Performance assessment of cascade control loops

The minimum variance control law for a cascade control system is derived fo r two cases: the primary process has stable and unstable inverses. The expr ession for the primary output variable under the minimum variance cascade c ontrol is shown to be feed-back-invariant. An estimate of the minimum achie vable variance in a cascade control loop can be obtained via multivariate t ime-series modeling of the primary and secondary measurements collected und er normal operation. The utility of the proposed performance assessment met hod is demonstrated using simulated and industrial data.