MMM Lecture 2

1. MMM Lecture 2

2. Inspection • Inspection is defined as a procedure in which a part or product characteristic, such as a dimension, is examined to determine whether it conforms to the design specification. • Basically inspection is carried out to isolate and evaluate a specific design or quality attribute of a component or product. • In inspection, the part either passes or fails. Thus, industrial inspection has become a very important aspect of quality control.

3. Inspection Inspection essentially encompasses the following: Ascertain that the part, material, or component conforms to the established or desired standard. Accomplish interchangeability of manufacture. Sustain customer goodwill by ensuring that no defective product reaches the customers. Provide the means of finding out inadequacies in manufacture. The results of inspection are recorded and reported to the manufacturing department for further action to ensure production of acceptable parts and reduction in scrap.

4. Inspection Purchase good-quality raw materials, tools, and equipment that govern the quality of the finished products. Coordinate the functions of quality control, production, purchasing, and other departments of the organizations. Take the decision to perform rework on defective parts, that is, to assess the \ possibility of making some of these parts acceptable after minor repairs. Promote the spirit of competition, which leads to the manufacture of quality products in bulk by eliminating bottlenecks and adopting better production techniques.

5. Measuring Instruments • Deflection and null type instruments • Analog and digital instruments • Active and passive instruments • Automatic and manually operated instruments • Contacting and non contacting instruments • Absolute and secondary instruments • Intelligent instruments.

6. DEFLECTION AND NULL TYPE • Physical effect generated by the measuring quantity • Equivalent opposing effect to nullify the physical effect caused by the quantity

7. ANALOG AND DIGITAL INSTRUMENTS • Physical variables of interest in the form of continuous or stepless variations • Physical variables are represented by digital quantities

8. ACTIVE AND PASSIVE INSTRUMENTS • Instruments are those that require some source of auxiliary power • The energy requirements of the instruments are met entirely from the input signal

9. Automatic and manually operated • Manually operated – requires the service of human operator • Automated – doesn't requires human operator

10. Contacting And Non Contacting Instruments • A contacting with measuring medium • Measure the desired input even though they are not in close contact with the measuring medium

11. Absolute and Secondary Instruments • These instruments give the value of the electrical quantity in terms of absolute quantities • Deflection of the instruments can read directly

12. Intelligent instruments • Microprocessors are incorporated with measuring instruments

13. Characteristics of Measuring Instrument • Sensitivity • Readability • Range of accuracy • Precision

14. Definition • Sensitivity- Sensitivity is defined as the ratio of the magnitude of response (output signal) to the magnitude of the quantity being measured (input signal) • Readability- Readability is defined as the closeness with which the scale of the analog instrument can be read

15. Definition • Range of accuracy- Accuracy of a measuring system is defined as the closeness of the instrument output to the true value of the measured quantity • Precision- Precision is defined as the ability of the instrument to reproduce a certain set of readings within a given accuracy

16. Sensitivity • If the calibration curve is liner, as shown, the sensitivity of the instrument is the slope of the calibration curve. • If the calibration curve is not linear as shown, then the sensitivity varies with the input.

17. Sensitivity This is the relationship between a change in the output reading for a given change of the input. (This relationship may be linear or non-linear.) Sensitivity is often known as scale factor or instrument magnification and an instrument with a large sensitivity (scale factor) will indicate a large movement of the indicator for a small input change.

18. Output, Vo (V) Force, F Slope = 5 V/kN Output, Vo Load Cell Input, Fi (kN) Block Diagram: Input, F (kN) Output, Vo (V) K Sensitivity, K = 5 V/kN

19. Example (1) A 0.01 W/A meter with 5 A fsd, Rm = W/A x A = 0.01 x 5 = 0.05 W Vmax across the Meter will be = 5 A x 0.05 W = 0.25 V for fsd. (2) A 0.1 W/A meter with 5 A fsd,will drop 2.5 V (i.e., it is 10 times less sensitive), which may bias the results

20. Readability • Readability is defined as the ease with which readings may be taken with an instrument. • Readability difficulties may often occur due to parallax errors when an observer is noting the position of a pointer on a calibrated scale

21. Readability What is the value ? What is the value ? What is the value ?

22. Accuracy • Accuracy = the extent to which a measured value agrees with a true value • The difference between the measured value & the true value is known as ‘Error of measurement’ • Accuracy is the quality of conformity

23. Example: Accuracy Who is more accurate when measuring a book that has a true length of 17.0 cm? A : 17.0 cm, 16.0 cm, 18.0 cm, 15.0 cm B :: 15.5 cm, 15.0 cm, 15.2 cm, 15.3 cm

24. Precision • The precision of a measurement depends on the instrument used to measure it. • For example, how long is this block?

