• For Example: Resistive, Inductive and Capacitive Transducers.

Signal Conditioner

• Its role comes into play when the output of transducer or primary sensing element is very low. It is used to amplify or modify the incoming signal from transducer according to output requirement.

• When noise is present in signal, filters need to be used to eliminate it.

• If the processor operates only on digital signal, A/D and D/A converters must be used at the input and output of the processor

• In other words Signal Conditioning is done to improve the quality of output of measurement system.

Signal Conditioner• This optional intermediate stage can be used to

increase

– The magnitude of the signal through amplification,

– Remove portions of the signal through some filtering technique,

– Provide mechanical or optical linkage between the transducer and the output range.

O/P stage• The output unit of a measurement system is

consists of a display and storage unit

• It is used to display or analyze the final output of the measurement system.

• The examples of Output unit can be any output device like CRO (Cathode Ray Oscilloscope) or XY recorder.

Comparison Digital Signal Analog Signal• Data Storage can be

easily done • Processing of digital

information is very easy• Will not interfere with

other signals, so less affected with Noise. Data transmission quality is good

• Repeaters are required for long distance communication

• Difficult to store the signal/information

• Processing of signal is difficult

• Will interfere with other signals, so affected with noise. Transmission quality is comparatively poor

• Repeaters are not required

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Performance characteristics of instruments

Performance characteristics of instruments

• A knowledge of the performance characteristics of an instrument is essential for selecting the most suitable instrument for specific measuring jobs.

• Performance characteristics of an instrument are mainly divided into two.

• Static characteristics

• Dynamic characteristics

STATIC CHARACTERISTICS

• The set of criteria defined for the instrument which are used to measure the quantities that are varying slowly with time or constant is called static characteristics.

• OR• The static characteristics of an instrument are

considered for instruments which are used to measure an unvarying process condition.

• Some criteria will be set to for the measurement of quantities that are either constant or vary slowly is called static characteristics

STATIC CHARACTERISTICS

• All the static performance characteristics are obtained by one form or another of a process called calibration.

• It provides a opportunity to check the instrument against a known standard and to find the errors and accuracy.

• Calibration involves comparison of an instrument with either primary standard or a secondary standard or an instrument with known accuracy

STATIC CHARACTERISTICS

• There are a number of related definitions (or characteristics) such as

• Accuracy & Precision• Sensitivity• Linearity & Hysteresis • Repeatability and Reproducibility • Resolution, • Drift,• Span • Threshold etc.

STATIC CHARACTERISTICS

• Accuracy: The degree of exactness (closeness) of a measurement compared to the expected (true) value.– It is expressed in terms of errors

• Static error = measured value – true value• Precision: A measure of the consistency or

reproducibility of measurements, i.e. successive readings does not differ. – (Precision is the consistency of the instrument output

for a given value of input).– Accuracy can be improved by calibration but not

precision

STATIC CHARACTERISTICS • Resolution: The smallest change in a measured

variable to which an instrument will respond.

• Sensitivity: The ratio of the change in output (response) of the instrument to a change of input or measured variable.

• Drift : Gradual shift in the meassured value ,over an extended period, when there is no change in input.

• Threshold: The minimum value of input for which the device just starts to respond

• Range/Span: The minimum and maximum value of quantity so that the device is capable of measuring

STATIC CHARACTERISTICS

Repeatability: A measure of how well the output returns to a given value when the same precise input is applied several times.

Or The ability of an instrument to reproduce a certain set of reading within a given accuracy.

Linearity

• Input output relationship of a device must be linear i.e, Y= mx +C• But practical systems shows small deviations from the

linear shape ( allowed within the specified limits)

Hysteresis• Input is increased from

negative value, output increases as indicated by curve 1

• Then the input is steadily decreased , output does not follow the same path , but lag by a certain value as indicated by curve 2

• The difference between the two curves is called Hysterisis

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DYNAMIC CHARACTERISTICS• The response of instruments or systems to dynamic

I/P s are also functions of time.

• Instruments rarely respond instantaneously to changes in the measured variables

• Instead, they exhibit slowness or sluggishness due to such things as mass, thermal capacitance, fluid capacitance or electric capacitance

DYNAMIC CHARACTERISTICS

The dynamic characteristics of an instrument are

• Speed of response

• Fidelity

• Time delay or lag

• Dynamic error

DYNAMIC CHARACTERISTICS

• Speed of Response:

It is the ability of a system to respond to a sudden changes in the input signal/quantity

• Fidelity:

It is the degree to which an instrument indicates the changes in the measured variable without dynamic error ( Indication of how much faithfully system responds to the changes in input).

