MEASUREMENT AND INSTRUMENTATION BMCC 4743

1. MEASUREMENT AND INSTRUMENTATIONBMCC 4743 LECTURE 2: PERFORMANCE OF STATIC CHARACTERISTICS IN MEASUREMENT AND INSTRUMENTATION Mochamad Safarudin Faculty of Mechanical Engineering, UTeM 2008

2. Contents • Definition of static • Requirement of static characteristics preservation • Static calibration • Passive & active sensors

3. Definition of Static • The steady state relationship between input and output of an instrument • Measurement of quantities that are constant or vary quite slowly with respect to time. • It does not involve differential equations. • All the static performance characteristics are obtained by one form or another via a process called static calibration.

4. Contents • Definition of static • Requirement of static characteristics preservation • Static calibration • Passive & active sensors

5. Requirement of static characteristics preservation • Precise & Accurate • Measurements that are close to each other are precise • Measurements that are close to the correct value are accurate • Measurements can be: • Precise but inaccurate • Neither precise nor accurate • Precise and accurate

6. Examples Three industrial robots were programmed to place components at a particular point on a table. The target point was the center of a circle shown below. The results are: • Low precision, • low accuracy (b) Precise not accurate (c) Precise and accurate

7. Accuracy Ideal device Output of device 40 20 Accuracy at % of FS 0 20 40 Value of measurand

8. Requirement of static characteristics preservation • Repeatability • The ability of an instrument to register readings accurately repeatedly • Linearity • The input and output relationship of a linear transducer can be represented by the following equation: • where y is theoutput of transducer, x is the input of transducer, m is theslope of curve (transfer function), c is the offset. • Often, the straight line approach is used for certain range of operation for a non-linear system. y = mx + c

9. Requirement of static characteristics preservation • Hysteresis • Hysteresis results in predictable error. May be due to internal friction, freeplay or looseness in the mechanism of an instrument. • Also in electrical phenomena (relation between the output voltage and the input field current in a d.c. generator) - the effect is due to magnetic hysteresis of the iron in the field coils. • The transfer functions differ with the increase and decrease of inputs as shown in the following figure.

10. Generalized graph of output/input relationship where hysteresis is present. (From P. H. Sydenham, Handbook of Measurement Science, Vol. 2, Chichester, U.K., John Wiley & Sons, 1983)

11. Linearity It is highly desirable that the measurement system has a linier relationship between input and output means that the change in output is proportional to the change in the value of the measurand Deviation from true linearity is called linearity error

12. Sensitivity • Sensitivity is the ratio of change in magnitude of the output to the change in magnitude of the measurand • Sensitivity=D(output)/D(input)

13. Sensitivity and Non Linearity

14. Contents • Definition of static • Requirement of static characteristics preservation • Static calibration • Passive & active sensors

15. Static Calibration • Imagine a situation in which all inputs (desired, interfering or modifying) except one are kept at some constant values. • The one input under study is varied over some range of constant values which causes the output(s) to vary over some range of constant values. • The input/output relationships developed in this way comprise a static calibration valid under the stated constant conditions of all other inputs. • The procedure may be repeated for other inputs for overall instrument static behaviour • Ultimate objective is to define measurement accuracy

16. Contents • Definition of static • Requirement of static characteristics preservation • Static calibration • Passive & active sensors

17. Passive & Active Sensors • Sensors are often transducer in that they are devices that convert input energy of one form into output energy of another form. • Categorisation of sensors: depends on how they interact with the environment they are measuring • Passive sensors: they do not add energy as part of the measurement process but may remove energy in their operation. E.g. thermocouple converts a physical temperature into a voltage signal • Active sensors: they add energy to the measurement environment as part of the measurement process. E.g. in a radar or sonar system, the distance to some object is measured by actively sending out a radio (radar) or acoustic (sonar) wave to reflect off some object and measure its range from the sensor

18. EXERCISE • What is the meaning of the following words: • Measurand • Physical quantity • Data • Parameter • Transducer • Actuator

19. ANSWER TO EXERCISE • Measurand: Physical quantity being measured • Physical quantity: Variable such as pressure, temperature, mass, length, etc • Data: Information obtained from the instrumentation/measurement system as a result of the measurements made of the physical quantities • Parameter: Physical quantity within defined (numeric) limits. • Transducer: A device that converts one form of energy to another • Actuator: Electronic transducer that converts electrical energy into mechanical energy

20. Next Lecture Analysis on Experimental Data End of Lecture 2