The control law of the main and auxiliary control bells and the choice of positive and negative effects

Abstract: The control law of the main and auxiliary control bells and the selection information of positive and negative effects are provided by excellent flowmeter and flowmeter production and quotation manufacturers. 1. Selection of control laws The selection principles of the control laws of the main and auxiliary controllers in the cascade control system are different. (1) The main controller usually chooses the proportional integral control law. The purpose of the cascade control system is to stabilize the main variable with high precision. The main variable is. For more flowmeter manufacturers to select models and price quotations, you are welcome to inquire. The following are the details of the main and auxiliary control bell control laws and the selection of positive and negative effects. 1. Selection of control laws The selection principles of the control laws of the main and auxiliary controllers in the cascade control system are different. (1) The main controller usually chooses the proportional integral control law. The purpose of the cascade control system is to stabilize the main variable with high precision. The main variable is the main control index of the production process, which is directly related to the quality of the product or the normal progress of the production, and the requirements for it in the process are relatively strict. Generally speaking, the variables do not allow residuals. Therefore, the main controller usually adopts the proportional integral control law to realize the error-free control of the main variable. Sometimes, the capacity lag of the object control channel is relatively large, such as the temperature object or the component object, etc. In order to overcome the capacity lag, the proportional integral derivative control law can be selected. (2) The secondary controller generally adopts the proportional control law. In the cascade control system, stabilizing the secondary variable is not the purpose. The purpose of setting the secondary variable is to ensure and improve the control quality of the primary variable. Under the influence of interference, in order to keep the primary variable unchanged, the secondary variable will change. The given value of the secondary variable changes with the output of the main controller. Therefore, in the control process, the requirements for the secondary variable are generally not very strict, and it is allowed to fluctuate. Therefore, the sub-controller generally adopts the proportional control law. In order to be able to track quickly, it is best not to use the integral action, because the integral action will make the tracking slow. The differential action of the sub-controller is also unnecessary, because when the sub-controller has a differential action, once the output of the main controller changes slightly, it is easy to cause a large change of the control valve, which is unfavorable to the stability of the system. 2. Selection of positive and negative action of the controller According to various situations, the selection method of the action direction of the main and sub-controllers is as follows: (1) The selection of the action direction of the sub-controller in the cascade control system is based on the requirements of process safety and so on. , After selecting the air-open and air-close type of the actuator, it is determined according to the principle of making the auxiliary control loop a negative feedback system. Therefore, the action direction of the sub-controller is related to the characteristics of the sub-object, the air-open and air-close types of the actuator, and the selection method is the same as the selection method of the positive and negative actions of the controller in the simple control system. The direction of action is only to use the output of the main controller as the given of the sub-controller. For example, in the secondary loop in the temperature-temperature cascade control system of the tubular heating furnace shown in Figure 8-1-2, if the supply of fuel oil is stopped when the gas source is interrupted to prevent the furnace from burning out, the actuator should be selected air valve, yes“just”direction. When the amount of fuel increases, the furnace temperatureθ2 (the secondary variable) is incremented, so the secondary object is“just”direction. In order to make the secondary loop form a negative feedback system, the secondary controller T2C should choose“opposite”direction of action. Only in this way can the output of T2C be reduced when the furnace temperature is disturbed and increased, so that the gas opening and closing valve can be reduced, the fuel amount can be reduced, and the furnace temperature can be lowered. In the cascade control system for the temperature and steam flow of the distillation column, as shown in Figure 8-1-5, if based on technical considerations, the gas shut-off valve is selected as the actuator. Then, in order to make the secondary loop a negative feedback control system, the action direction of the secondary controller FC should be selected as“just”effect. At this time, when the steam flow increases due to the fluctuation of the steam pressure, the output of the sub-controller will increase, so that the control valve is closed (because it is a gas shut-off valve), so as to ensure that the amount of heating steam entering the reboiler will not be affected. Or less affected by steam pressure fluctuations. In this way, the rapid effect of the auxiliary circuit to overcome the disturbance of steam pressure fluctuation is fully exerted, and the control quality of the main variable is improved. (2) The selection of the action direction of the main controller in the cascade control system can be carried out according to the following methods: when the main and auxiliary variables are increasing (or decreasing), if it is obtained from the process analysis, in order to reduce the main and auxiliary variables (or increase), when the action direction of the control valve is required to be the same, the main controller should be selected“opposite”role; otherwise, you should choose“just”effect. It can be seen from the above method that the selection of the action direction of the main controller in the cascade control system is completely determined by the process conditions, and has nothing to do with the air-open and air-close types of the actuator and the action direction of the sub-controller. Therefore, the selection of the main and auxiliary controllers in the cascade control system can be in the order of the auxiliary first and the main, that is, first determine the opening and closing type of the actuator and the positive and negative effects of the auxiliary controller, and then determine the action direction of the main controller. ;It can also be in the order of the main and the auxiliary, that is, first determine the action direction of the main controller according to the requirements of the process characteristics, and then select the open and close types of the actuator and the auxiliary controller according to the method of the general single-loop control system. direction of action

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