UNIT V MEASUREMEANT AND INSTRUMENTATION BEEE BE 3251.ppt
- 2. 2 Measurements Measurement of a given quantity is essentially an act or result of comparison between the quantity (whose magnitude is unknown) and predetermined or predefined standards. Two quantities are compared the result is expressed in numerical values.
- 3. 3 Basic requirements for a meaningful measurement The standard used for comparison purposes must be accurately defined and should be commonly accepted. The apparatus used and the method adopted must be provable (verifiable).
- 4. 4 Significance Of Measurement Importance of Measurement is simply and eloquently expressed in the following statement of famous physicist Lord Kelvin: ”I often say that when you can measure what you are speaking about and can express it in numbers, you know something about it; when you cannot express in it numbers your knowledge is of meager and unsatisfactory kind”
- 5. 5 Two major functions of all branch of engineering Design of equipment and processes Proper Operation and maintenance of equipment and processes.
- 6. 6 Methods of Measurement Direct Methods Indirect Methods
- 7. 7 DIRECT METHODS: In these methods, the unknown quantity (called the measurand ) is directly compared against a standard. INDIRECT METHOD: Measurements by direct methods are not always possible, feasible and practicable. In engineering applications measurement systems are used which require need of indirect method for measurement purposes.
- 8. 8 Instruments and Measurement Systems. Measurement involve the use of instruments as a physical means of determining quantities or variables. Because of modular nature of the elements within it, it is common to refer the measuring instrument as a MEASUREMENT SYSTEM.
- 9. 9 Evolution of Instruments. a) Mechanical b)Electrical c) Electronic Instruments. MECHANICAL: These instruments are very reliable for static and stable conditions. But their disadvantage is that they are unable to respond rapidly to measurements of dynamic and transient conditions.
- 10. 10 Contd ELECTRICAL: It is faster than mechanical, indicating the output are rapid than mechanical methods. But it depends on the mechanical movement of the meters. The response is 0.5 to 24 seconds. ELECTRONIC: It is more reliable than other system. It uses semiconductor devices and weak signal can also be detected.
- 11. 11 Classification Of Instruments Absolute Instruments. Secondary Instruments. ABSOLUTE: These instruments give the magnitude if the quantity under measurement terms of physical constants of the instrument. SECONDARY: These instruments are calibrated by the comparison with absolute instruments which have already been calibrated.
- 12. 12 Further its classified as Deflection Type Instruments Null Type Instruments.
- 13. 13 Functions of instrument and measuring system can be classified into three. They are: i) Indicating function. ii) Recording function. iii) Controlling function. Application of measurement systems are: i) Monitoring of process and operation. ii) Control of processes and operation. iii) Experimental engineering analysis.
- 14. 14 Types Of Instrumentation System Intelligent Instrumentation (data has been refined for the purpose of presentation ) Dumb Instrumentation (data must be processed by the observer)
- 15. 15 Elements of Generalized Measurement System Primary sensing element. Variable conversion element. Data presentation element. PRIMARY SENSING ELEMENT: The quantity under measurement makes its first contact with the primary sensing element of a measurement system. VARIABLE CONVERSION ELEMENT: It converts the output of the primary sensing element into suitable form to preserve the information content of the original signal.
- 16. 16 Contd.. DATA PRESENTATION ELEMENT: The information about the quantity under measurement has to be conveyed to the personnel handling the instrument or the system for monitoring, control or analysis purpose.
- 17. 17 Functional Elements of an Instrumentation System PRIMARY SENSING ELEMENT VARIABLE CONVER -SION ELEMENT VARIABLE MANIPULATI- ON ELEMENT DATA TRANSMISSIO -N ELEMENT DATA CONDITIONING ELEMENT INTERMEDIATE STAGE DETECTOR TRANSDUCER STAGE TERMINATING STAGE QUANTITY TO BE MEASURED DATA PRESENTA TION ELEMENT
- 18. 18 Calibration Calibration of all instruments is important since it affords the opportunity to check the instruments against a known standard and subsequently to find errors and accuracy. Calibration Procedure involve a comparison of the particular instrument with either a Primary standard a secondary standard with a higher accuracy than the instrument to be calibrated. an instrument of known accuracy.
- 19. 19 Standards A standard is a physical representation of a unit of measurement. The term ‘standard’ is applied to a piece of equipment having a known measure of physical quantity.
