UNIT-4.pptx for engineering for computer science

eng

1. UNIT – IV OPTICAL, PRESSURE AND TEMPERATURE SENSORS By Mr. E. SUBRAMANIAN, AP/CSE EINSTEIN COLLEGE OF ENGINEERING, TIRUNELVELI OAN 551 – SENSORS AND TRANSDUCERS (Regulation – 2017)

2. Photoconductive cell, photo voltaic, photo resistive, LDR – Fiber optic sensor Pressure – Diaphragm, Bellows, Piezoelectric – Tactile sensors Temperature – IC, Thermistor, RTD, Thermocouple, Acoustic sensors – flow and level measurement, radiation sensors Smart sensors – Film sensors, MEMS & Nano sensors, LASER sensors TOPICS

3. Light Sensors: Light sensors are photoelectric devices that converts light energy whether visible or infra-red light into an electrical signal. The light sensor is a passive devices and most commonly known as “Photo sensors” because it convert light energy(Photons) into electricity(electrons). Introduction

4. It is a two terminal semiconductor device whose terminal resistance will vary with the intensity of the incident light. It is also called a photo resistive device. Photoconductive materials: Cadmium Sulphide ( CdS ) and Cadmium Selenide ( CdSe ). Both materials respond slowly to changes in light intensity. The peak spectral response time is CdS – 100 ms & 10 ms for CdSe cells. Photoconductive Cell

5. Temperature sensitivity: Large change in resistance of CdSe cell with changes in ambient temperature, but the resistance of CdS remains relatively stable. Essential elements of Photoconductive cell: Ceramic substrate A layer of photoconductive material Metallic electrodes

6. Photoconductive cell circuit symbol and construction

7. Light sensitive material is arranged in the form of a long strip, zigzagged across a disc shaped base with photo sides. For added protection, a glass or plastic cover may be included. The two ends of the strip are brought out to connecting pins below the base.

8. Photoconductive cell circuit and Characteristics

9. Temperature variations cause substantial variations in resistance for a particular light intensity. Such cell not suitable for analog applications. Drawback of Photoconductive cell

10. Making street lights turn On and Off automatically according to the level of daylight. In the point of sale and inventory bar code reading devices. In alarm systems. In security devices such as motion sensing lights and cameras. Applications

11. The most common type of photoconductive device is the Photoresistor which changes its electrical resistance in response to changes in the light intensity. Photoresistor are semiconductor devices that use light energy to control the flow of electrons. The commonly used Photoconductive Cell is called the LDR. Light Dependent Resistor(LDR)

12. LDR is made from a piece of exposed semiconductor material such as Cadmium Sulphide that changes its electrical resistance from several thousands ohms in the dark to only a few hundred ohms when light falls upon it by creating hole-electron pairs in the material. The net effect is an improvement in its conductivity with a decrease in resistance for an increase in illumination. Photoconductive cells have a long response time requiring many seconds to respond to change in the light intensity.

13. Materials used as the semiconductor substrate include, lead sulphide ( PbS ), lead selenide( PbSe ), indium antimonide ( InSb ) which detect light in the infra-red range with the most commonly used of all photoresistive light sensors being Cadmium Sulphide . Cadmium Sulphide – Spectral response curve very closely matches that of human eye and can even be controlled using a simple torch as a light source. Visible spectral range – 560nm to 600 nm.

14. LDR symbol and Characteristics

15. LDR as Potential Divider

16. LDR as Light Sensitive Switch

17. It is a photo detector. “It generate a voltage which is proportional to electromagnetic radiation intensity”. It convert the EM energy into electric energy. It is a passive transducers, it does not require any external source to power them. Photovoltaic Cell

18. Photovoltaic Cell

19. The photovoltaic effect is a process that generates voltage or electric current in a photovoltaic cell when it is exposed to sunlight. These solar cells are composed of two different types of semiconductors a p-type and n-type that are joined together to create a p-n junction. An electric field is formed in the region of the junction as electrons move to the p-side and holes move to the n-side. Photovoltaic Effect

20. Photovoltaic effect

21. When light of a suitable wavelength is incident on these cells, energy from the photon is transferred to an electron of the semiconductor material, causing it to jump to a higher energy state known as the conduction band. In their excited state in the conduction band, these electrons are free to move through the material, and it is this motion of the electron that creates an electric current in the cell.

