Some properties of delayed controls (Smith regulator)†

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Abstract
This paper studies the feedback and filter properties of the Smith regulator and concludes that it does not in any way alleviate the sensitivity and disturbance attenuation limitations in feedback systems with non-minimum phase plants. Also, its single advantage is shown to be in the realization of one-degree-of-freedom feedback structures, wherein the non-minimum phase constraints preclude the achievement, in the typical conventional design, of a bandwidth (rise time) which may be within the plant's capabilities. Even then, this is secured only for command input tracking, but its sensitivity and disturbance regulation properties in the extra achieved band width are worse than in an open-loop design. They may also easily be much poorer in the low frequency range than in a conventional design. A modification exists for a sub-class of plants where the output of the nmp plant element- may be measured and used for feedback purposes. It has whatever advantage of the original and better sensitivity to the nmp elements and disturbance regulation.