Measurement & Control -...

Measurement & Control

MAC 17528

Program Outcomes (POs) PO 1. Basic knowledge: Apply basic mathematics, sciences and basic engineering knowledge PO 2. Discipline knowledge: Apply Mechanical engineering knowledge PO 3. Experiments and practice: Plan to perform experiments and practices to use the results to solve broad-based Mechanical engineering problems. PO 4. Engineering tools: Apply relevant Mechanical technologies and tools knowing their limitations. PO 5. The engineer and society: Assess societal, health, safety, legal and cultural issues and the consequent responsibilities relevant to practice in field of Mechanical engineering.

Measurement & Control

MAC 17528

Program Outcomes (POs) PO 6. Environment and sustainability: Apply Mechanical engineering solutions also for sustainable development practices in societal and environmental contexts. PO 7. Ethics: Apply ethical principles for commitment to professional ethics, responsibilities and norms of the practice also in the field of Mechanical engineering. PO 8. Individual and team work: Function effectively as a leader and team member in diverse/ multidisciplinary teams. PO 9. Communication: Communicate effectively in oral and written form. PO 10. Life-long learning: Engage in independent and life-long learning activities in the context of technological changes also in the Mechanical engineering and allied industry.

Measurement Introduction & Significance

Course Outcome: The students will be able to understand Terminology related to

measurement, various types of errors& concept of transducers.

18 Marks

MAC 17528 MAEER's MIT Polytechnic, Pune 4

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I n t r o d u c t i o n • Whatever exists, exists in some amount.

• The process or the act of measurement consists of obtaining a quantitative comparison between a predefined standard & a measurand.

• Standard

• Measurand

• Character should be same.

• Prescribed & defined by a legal or recognised organisation or agency like ISO or IBS.

MAC 17528 6 MAEER's MIT Polytechnic, Pune

Requirement of Meaningful Measurement

• Accurately defined & commonly accepted standard.

• Character should be same.

• The apparatus & method should be provable.


Measurand

(Unknown)

Result

(Readout)

Standard (Known)

Process of Comparison (measurement)

Significance & Application of Mechanical Measurement • Significance.

• Fundamental basis for all research, design & development.

• Monitoring & Process Control.

• Product Quality & Process Efficiency.

• Control Resources.

• Application. • Reaserch & Development Activities. • Automatic Control System. • Process Plants. • Testing. • Production, Sale, Purchase.


Fundamental Units & Measurement • Length

• Mass

• Time

• Electrical Current

• Temperature

• Luminous Intensity

• Matter


Fundamental Units & Measurement


Mechanical Measurement • Mechanics Type: Design tool for experiment, research & development

Empirical: Based on intuition & good engineering judgment

Rational: Based on scientific laws & relationship

Experimental: Based on correlated measurement of quantities involved

• Power Type: Tool for monitoring / operational measurement

Application: Automation & Control


Mechanical Measurement • Mechanics Type: Design tool for experiment, research & development

Empirical: Based on intuition & good engineering judgment

Rational: Based on scientific laws & relationship

Experimental: Based on correlated measurement of quantities involved

• Power Type: Tool for monitoring / operational measurement

Application: Automation & Control


Methods of Measurement • Direct Method

Measurand is directly compared with known standard.

Result is expressed in number followed by unit of measurement.

• Example: Steel Rule, Weight Balance, Tachometer

• Advantage: Simple, Inexpensive, Quick.

• Disadvantage: Inaccurate, Not applicable everywhere, Limitations due to human capability.


Methods of Measurement • Indirect Method

Value of Measurand is determined by indirect comparison.

Comparison is done with secondary standard through calibration.

Measurand is converted into analogous signal.

Saves primary & secondary standards from frequent & direct handling.


Methods of Measurement • Primary Measurement:

Doesn’t involve translation of unknown quantity.

Value of Measurand is determined by comparison with reference standards.

Information is obtained through sight or touch. • Example: Steel Bar, Colour Cards, Beam Balance


Methods of Measurement • Secondary Measurement: Involve only one translation of unknown quantity.

