This article deals with the level controls in a laboratory two-tank system. The plant is strongly nonlinear due to the basic dynamic equations and the characteristics of the valves. We develop a nonlinear control law which achieves robust approximately time-optimal control over the full range of operation conditions. The development is based on ideas from fuzzy control, but in contrast to usual fuzzy controller designs, most of the rules are not derived from heuristics but rather are mathematical formulas which, together with the standard fuzzy quantization of the system's variables, approximate the time-optimal control law. This approximation is improved by heuristic rules which were gained from the observation of the behavior of the controlled plant. The resulting nonlinear control law exhibits a performance which is neither attainable with standard linear control nor with classical time-optimal control.
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