Questions about the questions from the second week of 2011
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The topic is (figure omitted): “The following figure shows a cascade control system for the furnace outlet temperature and fuel oil flow rate. Please determine the positive or negative action of the primary and secondary controllers by ensuring that the amplification factors Kc for each stage are positive.” ” There is no issue with both the primary and secondary regulators being of the reactive type. This control system is a cascade control system, in which the output of the main controller in this cascade system serves as the setpoint for the secondary controller. We assume that the main controller operates in a positive-action mode; when the outlet temperature rises, the deviation of the main controller (measured value – setpoint) increases, leading to an increase in its output. This in turn increases the setpoint for the secondary controller, reducing the corresponding deviation (measured value – setpoint). Since the control valve operates in an air-open manner, in order to ensure negative feedback throughout the control system, it is necessary to reduce the opening degree of the control valve and thus lower the fuel flow rate. To reduce the output of the secondary controller, it should be set to operate in a positive-action mode. In my opinion, there are two ways to configure the primary and secondary regulators in this cascade control system: 1) both the primary and secondary regulators operate in a feedback mode ； 2. Both the main and auxiliary regulators are of positive action. It’s a bit confusing that the amplification factor Kc for each stage is positive; I hope the moderator can provide a detailed explanation. Thank you~ This is just my personal understanding for discussion purposes~~
This issue is about to be forgotten before it’s even discussed; I need to push for it~~~
Reply to 1# jlszhoujidai regarding “the magnification factor Kc for each stage is positive”. I understand that it means both the primary and secondary regulators are of the positive action type.
First of all, I need to make it clear that the original poster made a mistake: you cannot first determine the forward and reverse actions of the main controller before determining those of the secondary regulator. And regarding this issue, when the secondary regulator is set to positive action, think about it: if there are fluctuations in the fuel flow rate (for example, it increases), then the deviation also increases. Since the regulator operates in positive action and the control valve is a air-actuated valve, it will open wider, which immediately leads to an increase in the temperature at the raw material outlet. This actually goes against the purpose of cascade control, as the goal is to use the secondary regulator to eliminate certain disturbances; instead, you end up creating more disturbances, haha – that’s not right!
Reply to 4# ssln123: Thank you for your reply; what you said makes sense as well~~~ But this is a cascade control system. In the situation you described, when the secondary regulator is set to positive action, if there are fluctuations in the fuel oil flow rate (for example, it increases), then the deviation increases. Since the regulator operates in positive action and the control valve is a gas-actuated valve, the valve opens wider, resulting in an increase in the temperature at the raw material outlet. The deviation of the primary regulator also increases. As the primary regulator operates in positive action, its output increases as well. In a cascade control system, the output of the primary regulator serves as the setpoint for the secondary regulator; thus, the deviation of the secondary regulator decreases, its output decreases, the opening degree of the control valve reduces, and accordingly the fuel oil flow rate decreases along with the outlet temperature. The entire cascade control system operates on negative feedback principles. Haha, I’m not sure if my understanding is correct~~~
Reply to 5# jlszhoujidai: Hehe, the original poster should really figure out more carefully why cascade control is needed. I mentioned it a little earlier – why is it necessary to control the flow rate as well? The reason is that when there are fluctuations in the flow rate, these need to be eliminated in the inner loop so that they don’t affect the temperature. If, as you suggested, it’s the main controller that deals with these fluctuations after they occur, then cascade control serves no purpose at all. Think about it carefully – isn’t that the case?
This post was last edited by jcicwht on 2011-1-13 08:56. First, it is necessary to clarify the meaning of a positive amplification factor in each stage: For regulators, KC is defined as follows – if an increase in the measured value results in an increase in the output, then KC is negative (positive-action regulator); conversely, if an increase in the measured value leads to a decrease in the output, then KC is positive (negative-action regulator). The KV of a gas-open valve is positive, while that of a gas-close valve is negative. The magnification factor KO of an object is as follows: if the input increases, the output also increases, then KO is positive; if the input increases while the output decreases, then KO is negative. According to the regulations, if the regulator is set to positive action, then KC should be negative, which contradicts the given conditions; therefore, your statement is incorrect. In practical applications, if a positive-action regulator is used, an air-actuated shut-off valve should be selected for the control valve; this ensures that the product of the amplification factors is positive, thereby enabling negative feedback. As for whether the control valve can be set to air-operated shut, it must comply with the selection principles for control valves.
Reply to 6# ssln123: Thank you for your reply; I fully agree with your view. In such a cascade control system, both the primary and secondary controllers are set to operate in feedback mode, and that’s indeed what is used in practice. However, it’s possible you haven’t fully understood my intention. The purpose of my post was to raise doubts about the results of this problem, not to discuss the cascade control system itself. For example, there’s the issue of the amplification factors Kc being positive. Also, even if both the primary and secondary controllers are set to operate in forward mode, they can still perform regulation functions, but they may not be able to effectively suppress disturbances in flow rate. Poor parameter settings could even cause oscillations in the entire control system. Since this is a discussion topic, it should be analyzed; and based on the moderator’s evaluation, the analysis seems incorrect. That’s why I posted this question~~~
Reply to 7# jcicwht: I see, the answer is already given in the question? It is suggested that the moderator provide reference answers~~~