• Example: Thermometer, Bellow Pressure gauge


Methods of Measurement • Tertiary Measurement: Involve two translations.

• Example: Thermocoulple, Bourdan pressure gauge,


Methods of Measurement • Tertiary Measurement: Involve two translations.

• Example: Thermocoulple, Bourdan pressure gauge,


Generalized Measurement System

• Principal function of an instrument

1. Acquisition of information by sensing

2. Processing

3. Presentation

• For the purpose of analysis instruments are considered as system • System: Assembly of interconnected components organized to perform a specified function.

• These components are called as elements & perform certain definite functions.



• Elements of a Measurement System:

• Primary Sensor

• Signal Conditioning

• Readout



• Elements of a Measurement System:

• Primary Sensor

• Signal Conditioning

• Readout


Primary Sensor

Variable Conversion

Element

Data Transmission

Element

Variable Manipulation

Element

Data Presentation

Element

Bourdan Tube Link Gearing Pointer & Dial

Pressure

Force

Displacement

Errors

• Gross Error • Personal error: Errors due to gross blunders on the part of experimenter.

• Observational Error: Mistake in using instruments, recording data and calculating measurement results.

• Parallax Error

• Operational Error: Improper method of operation

• Random Error • An error that results from unpredictable variations from one or more

influenced quantities.

• Random errors are inconsistent and easily recognizable.


Error in the measurement of a physical quantity is its deviation from actual value.

Errors

• Systematic Error • These are inherrent to the apparatus.

• Constructional Error

• Errors in Reading / Observation

• Construction of the scale

• Pointer

• Parallax

• Observer

• Signal Transmission Error

• Environmental Error

• Temperature Variation, Effect of time, Electric & magnetic field


Error in the measurement of a physical quantity is its deviation from actual value.

Examples of Errors


1. A Vernier Caliper with main scale zero not matching with Vernier scale zero. 2. Pressure gauge is not connected to its flow path. 3. Air condition of test lab is not working. 4. Speed of pulley is reduced when tachometer is pressed against it. 5. Partly damaged transmission cables touch each other. 6. Operator anticipates a certain output & takes reading in a hurry.

1. Systematic Error ( Constructional)

2. Gross Error (Personal ) 3. Systematic Error ( Environmental) 4. Gross Error (Operational) 5. Systematic Error ( Transmission) 6. Gross Error ( Observational)

Examples of Errors


1. A The magnitude of a known voltage source of 100 V was measured with a voltmeter. Five readings were taken i.e. 101, 100, 102, 100 and 99.

2. The temperature of a hot fluid is 200oC. A glass bulb thermometer is used to measure the same for 5 times. The temp. indicated in each case is 180o C.

3. 5 students were asked to take the readings of a pressure gage. The readings noted by them were 1.5 N/m2 , 1.51 N/m2 , 1.49 N/m2 , 1.48 N/m2 and 1.5 N/m2

4. Due to fluctuation of the voltage source, the pointer of the voltmeter indicates maximum and minimum readings of 230 and 220 volts respectively but the reading taken by the experimenter is 203 V 1. Random Error

2. Systematic Error 3. Random Error 4. Gross Error

Instrument • Device which senses a physical parameter, process & translates it into

a format which can be understood by the observer.

• Mechanical

• Reliable for static & stable condition

• Not suitable for dynamic & transient condition

• Electrical

• Output of detector are more rapid

• Indicating device is mechanical meter

• Electronic

• Rapid response as no moving parts

• Semiconductor devices are used


Instrument • Device which senses a physical parameter, process & translates it into

a format which can be understood by the observer.

