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Measurement System and concept of measuring.pptx.pdf
- 1. Measurement System & Concept ofMeasurement & Methods of Measurement & Functional Elements of an Instruments First Lecture
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- 3. SI:fundamental Units Physical QuantityUnit Name Symbol length meter m mass kilogram kg time second s electric current ampere A temperature Kelvin K amount of substance mole mol luminous intensity candela cd
- 4. SI:Derived Units Physical Quantity area volume UnitName square meter cubic meter meter per second meter per second squared newton pascal joule Symbol m2 m3 speed m/s acceleration m/s2 weight, force pressure energy, work N Pa J
- 5. Supplementary units PhysicalQuantity UnitNameSymbol Planeangle Radian rad Solid angle Steradian sr
- 6. Standards • International standards •Primary standards • Secondary standards • Working standards
- 14. Scientists use manyskills as they investigate the world around them. They make observations by gathering information with their senses. Some observations are simple. For example, a simple observation would be figuring out the color or texture of an object. However, if scientists want to know more about a substance, they may need to take measurements. Measurement is perhaps one of the most fundamental concepts in science. Without the ability to measure, it would be difficult for scientists to conduct experiments or form theories. It is also essential in farming, engineering, construction, manufacturing, commerce, and numerous other occupations and activities.
- 15. 15 Measurements and measurementsystems: • Definition, significance of measurement, generalized measurement system. • Definitions and concept of accuracy, precision, calibration, threshold, sensitivity, hysteresis, repeatability, linearity, loading effect, system response-times delay. • Errors in measurement, classification of errors. • Transducers, transfer efficiency, primary and secondary transducers, electrical, mechanical, electronic transducers, advantages of each typetransducers.
- 16. MEASUREMENT SYSTEM: • Measurementmeans determination of anything that exists in some amount. • If those things that exist are related to mechanical engineering, then the determination of such amounts are referred to as mechanical measurements. • An engineer is not only interested in the measurement of physical variables but also concerned with their control. • These two functions are closely related because one must be able to measure a variable such as temperature, or flow in order to control it. • The accuracy of control is essentially dependent on the accuracy of measurement. Hence, a good knowledge of measurement techniques is necessary for the design of control systems. Definition of Measurement : Measurement is defined as the process or the act of obtaining a quantitative comparison between a predefined standard and an unknown magnitude. 16
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- 18. 18 SIGNIFICANCE OF MEASUREMENTSYSTEM • Measurement provides the fundamental basis for research and development. Development is the final stage of the design procedure involving the measurement of various quantities pertaining to operation and performance of the device being developed. • Measurement is also a fundamental element of any control process, which requires the measured discrepancy between the actual and the desired performances. • Many operations require measurement for proper performance. For example : In modern central power stations, temperatures, pressures, vibrational amplitudes etc., are monitored by measurement to ensure proper performance. • Measurement is also a bias of commerce, because the cost of the products are established on the basis of amounts of materials, power, expenditure of time and labour, and other constraints.
- 19. Contd.. 12 Measured Value: Anyvalue or any reading calculated from measurement system or measuring instrument. True value: Any value calculated from rated value known as True value ofActual Value. e.g. Motor Actual Speed Error : Any deviation of measured value from true value Measured Value-TrueValue Measuring Instrument True Value Measured Value
- 20. Why measure things? •Checkquality? • Checktolerances? Measurements require tools and provide scientists with a quantity. A quantity describes how much of something there is or how many there are. A good example of measurement is using a ruler to find the length of an object. The object is whatever you are measuring, the property you are trying to determine is the object’s length, and the standard you are comparing the object’s length to is the ruler.
