Measurement of Non-Electrical Quantities

Information about Measurement of Non-Electrical Quantities

Published on July 23, 2014

Author: islavatu

Source: authorstream.com

Content

Measurement of Non-Electrical Quantities : 1 Measurement of Non-Electrical Quantities Transducer: 2 Transducer Transducer is defined as a device which convert energy or information from one form to another. Transducer may be mechanical, electrical, magnetic, optical, chemical, thermal or combination of two or more of these. Electrical Transducers: 3 Electrical Transducers Most quantities to be measured are non-electrical such as temperature, pressure, displacement, humidity, fluid flow, speed, pH, etc., but these quantities cannot be measured directly. Hence such quantities are required to be sensed and changed into some other form of quantities. Therefore, for measurement of non-electrical quantities these are to be converted into electrical quantities (because these are easily measurable). This conversion is done by device called Electrical Transducer Factors influencing the choice of Transducers: Factors influencing the choice of Transducers Operating principle -resistive, inductive and capacitive Sensitivity - desired o/p for given i/p Operating range- wide range Accuracy - high and to meet industry specified Cross sensitivity- to check the sensitivity in different manner Transient and frequency -to meet the desired time domain specifications and frequency response Loading effects- high i/p impedance and low o/p impedance Environmental compatibility- work under specified environmental conditions Insensitivity to unwanted signals - allow only desired signals Usage and ruggedness -size and weight Electrical aspects- type of cable used Stability and reliability- high degree of stability and more reliable Static characteristics -low non linearity, low hysteresis loss, high resolution and high degree of repeatability 4 PowerPoint Presentation: 5 Classification of transducers Based on principle of transduction Active & passive Analog & digital Inverse transducer PowerPoint Presentation: 6 Based on principle used Thermo electric Magneto resistive Electro kinetic Optical PowerPoint Presentation: 7 Device which need external power for transduction from auxiliary power source Eg : resistive, inductive, capacitive Without power they will not work Passive transducer PowerPoint Presentation: 8 Active transducer No extra power required. Self generating Draw power from input applied Eg. Piezo electric x’tal used for acceleration measurement, photovoltaic cells Resistive Transducer: Resistive Transducer In this transducer, the resistance of the output terminal of the transducer gets varied according to the measurand. Some resistive transducers are:- Potentiometer Strain gauge Resistance Themometer PowerPoint Presentation: 10 RESISTIVITE POTENTIOMETERS A resistance element provided with a movable contact. This is very simple and cheap form of transducer and is widely used. It convert linear or rotational displacement into a voltage. The contact motion can be Linear rotation combination of the two such as helical PowerPoint Presentation: 11 Strain Gauges: 12 Strain Gauges It is a device which is used for measuring mechanical surface strain and one of the most extensively used electrical transducer. It can detect and convert force or small mechanical displacement into electrical signal. Many other quantities such as torque, pressure, weight and tension etc, which involve the effect of force or displacement can be measured with string gauge. Gauge Factor (G) = Change in resistance per unit strain. Strain Gauge can be of four types:- Wire strain gauge Foil strain gauge Thin film strain gauge Semiconductor strain gauge Unbonded metal strain gauges. : Unbonded metal strain gauges. Wires made of various copper nickel, chrome nickel or nickel alloys. Used almost all transducer applications Bonded wire strain gauges Used for both stress and strain analysis and or construction transducer. Resistance wire should have following characteristics High value of gauge factor Gf Resistance of strain gauge should be as high Low resistance temperature coefficient Free from hysteresis effects Have linear characteristics Frequency response is good 13 PowerPoint Presentation: Bonded metal foil strain gauges It is the extension of metal wire strain gauges. These have much greater heat dissipation capacity as compared with wire wound strain gauges. Used for both stress analysis and for