• Basic Requirments

• Ruggedness

• Linearity

• Repeatability

• Convenient Instrumentation

• High Stability & Reliability

• Low Intial & Maintenance Cost

• Safe Operation

• Functions • Indicating

• Recording

• Controlling


Classification of Instrument

• Method of Contact

• Contact

• Non Contact

• Mode of Operation

• Manual

• Automatic

• Output Display

• Null Type

• Deflection Type

• Arrangment of Physical Part

• Self Contended

• Remote Indicated

• Output Signal

• Analog Instrument

• Digital Instrument

• Source of Energy

• Self Operated (Active)

• Power Operated (Passive)


Classification of Instrument Contact & Non-Contact Instrument


Classification of Instrument Manual & Automatic Instruments


Classification of Instrument Null & Deflection Type Instruments


Classification of Instrument Self Contended & Remote Indicated Instruments


Classification of Instrument Analog & Digital Instruments


Classification of Instrument Active & Passive Instruments


Characteristics of Measurement Static Characteristics

Relnship between i/p & o/p when measurand is K or vary slowly with time.

Accuracy, Precision, Sensitivity, Reliability, Range, Span, Hystersis, Backlash, Drift, Sensitivity, Threshold, Resolution, Linearity, Reproducibility, Repeatability.

Dynamic Characteristics

Relnship between i/p & o/p when measurand varies rapidly with time.

Speed of Response, Measuring Lag, Dead Time, Dead Zone, Fidelity, Overshoot.


Static Characteristics of Measurement Range

The region between the limits within which an instument is designed to operate for measuring, indicating or recording a physical quantity.

Span The algebric difference between the upper & lower limits of instrument.


Range

00 Km/h to 260 Km/h

Span 260 - 0= 260 Km/h

Static Characteristics of Measurement Accuracy It is a measure of how close the output reading of the instrument is to the

correct value. (Degree of closeness / Confirmity / )

Precision It is a measure of repeatability of several number of independent measurement of desired input at the same reference condition.


Static Characteristics of Measurement Accuracy v/s Precision


Parameter Accuracy Precision

1 Definition Degree of correctness Degree of repeatability

2 Dependency Dependent on Systematic Errors Dependent on Random Errors

3 Method to Calculate Determined by proper calibration Determined by statistical analysis

4 Relation Accurate measurements are precise

Precise measurements are not always precise

Static Characteristics of Measurement

Reliability

It is the possibility that instrument will perform its assigned functions for a specific period of time under given conditions.

Repeatability

Closeness of output when the same input is applied repetitively

over a short period of time.

Same measurement condition, instruments, location, observer, location, time

Reproducibility

Closeness of output when the same input is applied repetitively

over a short period of time.

Different measurement condition, instruments, location, observer, location


Static Characteristics of Measurement Repeatability v/s Reproducibility


Repeatability Reproducibility

1 Variation in measurement taken by single observer with same instrument of same item under same condition.

Variation in measurement taken of same item with same instrument by different observer under different condition.

2 Conditions are unchanged Conditions are allowed to change within specific limits.

3 Standard deviation as part of precision Standard deviation depends on reliability

4 It refers condition of stability It refers condition of consistency

Static Characteristics of Measurement Sensitivity

The sensitivity of measurement is a measure of the change in instrument output that occurs when the quantity being measured changes by a given amount.


deflection producing measurand of Value

Deflection ScaleySensitivit

Element 1 Element 2 Element 3 X1 X2 X3 X4

K1 K2 K3

Overall Sensitivity= K1 . K2 . K3



Static Characteristics of Measurement Hysterisis

The maximum difference in output for the same measurand value within the transducer's range, one obtained by increasing from zero and the other by decreasing from a higher value of a measurand.


Causes IInd Law of Thermodynamics

Mechanical friction

Static Characteristics of Measurement Backlash (Dead Zone/ Lash/ Play)

• The range of different input values over which there is no change in output value.

• Maxm distance or angle through which any part of a mechanical system may be

moved in one direction without applying appreciable force or motion to the next part in a mechanical system.


Static Characteristics of Measurement Drift

• The undesired change / gradual variation in o/p over a period of time that is

unrelated to change in i/p & operating conditions.

• Changes start to occur after specified start up period.

Causes

Wear & tear of contact part

High stress developed in primary sensing element.

Corrosion & erosion of metal parts

Mechanical vibration

Stray electrostatic & magnetic fields


Static Characteristics of Measurement Zero Drift

• The whole calculation gradually drifts due to slippage or due to undue warming up of electronic circuit. Exp. Weight Scale

• Changes start to occur after specified start up period.