- 21. Need of Measurement •Establish standard • Interchangeability • Customer Satisfaction • Validate the design • Physicalparameter into meaningfulnumber • True dimension • Evaluate the Performance
- 22. Methods of Measurement •Direct method • Indirect method • Comparative method • Coincidence method • Contact method • Deflection method • Complementary method
- 23. Methods of Measurement DirectMethod The unknown quantity (measurand) is directly compared against a standard. The result is expressed as a numerical number and a unit. Direct methods are common for the measurement of physical quantities like length, mass and time Indirect Method In this method the comparison Is done with a standard through the use of a calibration s/m. These methods are used those cases where the desire parameter to be measured. E.g. Acceleration, power Method of Measurement 23 • Obtainedbymeasuringotherquantities – E x: Weight =LengthxBreadth xHeight xDensity
- 24. Direct method • Measurementsare directly obtained – Ex:Vernier Caliper, Scales
- 25. Direct Methods Clasifiedas: ❑Deflection methods Deflection method” includes the deflection of pointer on a scale due to the quantity to be measured. Example: Wattmeter, ammeter voltmeter ❑Comparison methods “Comparison method” include the comparison of the quantity under measurement with a pre-defined standard quantity which gives measurement. Example: potentiometer 25
- 26. Comparative Method • It’scompared with other knownvalue – Ex:Comparators
- 27. Coincidence method • Measurementscoincide with certain linesand signals Fundamental method • Measuring aquantity directly in relatedwith the definition of thatquantity Contact method • Sensor/Measuring tip touch the surfacearea
- 28. Complementary method • Thevalueof quantity to be measured is combined with known value of the same quantity – Ex:Volume determination by liquiddisplacement
- 29. Deflection method • Thevalueto be measured is directlyindicated by adeflection of pointer – Ex:Pressure Measurement
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- 31. Functional Elements ofan Instruments ❑Any instrument or measuring can be represented by block diagram, that indicates necessary elements and its functions. ❑The entire operation of the measuring system can be understand fro the bock diagram Primary sensing element Variable conversion element Variable manipulation element Data transmission element Data presentation element Qty. to be measured Data conditioning element Observer Data storage element 31
- 32. Take an example: ❑Justtake an example of an Analog meter (Ammeter) which measures current. Moving Coil Magnets and other components Mechanical Linkages Pointers and scale Current Data conditioning Observer Primary Sensing Data Transmission Force BASIC SCHEMATIC OF ANAMMETER 32
- 33. THE GENERALIZED MEASURINGSYSTEM • Most measuring systems fall within the framework of a general arrangement consisting of three phases orstages: • Stage 1. • Stage 2. A detection-transduction, or sensor-transducer, stage An intermediate stage, which we shall call the signal- conditioning stage • Stage 3. A terminating, or readout-recording, stage • Each stage consists of a distinct component or group of components that performs required and definite steps in the measurement. These are called basic elements: their scope is determined by their function rather than by their construction. Figure 1.2 outline the significance of each of thesestages. 33
- 34. Instrumentation Systems An instrumentation/measurementprocess can be viewed as a system whose input is the true value of the variable being measured and its output is the measured value. 34
- 35. The elements ofan instrumentation system 35
- 36. 36 First, or Sensor-Transducer,Stage • The primary function of the first stage is to detect or to sense themeasurand. • At the same time, ideally, this stage should be insensitive to every other possible input. • For instance, if it is a pressure pickup, it should be insensitive to say, acceleration; • if it is a strain gage, it should be insensitive totemperature; • if a linear accelerometer, it should be insensitive to angular acceleration; and so on. • Unfortunately, it is rare indeed to find a detecting device that is completely selective. • Unwanted sensitivity is a measuring error, called noise.
- 37. Sensor • This isthe element in contact with the process for which a variable is being measured. • It gives an output which depends in some way on the value of that variable. • For example, a thermocouple is a sensor which converts temperature changes into a small e.m.f. output which might be amplified to give a reading on a meter. 37
- 38. 38 Second, or Signal-Conditioning,Stage • The purpose of the second stage of the general system is to modify the transduced information so that it is acceptable to the third, or terminating stage. • In addition, it may perform one or more basic operations, such as selective filtering to remove noise, integration, differentiation, as may be required. • Probably the most common function of the second stage is to increase either amplitude or power of the signal, or both, to the level required to drive the final terminating device.
- 39. Signal Conditioner • Aswe have seen in previous lectures, this element converts the sensor output into a form which is suitable for further processing such as display or onward transmission in some control system. • For example, in the case of the resistance thermistor there might be a signal conditioner, a Wheatstone bridge, which transforms the resistance change into a voltage change, then an amplifier to make the voltage big enough for display. 39
- 40. 40 Third, or Readout-Recording,Stage • The third stage provides the information sought in a form comprehensible to one of the human senses or to a controller. • If the output is intended for immediate human recognition, it is, with rare exception, presented in one of the following forms: – As a relative displacement, such as movement of an indicating hand or displacement of oscilloscope trace – In digital form, as presented by a counter such as an automobile odometer, or by a liquid crystal display (LCD) or light-emitting diode (LED) display as on a digital voltmeter
- 41. Data presentation • Thispresents or displays the measured value in a suitable form for the operator. • This may be done using a pointer moving across the scale of a meter or by presenting information on a visual display. • Alternatively, the signal may be recorded, e.g. on the paper of a chart recorder or transmitted to some other system such as a control system. 41
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- 43. • To illustratea very simple measuring system, let us consider the familiar tire gage used for checking automobile tire pressure. Such a device is shown in Fig. (a). • It consists of a cylinder and piston, a spring resisting the piston movement, and a stem with scale divisions. • As the air pressure bears against the piston, the resulting force compresses the spring until the spring and air forces balance. • The calibrated stem, which remains in place after the spring returns the piston, indicates the applied pressure. • The piston-cylinder combination constitutes a force-summing apparatus, sensing and transducing pressure to force. • As a secondary transducer, the spring converts the force to a displacement. Finally, the transduced input is transferred without signal conditioning to the scale and index for readout. 43
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