lec 2_Real_time.pdf
- 2. Announcements https://sites.google.com/view/real-time-1819 Lab is independent (100%) Groups by the end of the day Seminars! (3-4 hours) <> Projects (22 group) Quizzes/HomeWorks/Attendance/Class and Lab participation/Seminar 10% (Quize 5, Seminar 2, Absence, participation, HW. 3) 2
- 3. 3
- 4. Sensors and Their Classification Sensors and actuators are examples of transducers A transducer is a device that converts one physical quantity into another Sensor: an input transducer (i.e., a microphone) Actuator: an output transducer (i.e., a loudspeaker)
- 5. widely used sensors include those that are: 1. Resistive 2. Capacitive 4. Piezoelectric 3. Inductive 5. Photoelectric 6. Pyroelectric 7. Hall effect 8. Thermocouple 5
- 7. 1.Resistive Sensors 1.1 Potentiometers: It is a resistance element with a sliding contact which is moved over the length of the element. Used for monitoring Linear or circular displacements. The fraction ratio (f ) is equal to f = xi/L = fi/ft If the total track resistance =Rp then the resistance between the sliding terminal and the reference terminal = f Rp
- 8. Potentiometers are linear elements (Vo is linearly proportional to Vs) but as a load is placed across the output linearity disappear and error is introduced. Req= f RpRL/ f Rp+RL Vo=Vs* Req/ Req+(1-f )Rp =Vs f a (Rp/RL)f (1-f )+1 Error=f Vs – Vo = Vs(Rp/RL)(f 2-f 3) d(error)/df =0 f =2/3 for max. error 1.Resistive Sensors
- 9. EX1: A potential resistor of 500W is connected with a multi-meter which has an internal resistor of 10KW calculate the error is the Vs=4v when f=0.5 (Vo=?) 𝐸𝑟𝑟𝑜𝑟 = 𝑣𝑠𝑅𝑝 𝑅𝑙 ∗ 𝑓2 − 𝐹3 𝐸𝑟𝑟𝑜𝑟 = 4 ∗ 500 10000 ∗ 0.52 − 0.53 = 0.025 HW: If a voltmeter of 10KW internal resistance is connected to a potentiometer of 500W total resistance which is connected to a 10V voltage source 1)find the error if the slide is a)at the middle b)at the position which produces the maximum error. C) derive the expression for the maximum error. 1.Resistive Sensors
- 10. 1.2 Resistance Temperature Devices (RTDs): RTDs are made of materials whose resistance changes in accordance with temperature a)Metals RT: R at t temperature R0: R at 0 Co α: Temperature coefficient Why Platinum is used widely? Stable (linear) Wide range of temperature 1.Resistive Sensors
- 11. Example A platinum resistance thermometer has a resistance of 100 ohm at 0C. Determine the change in resistance that will occur when the temperature rises to 30 C if the temperature coefficient is 0.0039K-1 𝑅𝑡 = 𝑅0 + 𝑅0 𝛼𝑇 Δ𝑅 = 𝑅0𝛼𝑇 = 100 ∗ 0.0039 ∗ 30 = 11.7W 11
- 12. Photo conductive: semiconductors used for their property of changing resistance when electromagnetic radiation is incident on them. They are often called CdS cells (Cadmium-Sulfide) or LDR (Light Dependent Resistor) 1.Resistive Sensors Lux values • Dark night (0.002) • Living room (50) • (32,000–100,000) Direct sun light
- 13. LDR circuits 13 1. Voltage divider 2. Switch circuit • Require a voltage source as it does not generate voltage (active)
- 14. They are p-n junctions which produce a change in current when electromagnetic radiation is incident on the junction. Faster and more sensitive LDR. Varying current rather than resistance. The output is either on or off Photo transistor
- 15. 1.3 Strain Gauge: consists of a resistance element in the form of a flat coil of wire 1.Resistive Sensors Resistance is related to length and area of cross-section of the resistor and resistivity of the material as By taking logarithms and differentiating both sides, the equation becomes Dimensional piezoresistance Δ𝑅 𝑅 = 𝐺𝐸 E: Strain G: sensitivity or gauge factor
- 16. Example An electrical strain gauge has a resistance of 120W and a gauge factor of 2.1. Find the change in resistance when a strain of 0.0005 is applied along the length. Δ𝑅 𝑅 = 𝐺𝐸 Δ𝑅 = 𝑅𝐺𝐸=120*2.1*0.0005=0.126 W 16
- 17. T1.Resistive Sensors 17 • ResistiveTouch The resistive touchscreen consists of a glass panel with a resistive coating plus a coversheet with a conductive coating. The two layers are separated by tiny insulating dots. When the screen is touched, the coversheet flexes to make electrical contact with the coating on the glass. The controller alternately drives the X andY axes on the glass layer with a +5V current and reads the resulting voltage from the cover sheet,
