Causal observability of nonlinear time-delay systems with unknown inputs

This paper investigates the problem of causal observability of the states and unknown inputs of nonlinear time-delay systems. Using the theory of non-commutative rings, the nonlinear time-delay system is transformed into a suitable canonical form to solve the problem. A necessary and sufficient condition is given to guarantee the existence of a change of coordinate leading to such a form. Then, algorithms are provided to check the possibility of obtaining causal estimations of the states and unknown inputs for the studied systems.