How is the selection of the primary and secondary controller determined in cascade control?
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How is the selection of the primary and secondary controller determined in cascade control? For example, there is a atmospheric pressure tower, where the bottom liquid level and the bottom flow rate form a cascade control system, and the material at the bottom of the tower enters a heater. How to determine the tonic and subdominant? If the fluctuations of both quantities are not significant, how can we determine the primary and secondary tones?
The main control parameters are those that need to be regulated in the process, while the secondary control parameters are the process parameters that affect the main parameters. For example, in the case you mentioned where the bottom of the tower liquid level and the bottom flow rate form a cascade control system, the liquid level is the primary parameter while the flow rate is the secondary parameter.
The secondary loop needs to be able to encompass as many disturbances as possible, that is, as many interferences as possible! The corresponding speed of the secondary circuit should be faster than that of the primary circuit. The secondary loop is key to ensuring the stability of the main parameters! Common cascade configurations include level-flow cascade, pressure-flow cascade, and temperature-flow cascade; in these setups, flow rate is usually used as the secondary loop!