22. A photovoltaic cell is the specially treated semiconductor layer. It is comprised of two distinct layer(p-type and n-type). It converts the Sun’s energy into useful electricity through a process called the “Photovoltaic effect”. Basic Operation of PV-Cell

23. The photovoltaic light sensor is the Solar Cell. It convert light energy directly into DC electrical energy in the form of voltage or current to power a resistive load such as Light or Battery. Applications – Alternative power source from conventional batteries, such as in calculators, satellites and now in homes offering a form of renewable power. It is made from single crystal silicon PN junction. Solar Cell

24. Solar Panel

25. Characteristics of Photovoltaic Solar Cell

26. When illuminated the light energy causes electrons to flow through the PN junction and individual solar cell can generate an open circuit voltage of about 0.58 Volt. Individuals solar cells can be connected together in series to form solar panels which increases the output voltage. Commercially available solar panels are rated in Watts.

27. Its efficiency generally very low about 15 to 20%. To increase the overall efficiency of the cell commercially available solar cells use polycrystalline silicon or amorphous silicon. It can generate currents of between 20 to 40 mA per square cm.

28. Fibre optic sensors are used for sensing different types of variables such as, Temperature Liquid level Fluid flow Magnetic field Acoustic parameters Fibre Optic Sensors

29. Optical radiation happens to be the energy source in these applications with the fibre acting as medium as well as a sensor. Fibre has been divided into two groups: Active: The fibre is exposed to the energy source that affects the measurand and a consequent change in the optical propagation in the fibre is detected and related to the measurand . Passive: Light transmitted through a fibre , called input fibre , is first modulated by a conventional optical sensor and this intensity-modulated light is propagated through a second fibre called the output fibre and then dectected and corrected with the measurand .

30. Optical fibre can be used for distributed temperature sensing. Optical pulse from a pulsed laser source is sent along a fibre over a distance covering few km. Any localized change in temperature somewhere along the fibre changes its backscattered intensity ratio. This backscattered light is filtered and Raman components are detected by photodetectors from which the temperature can be known. From the pulse delay time, the location can also be identified. Temperature Sensors

31. Temperature sensing using backscatter in optical fibre

32. Usually, light propagates through a fibre by total internal reflection with appropriate cladding or even without that, if the light incident angle is properly chosen. The refractive index of air is such, with respect to that of the fibre , that no refraction can take place. If the fibre is placed in a liquid medium of a different refractive index, it is possible that light refracts into the liquid and total internal reflection inside the fibre stops, stopping light propagation in it. Liquid Level Sensing

33. The bottom end of the fibre is shaped like a prism so that with large difference in refractive indices of the fibre and the medium like air, there is internal reflection and the light travels to be detected fig.(a). When the liquid level rises to cover the bottom of the fibre , light refracts into the liquid and the detector fails to show any output fig.(b). The single position level detection has been extended for discrete multistep detection covering the entire height of the tank. Liquid Level Sensing

34. In this, a step-index multimode fibre is used and the fibre goes down carrying the light but in the return upward path, its cladding is exposed and the fibre is also given zig-zag rise with small bend radius at regular intervals in length. When no liquid is there, cladding mode operation continues and a detector at the end of the return path of the fibre shows full intensity. But with liquid rising in the tank, refraction of light into liquid occurs at each bend and the intensity detected by the detector becomes less. Liquid Level Sensing

35. Level detector using Optical fibre (a) level below sensor (b) level covering sensor

36. Acoustic pressure sensing can be done by the micro bending of a multi mode fibre shown below how light loss occurs in micro bends of a fibre . Micro bend Sensors

37. Optical fibre is placed in two corrugated plates to form a transducer as shown applied force causes micro bending in the fibre . Consequently more light is lost and the receiver detector indicates less intensity. A calibration of force in terms of the intensity of detected light.

