A cascades approach to a nonlinear separation principle

We provide a separation principle for nonlinear time-varying systems. The only assumption we make on the state feedback control law is that it does not vanish as the states grow unboundedly. For the observer, we propose a design method based on the assumption that the system is transformable into a form affine the unmeasured variable. However, in contrast to most works relying on this assumption we do not impose the condition that the vector field multiplying the unmeasured variable is separated from zero for all state values and all times. Our separation principle relies on the stability results for cascades systems.