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Introduction to Mechanical Measurement


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Please refer this file just as reference material. More concentration should on class room work and text book methodology.
Introduction to Mechanical Measurement

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Introduction to Mechanical Measurement

  1. 1. CHAPTER 1 Mechanical Measurement Presentation Prepared by Prof. Naman M. Dave Assistant Prof. (Mechanical Dept.) Gandhinagar Institute of Technology
  2. 2. Please refer this file just as reference material. More concentration should on class room work and text book methodology.
  3. 3. Measurement and Metrology  Science of measurement  Depending on field of application  Today (in broader sense) with some practical terms
  4. 4. Metrology  Practical terms related with measurement: o Errors in measurements o Methods of measurements o Measuring Instruments o Units of measurement and their standards o Industrial inspection and its different techniques o Measuring instruments and accuracy
  5. 5. Needs of Measurement and Metrology  To ensure that the part to be measured conforms to the established standard.  To meet the interchangeability of manufacture.  To provide customer satisfaction by ensuring that no faulty product reaches the customers.  To coordinate the functions of quality control, production, procurement & other departments of the organization.  To judge the possibility of making some of the defective parts acceptable after minor repairs.
  6. 6. Objectives of Measurement and Metrology  Although the basic objective of a measurement is to provide the required accuracy at a minimum cost, metrology has further objectives in a modem engineering plant with different shapes which are:  To minimize the cost of inspection by efficient and effective use of available facilities,  To minimize the cost of rejection and re-work through application of statistical quality control techniques.  To maintain the accuracies of measurement.  To determine the process capabilities and ensure that these are better than relevant component tolerances.  To do complete evaluation of newly developed products.
  7. 7. Modes of Measurement  Based upon the number of conversions, three basic categories of measurements have been developed.  They are; 1. Primary measurement 2. Secondary measurement 3. Tertiary measurement
  8. 8. Modes of Measurement 1. Primary measurement  Direct observation and comparison  Not involvement of any conversion Ex. Length, Height, Depth or Width etc. measurement.
  9. 9. Modes of Measurement 2. Secondary measurement  >Indirect method >Involvement of one conversion  Ex. Pressure or Temperature measurement 3. Tertiary measurement  >Indirect method >Involvement of 2 conversion  Ex. Measurement of rotating shaft
  10. 10. Methods of Measurement
  11. 11. Methods of Measurement Type of Method Technique to measure With contact Instrument is placed in contact with the object. For ex. vernier calliper Without contact Instrument not placed in contact with the object. (use of sensor) Absolute or Fundamental Based on the measurements of base quantities entering into the definition of the quantity.
  12. 12. Methods of Measurement Type of Method Technique to measure Comparative Based on the comparison of the value of a quantity to be measured with a known value of the same quantity. Null measurement Here, difference between measurand value and known value of same quantity with which it is compared is brought to zero.
  13. 13. Methods of Measurement Type of Method Technique to measure Substitutional Method Quantity to be measured is replaced by a known value of the same quantity, so selected that the effects produced in the indicating device by these two values are the same (a type of direct comparison). Complementary Method The value of quantity to be measured is combined with known value of the same quantity. Ex: Volume determination by liquid displacement.
  14. 14. Methods of Measurement Type of Method Technique to measure Transposition Quantity to be measured is first balanced by a known value and then balanced by an other new known value. Value of quantity measured is first balanced by an initial known value A of same quantity, then measured by quantity is put in place of this known value. Then, it is balanced again by another value B. If the position of element in equilibrium.
  15. 15. Methods of Measurement Type of Method Technique to measure Coincidence Measurements coincide with certain lines and signals. Ex. Callipers Deflection The value of the quantity to be measured is directly indicated by the deflection of a pointer on a calibrated scale
  16. 16. Generalized Measurement System
  17. 17. Generalized Measurement System Elements of measuring system 1. Primary sensing element  Quantity under measurement makes its first contact with primary sensing element.  Sense the condition, state or value of the process variable by extracting a small part of energy from the measurand,  and then produce an output which reflects this condition, state or value of measurand.
  18. 18. Generalized Measurement System
  19. 19. Generalized Measurement System Elements of measuring system 2. Variable conversion (transducer) element  Convert one physical form into another form without changing the information content of the signal.
  20. 20. Generalized Measurement System Elements of measuring system 3. Variable manipulation element  Modifies the signal by amplification, filtration or other means so that desired output produced according to some mathematical rule for Ex. i/p x constant = o/p
  21. 21. Generalized Measurement System Elements of measuring system 4. Data transmission element  Transmits the signal from one location to another without changing its information contents.
  22. 22. Generalized Measurement System Elements of measuring system 5. Data processing element  modifies the data before it displayed or finally recorded • perform mathematical operation such as addition, subtraction, multiplication, division, etc. • to calculate average, statistical and logarithmic values • to convert data into desired form. • to separate undesired signal from output signal. • to provide correction on the output signal.
  23. 23. Generalized Measurement System
  24. 24. Generalized Measurement System Elements of measuring system 6. Data presentation elements  Provides a record or indication of the output (i) Transmitting information (measured quantity) to another location or devices. (ii) Signaling : To give a signal that the pre-defined value has been reached. (iii) Recording : To produce a continuous record of measured quantity in written form. (iv) Indicating : To indicate the specific value on calibrated scale.
