4/18/2012 2:28 PM | |
Joined: 1/28/2009 Last visit: 9/10/2024 Posts: 6849 Rating: (1365)
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Hello friends, **Only Continuous control loops Selecting an optimum sampling time of PID controllers is crucial when Processing resources of CPU is limited and several loops should be processed.There are some practical tips such as dividing processes to Slow,Medium and Fast ( Table 8.1of Controlling with SIMATIC) .But there is always degree of ambiguity in categorizing. But,there is an generally accepted rule of thumb in control literature (also mentioned in Controlling with SIMATIC) :
What do you think?Is there a way to determine the dominant time constant of a process by SIMATIC products? Best regards, Hamid Hosseini |
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4/18/2012 4:41 PM | |
Joined: 10/10/2005 Last visit: 8/29/2024 Posts: 1917 Rating: (136) |
Hello, there are some possibilities to delay the output value in refer to the input value. See the examples of the FAQ: http://support.automation.siemens.com/WW/view/de/1005801 With best regards |
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4/18/2012 5:00 PM | |
Joined: 1/28/2009 Last visit: 9/10/2024 Posts: 6849 Rating: (1365)
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Hello Cicero, This is not a question to be answered by just copying FAQ links. The link you pointed us to that is a pure delay function which is far from concept of "Time Constant". http://en.wikipedia.org/wiki/Time_constant Here, I intended to start an open discussion to find a method to estimate the dominant time constant of process .I have some possibilities in my mind but needs to be developed a bit more. Share your ideas. Best regards, Hamid Hosseini |
5/7/2012 3:46 PM | |
Posts: 2348 Rating: (264)
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Well, if you have 2 groups of PID-loops but only OB35 available (no other OB3x) such a solution can provide proper grouping |
5/7/2012 5:08 PM | |
Joined: 2/8/2007 Last visit: 9/9/2024 Posts: 12 Rating: (10) |
hello, I calculate the optimal cycle time with this formula: CT = ((T75-T0) - 1.25*(T75-T25))/10 CT = Cycle Time T75 = Time at 75% of change of PV T25 = Time at 25% of change of PV T0 = Time at 0% of change of PV the rules that use are: put the control in manual (example: output = 15%) wait for the stabilization increase of 10% (example: output = 25%). save T0 wait for the stabilization T25 and T75 calculated as the time to reach 25% and 75% of change I use also this formula form my on-line Tune |
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5/8/2012 9:04 AM | |
Joined: 2/8/2007 Last visit: 9/9/2024 Posts: 12 Rating: (10) |
i share, in my site, a implementation of FB41 with a sample time programmable and a parameters switch.... link to download: http://blog.ranghetto.com/?p=167 i use a "timer" maked withsystem tick (sfc64) also i make a cascade controller look inside my library ;) |
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