1. Hierarchical Structure:
• Primary (Outer) Loop: Manages the main process variable (e.g., temperature of a tank).
• Secondary (Inner) Loop: Manages a faster, intermediate variable (e.g., the flow rate of the heating steam).
2. Dynamic Requirements: For cascade control to be effective, the inner loop
must be significantly faster than the outer loop. This ensures that disturbances in the secondary variable (like steam pressure fluctuations) are corrected by the inner controller before they can significantly affect the primary variable.
3. Frequency Domain Interpretation: In the frequency domain, the speed of a loop is often characterized by its "cross-over frequency" (the frequency at which the gain drops to 1). A higher cross-over frequency indicates a faster-responding system. Therefore, in a properly designed cascade system, the secondary loop always has a higher cross-over frequency than the primary loop.