Variable structured properties in fuzzy logic control

Both qualitative and quantitative studies in the paper show that the conventional fuzzy logic control (FLC) has the feature of variable structure control (VSC). The mathematical models of FLC, including both fuzzy two-term control and fuzzy PID control are derived for the quantitative analysis. These models clearly show the VSC feature of FLC, and reveal that this feature can be adjusted by scaling gains and membership functions (MFs). Large gains and more MFs result in stronger VSC effect and vice versa. Simulation confirms these theoretical results. As VSC is usually better for the complex process, its influence on FLC design and tuning is significant. The VSC feature of FLC will help to understand why FLC can achieve a robust performance for the complex process.