Sensitivity Drift (scale factor drift)

• It is the amount by which an instrument’s sensitivity of measurement varies as ambient conditions change.




Static Characteristics of Measurement Threshold

• Minimum value of input necessary to cause a detectable change from zero output.

• It starts from zero value.

Resolution (Discrimination)

• Smallest increment in input which is detected by an instrument.

• It starts from non zero value.


Static Characteristics of Measurement Linearity

• Ability of instrument to give output linearly proportional to input.


Dynamic Characteristics of Measurement

Speed of Responce

• Rapidity with which an instrument respond to a change in the value of measurand.

Measuring Lag

• Retardation or delay in the response of a measuring system to change in measured quantity.


Dynamic Characteristics of Measurement Dead Zone

• Range of values of a measured variable to which instrument does not respond.

• Causes: Friction, Backlash, Hystersis.

Dead Time

• Time required by the measurement system to begin

to respond to a change in the measurand.

• Causes: Friction, Backlash, Hystersis.


Dynamic Characteristics of Measurement Fidelity

• Degree of closeness with which system indicates/record the signal impressed on it.

Overshoot

• Maximum amount by which pointer moves beyond steady state.


Transducers Elements of Measurment System

• Sensing (Input), Signal Conditioning, Readout (Output).

Transducer

Device which when actuated transform energy from one form to other.

Device which converts non electrical quantity into proportional electrical quantity.

Sensing Element ( Detector)

Transductional Element

Photoconductor ( Light Resistance)

Thermocouple ( Temperature Voltage)

Strain Gauge ( Force Resistance)


Transducers Mechanical

• Elastic ( Springs, Bourdan tube) • Thermal ( Thermometer, Bimetal) • Hydro-Pneumatic ( Hydrometer, Venturi, Orifice Plate)

Electrical • Passive Resistance ( RTD, Thermistor) Capacitive ( Liquid Level Measurement, Microphone) Inductive ( LVDT, Inductive Pick Up) Voltage & Current ( Photo Emmissive Cell) • Active Thermocouple Photo Voltaic Piezoelectric Pick up


Transducers Primary transducers

• Converts one physical parameter from one form to another.

Secondary transducers • Converts physical parameter into suitable electrical signal.


Transducers Primary transducers & Secondary transducers


Comparison Transducers Primary Transducer v/s Secondary Transducer


Primary Transducer Secondary Transducer

1 First element in the measurement system Second element in the measurement system

2 In direct contact with measurand Not in direct contact with measurand

3 Working doesn’t depends on secondary . Working depends on primary transducer.

4 Mostly mechanical devices Mostly electrical devices

5 Exp: Bourdan Tube LVDT (Linear Voltage Differential Transformer)

Mechanical Transducers


Contacting Spindle, Pin

Proving Ring, Spring

Bourdan Tube, Bellows, Diaphragm

Manometer

Thermocouple

Bimaterial

Float

Hydrometer

Orifice, Venturi, Pitot Tube

Vanes

Turbines

Displacement Displacement

Force Displacement

Pressure Displacement

Pressure Displacement

Temperature Electric Current

Temperature Displacement

Fluid Level Displacement

Specific Gravity Displacement

Velocity Pressure

Velocity Force

Linear Velocity Angular Velocity

Electrical Transducers


Sensing device which converts any physical or mechanical parameter into an analogus or proportional electric voltage or current.

Advantage: 1. Electrical amplication & attenuation is easy. 2. Mass inertia effects are minimized. 3. Effect of friction is minimised. 4. Power requirement is low. 5. Telemetry. 6. Electrical output can be easily used, transmitted & processed . 7. Can be fed to number of instrument.

Active Transducers


Sensing device which doesn’t need external power source for its operation. Work on energy conversion principle. Energy required for production of output is obtained from measurand. Convert external energy into proportional electrical energy.

Exp: 1. Thermocouples ( temperature to voltage) 2. Piezo electric crystal ( force to voltage)

Passive Transducers


Sensing device which need external power source for its operation. Work on energy contol principle. Measurand converted into passive parameters (Resistance, Inductance,

Capacitance).

