Dual features of conventional fuzzy logic control

Fuzzy logic control (FLC) based on tracking error feedback is called conventional FLC in this paper. A thorough study of the conventional FLC on its dual features is presented. Both linguistic and numerical analyses show that conventional FLC combines properties of its linear control and variable structure control (VSC) components. These features can be adjusted by tuning the scaling gains and membership functions (MFs). Using large gains or more MFs result in stronger VSC effect but less linear control effect, and vice versa. Simulation results have confirmed this analysis. As VSC is usually advantageous for complex processes while linear control is better for simpler ones, the influence of these dual features on FLC design and tuning is significant. Thus, initial gains of FLC are suggested to be designed based on the well-tuned linear control structure for a simple process, and they can be determined based on the VSC theory for a complex process. These dual features can also help understand the robustn ess of the conventional FLC systems.