DYNAMIC CHARACTERISTICS• Lag:

It is the retardation or delay in the response of an instrument to changes in the measured variable.

Two types : Process lag(process) and Control lag (Instrument)

• Dynamic Error: It is the difference between the true values of a

quantity changing with time and the value indicated by the instrument, if no static error is assumed.

• NOTE : The dynamic and transient behavior of the instrument is as important as the static behavior.

DYNAMIC CHARACTERISTICS

Inputs used to study characteristics of a system are Impulse signal Step Signal Ramp signal Exponential signal (sinusoidal signal)

Transient ResponseResponse exhibited by the system suddenly after an input change

Steady State responseResponse exhibited by the system at infinite time after an input change

Time Response of a System

• Peak Time Time taken to reach the maximum overshoot•Delay Time Time taken to reach 50% of the final expected value at the first timeTime constant Time required to for the output to reach 63.2% of its final value• Settling Time Time taken for the output oscillations are died out completely or diminished within the allowed limits • Rise Time Time taken by the system to reach the desired value first time in the transient stage, when the input is changed from one state to another• Over shoot

Maximum deviation of the output from input in the transient stage.Percentage of overshoot = (Max. Overshoot/ Final expected value)*100

Time Response of a System

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JOBY JOHN 32

Error

Error• Error is the difference between the true value of

the variable and the measured value.• Errors are classified as 1. Gross error /Human error (human mistakes

and instrument malfunctions)

2. Random errors (Noise/Interference)

3. Systematic errors (which may be either constant or variable)-Due to shortcoming of the instruments

Random Errors

Associated to any measurement or electronic signal we find random, non-deterministic variations as the result of different sources:

• Electronic noise (Johnson, shot,..)• Interference• Even though interference is systematic ,for

the easiness of modeling, it can be rendered as random.

All the random sources are independent.

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Gross error • Instrumentation misuse, calculation errors and other human mistakes (mistakes in reading,

recording )are the main source of Gross errors. • Gross error mainly occur due to carelessness or

lack of experience of a human being or incorrect adjustments of instruments

• These errors can be minimized by – 1.Taking great care while taking reading, recordings

and calculating results.– 2. Taking multiple readings preferably by different

persons.

Systematic errors

A constant uniform deviation in the operation of an instrument is known as systematic error.

• There are three types of systematic errors as – Instrumental errors– Environmental errors–Observational errors

Systematic Errors Instrumental errorsThese errors are mainly due to following three

reasons• Short-comings of instrumentThese are because of the mechanical structure of

the instruments eg. Friction in the bearings of various moving parts, irregular spring tensions, hysteresis, gear backlash, variation in air gap etc.

• Ellimination.– Selecting proper instrument and the transducer for

the measurement.– Recognize the effect of such errors and apply the

proper correction factors.– Calibrate the instrument carefully against standard.

Systematic errors INSTRUMENTAL ERRORS• Misuse of instrument A good instrument if used in abnormal way gives

misleading results. Poor initial adjustments, Improper zero setting, Using leads of high resistance.Elimination: Use the instrument intelligently & Correctly• Loading effects Loading effects due toImproper way of using the instrumentElimination: Use the instrument intelligently & Correctly

Systematic Errors Observational Errors Error introduced by the observer Few souces are:• Parallax error while reading the meter,• wrong scale selection, • habits of individual obsever• EliminationUse the • instrument with mirrors,• instrument with knife edge pointers, • Instrument having digital display

Systematic Errors Environmental Errors (due to the External Conditions)• The various factors : Temperature changes,

Pressure, vibratons, Thermal emf., stray capacitance, cross capacitance, effect of External fields, Aging of equipments and Frequency sensitivity of an instrument.

Elimination• Using proper correction factors and using the

instrument Catalogue• Using Temperature & Pressure control methods

etc.• Reducing the effect of dust, humidity on the

components in the instruments. • The effects of external fields can be minimized by

using the magnetic or electrostatic shields of screens.

Error due to Other Factors • Effect of the Time on Instruments

– There is a possibility of change in calibration error in the instrument with time. This may be called ageing of the instrument.

• Mechanical Error Friction between stationary and rotating parts and

residual torsion in suspension wire cause errors in instruments. So, checking should be applied. Generally, these errors may be checked from time to time.