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Unit 3 Transducers Engineering (Instrumentation).pdf
- 2. Resistive Transducers The transducer whose resistance varies because of the environmental effects such type of transducer is known as the resistive transducer. The resistive transducer is used for measuring the physical quantities like temperature, displacement, vibration etc. Working Principle of Resistive Transducer The resistive transducer element works on the principle that the resistance of the element is directly proportional to the length of the conductor and inversely proportional to the area of the conductor. Where R – resistance in ohms. A – cross-section area of the conductor in meter square. www.rejinpaul.com
- 4. Potentiometer & Loading Effect Potentiometer is also called as pots and it one of the most commonly used devise for measuring the displacement of the body. The potentiometer is the electrical type of transducer or sensor and it is of resistive type because it works on the principle of change of resistance of the wire with its length. It converts either linear or angular displacement into an output voltage by moving a sliding contact along the surface www.rejinpaul.com
- 5. Potentiometer & Loading Effect www.rejinpaul.com
- 6. Construction Of Potentiometer A resistance potentiometer or a pot consist of resistive element provided with a sliding contact. The sliding contact is called a wiper. The motion of the sliding contact may be translatory or rotational. Some pot uses the combination of two motions i.e. translational as well as rotational Working video: https://youtu.be/v3ag-bGVt9I www.rejinpaul.com
- 7. Loading Effect in Potentiometer The output of a potentiometric transducer is normally connected to an amplifier or a recorder or a meter which has a definite input impedance and hence a current will be drawn by this meter or recorder or amplifier. www.rejinpaul.com
- 10. Applications Calibration of voltmeter Calibration of ammeter Measurement of resistance Linear displacement measurement Liquid level measurements using floats Rotary displacement measurement Brightness control Volume control www.rejinpaul.com
- 11. ADVANTAGE: They are cheap. It is easy to use and useful in many applications where requirements are not severe. It gives sufficient output that does not require further amplification. Potentiometer efficiency is high. They are useful for the measurement of large displacement. The resolution is infinite in cermet and metal film potentiometers. www.rejinpaul.com
- 12. DISADVANTAGE: Wear. Most potentiometers last only a few thousand ro Noise. In new pots, this noise is inaudible, but it can get worse with agetations before the materials wear out Limited Power. Out of necessity, most potentiometers can dissipate only a few watts of power at most. Inertia.The friction between the potentiometer’s wiper and resistive element creates a drag or inertia that the pot must overcome before it turns. www.rejinpaul.com
- 13. Excercise Q. What are the factors while selecting a potentiometer? Ans. The important parameters that are important while selecting a potentiometer are Humidity Operating temperature Contamination and seals Shock and vibration lifecycle www.rejinpaul.com
- 15. STRAIN GAUGE A strain gauge is a device used to measure strain on an object. As the object is deformed, the foil is deformed, causing its electrical resistance to change. This resistance change, usually measured using a Wheatstone bridge, is related to the strain by the quantity known as the gauge factor. www.rejinpaul.com
- 16. PRINCIPLE A strain gage (sometimes referred to as a Strain gauge) is a sensor whose resistance varies with applied force. It converts force, pressure, tension, weight, etc., into a change in electrical resistance which can then be measured www.rejinpaul.com
- 17. Types of Strain Gauge Bonded and unbonded metal type Wire type Foil type Frame type Sheath type www.rejinpaul.com
- 18. Bonded type strain gauge www.rejinpaul.com
- 19. APPLICATION Strain Gauges are used for analysing the dynamic strains in complex structures like roads, bridges, buildings, etc. It is used for measuring tension, force, torque and stresses in structures. It is used for the measurement of force by the strain produced in load rings. www.rejinpaul.com
- 20. EXERCISE Q. What are Rosettes? Ans. The cluster of strain gauges is called the Rosettes. Practically, stress can be applied in any direction and it is not possible to orient the strain gauges along the direction of principle stress. So with the help of a combination of strain gauges, it is possible to find the strain and stress values without actually knowing their directions. It is used for specific stress analysis or transducer applications. What is the selection criterion of strain gauge? Ans. Operating temperature, Stability requirements and Nature of the strain to be detected are the three primary considerations in strain gauge selection. Also, choosing the right carrier material, adhesive, grid alloy, and protective coating plays an important role in the particular application. Why are strain gauges called the piezoresistive strain gauges? Ans. The resistance of conductor under strain is also changed due to change in resistivity of the conductor. This property is known as piezo-resistive effect and hence strain gauges are also called as piezoresistive strain gauges. www.rejinpaul.com
