How does the reset in the diagram work?
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As shown in the image, this is a post previously shared by a forum member, but there are no satisfactory answers available; please offer some guidance.
This post was last edited by blued simple life on 2023-6-16 at 17:19. Isn’t it actually just a normal cascade control for liquid level and flow rate?
There is something wrong with this picture. How can the controller LIC output be sent to both the controller FIC and the control valve at the same time? ? Does the control valve operate based on FIC? Still based on LIC actions? If the output of LIC is fed to the given terminal of FIC, a level-flow cascade control system is formed. In addition to the meaning of reset, RESET also implies re-setting, that is, LIC resets the value assigned to FIC, thereby adjusting the value assigned to FIC again.
Thank you for your advice. 1. Consider the signal line from the control valve to the FIC as being identical to that from the LIC to the FIC; this is essentially cascade control. 2. Regarding reset, I understand that it also means to set the value again. What I don’t quite understand is why/under what circumstances is this reset needed? (I also checked other parts of the flowchart; not all the level-flow cascade control sections have a reset function, so there must be a purpose behind this.)
Marking RESET here may be to emphasize the readjustment of the flow rate controller based on the liquid level. A long time ago, integral control was also referred to as RESET control; perhaps this was to emphasize that the LIC has an integral function, which adjusts the setpoint of the FIC controller, and that the LIC indeed possesses an integral capability. However, ‘RESET control function’ is an old term.
Regarding the wiring issue: 1. The output of the LIC controller is a signal line with a small circle, indicating that it is a data link; in other words, the data is transmitted to the FIC in digital form； 2. The output terminal of FIC and the given terminal are not at the same point, so they cannot be drawn together ； 3. The signal sent to the electrical valve positioner of the control valve should be an electrical signal (usually 4~20mA); this electrical signal is represented by a dashed line and is different from the data link.
Thank you; I think what you say is correct. I asked another colleague for the flowchart of another unit in the same project, and in each case where there was cascade control, the word “reset” was indicated. As mentioned in my previous reply, those individual cases that were not marked as reset can be considered as omissions for now. (Also, the flowchart I’m looking at was produced by the design institute last year; I don’t know why such ‘old’ annotations are still being used – they come from foreign process packages.)