- 18. Wheatstone bridge: Consists of 4 resistors in a diamond orientation, with a resistive transducer in one or more legs. Wheatstone bridge 0 , if 4 2 3 1 4 2 2 3 1 1 2 2 1 1 4 2 4 2 3 1 3 1 = = + − + = − = + = = + = = o S S o o S S V R R R R V R R R V R R R V R I R I V R R V I I R R V I I 𝑅1 = 𝑅2𝑅3 𝑅4 When Vo=0 Used to measure resistance accurately
- 19. If R4 is a sensor its resistance is changed to be R4+ΔR4 therefore Vo is changed to Vo+ΔVo Wheatstone bridge as a sensor 3 1 1 1 1 3 1 1 3 1 1 1 1 4 2 2 3 1 1 1 1 4 2 2 3 1 1 if , R R R V V R R R R R R R R R R V V R R R R R R R R V V V R R R R R R V V S o S o S o o S o + + − + + + = + − + + + = + + − + = For Strain gauge Δ𝑅 𝑅 = 𝐺𝐸 Δ𝑉 𝑜 = 𝑉𝑠𝑅1𝐺𝐸 𝑅1+𝑅3
- 20. Load cell 20
- 21. 21 HW. Show that the output voltage for a Weatstone bridge with a strain gauge which has identical platinum resistors is equal to: 𝑉0 = 𝑉𝑠𝐺𝐸 4
- 22. Different Types of Sensors
- 23. 2.Capacitive Sensors Input being measured is transformed into a capacitive change. The capacitive (C) of a parallel plate capacitor depends on the area (A), separation distance (d) and the relative permittivity (ε) of the materials between them given as: Vacuum permittivity ε0= 8.85×10−12 F/m.
- 24. 2.1 Displacement sensor: if the distance d is increased by displacement x then: This is a non-linear relationship and can be overcome by using a push-pull displacement sensor 2.Capacitive Sensors
- 25. 25 An electrolytic capacitor is made of Aluminum evaporated on either side of a very thin plastic film (or electrolyte) • Cheap • High value It is constructed of two or more alternating layers of ceramic and a metal layer acting as the electrodes. • high frequency • stable
- 26. Capacitive Touch Screen Capacitive touchscreens work by sensing the conductive properties of an object, usually the skin. When a capacitive panel is touched, a small amount of charge is drawn to the point of contact. A controller measures the current from different corners to determine the location 26
- 27. Example: A capacitive sensor consist of two plates in air, the plates being 50mm square and separated by a distance of 1mm.A new sheet of dielectric material of thickness 1mm and 50mm square can slide between the plates. Determine the capacitance of the sensor when the sheet has been displaced so that half of it is between the capacitor plates.The dielectric of the new sheet is 4 and it can be presumed as 1 for the air. 𝐶𝑇𝑜𝑡𝑎𝑙 = 𝐶𝑎𝑖𝑟 + 𝐶𝑛𝑒𝑤 𝐶𝑎𝑖𝑟 = EoErA D = 8.85 ∗ 10−12 ∗ 1 ∗ 50 ∗ 25 ∗ 10−32 1 ∗ 10−3 =1.106*10-11 =11.06 pf 𝐶𝑛𝑒𝑤 = EoErA D = 8.85 ∗ 10−12 ∗ 4 ∗ 50 ∗ 25 ∗ 10−32 1 ∗ 10−3 =4.425*10-11 =44.25 pf 𝐶𝑇𝑜𝑡𝑎𝑙=11.06+44.25=55.31 pf 27
- 28. Humidity Sensor Capacitive:A capacitive humidity sensor measures relative humidity by placing a thin strip of metal oxide between two electrodes.The metal oxide’s electrical capacity changes with the atmosphere’s relative humidity. Weather, commercial and industries are the major application areas Resistive: Resistive humidity sensors utilize ions in salts to measure the electrical impedance of atoms.As humidity changes, so does the resistance of the electrodes on either side of the salt medium. 28
- 29. If a voltmeter of 10KW internal resistance is connected to a potentiometer of 500W total resistance which is connected to a 10V voltage source 1)find the error if the slide is a)at the middle. b) at the positions of the minimum error. 29
- 30. HW: If a voltmeter of 10KW internal resistance is connected to a potentiometer of 500W total resistance which is connected to a 10V voltage source 1)find the error if the slide is a)at the middle b)at the position which produces the maximum error. C) derive the expression for the maximum error. 𝐸𝑟𝑟𝑜𝑟 = 𝑣𝑠𝑅𝑝 𝑅𝑙 ∗ 𝑓2 − 𝐹3 𝐸𝑟𝑟𝑜𝑟 = 10 ∗ 500 10000 ∗ 0.52 − 0.53 = 0.0625 𝐸𝑟𝑟𝑜𝑟 = 10 ∗ 500 10000 ∗ 2 3 2 − 2 3 3 = 0.074 𝑑 𝑒𝑟𝑟𝑜𝑟 𝑑𝑓 = 2𝑓 − 3𝑓2 = 0 ≫ 𝑓 2 − 3𝑓 = 0 ≫ 𝑓 = 2 3 HW solution
- 31. 3.1 Variable reluctance sensor: in a similar way that a electromotive force drives current through a resistance a magneto-motive force drives flux though a reluctance m.m.f=flux (f)*reluctance(S) Reluctance(So) =L/ μr μoA Reluctance of air gap (Sa)=2d/ μoA ST= So+ Sa ST= L/ μr μoA+ 2d/ μoA 3.Inductive Sensors