- 20. 20 Types of Standards –International Standards (defined based on international agreement ) –Primary Standards (maintained by national standards laboratories) –Secondary Standards ( used by industrial measurement laboratories) –Working Standards ( used in general laboratory)
- 22. MOVING COIL INSTRUMENT – PERMANENT MAGNET TYPE Principle : when a current carrying conductor is placed in magnetic field it is acted upon by a force which tends to move it to one side and out of the field. This movement of coil is used to measure current or voltage.
- 23. Construction This instrument consists of a permanent magnet and a rectangular coil of many turns wound on a light aluminum or copper former inside which is an iron core • The sides of the coil are free to move in the two air gaps between the poles and core • To the moving coil spindle is attached, a pointer is attached to the spindle to move over a calibrated scale.
- 24. Working A magnetic field of sufficient density is produced by the permanent magnet. The moving coil carries the current or a current proportional to the voltage to be measured. Hence, an electromagnetic force is produced which tends to act on the moving coil and moves it away from the field. This movement makes the spindle move and so the pointer gives a proportionate deflection
- 25. Deflecting torque : It is directly proportional to the current or the voltage to be measured. So, the instrument can be used to measure direct current and dc voltage. Control torque : Spring control. Damping torque : Eddy current damping.Damping is electromagnetic by eddy currents induced in the metal frame over which the coil is wound. Since the frame moves in an intense magnetic field, the induced eddy currents are large and damping is very effective.
- 26. The permanent-magnet moving coil (PMMC) type instruments have the following advantage and disadvantages: ADVANTAGES 1. They have low power consumption 2. Their scales are uniform and can be designed to extend over and arc of 1700 degree or so 3. They possess high (torque/weight) ratio. 4. They can be modified what the help o f shunts and resistances to cover a wide range of currents and voltages. 5. They have no hysteresis loss. DISADVANTAGES 1. Due to delicate construction and the necessary accurate machining and assembly of various parts, such instruments are somewhat costlier as compared to moving iron instruments. 2. Some errors are set in due to the ageing of control springs and the permanent magnets.
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- 29. MOVING IRON INSTRUMENTS – ATTRACTION TYPE Principle A soft iron piece gets magnetized when it is brought into a magnetic field produced by a permanent magnet. The same phenomenon happens when the soft iron piece is brought near either of the ends of a coil carrying current. The iron piece is attracted towards that portion where the magnetic flux density is more. This movement of soft iron piece is used to measure the current or voltage which produces the magnetic field.
- 30. Construction A soft iron disc is attached to the spindle To the spindle, a pointer is also attached, which is made to move over calibrated scale The moving iron is pivoted such that it is attracted towards the center of the coil where the magnetic field is maximum
- 31. Principle When the current to be measured is passed through the coil or solenoid, field is produced which attracts the eccentrically mounted disc inwards, thereby deflection the pointer which moves over a calibrated scale
- 32. Deflecting Torque Produced by the current or the voltage to be measured. It is proportional to the square of the voltage or current. Hence, the instrument can be used to measure d.c. or a.c. Scale is non- uniform Control torque : Spring or gravity Damping : Air friction damping
- 33. MOVING IRON INSTRUMENT - REPULSION TYPE Principle Two iron piece kept with close proximity in a magnetic field get magnetized to the same polarity. Hence, a repulsive force is produced. If one of the two piece is made movable, the repulsive force will act on it and move it on to one side. This movement is used to measure the current or voltage which produces the magnetic field.
- 34. Construction There are two iron pieces-fixed and moving. The moving iron is connected to the spindle to which is attached a pointer. It is made to move over a calibrated scale.
- 35. Working When the current to be measured is passed through the fixed coil it sets up its own magnetic field which magnetizes the two rods similarly the adjacent points on the lengths of the rods will have the same magnetic polarity. Hence, they repel each other with the result that the pointer is deflected against the controlling torque of a spring or gravity. The force of repulsion is approximately proportional to the square of the current passing through the coil Whatever be the direction of current in the coil, the two irons are always similarly magnetised.
- 36. Deflecting torque Produced by the current or the voltage to be measured. It is proportional to the square of the voltage or current. Hence, the instrument can be used to measure d.c. or a.c. Control torque : Spring or gravity Damping : Air friction damping
- 37. MI Instrument 37
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- 39. Advantages and disadvantages: • The instruments are cheap ,reliable and robust The instruments can be used on both A.C and D.C They cannot be calibrated with high degree of precision with D.C on account of the effect of hysteresis in the iron rods or vanes .