- 22. PRINCIPLE A load cell is a transducer that is used to create an electrical signal whose magnitude is directly proportional to the force being measured. A load cell usually consists of four strain gauges in a Wheatstone bridge. The change in resistance of the strain gauge can be utilized to measure strain accurately when connected to an appropriate measuring circuit configuration. The electrical signal output is typically very small in the order of a few millivolts. It is amplified by an instrumentation amplifier before sending it to the measurement system. www.rejinpaul.com
- 24. ADVANTAGES Rugged and compact construction No moving parts Can be used for static and dynamic loading Highly Accurate Wide range of measurement Can be used for static and dynamic loading www.rejinpaul.com
- 25. DISADVANTAGES Mounting is difficult. Calibration is a tedious procedure. It cannot be used for the pressure measurement of highly reactive or corrosive materials because they can damage the gauge. It requires continuous electric energy for the production and display of signals. It cannot be used for the measurement of very high pressure if the diaphragm use is of plastic www.rejinpaul.com
- 26. APPLICATIONS DNA SYNTHESIS COPTER PEDAL TESTING BOLT FASTENING CATHETAR FORCE MEASUREMENT www.rejinpaul.com
- 28. The proving ring is a device used to measure force. It consists of an elastic ring of known diameter with a measuring device located in the center of the ring. Proving rings can be designed to measure either compression or tension forces. Some are designed to measure both. www.rejinpaul.com
- 30. WORKING The basic operation of the proving ring in tension is the same as in compression. However, tension rings are provided with threaded bosses and supplied with pulling rods which are screwed onto the bosses. Proving rings can be designed to measure either compression or tension forces. Some are designed to measure both. The basic operation of the proving ring in tension is the same as in compression. However, tension rings are provided with threaded bosses and supplied with pulling rods which are screwed onto the bosses www.rejinpaul.com
- 31. APPLICATIONS Proving-ring technology is used to measure thermally induced displacements in large boreholes in rock Proving ring is used to measure force with its elastic ring of know diameter www.rejinpaul.com
- 32. MERITS AND DEMERITS MERITS – It is easy to handle It gives accurate values Can be used in all condition DEMERITS – It gives a zero error Parallel axes error may occur www.rejinpaul.com
- 34. Working principle A resistance temperature detector (RTD) can also be called a resistance thermometer as the temperature measurement will be a measure of the output resistance. Resistance temperature detectors (RTDs), are sensors used to measure temperature The main principle of operation of an RTD is that when the temperature of an object increases or decreases, the resistance also increases or decreases proportionally www.rejinpaul.com
- 36. Applications- Air conditioning and refrigeration servicing Food Processing Stoves and grills Textile production Plastics processing Petrochemical processing Micro electronics Air, gas and liquid temperature measurement Exhaust gas temperature measurement www.rejinpaul.com
- 37. Advantages Very stable output Linear and predictable Easy to verify and recalibrate High accuracy No special wire required for installation www.rejinpaul.com
- 38. Disadvantages- More limited temperature range (-200 deg C to 500 deg C) High initial price Slower response time than a thermocouple www.rejinpaul.com
- 39. EXERCISE Q. Why is platinum considered most suitable material for RTDs for most laboratory work and for industrial material measurements of high accuracy? Ans. Platinum is the most suitable material for RTDs for most laboratory work and for industrial measurements of high accuracy because Platinum has optimum characteristics for service over a wide temperature range. Platinum is available in pure form and is stable under varying environmental conditions. Platinum’s resistance-temperature curve is simple and holds over a wide range of temperature (-263 °C to 545 °C) with high precision. Its resistivity tends to increase less rapidly at a higher temperature than for the other metals. Platinum can withstand high temperature while maintaining stability. www.rejinpaul.com
- 41. PRINCIPLE A thermistor is a type of resistor whose resistance strongly depends on temperature.The word thermistor is a combination of word ‘thermal’ and ‘resistor’ www.rejinpaul.com
- 42. Advantages High sensitivity Can be used at normal room temperature. Measurements upto high temperatures. Small in size Low cost www.rejinpaul.com
- 43. Disadvantages High sensitivity allows the thermistor to work at low temperature range. Need to be sterilized Require form of power like batteries www.rejinpaul.com
- 44. Applications Applications include temperature measurements , compensation and control. Used In detection of fire alarms. www.rejinpaul.com