25. How big is the beetle? Measure between the head and the tail! Between 1.5 and 1.6 in Measured length: 1.54 in The 1 and 5 are known with certainty The last digit (4) is estimated between the two nearest fine division marks.

26. Example: Precision Who is more precise when measuring the same 17.0 cm book? A: 17.0 cm, 16.0 cm, 18.0 cm, 15.0 cm B :: 15.5 cm, 15.0 cm, 15.2 cm, 15.3 cm

27. High Accuracy High Precision High Precision Low Accuracy Accuracy vs. Precision

28. The person hit the bull's-eye? Three targets with three arrows each to shoot. Precise but not accurate Both accurate and precise Neither accurate nor precise How do they compare? Can you define accuracy vs. precision?

29. Uncertainty • The word uncertainty casts a doubt about the exactness of the measurement results • True value = Estimated value + Uncertainty

30. Performance of Instruments • All instrumentation systems are characterized by the system characteristics or system response • There are two basic characteristics of Measuring instruments, they are • Static character • Dynamic character

31. Static Characteristics • The instruments, which are used to measure the quantities which are slowly varying with time or mostly constant, i.e., do not vary with time, is called‘static characteristics’.

32. STATIC CHARACTERISTICS OF AN INSTRUMENTS Dead zone Backlash True value Hysteresis Linearity Range or Span Bias Tolerance Stability • Accuracy • Precision • Sensitivity • Resolution • Threshold • Drift • Error • Repeatability • Reproducibility

33. Resolution This is defined as the smallest input increment change that gives some small but definite numerical change in the output.

34. Threshold This minimum value of input below which no output can be appeared is known as threshold of the instrument. Output input

35. Drift Drift or Zero drift is variation in the output of an instrument which is not caused by any change in the input; it is commonly caused by internal temperature changes and component instability. Sensitivity drift defines the amount by which instrument’s sensitivity varies as ambient conditions change.

36. Output Output sensitivity drift zero drift input input sensitivity drift Output zero drift input

37. Error – The deviation of the true value from the desired value is called Error • Repeatability – It is the closeness value of same output for same input under same operating condition • Reproducibility - It is the closeness value of same output for same input under same operating condition over a period of time

38. Range • The ‘Range’ is the total range of values which an instrument is capable of measuring.

39. Measured Value Ideal Actual/ Input Value Hysteresis This is the algebraic difference between the average errors at corresponding points of measurement when approached from opposite directions, i.e. increasing as opposed to decreasing values of the input. Hysteresis is caused by energy storage/ dissipation in the system.

40. Dead band This is the range of different input values over which there is no change in output value.

41. Linearity- The ability to reproduce the input characteristics symmetrically and linearly

42. Backlash – Lost motion or free play of mechanical elements are known as backlash • True value – The errorless value of measured variable is known as true value • Bias – The Constant Error • Tolerance- Maximum Allowable error in Measurement

43. Dynamic Characteristics • The set of criteria defined for the instruments, which are changes rapidly with time, is called ‘dynamic characteristics’.

44. Dynamic Characteristics • Steady state periodic • Transient • Speed of response • Measuring lag • Fidelity • Dynamic error

45. Steady state periodic – Magnitude has a definite repeating time cycle • Transient – Magnitude whose output does not have definite repeating time cycle • Speed of response- System responds to changes in the measured quantity

46. Measuring lag • Retardation type :Begins immediately after the change in measured quantity • Time delay lag : Begins after a dead time after the application of the input • Fidelity- The degree to which a measurement system indicates changes in the measured quantity without error • Dynamic error- Difference between the true value of the quantity changing with time & the value indicated by the measurement system