38. They are compatible with fibre optic communication system if the data is being transmitted through aforesaid medium. It does not conduct electric current , this sensors are very useful in high voltage equipment's and explosive environment. Optical fibre sensors are free from noise of any type. Also used to measure electrical quantities . Advantages of fibre optic Sensors

39. Introduction: Pressure measurements are most important in industry such as chemical processing and manufacturing. The principles used in measurement of pressure are also applied in the measurement of temperature, flow and liquid level. Pressure = Force/Area ; The measurement of force per unit area, i.e. pressure in fluid systems. Fluid pressure is on account of exchange of momentum between the molecules of the fluid and a container wall. The total exchange of momentum is dependent upon the total number of molecules striking the wall per unit time and the average velocity of molecules. Pressure Sensors

40. To monitor fluid pressure in industrial processes involve the monitoring of the elastic deformation of diaphragms, capsules, bellows and tubes. Types of pressure measurement: Absolute pressure where the pressure is measured relative to zero i.e. a vaccum . Differential pressure where a pressure difference is measured. Gauge pressure where the pressure is measured relative to the barometric pressure. Fluid Pressure

41. For a diaphragm, when there is a difference in pressure between the two sides then the centre of the diaphragm becomes displaced. Corrugations in the diaphragm result in a greater sensitivity. This movement monitored by displacement sensor, i.e. strain gauge. Two strain gauge used for measuring the strain in a circumferential direction while two measure strain in a radial direction. Four strain gauges are then connected to form the arms of a Wheatstone bridge.

42. With the Motorola MPX pressure sensors, the strain gauge element integrated, together with a resistive network, in a single silicon diaphragm chip. When a current passed through the strain gauge element and the pressure applied at right angles to it, a voltage is produced. The output voltage is directly proportional to the pressure.

43. Diaphragm pressure gauge

44. A stack of capsules is just a bellows and even more sensitive. It is made of sealed chamber that has multiple ridges like the pleats of an accordion that are compressed slightly when the sensor is manufactured. When pressure is applied to the chamber, the chamber will try to expand and open the pleats. It uses a spring to oppose the movement of the bellows. Bellows Pressure Sensor (a) Capsule (b) Bellows

45. In low-pressure bellows sensors(<30 psi), spring not required. The travel of bellows can be converted to linear motion so that the switch can be activated. The bellows sensor is also used to make a differential pressure sensor. In this two bellows are mounted in housing so that the movement of each bellows opposes each other. This will cause the overall travel of the pair to be equal to the difference of pressure that is applied to them. Bellows Pressure Sensor

46. Working of Bellows Pressure Sensor

47. LVDT with Bellows

48. A different form of deformation is obtained using a tube with an elliptical cross section fig.(a) Increasing the pressure in such a tube causes it to tend to a more circular cross-section. When such a tube is in the form of C-shaped tube, generally known as a Bourdon tube fig.(b) A helical form of such a tube gives a greater sensitivity fig.(c) Tube Pressure Sensor

49. Piezoelectric materials when stretched or compressed generate electric charges with one face of the material becoming positively charges and the opposite face negatively charged. As a result voltage is produced. The displacement is proportional to the applied force F. q = kx = SF Where, k – constant, S – Charge sensitivity. Piezoelectric Sensor

50. Pie Piezoelectric Effect is  the ability of certain materials to generate an electric charge in response to applied mechanical stress. Piezoelectric Effect

51. Piezoelectric Sensor

52. A tactile sensor is a particular form of pressure sensor. A tactile sensor is a device it measures the coming information in response to the physical interaction with the environment. Used for Fingertips of ROBOTIC hands to determine when a hand has come into contact with an object. Tactile Sensor

53. Tactile Sensor – Point of Contact

54. Used for Touch Display screens where a physical contact has to be sensed. Tactile Sensor

55. The sense of touch in humans is generally modeled as Cutaneous sense and the Kinesthetic sense. Cutaneous touch has a capability of detecting the stimuli resulting from the mechanical simulation, pain and temperature. Kinesthetic touch receives sensor inputs from the receptors present inside the muscles, tendons and joints. Tactile Sensor

56. Force/torque sensor – Force control Dynamic sensor – Smaller accelerometers at the finger strips Thermal sensor – Temperature and thermal conductivity of an object Types of Tactile Sensor

57. The robotic thermal sensors are involved in the Peltier junctions in combination with the thermistor. One form of tactile sensor uses piezo electric Polyvinylidene Fluoride film. Thermal Sensor

58. Two layers of the film are used and are separated by a soft film which transmits vibrations. The lower PVDF film has an alternating voltage applied to it and this result in mechanical oscillations of the film. The intermediate film transmits these vibrations to the upper PVDF film. Due to piezoelectric effect, these vibrations cause an alternating voltage to be produced across the upper film. When the pressure applied to the upper PVDF film its vibrations are affected and the output voltage is changed. Thermal Sensor

59. Robotics Computer hardware Security system Touch screen devices on mobile phone Computing Tactile Sensor Applications

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