CAD.Reference:1 - What the language helps obtain

A controller acts on information that can be partial, old or costly to obtain. Its chosen action must affect the plant before another decision is due, fit the available actuation and leave an acceptable continuation. When planning and feedback are separated, their different models and time scales add another coupling: an attractive nominal command may be impossible for the lower loop to realize.

The useful result is a control architecture whose information, command meanings, error allowances and execution conditions are mutually consistent for the required task. The result may instead be a reasoned rejection: no tested realization fits, a sufficient enclosure is too coarse, or indistinguishable states make the required action impossible. These conclusions call for different next work.

There are two directly useful branches. CAD.5 constructs the information available to a specified controller. CAD.1–CAD.4 develop a planning-and-feedback arrangement when separation has a useful reason. Their quantitative results connect where an estimate, command or timing claim is actually used. The language does not require every design to use both branches.

Its recurring tensions are concrete. A precise delayed sensor leaves recent disturbances unknown. A larger model can reduce modeling error while missing the deadline. A gain that reduces tracking error can consume more input authority. Conservative sets can protect a guarantee while excluding a useful design. All of these must be judged against the same task; a smaller isolated error measure is not automatically a better control result.