- 40. MEASUREMENT OF THREE PHASE POWER 40
- 41. The dynamometer type 3-phase wattmeter consist of two separate wattmeter movements mounted together in one case with the two moving coils mounted on the same spindle. The current and the pressure coil mounted together is known as an element. Therefore a 3phase wattmeter has two elements. The connection of two elements of 3 phase wattmeter is shown in diagram. 41
- 42. Two Element Energy Meter 42 The construction of a three-phase induction type energy meter appears as an assembly of two single-phase induction type energy meters in one case, having a common spindle and registering mechanism. The schematic diagram of the meter is shown in the below figure.
- 43. Two Element Energy Meter 43
- 44. Working of Three Phase Energy Meter : The principle of working of a 3-phase energy meter is similar to the single-phase energy meter. When the load is connected to the meter both the pressure coil and the current coil of the two elements mounted on the shunt and series magnet produces magnetic flux. This flux when links with the discs cause an eddy current to flow in it. Interaction of eddy currents with the flux imposed by the two coils causes the production of torque on discs. Since two discs are attached to one spindle, the torque exerted on the two discs added mechanically. Hence, rotation of the shaft gives the 3-phase energy consumed. 44
- 45. • The essential parts of the 3-phase energy meter are, • Driving system • Moving system • Braking system • Registering system. 45
- 46. Driving System: The combination of a shunt and series magnet is called an element. So, it consists of two elements. The connections for the winding of these electromagnets are shown in the figure. The shunt magnet windings are provided such that, during no-load conditions, the torque developed by both the shunt magnets is opposite in nature. In order to make the resultant driving torque equal to zero, a magnetic shunt is provided in the meter. Its position is adjusted until the discs stop rotating at no-load. The necessary driving torque is obtained due to the interaction between the shunt and series magnetic fields on the disc in each element. 46 Moving System : The moving system consists of two aluminum discs (one for each element) which are mounted over a single spindle. The torque developed by each disc will be added up and as a result, the total torque will be proportional to the 3-phase power consumed by the load.
- 47. Braking System : This system provides the necessary braking action on the discs. An individual permanent magnet is provided for each disc. There is a provision for adjusting the position of each brake magnet in order to vary the braking torque. Registering System : This system is attached to the moving system through the pinion and a gear train. It continuously counts or registers the number of revolutions made by the discs. It means it integrates the power consumed by the three-phase load over a considered period of time which is nothing but energy. 47
- 48. Instrument Transformer These are generally classified as • Current Transformer • Potential Transformer 48
- 50. 50 What is Instrument Transformer ? A transformer that is used in conjunction with a measuring instrument. It utilizes the current-transformation and voltage transformation properties to measure high ac current and voltage.
- 51. 51 Importance of Instrument transformers In dc circuits for current and voltage measurement we use ammeters and voltmeters. For measurement of high current ,it is usual to use low range ammeter with suitable shunt. For measurement of high voltage, low range voltmeter are used with high resistance connected in series. But for measurement of high A.C. current and voltage we cannot use these methods. We use specially constructed instrument transformers.
- 52. 52 Types of instrument transformers These instrument transformers are of two types:- 1. Current transformers 2. Potential transformers
- 53. 53 Current Transformers Current transformer normally known as c.t. is a step up transformer. These are used with low range ammeter to measure current in high voltage alternating circuits where it is not practical to connect instrument and meters directly to lines. This is step up transformer because when we step up the, voltage increases and current decreases. The current is step down in a known ratio called current ratio.
- 54. 54 Construction of C.T. C.T. has a primary coil of one or more turns of thick wire connected in series with the line whose current is to be measured. The secondary consist of large number of turns of fine wire, is connected across the ammeter terminals.
- 55. 55 Working If a current transformer has primary to secondary current ratio of 100:5 then it step up the voltage 20 times and step down the current 1/20 times of its actual value. If we know the current ratio I1/I2 and the reading of a.c. ammeter, the current can be calculated. Current = ratio × ammeter reading
- 56. 56 Importance of short ckt. Ammeter resistance is very low ,the current transformer normally works short circuited. If for any reason the ammeter is taken out of secondary winding then the secondary winding must be short ckted with the help of short ckt switch s. If this is not done, then due to high m.m.f. will set up high flux in the core and it will produces excessive core loss which produce heat and high voltage across the secondary terminals Hence the secondary of current transformer is never left open.
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- 58. 58 Potential transformer A PT is a step down transformer having many primary turns but few secondary turns. In step down the voltage decreases and current increases, thus voltage can be easily measured by using low range voltmeter. The voltage is stepped down in known ratio called voltage ratio.
