Zero-reference tracking control of a disturbance observer applied time-delay system based on the time-domain stability analysis

This paper proposes a disturbance observer (DOB) that can be applied for the zero-reference control of a time-delay system. The system delay might be uncertain or variable within a known bound range. Time-domain stability analysis was introduced to apply the DOB to systems with an unfixed time delay. The internal paths of the DOB are modified based on the time-domain stability analysis together with a robust condition, which guarantees the structural stability and minimizes the influence of the worst disturbance effect over the whole frequency range. Introducing time-domain stability analysis allows the proposed DOB to be applied to uncertain or varying delayed systems and well-developed time-domain based techniques can be applied. In zero-reference tracking simulations, the proposed DOB stably eliminated the disturbance effect when a system had uncertain or varying time delays.