Engineering Measureme

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Introduction to Mechanical Measurements

Why

Measure?

GenerateData forDesign

Process Improvement

Generating design data• Reliable design of a machine component is possible if it’s

behavior can be analyzed during operating condition.

• Such an activity needs data regarding machine components viz. material properties made available through measurements

Process Improvement• Process Improvement theories can be validated by using

measured data

• Critical to satisfaction parameters are identified and process improvement is validated based on measured data.

Scope of metrology

• Establishing units of measurement, reproducing units in the form of standards, ensuring the uniformity of measurements.

• Developing methods of measurement.

• Analyzing accuracy of measurement methods, establishing uncertainty of measurement, researching into causes of errors, and error elimination.

Standardization and Standardizing Organizations• Diversity should be minimized and interchangeability

encouraged for higher economy, efficiency and productivity in a factory/country.

• Organizations to handle the complexities of standardization have evolved in each of the countries.

• In India, Bureau of Indian standards is responsible for evolving standards on metrological instruments etc.

• There exits a separate Engineering Metrology Sectional Committee under Mechanical Engineering Division Council, EDC, of the BIS.

Other Standardizing Organizations• International Organizationof Weights and measures• General conference of Weights and Measures• International Organization of Standardization

International Standard of Units (SI)• Established in 1960 by the General Conference of

Weights and Measures(CGPM).• System based on decimal Arithmetic. Units of different

sizes are formed by multiplying or dividing a single base value by powers of 10.

• SI provides only one basic quantity for each physical quantity.

Physical Measurement• Defined as the act of deriving quantitative information

about a physical action by comparison with a reference

Elements of a measurement• Measurand :The physical property such as length, angle

being measured.

• Comparator: Means of comparing measurand with some reference to render a judgment.

• Reference: The physical quantity to which the quantity comparisons are made

Important Terms in measurements1. Measuring range: • Range of values of measured quantity for which

error under normal conditions do not exceed maximum permissible.

2. Sensitivity: • Defined as the ratio of increase in observed variable

(indicated by pointer/scale) and the corresponding increase in measured quantity.

• Also defined as length of any scale division divided by the value of that division expressed in terms of the measured quantity.

3. Response time• Time which elapses after a sudden change in the

measured quantity until the instrument gives an indication differing from the true value within permissible error.

• The figure below represents a second order response of an instrument.

4. Repeatability:• Ability of the measuring instrument to give the same

value every time measurement of a given measurand is repeated.

• The method of measurement, observer, measuring instrument, locations etc are the same for all measurements.

• Repeatability is characterized by dispersion of indications of the same quantity described by two limiting values.

• Expressed either as maximum difference between output readings or percentage of full scale output.

5. Reproducibility• Quantitative measure of the closeness of the agreement

between results of measurements of the same measurand where individual measurements are carried out by changing method, instrument, location etc.

• Expressed as the dispersion of results. 6. Magnification• Human limitations to read instruments places limits on

sensitivity of measuring instruments.• Magnification of signal from measuring instruments can

make it better readable.• Possible with the use of mechanical, electronic, optical,

electrical principles.

7. Calibration• Calibration of measuring equipment is required in

order to eliminate errors arising due to variation in conditions.

• Achieved by making adjustments such that device produces zero output for zero measured input, and display an output equivalent to known measurand input near full scale input value.

Measurement System

• A measurement system is made up of five basic elements:

1. S-Standard2. W-Work piece3. I-Instrument4. P-Person5. E-Environment • SWIPE is an acronym for the basic elements in a

measurement system.

Precision and Accuracy• Precision is defined as the repeatability of a measuring

process.• Accuracy is the agreement of the result with the true

value of the measured quantity.• Precision is concerned with set of measurements.• Poor repeatability is a sure sign of poor Accuracy.• Good repeatability of instruments is necessary but not

sufficient condition of good Accuracy.• Accuracy=√((Repeatability)2 + (Systemmatic error*)2) *systemmatic error=|(mean of readings) - (true value)|

Factors affecting accuracy • The Accuracy of a measurement system is affected due

to effects in the Elements of a Measurement System.1. Factors affecting Standard • Coefficient of thermal expansion• Elastic properties• Geometric compatibility

2. Factors affecting Workpiece• Cleanliness, surface finish, scratches, surface defects etc.• Hidden Geometry• Adequate datum on the workpiece

3. Factors affecting characteristics of Instrument• Methods of Amplification• Effect of friction, backlash etc.• Calibration errors

4. Factors affecting person• Sense of sight/Sense of hearing/ Sense of touch of the

operator• Attitude towards personal accuracy.• Ability to select measuring instruments and standards.

5. Factors affecting Environment• Temperature, Humidity etc• Clean surrounding and minimum vibration enhance

precision.• Thermal Expansion due to heat radiation from light,

heating elements etc.

High Accuracy is achieved by controlling all the sources of error affecting the above elements.

General Rules of Accurate measurements• Equipment chosen for a particular measurement must

bear some relation to the desired accuracy. eg. If a measurement is desired to an accuracy of 0.01mm, an instrument with accuracy 0.001 must be used.

• Use of highly sensitive, consistent and correctly calibrated instrument.

• Incorporating magnifying devices in the instrument.

• Determining knowable sources of error by aid of superior instruments and calibrating the instrument accordingly.

References• Metrology – M. Mahajan• Engineering Metrology – R.K Jain