Eigenstructure view of cascade control

Cascade control is one of the early and widely successful multi-input control techniques used in the process industry. The technique essentially consists of nesting a secondary control loop inside a primary control loop and considering the primary controller to produce a setpoint for the secondary controller. When will cascade control be effective. Success is generally thought directly related to the ratio of time constants in the two loops. A fast secondary loop and slow primary loop indicates high probability of success. An eigenstructure analysis of the cascade configuration reveals that the major benefits of cascade control actually occur in the presence of slow secondary loops. The eigenstructure control strategy provides a straightforward procedure for calculating controller gains with the cascade configuration and other more general multi-input, multi-output configurations