- 59. 59 Construction and working of P.T. Construction A potential transformer has many primary windings but few number of secondary windings that makes it step down transformer. Voltmeter is connected to secondary winding usually voltmeter of 150 v is suitable. Working Primary terminals are connected across the line to which the voltage is to be measured. The voltmeter gives the transformed value of voltage at secondary. The deflection of voltmeter when divided by transformed ratio gives the actual voltage at primary. Line voltage = deflection / trasf. Ratio Where transformation ratio = V2/V1
- 60. 60 Precaution for P.T. Since the secondary of p.t. is connected to relays, their ratings are usually 40 to 100 Watts. For safety purpose the secondary should be completely insulated from the high voltage primary and should be in addition grounded.
- 61. 61 Types of P.T. Some types of p.t. are 1. Shell type 2. Dry type 3. Oil type Rating Type below 5000 v Shell type 5000-13800 v Dry type and oil type above 13800 v only oil type
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- 63. 63 Digital Storage Oscilloscope Definition: The digital storage oscilloscope is defined as the oscilloscope which stores and analysis the signal digitally, i.e. in the form of 1 or 0 preferably storing them as analogue signals. The digital oscilloscope takes an input signal, store them and then display it on the screen. The digital oscilloscope has advanced features of storage, triggering and measurement. Also, it displays the signal visually as well as numerically.
- 64. 64 Working Principle of Digital Storage Oscilloscope The digital oscilloscope digitizes and stores the input signal. This can be done by the use of CRT (Cathode ray tube) and digital memory. The block diagram of the basic digital oscilloscope is shown in the figure below. The digitization can be done by taking the sample input signals at periodic waveforms.
- 65. 65 Digital Storage Oscilloscope The maximum frequency of the signal which is measured by the digital oscilloscope depends on the two factors. Theses factors are the 1. Sampling rate 2. Nature of converter.
- 66. Sampling Rate – For safe analysis of input signal the sampling theory is used. The sampling theory states that the sampling rate of the signal must be twice as fast as the highest frequency of the input signal. The sampling rate means analogue to digital converter has a high fast conversion rate. Converter – The converter uses the expensive flash whose resolution decreases with the increases of a sampling rate. Because of the sampling rate, the bandwidth and resolution of the oscilloscope are limited. The need of the analogue to digital signal converters can also be overcome by using the shift register. The input signal is sampled and stored in the shift register. From the shift register, the signal is slowly read out and stored in the digital form. This method reduces the cost of the converter and operates up to 100 mega sample per second. 66 The only disadvantage of the digital oscilloscope is that it does not accept the data during digitization, so it had a blind spot at that time.
- 67. 67 Waveform Reconstruction For visualizing the final wave, the oscilloscopes use the technique of inter-polarization. The inter-polarization is the process of creating the new data points with the help of known variable data points. Linear interpolation and sinusoidal interpolation are the two processes of connecting the points together.
- 68. Applications The applications of the DSO are • It checks faulty components in circuits • Used in the medical field • Used to measure capacitor, inductance, time interval between signals, frequency and time period • Used to observe transistors and diodes V-I characteristics • Used to analyze TV waveforms • Used in video and audio recording equipment’s • Used in designing • Used in the research field • For comparison purpose, it displays 3D figure or multiple waveforms • It is widely used an oscilloscope 68
- 69. Advantages The advantages of the DSO are • Portable • Have the highest bandwidth • The user interface is simple • Speed is high Disadvantages The disadvantages of the DSO are • Complex • High cost 69
- 70. Data Acquisition System: A typical Data Acquisition System consists of individual sensors with the necessary signal conditioning, data conversion, data processing, multiplexing, data handling and associated transmission, storage and display systems. In order to optimize the characteristics of the system in terms of performance, handling capacity and cost, the relevant sub systems can be combined together. Analog Data Acquisition System is generally acquired and converted into digital form for the purpose of processing, transmission, display and storage. 70
- 71. Introduction A data acquisition system is a collection of software and hardware that allows one to measure or control physical characteristics of something in the real world. Data acquisition systems can be classified into the following two types. Analog Data Acquisition Systems The data acquisition systems, which can be operated with analog signals are known as analog data acquisition systems. Digital Data Acquisition Systems The data acquisition systems, which can be operated with digital signals are known as digital data acquisition systems. So, they use digital components for storing or displaying the information 71
- 72. The components of DAS include: – Sensors that convert physical parameters to electrical signals. – Signal conditioning circuitry to convert sensor signals into a form that can be converted into digital values. – Analog-to-digital converters that convert conditioned sensor signals to digital values. 72
- 73. Data Acquisition System Block Diagram: 73
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