| Because of its small size, powerful data processing ability and low price,Single Chip Microcomputer has been widely applied. Most of the SCMcontrol systems use software arithmetic, which is more flexible than analogcontrol to achieve control projects; What is more, it has high reliability andlow disturbance due to fewer components and digital signal processing, whichavoids the aberration and distortion producing by the transmission of analogsignals; In addition, it may set the parameter simply, and makes the systemdebugging convenient. Therefore, the single chip control systems are verysuitable for control systems of application areas. It is why we use SCM as thekernel part for controlling.In recent decades, the development of optical communication technologygreatly speeds up the information transmission, lengthens the transfer distanceand heightens the transfer reliability. With the development of optical fibercommunication, optical amplifier plays a more and more important role. Asthe kernel part of optical amplifier, semiconductor laser diode (LD, LaserDiode) owns small size, light weight, simple structure, long life and otherfeatures, it may modulate easily. The output wavelength and light intensity canbe modulated by changing the injection current. But the output wavelength ofsemiconductor laser diode also has a great relationship with the temperature.When the internal temperature of semiconductor laser diode increases, outputwavelength increases. With the widely application of semiconductor laserdiode in measure, optical fiber communication, optical fiber transducer,especially the rapid development of WDM and DWDM opticalcommunications, the output wavelength of semiconductor laser diode is required to be very stable. In addition, the output power of semiconductor laserdiode is also influenced by temperature. Therefore, the internal temperature ofsemiconductor laser diode must be strictly controlled.Negative feedback control theory is used in temperature control systembased on the 89C51. Negative temperature coefficient temperature sensor isused to sample the temperature of semiconductor laser diode. Andsemiconductor cooler is used as the implementation device of temperaturecontrol system. This paper begins from the practical meaning of temperaturecontroller of semiconductor laser diode. Then, introduce the common principleof the control system. And then, introduce the technologies such as the designof the front panel,temperature sampling,the application of the A/D chipMAX1241 and the D/A chip DAC7512,the TEC driving circuit and the PItemperature control. This paper involves the knowledge of electricity,optics,technology of semiconductor refrigeration and so on.In this paper, the overall design of the constant temperature controlsystem is as follows: Internal temperature change of semiconductor laser diodeis got by the temperature sensor, and then converted into electrical signal. Thiselectrical signal is compared with preset temperature signal after sampling andAnalog to Digital converting. The difference between the two signals will beprocessed by SCM with PI control theory. The output of PI control isamplified by power amplifier after Digital to Analog converting. The outputsignal drives TEC and controls the value and direction of driver current. Andthen the heating and refrigeration operation of semiconductor laser diode areachieved. Several temperature sensors are introduced first in the temperaturecollection part. The nonlinear of Temperature Negative Coefficient (NTC)Resistor Temperature is modified. Because temperature is changed intoresistance by NTC and the resistance is difficult to be measured, we expect toobtain thermal resistance by providing a constant current for thermal resistance,and then measure the voltage on thermal resistor. And temperature can beobtained from the thermal resistance by using R T curve.The serial 12 bit chip MAX1241 is used for Analog to Digital Converter.And it is discussed in details in this paper.SCM processes the differences between the preset temperature and thepractical temperature with PI control theory. PID theory is a mature controltheory of industry. It is made up of three parts: proportion,integral anddifferential. It gets satisfactory control effect by combining them perfectly.The proportion tache is used to shorten the respond time. The integral tache isused to minish steady state error. The differential tache is used to lower theovershoot. To the temperature control system, temperture is a physicalquantity which changes slowly, so the differential tache which aims at thefast change process can be ignored. To get better control ability, we importFuzzy control principles. By combining Fuzzy control theory and theory ofPID control, the CPU can simulate human experience and make a reasonablejudgment and reasoning according to the performance indicators. The fuzzyPID principle in this design is to find out the relationship between parametersof PID control and the error of preset temperature and the actual temperature.The error is testing all the time. And the three parameters are auto tune in real time based on Fuzzy PID control principles, so that the object has a gooddynamic and static characteristics.In the constant temperature control system, cooling quantity of TEC ismainly determined by the drive signal. TEC is a current driven device, so itcan be driven by voltage controlled current source. TEC can heat or cool asthe direction of current changes. Because this temperature control system isdesigned for high power laser diode, the current should be large. But existingspecialized TEC chip cannot achieve great current, thus a bridge drive circuitfor power amplifying is designed by using Darlington. The driven current Thecontrol voltage of Darlington is generated by Digital to Analog Converter.AD7521 chip is selected as Digital to Analog Converter.In the designing scheme of this system, we consult advancedapplications in the aspect of constant temperature control and use the primedesign idea as reference, to complete the digital constant temperature controlsystem. This constant temperature control system can be applied not only forthe temperature control in semiconductor laser diode, but also for other caseswhen temperature control is necessary.This paper has positive significance tothe wide application of semiconductor laser diode. |
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