The Research Of Constant Temperature Water-bath Controller Based On Fuzzy PID Control

In the laboratories of the chemical industry, Medical institutions, colleges and universities, scientific research departments and industrial and mining enterprises etc, various reagents always need to be distilled, dried, concentrated and heated up, so the water-bath temperature controller has been widely used. At present, the water-bath temperature control using the traditional control method, mainly has two defects:at first, it has a low control precision, which often cannot meet the requirements of high-precision detecting. Secondly, it has low efficiency. Due to the long time of reaching a set temperature and entering a stable state, it has wasted a lot of resource and time.According to this situation, this paper puts forward a kind of fuzzy PID control method. The traditional PID algorithm has the advantage of simple operation, high reliability and good robustness features, but the time-varying of the bath system, the nonlinear character and the difficult to build mathematical model make it unavailable to control with the conventional PID controller because it cannot achieve ideal control effect. Therefore, it is the fuzzy PID algorithm that is the method to control the water-bath system. This method which has the advantage of a fuzzy controller without the requirement of mathematical model, but having the good robustness, the quality of dynamic response and strong adaptability, and the advantages of a PID controller which has a good dynamic tracking quality, high stable precision and other characters, has effectively slowed the fluctuation and improved the performance of the water-bath temperature controlling system.The system takes the microcontroller STC89C51 as the core, the single bus serial digital temperature sensor DS18B20 as the testing equipment, the LCD1602 as the display device, and uses the solid relay to control the heating equipment. Therefore, considering the experiment hardware design, the whole system mainly divided into four parts:the keyboard input module, LCD module, the temperature monitoring module and heating module. The keyboard input module inputs the temperature which needs to be displayed with the liquid crystal display module. According to the reflection of the temperature monitoring module, MCU is ware of the real-time water temperature, which also needs to be displayed with LCD module. When the input water temperature is higher than the real-time water temperature, MCU will control to heat the water with the heating module.Experiments plans to use the fuzzy PID controller to control the water-bath temperature. However, considering the actual situation and the complexity of the fuzzy PID control, when the deviation between the set temperature and the real temperature is very big, the experiment will use the method of the PID control. While the deviation reduces to a certain degree, then the system adds the fuzzy control function, which means using the method of the fuzzy PID control. The fuzzy control is applied to the microscopic adjustment. This plan not only has no affect to the precision of the water bath temperature, but also improves the system efficiency. The experiment which uses the fuzzy influencing the adjustment of three parameters of the PID control, realizes the online auto-tuning of the PID control parameters Kp, Ki and Kd. Then according to the improved parameters, it is not difficult to obtain a control amount applying the PID control, which can control heating power with the PWM method.Finally, the Water-bath temperature control realized the constant temperature control ranging from the room temperature to 100℃. It is available to adjust the needing temperature, and display with the liquid crystal display. Simultaneously the method of the fuzzy PID control finally achieved to control the water-temperature with the deviation less than 0.5℃between the actual temperature when the system worked stably and the set temperature.