Automatic control problems

When selecting the function of the secondary loop regulator in a cascade system, focusing only on the secondary loop, negative feedback is required. When selecting the function of the main-loop regulator in a cascade system, only the secondary-loop regulator is considered part of the main loop, and positive feedback is required!

In my opinion, the direction of the feedback actions for the primary and secondary controllers in a cascade control system should be determined based on the requirements of the process design – that is, it depends on whether there is a positive or negative deviation after considering PV-SP And since the process requirements vary, sometimes it is necessary to open the valve for a positive deviation, while other processes require the valve to be closed; moreover, the inherent positive or negative action of the valve itself can also affect the choice of positive or negative action for the regulator.

The direction of action of the auxiliary regulator is related to the characteristics of the controlled process and whether the control valve is of the open-air or closed-air type. The method for selecting it is the same as that used for choosing between positive and negative feedback in simple control systems; it is determined based on the principle of making the auxiliary loop a negative feedback system. The selection of the operating direction for the main regulator can be carried out as follows: when the primary and secondary variables increase (or decrease), if it is required that the actuation direction of the control valve remains consistent, then a reverse-acting main regulator should be chosen; otherwise, a direct-acting one should be selected

In a cascade control system, it is necessary to select the operating direction of the primary and secondary controllers separately according to various circumstances, and the selection method is as follows. (1) Selection of the action direction of the secondary controller in a cascade control system. It is determined based on requirements such as process safety; after selecting the air-open or air-close type of actuator, it is done in accordance with the principle of making the secondary control loop a negative feedback system. 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. Its selection method is the same as that for selecting 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 main controller – simply using the output of the main 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, the direction of operation of the main controller in a cascade control system 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 done in the order of secondary first and then primary: first determine the on/off mode of the actuator as well as the forward/reverse action of the secondary controller, and then determine the direction of action of the primary controller ； 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, and then use the methods of conventional single-loop control systems to select the on/off mode of the actuator as well as the function direction of the secondary controller. (3) When, due to the requirements of the process, the control valve is changed from air-open to air-close, or from air-close to air-open, it is sufficient to change the positive/negative action of the secondary controller without having to alter that of the primary controller.

The second sentence is incorrect. When adjusting the main loop, the secondary loop can be regarded as a element with a positive amplification factor; it’s not sufficient to consider only the secondary regulator. Moreover, the main loop must also have negative feedback

I have distilled this into two sentences: 1. The larger the measurement value, the greater the output required from the controller to achieve the control target value; in such cases, a positive action controller should be used. 2. The larger the measurement value, the smaller the output required from the controller to achieve the control target value; in such cases, a negative action controller should be used. It is important to note that regardless of whether the valve is of the air-open or air-close type, what matters is determining whether the controller needs to increase or decrease its signal based on the effect of an increased signal on the controlled parameter

It is necessary to consider whether it is the water supply valve or the drain valve that needs to be regulated, whether the control valve selected for the cascade loop should operate in air-open or air-close mode, and other control variables – all of these need to be determined based on the conditions of the production process.

It’s too complicated; think of them as two separate cascade systems and set the positive and negative feedback for each one