construction of transducers. Used over large temperature ranges(-269 to 816 degrees) 14 Thin film metal strain gauges: Thin film metal strain gauges Used for the fabrication transducers Exhibit better time and temperature stability. Semiconductor strain gauges: used where high degree of accuracy needed Merits: High gauge factor Accuracy limit is 0.05% Used for measurement of local strain analysis Disadvantages Very sensitive to temperature changes Poor linear characteristics More expensive 15 PowerPoint Presentation: 16 PowerPoint Presentation: 17 PowerPoint Presentation: 18 PowerPoint Presentation: 19 Inductive transducers are those in which SELF INDUCTANCE of a coil or the MUTUAL INDUCTANCE of a pair of coil is altered due to variation in the measurand. Change in inductance ∆L is measured. INDUCTIVE TRANSDUCERS The self inductance of a coil refers to the flux linkage within the coil due to current in the same coil. Mutual inductance refers to the flux linkages in a coil due to current in adjacent coil. PowerPoint Presentation: 20 PowerPoint Presentation: 21 A capacitor is an electrical component which essentially consists of two plates separated by an insulator. The property of a capacitor to store an electric charge when its plates are at different potential is referred to as capacitance. CAPACITIVE TRANSDUCERS PowerPoint Presentation: 22 Capacitance C = . If the capacitance is large, more charge is needed to establish a given voltage difference. The capacitance between two parallel metallic plates of area PowerPoint Presentation: 23 PowerPoint Presentation: 24 Linear Variable Differential Transformer (LVDT) : 25 Linear Variable Differential Transformer (LVDT) PowerPoint Presentation: 26 PowerPoint Presentation: There is one primary winding connected to an ac source (50 Hz – 20 kHz), excitation 3 – 15 V rms . Core is made of high permeability soft iron or nickel iron. Two secondary windings are connected in series opposition PowerPoint Presentation: 28 PowerPoint Presentation: Geometric centre of coil arrangement is called the NULL position. The output voltage at the null position is ideally zero. However it is small but nonzero (null voltage). Why? Harmonics in the excitation voltage and stray capacitance coupling between the primary and the secondary Manufacturing defects. PowerPoint Presentation: 30 Advantages Wide range of displacement from µm to cm. Frictionless and electrical isolation. High output. High sensitivity [sensitivity is expressed in mV (output voltage)/ mm (input core displacement)]. PowerPoint Presentation: 31 Disadvantages Sensitive to stray magnetic fields. Affected by vibrations. Dynamic response is limited mechanically by the mass of core and electrically by frequency of excitation voltage. Tpes of Pressure Measurement Devices: Tpes of Pressure Measurement Devices Mechanical instruments(manometers, belt type of gauges) Electro-mechanical instruments Electronic instruments 32 Pressure Measurement: 33 Pressure Measurement The measurement of force or pressure can be done by converting the applied force or pressure into displacement by elastic element ( such as diaphragam, capsule, bellows or bourdon tube) which act as primary transducer. This displacement, which is function of pressure is measured by transducer which act as secondary transducer (these may be potentiometer, strain gauge, LVDT, piezoelectric,etc.). Resistive transducers: Resistive transducers 34 Inductive transducers: Inductive transducers 35 Capacitive transducers: Capacitive transducers 36 Photoelectric transducers: Photoelectric transducers 37 Measurement of Torque: Measurement of Torque 38 Inductive transducer: Inductive transducer 39 Digital methods: Digital methods 40 Magneto-strictive transducers: Magneto-strictive transducers 41 Measurement of linear velocity: Measurement of linear velocity Electromagnetic transducers Moving magnet type Moving coil type 42 Moving magnet type : Moving magnet type Advantages: requires less maintenance o/p voltage is linearly proportional to velocity Used as event markers Disadvantages : Affected by stray magnetic fields Limited frequency response Not very useful for measurements of vibrations 43 Moving coil type velocity: Moving coil type velocity 44 Advantages: This systems forms a closed magnetic circuit with constant air gap Device is placed in antimagnetic case reduces the stray magnetic fields Seismic type velocity transducer: Seismic type velocity transducer Frequency of output is equal to frequency