Exp: 1. Strain Gauge ( force to voltage) 2. Linear Variable Differential Transformer ( force to voltage)

Types: 1. Resistive 2.Inductive 3. Capacitive 4. Piezo resistive 5. Thermo resistive

Comparison of Transducers


Active Transducer Passive Transducer

1 Energy conversion principle Energy control principle

2 External power supply not required

External power supply is essential

3 Exp: Thermoelectric transducer, Piezoelectri transducer, Photovoltaic transducer

Exp: Resistive transducer, Inductive transducer, Capacitive transducer

Resistive Transducers


Resistance changes with change in physical phenomenon. Resistance of metal conductor

Types: 1. Potentiometer transducer : Change in length to change in resistance. 2. Resistance thermometer : Change in temperature to change in resistivity. 3. Strain gauge : Change in length to Change in resistance.

A

ρ.LR

Inductive Transducers


Based on magnetic characteristics of an electrical circuit. 1.Change in Self Inductance:

R

NL

2

.AR

A..NL

2



Based on magnetic characteristics of an electrical circuit. 1.Change in Mutual Inductance: L1 & L2: Self inductance of two coils k: coefficient of coupling

If coils are connected in series then

Inductance will vary between L1+L2-2M to L1+L2+2M

21.LLk.M



Based on magnetic characteristics of an electrical circuit. 1.Production of Eddy Current:

• a/c current passing through a coil produces eddy current in the conducting plate. • Eddy current produces its magnetic field. • This magnetic field opposes the magnetic field of the coil. • In the process the inductance of the coil is reduced.

Capacitive Transducers


Based on capacitance of a parallel plate capacitor.

Capacitance

A: Overlapping area of plates d: Distance between plates

€: Permittivity of the medium

A

q

d

.A.

d

.AC or

Piezo resistive Transducers


If the dimensions of the crystal are changed by the application of

mechanical force, an electric potential appears across surface.

1. Rochelle salt 2. Ammonium Dihydrogen Phospate

3. Lithim Sulphate 4. Quartz 5. Ceramics A & B

Thermo resistive Transducers


Resistance of conductor varies with temperature.

1. Resistance Temperature Detector

Transducers Selection


Factors influencing the choice of transducer:

Usage & Ruggedness

Operating Principle

Sensitivity

Operating Range

Accuracy

Errors

Loading Effect

Environmental Compatability

Insensitivity to Unwanted Signals

Stability & Reliability

Static Characteristics


Calibration


• Process of comparing the output of the instrument or sensor under test against the output of an instrument of known accuracy when the same input (the measured quantity) is applied to both instruments & making adjustments.

• The act of making adjustments on scale so that the instrument readings conforms to accepted standards.

Procedure is carried out for a range of inputs covering the whole measurement range of the instrument or sensor in both ascending & descending order .

• Need: Changes in instrument characteristics are brought about by mechanical wear,

and the effects of dirt, dust, fumes, chemicals and temperature changes in the operating environment.

Calibration


Significance:

Calibration ensures the measuring accuracy of all instruments and sensors used in a measurement system. Hence errors are eliminated & accuracy is improved.

Precautions: Standard calibration instruments must be totally separate.

Calibration area should be a separate room. ( better environmental control & better protection against unauthorised handling of calibration instrument )

Approved & Dedicated persons should be provided.

ISO 9000 ( Training )

Calibration


Primary Standard: ( NSO)

Extremely accurate & absolute unit.

Secondary Standard: (IBS)

Reference standard designed & constructed from absolute standard.

Working Standard:

Standards needed by industries, labs having one order of accuracy lower than the secondary standard.

Calibration


Dead Weight Tester


Fluid Force = Gravitational Force + Frictional Drag P.A= M.g + F Thus P= ( M.g + F )/ A

Dead Weight Tester


Instrument Selection


Factors influencing the choice of instrument:

Intial Cost, Operating Cost, Maintenance Cost

Operating Range

Accuracy

Calibration

Nature of input signal

Ease of handling


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