Engineering topics strange gages report ppt
- 1. EE - 304 . 56 1 Name of the Faculty : B.V.Ramadevi Designation : Lecturer Branch : Electrical Institute : Govt. Polytechnic , Gudur Year/Semester : III semester Subject : Electrical Measurements and Batteries Code :EE-304 Topic : Transducers and Sensors Sub topic : Transducers Duration : 50 minutes Teaching Aids : PPT DEPARTMENT OF TECHNICAL EDUCATION ANDHRA PRADESH
- 2. RECAP Q) Name the devices used for measurement of Temperature Answer : Thermistors and Thermocouples Q) Which type of thermistors are most commonly used ? a) NTC type b) PTC type c) Both d) None 2 EE - 304 . 56
- 3. Objectives After completion of this period you should be able to know about Strain transducers Factors influencing the choice of transducers 3 EE - 304 . 56
- 4. Strain gauge Strain serves as an index of pressure Strain gauge are classified as Mechanical, Optical and Electrical Strain transfers the from the specimen to the basic gauge element Electrical strain gauge measure the changes that occurs in Resistance, Inductance and Capacitance The most versatile type electrical strain gauge is the Resistance type. 4 EE - 304 . 56
- 5. High accuracy Reliable data and known performance Flat frequency response from steady state to several thousand Hertz Standardized signal conditioning device Advantages of using Electrical Strain Gauge: 5 EE - 304 . 56
- 6. Experimental stress analysis of machine structures Construction of force , torque , pressure , flow and acceleration transducers Applications of Strain Gauges: 6 EE - 304 . 56
- 7. Types of Strain Gauges: 1. Unbonded resistance wire strain gauge 2 . Bonded resistance wire strain gauge 3. Bonded metal foil strain gauge 4. Bonded semiconductor strain gauge 5. Vacuum deposited thin metal film strain gauge 6. Sputter deposited thin metal strain gauge 7. Diffused metal strain gauge 7 EE - 304 . 56
- 8. Strain Gauge Measurement Technique A metal wire or the foil whose strain is to be measured is attached to the element The element deforms as soon as stress is applied. As strain is a fractional change in resistance reflects strain of gauge and element Wheatstone bridge has very high sensitivity for deflecting very small change in resistance 8 EE - 304 . 56
- 9. Unbonded Resistance Strain Gauge Construction : It consists of a wire stretched between two points in an insulating medium such as air The diameter of the wire is about 25µm This wire is kept under tension so that there is no sag and no free vibration Unbonded strain gauges are usually connected in a bridge circuit 9 EE - 304 . 56
- 10. Working : apply load Stress produced strain changes resistance of one arm of the bridge bridge gets unbalanced and produces O/p O/p is proportional to strain 10 EE - 304 . 56
- 11. Bonded Resistance Wire Strain gauge The wires of resistive elements are exposed to air and is cemented to the base The base may be a thin sheet of Paper , Bakelite or Teflon The resistance element may be in the form of wire (zigzag shape ),foil or film of the material 11 EE - 304 . 56 Bonded type strain gauge Base
- 12. Metallic Bonded Strain Gauge Construction: A fine wire element about 25 µ m or less in diameter is looped back and forth on a base or mounting plate The base (carrier ) is connected to the member subjected to stress. The grid of line wire is connected on a carrier Carrier may be sheet of paper , Bakelite , Teflon 12 EE - 304 . 56
- 13. Working : tensile strength is applied to the structure (to which gauge is fixed) Wire elongates wire length increases and diameter decreases resistance increases increase in a resistance is function of strain 13 EE - 304 . 56
- 14. Gauge Factor The measurement of sensitivity of a material to strain is called gauge factor It is the change ratio of change in resistance ∆R ⁄ R to the change in length ∆l ⁄ l ∆R ⁄ R K = ∆l ⁄ l where ∆R = change in resistance in Ω R = resistance of the gauge without strain in Ω ∆l = change in the length in metres l = initial length without strain in metres 14 EE - 304 . 56