of vibrations Disadvantage: Both moving coil and seismic transducers are connected moving member 45 Measurement of angular Velocity: Measurement of angular Velocity Electrical Tachometers Electromagnetic Tachometer generators D.C. t achometer generators A.C. tachometer generators 46 D.C.Tachometer Generators: D.C.Tachometer Generators 47 Voltage generated is proportional to speed. Polarity of output voltage indicates direction of rotation D.C.Tachometer Generators: D.C.Tachometer Generators Advantages: The direction of rotation directly indicated by the polarity of the output voltage The output voltage us 10 mv/rpm & can be measured with conventional type voltmeters Disadvantages Maintenance problems due to brushes Input resistance of the generator is high as compared to output resistance 48 A.C. Tachometer Generators: A.C. Tachometer Generators 49 A.C. Tachometer Generators: A.C. Tachometer Generators Consists of a rotating magnet and coil is wound on stator part The rotation of the magnet causes an emf induced in the stator coil. The amplitude or frequency of this emf is proportional to the speed of rotation. The output is rectified and measured by PMMC Limitations: Large no.of poles required to produce high value of o/p at low speed The frequency pulses are measured by electronic counters 50 Drag Cup Rotor A.C. Tachometer: Drag Cup Rotor A.C. Tachometer Stator windings are displaced by 90 degrees with each other. Reference winding Quadrature winding Rotor is a thin aluminum cup having low inertia called drag cup that rotes in the field of air gap. 51 PowerPoint Presentation: 52 The emf induced in the quadrature coil is proportional to rotor speed and with voltage applied to rotor speed and in phase with the voltage applied reference coil. Hence voltage o/p available at the terminals of quadrature coil is a function of speed Advantages : The linear relationship between i /p and o/p Disadvantages: The input voltage to be maintained constant Difficult to calibrate At high speed there should be a non linearity between i /p and o/p Digital methods for speed measurement : Digital methods for speed measurement High speed measurements is possible with digital pickups in conjunction with digital frequency meter Digital pickups- no physical contact required with the shaft Two types of digital pickups Photoelectric pickups Inductive pickups 53 Photoelectric tachometer: Photoelectric tachometer An opaque disc on the rotating shaft with equidistant holes on its periphery. One side of the disc a light source and other side a light sensor(photo transistor) Advantages: Output format is digital The pulses amplitudes are constant. disadvantages: Light sourse is to be replaced Accuracy is depend on time period of pulses 54 Toothed rotor variable reluctance tachometer: Toothed rotor variable reluctance tachometer Consists of a metallic toothed rotor mounted on the shaft whose speed is to be measured. The magnetic pickup is placed near the toothed rotor. Magnetic pickup consists of a small permanent magnet with a coil wound round it. 55 PowerPoint Presentation: When the rotor rotates, the reluctance of the airgap between pickup and the toothed rotor changes giving rise to an induced e.m.f in the pickup coil. The frequency of the pulsed induced voltage will depend uponthe number of teeth of the rotor and its speed of rotation. Suppose the rotorhas T teeth, the speed of rotation is n rps and number of pulses per second is P. Number of pulse per revolution = T Hence speed n = pulses per second / no. of teeth = P / T rps Advantages : Simple and rugged construction Maintenance free Easy to calibrate Information from the device easily transmitted 56 Stroboscope and stroboscopic methods : Stroboscope and stroboscopic methods Simple, portable manually operated device which may be used for measurement of periodic or rotary motions. instrument is source of variable frequency flashing light, the flashing frequency being set by the operator. The speed is measure by adjusting the frequency so that the moving objects are visible only at specific intervals of time Stroboscope consists of a source of flashing light whose frequency can be varied and controlled this source is called strobotron. 