Robust control of cascaded nonlinear systems

Robust control is designed for nonlinear uncertain systems, which can be decomposed into two cascaded subsystems, that is, a series connection of two nonlinear subsystems. For such a system, a recursive design is used to include a second subsystem in robust control. The recursive design procedure is constructive and contains two steps. First, a fictitious robust controller for the first subsystem is designed as if the subsystem had an independent control. As the fictitious control, nonlinear H_∞ control using the energy dissipation is designed in the sense of L₂-gain attenuation from the disturbance caused by uncertainties to performance. Second, the actual robust control is designed recursively by Lyapunov´s second method. The designed robust control is applied to a robotic system with actuators, in which physical control inputs are not the torque to robot links but electrical signals to actuators