- 15. Strain is defined as the change in length divided by the original length strain ( ) = ∆l ⁄ l Then K = {∆R ⁄ R} Gauge factor K = 2 for most strain gauges Initial resistance R = 120 Ω Strain Gauge is normally formed as one arm of the bridge. The bridge may be a.c. or d.c. actuated 15 EE - 304 . 56
- 16. Foil Strain Gauge Strain is detected by using foil Nichrome Constantan Isoelastic, Nickel and Platinum metals are used for the foil and wire Foil can be etched on the base Foils are thin(2 mm thickness) and flexible Resistance value of the foil gauges vary 50 to 1000 Ω Foil type Strain gauge 16 EE - 304 . 56
- 17. Due to large surface area foil gauges have a much greater dissipation capacity Foil strain gauges can be used at a higher operating temperature range The characteristic of wire type and foil type strain gauge are similar Strain gauge factor is also same for wire type and foil type strain gauges These strain gauges can be mounted in remote and restricted places and especially on curved surfaces They can be fabricated on a large scale and in any shape 17 EE - 304 . 56
- 18. Semiconductor Strain Gauge The basic principle of operation is Piezoresistive effect When strain applied, resistivity of the semiconductor changes corresponding to it resistance changes ( In metallic gauges the change in resistance is mainly due to dimension strained) 18 EE - 304 . 56
- 19. Construction Germanium and silicon are used as semiconductor material Thickness of semiconductor wafer of filaments is 0.05 mm They are bonded on insulating material Teflon Gold leads are used for making contacts These strain gauges are fabricated along with an IC OP-AMP which act as a pressure transducer 19 EE - 304 . 56
- 20. semiconduct or Gold leads Electrodes Base Diagram of Semiconductor Strain Gauge 20 EE - 304 . 56
- 21. Semiconductor strain gauges are used when a very high value of gauge factor is desired The gauge factor of these are about130+10% for a unit of 350 Ω The gauge is stable and can be operated with conventional indicating and recording systems 21 EE - 304 . 56
- 22. Applications of Semiconductor Strain Gauges Measurement of strain and the associated stress in experimental stress analysis Measurement of pressure, acceleration and Temperature Investigate strain in many structural materials over a wide range of environmental conditions In medical field investigation 22 EE - 304 . 56
- 23. Advantages : High gauge factor Long life (more than 10 X 106 operations ) Excellent frequency response Very small in size ranging from 0.7 to 7 mm Disadvantages: 1. Very sensitive to change in temperature 2. The gauge is more expensive 3. The linearity of the gauge is poor 4. The gauge factor varies with strain 23 EE - 304 . 56
- 24. Selection of a transducer or Factors influencing the Choice of Transducer Selection of an appropriate transducer is an important decision to be taken for obtaining accurate results This is because ,it is transducer through which the I/p signal is transformed to a proportional electrical signal 24 EE - 304 . 56
- 25. The following points are to be considered while selecting a particular transducer 1. The nature of the physical quantity to be measured Operating principle Operating range Sensitivity Accuracy Electrical parameters Transient and frequency response Environmental considerations Usage and ruggedness Errors 25 EE - 304 . 56
- 26. 11. Loading effects 12. Stability and Reliability 13. Cost and availability 26 EE - 304 . 56
- 27. SUMMARY In this period , we have discussed about • Types of strain tranducers • Factors influencing the choice of tranducers
- 28. Quiz Q) What is the most versatile type of Electrical Transducer a) Resistance type b) Inductance type c) Capacitance type d) None Q) Identify the insulating materials used for base in Strain gauges a)Paper b)Bakelite c)Teflon d)All the above 28 EE - 304 . 56
- 29. Questions 1. What is a Strain Gauge ? 2. List various types of Strain Gauges. 3 . What are the factors to be considered while selecting the Transducers ? 4. Explain briefly about unbonded and bonded resistance wire strain gauge. 29 EE - 304 . 56