Crc Press Mechatronics Handbook 2002 By Laxxuss Episode 2 Part 7 pps

spectral response, which peaks at a wavelength about hc/E g . Photoresistors and junction detectors are discussed in more detail in the following sections. Photoresistors The electrical conductivity of a semiconductor is the sum of two terms [5], one contributed by electrons and the other by holes, as follows: (19.77) Each term is proportional to n(p) the number of electrons (holes) per unit volume in the conduction (valence) band, the electron (hole) mobility µ n ( µ p ), and the magnitude of the charge of the electron e. The increase in conductivity, caused by the absorption of photons increasing n and p, is the basis for the operation of the photoresistive detector. This consists of a slab of semiconductor material on the faces of which electrodes are deposited to allow the resistance to be monitored, as illustrated in Fig. 19.103. The photon-induced current is proportional to the length of the electrodes and inversely proportional to their separation, hence the typical comb-like electrode geometry of photoresistors, shown in Fig. 19.73. Because the resistance R C is inversely proportional to conductivity, the variation of R C with incident power P D is very nonlinear and is often expressed in the form (19.78) where a and b are constants. Cadmium sulfide is commonly used as a detector of visible radiation because it is low cost and its response is similar to that of the human eye. Other photoconductive materials include lead sulfide, with a useful response from 1000 to 3400 nm, indium antimonide with a useful response out to 7000 nm, and mercury cadmium telluride with peak sensitivity in the range 5000–14,000 nm. The wavelength range 5000–14,000 nm is of importance because it covers the peak emission from bodies near and above ambient temperature and also corresponds to a region of good transmission through the atmosphere. Photoconductive devices used for the detection of long wavelength infrared radiation should be cooled because of the noise caused by fluctuations in the thermal generation of charge. As a rough rule of thumb, because of the Boltzmann factor, a detector with energy gap E g should be cooled to a temperature less than E g /25k. Junction Detectors In photoresistors, the rate of generation of electron–hole pairs by the absorption of radiation, combined with recombination at a rate characteristic of the device, results in an increase in free charge and therefore electrical conductivity. In junction photodetectors [6], such as photodiodes and phototransistors, newly generated electron–hole pairs separate before they can recombine so that a photon-induced electric FIGURE 19.103 A simple light detector circuit employing a photoresistor is shown. An increase in light illumination causes the resistance of the photoresistor to decrease and the output voltage to increase. The comb-like pattern typically employed in photoresistors gives a relatively large active area of photoconducting material and a small electrode spacing resulting in high sensitivity. R L Incident radiation Output voltage Photoresistor Bias voltage Evaporated metal electrodes Photoconducting material σ ne µ n pe µ p += log 10 R C ablog P D –= 0066_frame_C19 Page 124 Wednesday, January 9, 2002 5:32 PM ©2002 CRC Press LLC spectral response, which peaks at a wavelength about hc/E g . Photoresistors and junction detectors are discussed in more detail in the following sections. Photoresistors The electrical conductivity of a semiconductor is the sum of two terms [5], one contributed by electrons and the other by holes, as follows: (19.77) Each term is proportional to n(p) the number of electrons (holes) per unit volume in the conduction (valence) band, the electron (hole) mobility µ n ( µ p ), and the magnitude of the charge of the electron e. The increase in conductivity, caused by the absorption of photons increasing n and p, is the basis for the operation of the photoresistive detector. This consists of a slab of semiconductor material on the faces of which electrodes are deposited to allow the resistance to be monitored, as illustrated in Fig. 19.103. The photon-induced current is proportional to the length of the electrodes and inversely proportional to their separation, hence the typical comb-like electrode geometry of photoresistors, shown in Fig. 19.73. Because the resistance R C is inversely proportional to conductivity, the variation of R C with incident power P D is very nonlinear and is often expressed in the form (19.78) where a and b are constants. Cadmium sulfide is commonly used as a detector of visible radiation because it is low cost and its response