In a cascade control system, how to choose the forward and reverse action of the controller
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In a cascade control system, since there are two controllers, how should their positive and negative actions be chosen? What are the principles for making such choices? What if we first determine the forward and reverse action of the secondary control loop, and then choose the forward and reverse action of the primary control loop? Would that work?
First, determine the positive and negative action of the secondary controller. Then treat the secondary circuit as an FC valve, and determine the forward and reverse action of the main controller.
First, select the direction of action for the secondary controller; then treat the secondary control loop as a single element, and finally determine it by examining the relationship between the output of the primary controller and the secondary controlled variable.
Main controller operation mode selection: (Main controller ±) (Secondary object ±) (Main object ±) = (—). Therefore, when the main and secondary variables change in the same direction, the main controller should use the reactive mode; when they change in opposite directions, the active mode should be used. Secondary controller operation mode selection: (Secondary controller ±) (Control valve ±) (Secondary object ±) = (—)
I remember a simple method taught by my teacher back in school; I’m not sure if I remember it correctly. To determine the effect of changes in the auxiliary parameter on the valve, one can first figure out how the auxiliary control operates, and then determine the effect of changes in the main parameter on the valve to find out how the main control functions. If the auxiliary control is of the direct-acting type, then the main control should operate in the opposite manner. In simple terms, it means determining the primary and secondary controllers separately; if the secondary controller has a direct effect, then the corresponding primary controller’s value is inverted
The approach taken by the original poster is feasible: first determine whether the control valve should be air-operated to close or open based on the process requirements, and then decide on the forward or reverse action of the secondary regulator according to whether the control valve operates in an open or closed manner. Then, based on the control valve and the type of secondary regulator, the direct or reverse action of the main regulator is selected.
Why use a cascade? It is mainly to include all disturbances in the secondary loop, and also to reduce the impact of variable lag on the primary loop. When determining the forward and reverse action, it is necessary to first decide on the operating mode of the main and auxiliary circuit control valves in accordance with safety requirements, and then determine the forward and reverse action of the controller. Generally, one starts with the secondary circuit and evaluates it according to the standards for single-circuit systems, with the aim of creating a negative feedback system; afterwards, the primary circuit is evaluated in the same way, also with the requirement of having a negative feedback system. In fact, the positive and negative actions of the controller have nothing to do with the operating principle of the control valve. A humble opinion