For cascade control to be worthwhile the following need to be satisfied:-
- There needs to be a secondary measured variable available
- The secondary variable needs to have a causal effect on the primary controlled variable
- The secondary variable needs to be affected by a significant disturbance
- The secondary process must be faster than the primary process. As a rule of thumb the secondary needs to be around five times faster (i.e. the open loop time constant needs to be one fifth that of the primary process) for cascade control to show significant improvements.