How to determine the forward and reverse actions of the primary and secondary controllers in a cascade control system?

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How to determine the forward and reverse actions of the primary and secondary controllers in a cascade control system? (I brought it here by myself; thanks to this friend!) ) In a cascade control system, it is necessary to determine the operating direction of the primary and secondary controllers separately, depending on various circumstances. The method for making this choice is as follows: (1) The operating direction of the secondary controller in a cascade control system is determined by considering requirements such as process safety; after selecting whether the actuator should operate in an open-or-close mode based on air pressure, the principle of turning the secondary control loop into a negative feedback system is applied. Therefore, the direction of action of the secondary controller is related to the characteristics of the secondary object and whether the actuator operates in an air-open or air-close mode. The method of selection for it is the same as that used for determining the forward and reverse actions of a controller in a simple control system; in this case, there is no need to consider the direction of action of the primary controller – simply using the output of the primary controller as the input for the secondary controller is sufficient.   (2) The selection of the action direction for the main controller in a cascade control system can be carried out as follows: when both the primary and secondary variables are increasing (or decreasing), if process analysis indicates that, in order to reduce (or increase) these variables, the control valve must move in the same direction, then the main controller should be set to “reverse” action ； Conversely, the “positive” direction should be chosen.  As can be seen from the above methods, in a cascade control system, the direction of operation of the main controller is determined entirely by the process conditions, and it has nothing to do with whether the actuator is of air-open or air-close type, nor with the direction of operation of the secondary controller. Therefore, in a cascade control system, the selection of the primary and secondary controllers can be carried out as follows: 1. First, determine the on/off mode of the actuator (taking process safety into account) and the forward/reverse action of the secondary controller (so that the secondary loop provides negative feedback), and then determine the direction of action for the primary controller ； 2. It is also possible to follow a primary-then-secondary order: first, determine the function of the main controller based on the requirements of the process characteristics (refer to point 2 above), and then use the methods typical of single-loop control systems to select the on/off mode of the actuator as well as the function of the secondary controller. Attach a link to an auto-controlled learning video: Video 1: https://v.youku.com/v_show/id_XOTAzNzU2NTAw.html?spm=a2h0k.11417342.soresults.dtitle Video 2: https://v.youku.com/v_show/id_XOTE3NjcyMzk2.html?spm=a2h0k.11417342.soresults.dtitle