7/1/2012 5:36 PM | |
Joined: 1/28/2009 Last visit: 10/26/2024 Posts: 6852 Rating: (1365) |
Hello , Cascade structure is primarily used to speed up a slow process(temperature) by a fast process(flow). We have a setpoint and one LMN for that structure.Setpoint of the first process achieved by output of the 2nd controllers' output.See the attachment for sample program and trends. Check this topic also: Need help in Cascade control method...Hope this helps Best regards Hamid Hosseini Attachmenthd_cascade.zip (117 Downloads) |
Last edited by: hdhosseini at: 7/1/2012 5:38 PM |
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7/2/2012 1:39 AM | |
Joined: 9/27/2006 Last visit: 11/4/2024 Posts: 12293 Rating: (2690) |
Hello maher; Cascade controll does not automatically make a better loop response than a simple loop; it all depends on the requirements of the loop controllers. Here is an explanation of when a cascade controller is required: Cascade control can improve control system performance over single-loop control whenever either: (1) Disturbances affect a measurable intermediate or secondary process output that directly affects the primary process output that we wish to control; or (2) the gain of the secondary process, including the actuator, is nonlinear. A typical candidate for cascade control is the shell and tube heat exchanger The author then proceeds to explain the shell and tube heat exchanger system, which should remind you of your own furnace cooling system, and the cascade control to be implemented. Have a look here: http://www.bgu.ac.il/chem_eng/pages/Courses/oren%20courses/Chapter_10.pdf Hope this helps, Daniel Chartier |
7/2/2012 9:38 AM | |
Posts: 137 Rating: (2) |
thanks alot for your information, |
7/2/2012 2:48 PM | |
Joined: 9/27/2006 Last visit: 11/4/2024 Posts: 12293 Rating: (2690) |
Hello maher; See this docment from Emerson Process (makers of Fisher valves) that gives a good explanation of the term: http://www.documentation.emersonprocess.com/groups/public/documents/articles_articlesreprints/headleyrev_spr03_valvemag.pdf If your control valve is of butterfly type for example, then in low-flow control it is non-linear. If you open the valve 20% you get about 50% of the flow. When you open 50%, you get 90% of the flow... However, it is not only the valve itself that shows a linear/non linear flow characteristic, it is the whole control loop design, including the controller, the actauator, the valve manifold... Hope this helps, Daniel Chartier |
Last edited by: dchartier at: 7/2/2012 4:38 PMLast edited by: dchartier at: 7/2/2012 2:48 PM |
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