- 45. EXERCISE: Q. Why is thermistor well suited to precision temperature measurement, control and compensation? Ans. Thermistors are well suited to precision temperature measurement, control and compensation because they are rugged in construction, cheap and highly sensitive devices. Q. What is the application of PTC thermistors? Ans. PTC thermistors can be used as heating elements in small temperature controlled ovens. Q. What are the application of NTC type thermistors? Ans. NTC thermistors can be used as inrush current limiting devices in power supply circuits. It is also used for temperature compensation i.e. for cancelling the effect of temperature on other electronic devices. It is also used for measurement and control of liquid flow, temperature, gas flow, etc. Q. Why most of the thermistors have negative temperature coefficient? Ans. The reason is that when thermistors temperature is increased, the concentration of charge carriers increases resulting in a decrease in resistance. www.rejinpaul.com
- 47. PRINCIPLE A gyroscope is a spinning wheel or disc in which the axis of rotation is free to assume any orientation by itself. When rotating, the orientation of this axis is unaffected by tilting or rotation of the mounting, according to the conservation of angular momentum. Because of this, gyroscopes are useful for measuring or maintaining orientation www.rejinpaul.com
- 48. WORKING Gyroscope works on the principle that Angular momentum changes in the direction of Torque. M=mass of flywheel W=Mg N=W=Mg while flywheel is rotating with angular velocity ωs, in anticlockwise direction. Now the torque due to weight of flywheel is in positive y direction. Since angular momentum goes in the direction of torque so flywheel will go to y axis changing direction of Ls from x axis to y axis. so, dLs=lW.dt Where dLs/dt is torque by weight of flywheel. We can now find the angular frequency of pivot Ω. ΩLs=lW (balancing torques) We get Ω=lWIsωs where Is is moment of inertia of flywheel. That is the reason why rotating gyroscope does not behave like pendulum when it is in normal gravitational field. Same concept will be applied when flywheel moves in clockwise direction; now flywheel will move in negative y direction, as angular momentum points toward pivot. www.rejinpaul.com
- 49. Advantages and Disadvantages of Gyroscope ADVANTAGES : They really make smaller stabilized system. They impart greater Stabilization. They are Accurate and Easy to understand. `DISADVANTAGES : The Gyroscopes are really expensive, but not in the terms of camera stabilization. They are noisy if you are concerned about sound. Pan and tilt speed is limited. They take too much time to get up the speed. They require another cable, battery and an inverter to work. www.rejinpaul.com
- 50. Application of Gyroscope Coriolis Force Gyros Lateral movement induced by rotation. Torsional Resonator Miniature Gyroscope in a chip Wine glass Gyroscope Conservation of momentum- Basic Gyroscope and Basic Rate Gyroscope. Centripetal Force- Accelerator used for Rotation Sensing. www.rejinpaul.com
- 52. HOT WIRE ANEMOMETER It is used for measurement of velocity of flow Resistance wire is used as basic sensor which heated initially by passing an electric current These resistance wire are mounted on probe is cooled because of fanning effect The amount of cooling depends on the velocity variation how velocity www.rejinpaul.com
- 53. TYPE OF HOT WIRE ANEMOMETER 1.CONSTANT TEMPTYPE (CCT) 2.CONSTANT CURRENT TYPE (CTT) Principle: Sensor resistance is kept constant by servo motor Advantages: Easy to use Accepted standard Disadvantages: More complex circuit Principle: Current through sensor is kept constant Advantages: High frequency response Disadvantage: Difficult to use Risk of probe burnout www.rejinpaul.com
- 55. A fine resistance wire carries fixed current is exposed to the flow velocity The wire attains an equilibrium temp when I2r heat generated in it is just balanced by the convective heat loss from its surface and it is essentially constant Therefore the wire temp must adjust itself to change the convective heat until equilibrium is reached This wire temp is measure of the flow velocity which is measured in terms of electrical resistance www.rejinpaul.com
- 57. Current through the wire is adjusted to keep the wire temp constant .the current required to keep temp constant is a measure of the velocity It is most widely used method for measurement of velocity Steady flow It can only be used for steady flow not for large fluctuation in the velocity of flow It can be used for measuring both average and fluctuating component of velocity by balancing the bridge operation automatically With zeroflow velocity the bridge excuition is shut off (iw=0),the hot wire assume the fluid temp www.rejinpaul.com
- 58. R3 is manually adjusted so that R3>Rw to unbalance the bridge If excitation current is turned on unbalanced bridge produces an unbalanced voltage be which is applied to high gain current amp So the current flow through Rw increase its temp thus its resistance Rw increases ,Ee decreases . Since the amp has limited voltage so voltage cannot become zero Frequency response = 17000 cps (cycle per second ) Average flow velocity = 9 m/s 30000cps for 30 m/s 50000cps for 90m/s www.rejinpaul.com