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s7-300 pid

Created by: danac at: 8/9/2010 2:42 PM (6 Replies)

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8/9/2010 2:42 PM	Rate (0)
danac Posts: 10 Rating: (0)	Hy!!! I want to regulate hidraulic cilindarwiths7-300 (CPU 312c).In one programmI must create threecontroll loops (p, pi and pid), therefore, whenI press button one (I 0.0) P controll is started, when I press second button (I 0.1) PI controll is started, and when I press third button (I 0.2) PID controll is started. Can someono tell me how to do this?!? Is it posible with only one functional block (FB41)?? thanks!!
danac Posts: 10 Rating: (0)
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8/9/2010 2:56 PM	Rate (0)
dchartier Grandmaster Joined: 9/27/2006 Last visit: 1/11/2026 Posts: 12480 Rating: (2722)	Hello danac; Do you mean that you want to control the same output with three different loops (FB41) but that you want to activate a differently parametrized FB41 based on different conditions? Of course this is possible, if you simply call the FB41s conditionally and make sure the calls are mutually exclusive (so that you don't call two versions of the FB41 active at the same time). A conditional call of a function is simple: just put one (or more) inputs instructions ( ---\| \|--, --\|/\|--, COMPARE instructions...)on the network, to the left, and the FB41 call at the far right of the network; conditions of the inputs must be true for the FB to be callled. Since you are planning to apply 3 different values to the output variable, with the 3 versions of your FB41, I would plan some sort of bumpless transfer when switching from one controller to the next, so that the output controller is not jerked if the outputs calculated by each FB41 is very different from the one actually controlling it. This implies maintaining the actual control value for a little while after switching controllers, and then ramping to the new value. Hope this helps, Daniel Chartier

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8/9/2010 3:38 PM	Rate (0)
danac Posts: 10 Rating: (0)	thank you Daniel!! it helps! ifI use three differently parametrized FB41,does this meanthat I must use three different data blocks? Can I call all three FB41 in only one OB35??How?
danac Posts: 10 Rating: (0)
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8/9/2010 3:38 PM	Rate (0)
Aret Posts: 2348 Rating: (265)	Daniel, i think there would be a problem in transferring internal sums for I component on changing PI to PID and visa versa - and you already mentioned that, as well as there must be a bumpless transfer. Maybe we can just turn I and D components on and off? I have never experienced with such a task, it's just a proposal to save internal variables' condition....
Aret Posts: 2348 Rating: (265)
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8/9/2010 9:24 PM	Rate (0)
dchartier Grandmaster Joined: 9/27/2006 Last visit: 1/11/2026 Posts: 12480 Rating: (2722)	Hello Aret; I understand you concerns, I would be wary of using this kind of control strategy in realworld application. One generally selects one form of PID control depending on the output curve requirements. In some applications, a PI controller will give fast enough response without too much overshoot, a good compromise; except if there is an integral component in the process, then you need the derivative control to smooth it out. But the derivative may cause oscillations, if the process has a tendancy to instability... The only time I did anything like this was in a demo application, on a simulation process, and I decided to go with 3 conditonnal calls of FB41 in OB35, each FB41 having its own IDB. This allowed the trainees to test individual control strategies, and test open/closed loops, with different tuning parameters (in each IDB). They would be able to see the results of their technical choices, and since this was a very (!) fast level simulator, the bumpless transitions were made evident by the mopping up they had to do when they did not pay attention to output tracking between loop changes. We also trained the students on cascade control, long deadtime process controland split-range techiques on the same simulators. A lot of work for all of us, but a lot of fun too. This being said, i repeat that I would make sure that tuning of each loop br done as close to perfect as possible and that bumpless transfer be impleented correctly, before applying such a control stratagem to a real application. Hope this helps, Daniel Chartier

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8/10/2010 6:10 AM	Rate (0)
Aret Posts: 2348 Rating: (265)	Well, i can't agrue withconfirmed experience :^) Can you tell a bit more about bumpless transfer algorithm? Is it done by setting output value of 2nd IDB equal to output value of 1st IDB or there was more interesting details? :^)
Aret Posts: 2348 Rating: (265)
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8/14/2010 4:51 PM	Rate (0)
hdhosseini Diamond Expert Joined: 1/28/2009 Last visit: 1/5/2026 Posts: 6892 Rating: (1368)	Hi For bumpless switchover from manual to auto or bumpless set point change or any cases which create steps, Rate limiter converts steps to ramps.Check the following attachment for rate of change limiter function. regards hdhosseini Attachment RATE.zip (505 Downloads)
	Demo Channel on Youtube
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