Technical Report - Designing a Temperature Controller for...

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Designing a Temperature Controller for an Alexandrite Laser Rod Grant Corley, Albert Ho A&EP Lasers, Inc. 245 Clark Hall Ithaca, NY 14850 April 10, 2006 Abstract A temperature controller for an Alexandrite laser rod can be created using a computer in conjunction with a thermistor, heater, and an analog to digital converter. Utilizing the exponential relationship between the thermistor resistance and temperature, a computer is calibrated to provide accurate measurements of the temperature. After calibration, a computer can control a heater to precisely control the temperature of an Alexandrite laser rod between 30 and 100°C to within ±3°C. The prototype temperature controller is capable of meeting these requirements, but works optimally between 30 and 80°C. This simple and inexpensive technique for controlling temperature will be important for our company’s fall product line. Introduction The goal of this project is to develop an inexpensive scheme to maintain the selected temperature of an Alexandrite laser rod between 30 and 100°C to within an accuracy of ±3°C. A temperature controller can be built using a thermistor, an inexpensive semiconductor device whose resistance varies as a function of temperature, in conjunction with a heater and an analog to digital converter interfaced to a computer. This temperature controller will be installed in the A&EP Lasers, Inc. fall product line of Alexandrite laser rods, so its implementation must be simple and inexpensive. In order to operate correctly, the Alexandrite laser rod must be maintained at some specified temperature between 30 and 100°C. This is achieved by connecting a heater, operated by a temperature controller, to the device. The operation of the temperature controller begins by taking a temperature reading from the Alexandrite laser rod. A computer then compares the temperature to the user’s target temperature. It will turn the heater off when the rod temperature exceeds the target temperature, and turn it on when it cools to a temperature below the target. The results of the temperature controller development efforts show that the temperature of the Alexandrite laser rod can be maintained within ±2°C of the target temperature. This achieves the goals of the project, since the product maintains a very accurate temperature using an inexpensive method.
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The following report first delves into a short discussion of the theory before presenting the apparatus and method, results and conclusions. An explanation of error analysis can be found in Appendix A, a collection of charts and graphs related to the project in Appendix B, a list of programs in Appendix C, and numerical results of the development and testing efforts in Appendix D. Theory The temperature of the Alexandrite laser rod can be measured by connecting a thermistor and applying a voltage across it in a resistor network (see Figure 1). The thermistor resistance is computed by solving the voltage divider formula. It varies exponentially
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