  25. 25. Performance characteristics  Important to select most suitable instrument for specific measurement.  Static Performance characteristics: • Desired input to the instrument not change w.r.t time.  Dynamic Performance characteristics:
  26. 26. Static Performance characteristics  Readability: This term indicates the closeness with which the scale of the instrument may be read.  Susceptibility of device to have its indications converted into meaningful number.  Least count: It is the smallest difference between two indications that can be detected on the instrument scale.
  27. 27. Static Performance characteristics  Range: It represents the highest possible value that can be measured by an instrument or limits within which instrument is designed to operate.  Linearity: A measuring system is said to be linear if the output is linearly proportional to the input.
  28. 28. Static Performance characteristics  Repeatability: It is defined as the ability of a measuring system to repeat output readings when the same input is applied to it consecutively, under the same conditions, and in the same direction.  Reproducibility: It is defined as the degree of closeness with which the same value of a variable may be measured at different times.  System response: Response of a system may be defined as the ability of the system to transmit & present all the relevant information contained in the input signal.
  29. 29. Static Performance characteristics  Threshold: Min. value of i/p required to cause a detectable change from ‘0(zero)’ o/p.  If i/p increased gradually from ‘0(zero)’, there will be some min. value below which no o/p change can be detected.
  30. 30. Static Performance characteristics  Hysteresis:  Hysteresis is the maximum differences in two output (indicated values) at same input (measurand) value within the specified range when input is continuously increased from zero and when input is continuously decrcased for maximum value.  Maximum diff. between increasing input value and the decreasing input value at the same output.
  31. 31. Static Performance characteristics  Calibration: Procedure of making, adjusting or checking a scale so that readings of an instrument conforms to an accepted standard.  Sensitivity: Ratio of o/p response to a specific range in i/p.  Dead zone: Largest change of i/p quantity for which instrument does not indicate output.
  32. 32. Static Performance characteristics  Drift: gradual variation or undesired change in o/p during constant i/p. Zonal Drift: Combination of both Zero Drift: whole calibration gradually shifts due to slippage • Span: proportional change in the indication or change along the upward scale
  33. 33. Static Performance characteristics  Loading effect: Any instrument, invariably extracts energy from system, the original signal should remain undistorted. This is incapability of system to faithfully measure signal in undistorted form.
  34. 34. Static Performance characteristics  Accuracy: It is degree to which the measured value agrees with true value. Max. amount by which result differs from the true value.  Precision: It is repeatability or reproducibility of the measurement. If instrument is not precise, great difference in dimension measured again and again.
  35. 35. Measurement Errors  What is Error ?  It is difference between indicated or measured value and true value.  It is impossible to made measurement with perfect accuracy
  36. 36. Measurement Errors  Types (Classification of Errors)
  37. 37. Measurement Errors  Gross errors • Human mistakes • Careless readings, mistake in recordings, • improper application of instrument • Can not treated mathematically • Can be avoided only by taking care in reading and recording
  38. 38. Measurement Errors  Systematic error • Have definite magnitude and direction. • Can be repeated consistently with repetition of experiments. • To locate these errors: repeated measurements under different conditions or with different equipment or possible by an entirely different methods.
  39. 39. Measurement Errors  Instrumental error  Due to design or construction /assembly of instruments  Limiting accuracy  Improper selection of instrument  Poor maintenance  For Ex. Errors due to friction, wear, slips, vibration  Errors due to incorrect fitting of scale at zero, non-uniform division of scale, bent pointer.
  40. 40. Measurement Errors  Operational error  Misuse of instrument  Poor operational techniques  For Ex. Errors in flow measurement if flow- meter is placed immediately after a valve or a bend.
  41. 41. Measurement Errors  Environmental errors  due to conditions external to the measuring instrument, including conditions in the area surrounding the instrument,  such as effects of change in temperature, humidity, barometric pressure, or magnetic or electrostatic fields.  For ex. Buoyant effect of the wind causes errors on precise measurement of weights by pan balance.
  42. 42. Measurement Errors  These errors may be avoided by  (i) Use instrument under conditions for which it was design and calibrated. This atmospheric condition can be maintain by air conditioning.  (ii) Provide sealing certain components in the instrument.  (iiii) Make calibration of instrument under the local atmospheric conditions  Environmental errors
  43. 43. Measurement Errors  System interaction errors  Interaction between system (to be measured) and instrument body. So it change the condition of the system.  For Ex. A ruler pressed against a body (system) resulting the deformation of the body.
  44. 44. Measurement Errors  Observation errors :  Due to poor capabilities and carelessness of operators. i. Parallax : These errors may arise when the pointer and scale not in same plane or line of vision of observer is not normal to the scale.
  45. 45. Measurement Errors  Observation errors :  Due to poor capabilities and carelessness of operators. ii. Personal bias: Observer tendency to read high or low, anticipate a signal and read too iii. Wrong reading, wrong calculations, wrong recording data, etc.
  46. 46. Measurement Errors  Random Error  Accidental in their incidence  Variable in magnitude and usually follow a certain statistical (probability) law.  Friction and stickiness in instrument  Vibration in instrument frame or supports  Elastic deformation  Large dimensional tolerances between the mating parts.  Supply power fluctuations  Backlash in the movement.
  47. 47. CHAPTER 1 Mechanical Measurement