57 Strobotron : Strobotron It is hot cathode gaseous discharge tube Has cathode, anode and two grids(inner & outer) The potential odf outer grid is increased or inner grid decreased beyond certain limit, the conduction starts. Flashing of light is started by a signal from an oscillator or multivibrator. Has maximum flashing rate of 300 per second with effective discharge of 10 to 100 micro sec. the discharge current may be of 300 A. 58 Shaft speed measurements: Shaft speed measurements A distinctive mark is made on the shaft or on a disc attached to the shaft. Stroboscope is made to flash light directly on yhe mark. Under these conditions the speed is equal to flashing frequency. The scale of stroboscope is calibrated in terms of speed. 59 PowerPoint Presentation: 60 When the disc appears to be stationary, the speed of rotating machine is given by n = f / m Where f = number of flashes per second And m = number of point marks. PowerPoint Presentation: Advantages : This method imposes no load on the shaft It requires no special attachments with the shaft Particularly useful where it is impossible to make contact with the shaft Convenient to use for spot checks on machinery speeds and laboratory work. Disadvantages : Less accurate than using of digital meters It requires subdued lighting conditions for efficient operation Commercial models of stroboscopes are available in the speed range of 600 to 20000 rpm 61 Measurement of vibrations: Measurement of vibrations Vibration measurements are frequently carried out on rotating and reciprocating machinery for analysis, design and trouble shooting purposes Vibration monitoring is carried out on such important machines as power station turbines and generators to give an early warning of impending conditions which may develop and lead to complete failure and destruction of the equipment . Nature of vibrations: Most vibrations are sinusoidal displacement of the vibrating member about to its mean position. For sinusoidal vibration, the displacement is given by Velocity Accelration 62 Seismic type transducers: Seismic type transducers Also called seismic accelerometers Used in two modes i) displacement ii) acceleration mode 63 Types od accelerometers: Types od accelerometers Potentiometric type 64 Inductive type: Inductive type 65 Advantages: Used for measurement of vibrations of higher frequencies It is a contact less device Offers low resistance to the motion than potentiometer and is capable of much better resolution LVDT accelerometers are used for steady state and low frequency vibrations measurements Piezo-electric type transducer: Piezo-electric type transducer 66 PowerPoint Presentation: 67 Output of LVDT Thermo-couple: 68 Thermo-couple The thermocouple is one of the most commonly used method for measuring the process temperature. The operation is based on seebeck effect. Thermo-couple consists of two dissimilar metals joined together as shown. It forms two junctions 1 and 2 in which one junction is hot and other is cold. Due to this difference in temperature, an e.m.f. is generated and electric current flow in circuit. 1 2 2 1 2 Flow Measurement: 69 Flow Measurement Electromagnetic Flow meter:- This is suitable for measurement of slurries, sludge and any electrical conducting liquid. flow PowerPoint Presentation: 70 Electromagnetic flow meter consist of insulated electrodes pair buried in opposite sides of non conducting pipe placed in magnetic field of electromagnet. The voltage induced across electrodes is E=Blv volts Liquid Level Measurement: 71 Liquid Level Measurement Gamma Ray Method Geiger Muller tube Source of Gamma rays The liquid level can be measured with ultrasonic method and by using float also PowerPoint Presentation: 72 PIEZOELECTRIC AND HALL EFFECT TRANSDUCERS PowerPoint Presentation: 73 Piezoelectricity Phenomenon of generating an electric charge in a material when subjecting it to a mechanical stress (direct effect). and Generating a mechanical strain in response to an applied electric field (converse effect). Piezoelectric materials are Anisotropic – Electrical and mechanical properties differ along different directions PowerPoint Presentation: 74 PowerPoint Presentation: 75 There are two families of constants: ‘g’ constants and ‘d’ constants. In the constants the first subscript refers to the direction of electrical effect and the second to that of the mechanical effect according to the axis systems. PowerPoint Presentation: 76 PowerPoint Presentation: 77 PowerPoint Presentation: 78 Commercially available Hall generators made of : Bulk Indium Arsenide (InAs) Thin Film InAs Gallium  Arsenide (GaAs) Indium Antimonide (InSb).   Summary: 79 Summary PowerPoint Presentation: 80 Thank you

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