is similar to that of the human eye. Other photoconductive materials include lead sulfide, with a useful response from 1000 to 3400 nm, indium antimonide with a useful response out to 7000 nm, and mercury cadmium telluride with peak sensitivity in the range 5000–14,000 nm. The wavelength range 5000–14,000 nm is of importance because it covers the peak emission from bodies near and above ambient temperature and also corresponds to a region of good transmission through the atmosphere. Photoconductive devices used for the detection of long wavelength infrared radiation should be cooled because of the noise caused by fluctuations in the thermal generation of charge. As a rough rule of thumb, because of the Boltzmann factor, a detector with energy gap E g should be cooled to a temperature less than E g /25k. Junction Detectors In photoresistors, the rate of generation of electron–hole pairs by the absorption of radiation, combined with recombination at a rate characteristic of the device, results in an increase in free charge and therefore electrical conductivity. In junction photodetectors [6], such as photodiodes and phototransistors, newly generated electron–hole pairs separate before they can recombine so that a photon-induced electric FIGURE 19.103 A simple light detector circuit employing a photoresistor is shown. An increase in light illumination causes the resistance of the photoresistor to decrease and the output voltage to increase. The comb-like pattern typically employed in photoresistors gives a relatively large active area of photoconducting material and a small electrode spacing resulting in high sensitivity. R L Incident radiation Output voltage Photoresistor Bias voltage Evaporated metal electrodes Photoconducting material σ ne µ n pe µ p += log 10 R C ablog P D –= 0066_frame_C19 Page 124 Wednesday, January 9, 2002 5:32 PM ©2002 CRC Press LLC 20 Actuators 20.1 Electromechanical Actuators Introduction • Type of Electromechanical Actuators—Operating Principles • Power Amplification and Modulation—Switching Power Electronics 20.2 Electrical Machines The dc Motor • Armature Electromotive Force (emf) • Armature Torque • Terminal Voltage • Methods of Connection • Starting dc Motors • Speed Control of dc Motors • Efficiency of dc Machines • AC Machines • Motor Selection 20.3 Piezoelectric Actuators Piezoeffect Phenomenon • Constitutive Equations • Piezomaterials • Piezoactuating Elements • Application Areas • Piezomotors (Ultrasonic Motors) • Piezoactuators with Several Degrees of Freedom 20.4 Hydraulic and Pneumatic Actuation Systems Introduction • Fluid Actuation Systems • Hydraulic Actuation Systems • Modeling of a Hydraulic Servosystem for Position Control • Pneumatic Actuation Systems • Modeling a Pneumatic Servosystem 20.5 MEMS: Microtransducers Analysis, Design, and Fabrication Introduction • Design and Fabrication • Analysis of Translational Microtransducers • Single-Phase Reluctance Micromotors: Microfabrication, Modeling, and Analysis • Three-Phase Synchronous Reluctance Micromotors: Modeling and Analysis • Microfabrication Aspects • Magnetization Dynamics of Thin Films • Microstructures and Microtransducers with Permanent Magnets: Micromirror Actuator • Micromachined Polycrystalline Silicon Carbide Micromotors • Axial Electromagnetic Micromotors • Conclusions 20.1 Electromechanical Actuators George T C. Chiu Introduction As summarized in the previous sections, a mechatronics system can be partitioned into function blocks illustrated in Fig. 20.1. In this chapter, we will focus on the actuator portion of the system. Specifically, we will present a general discussion of the types of electromechanical actuators and their interaction George T C. Chiu Purdue University C. J. Fraser University of Abertay Dundee Ramutis Bansevicius Kaunas University of Technology Rymantas Tadas Tolocka Kaunas University of Technology Massimo Sorli Politecnico di Torino Stefano Pastorelli Politecnico di Torino Sergey Edward Lyshevski Purdue University Indianapolis 0066_Frame_C20 Page 1 Wednesday, January 9, 2002 5:41 PM ©2002 CRC Press LLC . electrodes Photoconducting material σ ne µ n pe µ p += log 10 R C ablog P D –= 0066_frame_C19 Page 124 Wednesday, January 9, 20 02 5: 32 PM 20 02 CRC Press LLC 20 Actuators 20 .1 Electromechanical Actuators Introduction • Type of. electrodes Photoconducting material σ ne µ n pe µ p += log 10 R C ablog P D –= 0066_frame_C19 Page 124 Wednesday, January 9, 20 02 5: 32 PM 20 02 CRC Press LLC spectral response, which peaks at a wavelength about hc/E g Sergey Edward Lyshevski Purdue University Indianapolis 0066_Frame_C20 Page 1 Wednesday, January 9, 20 02 5:41 PM 20 02 CRC Press LLC