Design of a fuzzy logic controller for regulating the temperature in industrial polyethylene fluidized bed reactor

摘要

Proportional integral (PI) controllers are the most commonly used controllers in industrial applications because they are relatively easy to design, have simple control structure and inexpensive cost. However, they exhibit poor performance when exposed to unknown disturbances Such as dead zones. A successful way of dealing with such non-linearity is to use fuzzy logic, which features an improvement in the transient characteristic of the control performance. On the other hand, it is very difficult to establish a systematic design method for fuzzy control since it is non-linear and as such, has no mathematical design method to fall back on. Due to the superior performance of fuzzy control in transient state and high accuracy of PI control in the steady state, a combination between the two would present a very attractive solution. Fuzzy logic controllers based on the Takagi-Sugeno inference method has been applied for the regulation of the reaction temperature of the industrial bubbling fluidized bed reactors for polyethylene production. The simulation results suggest that the conventional fuzzy logic controller produces oscillations in the process response. To improve the performance of the conventional scheme, implementation of a hybrid control scheme is proposed. Significant improvements in the controller performance could be achieved by combining these approaches. The hybrid control scheme reduces the severe oscillations of the conventional method and enhances control precision. Comparison between Mamdani fuzzy logic and Takagi-Sugeno type fuzzy controller has been explored. Results reveal that Mamdani fuzzy logic is very easy to build, by contrast, it is too simple to control the process quickly and only suited to the long delay system. Takagi-Sugeno controller is ideal for acting as multiple linear controllers to operate dynamic non-linear systems that mean it can be used to control the process that changes very quickly and has high frequent input signals.