- 32. 3. Inductive Sensors Applications Detection of ferrous metals - steel, iron, cobalt, nickel Determine the position of a mechanical moving object Speed calculator Coil and transformer production 32
- 33. Radio-frequency identification (RFID) uses electromagnetic field to automatically identify and track tags attached to objects. Passive tags collect energy from a nearby RFID reader. Active tags have a local power source (such as a battery) and may operate hundreds of meters from the RFID reader. 33
- 34. Applications 1 2 3 …. 34
- 35. Wireless charging Advantageous Protected connections Used in medical devices Disadvantages Slow charging More expensive 35
- 36. Piezoelectricity: some dielectric materials when stretched its surfaces become charged 4.Piezoelectric Sensors Strain causes a redistribution of charges and results in a net electric • A piezoelectric material produces voltage by distributing charge (under mechanical strain/stress)
- 37. 37 • In a microphone, we need to convert sound energy (waves of pressure traveling through the air) into electrical energy • power source is not necessary hence can be used to convert kinetic energy to electricity Energy harvesting
- 38. The other way round! If you pass electricity through the same crystals, they "squeeze themselves" by vibrating back and forth! Can be used in ultrasound equipment, a piezoelectric transducer converts electrical energy into extremely rapid mechanical vibrations. In quartz clock 38
- 40. 85 to 180 Hz 165 to 255 Hz 40
- 43. Multi-sensors 43
- 44. Fire Alarm An optical smoke detector contains a source of infrared, visible, or ultraviolet light, a lens, and a photoelectric receiver. All of these components are arranged inside a chamber. Piezo-electric loud speaker is used. 44
- 45. 45
- 46. Carbon monoxide detector 46 • Carbon monoxide is an odorless, colorless, and tasteless gas that is near impossible to identify without a proper detector. • It is caused by fuels not burning completely • Metal-oxide detectors have open chambers containing sensors made of metal (tin or platinum) oxide. • Usually the percent is 0.2 ppm (particle per million) for clear air • It shouldn’t exceed 35 ppm for more than one hour
- 47. Carbon monoxide detector • When there's carbon monoxide around, the metal oxide reacts with it: the carbon monoxide "steals" oxygen from the metal oxide, converting itself into carbon dioxide, turning the metal oxide into pure metal, and producing heat at the same time. • An electronic circuit monitors the temperature inside the chamber and sounds the alarm if too much heat is produced too quickly. 47
- 48. Pyroelectricity can be described as the ability of certain materials to generate a temporary voltage when they are heated or cooled. When the pyroelectric material is exposed to infrared radiation its temperature rises and the amount of polarization is reduced. Charge decreases as temperature increases. Used to measure sensitive temperature changes. Stable and can tolerate severe conditions (flame detector) Near-infrared detector (trap camera) 6.Pyroelectric Sensors
- 50. Passive InfraRed Sensor (PIR) Passive infrared (PIR) sensors are sensitive to a person's skin temperature through emitted body radiation at mid-infrared wavelengths This sensor detects quick heat change. No energy is emitted from the sensor, thus the name passive infrared 50
- 51. Passive InfraRed Sensor (PIR) Applications: 51 • Security • Light control system • Motion activated camera • Pyrometer
- 52. Piezo-electric vs Pyro-electric 52 Mechanical Electrical Thermal Piezo-Electric Pyro-Electric/thermocouple Thermo-Elastic
- 53. The action of a magnetic field on a flat plate carrying an electric current generates a potential difference which is a measure of the strength of the field. A beam of charged particles can be deflected by a magnetic field (Hall effect). 7.Halleffect Sensors
- 54. Thermocouple: connecting two different metals produces a potential difference across the junction V α (T1 – T2) 1.The e.m.f depends only on the temperature of the junction A thermocouple is an electrical device consisting of two dissimilar electrical materials. The junctions has to be at different temperature to produce a voltage 8. Thermoelectric Sensors