Analyzing limits of one type of disturbance observer based PI control by the performance portrait method

This paper deals with a performance portrait based robustness and performance analysis of one of the first disturbance observer (DO) based PI controllers applied to the first order plants. The chosen structure has been established as a PI control augmented by a DO with the aim to improve the disturbance response and to increase the loop robustness. Thereby, three possible DO filters have been introduced, which should guarantee a gradually increasing loop robustness. This expectation will be discussed in dealing with control loops under high performance requirements on the step responses shapes. A special attention is paid to the unmodelled dynamics limiting significantly the achievable control performance.