  This note examines the robustness properties of the nonlinear PI control
method to ignored actuator dynamics. It is proven that global boundedness and
regulation can be achieved for sector bounded nonlinear systems with unknown
control directions if the actuator dynamics are sufficiently fast and the
nonlinear PI control gain is chosen from a subclass of the Nussbaum function
class. Simulation examples are also presented that demonstrate the validity of
our arguments.

Systems and Control;Optimization and Control