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Scan time vs cycle time

Created by: BCNC at: 4/15/2014 9:31 AM (5 Replies)

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4/15/2014 9:31 AM	Rate (0)
BCNC Posts: 7 Rating: (0)	Hi all I have a question about the cycle time of FB41 and the scan time of OB35. So my project contains FB41 (multuiple blocks) in OB35. Now, i've read that the OB scan time has to be the same as the FB41 cycle time. In my case, OB35 = 100ms and FB41 = 1s So this will have influence on TI and TD. But can anyone explain to me what the influence will be? Or are the settings right this way? Kind Regards.
BCNC Posts: 7 Rating: (0)
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4/15/2014 11:14 AM	Rate (0)
Jazz_M Posts: 1121 Rating: (86)	Hi Well you can call the block in OB 32 for 1 sec, See the Cycle time given in the FB41 is written as "The time between the block calls must be constant. The "sampling time" input specifies the time between block calls." The sampling time is the time taken for processing the block, if you have say five instance all the instance should be in the same OB and not in different cycle. What effect it will have on the Ti and Td is not know to me, Hope this helps.
Jazz_M Posts: 1121 Rating: (86)
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4/15/2014 1:46 PM	Rate (0)
Sydney Gold Member Joined: 1/4/2008 Last visit: 12/2/2025 Posts: 703 Rating: (98)	Hello, the influence is pretty simple - the blockperforms thePID algorithm and as the TI and TD are the time constants,the block evaluates the "real" time from the cycle parameter, in other words - it assumes that between twoconsecutive call cycles elapses the time specified in the cycle parameter. So if you provide a bad value, the calculation according the PID algorithm is wrong. Basicaly many people don't care the meaning of the PID constants so if theyhave any process controlled by PIDput there some numbers, check result and change the values accordingly - in this case it doesn't matter what number is in the cycle parameter and PID algorithm works somehow. But if you have exact process and somebody specifies the requested PID parameters (for example from known process dynamic behavior, transient response etc.) you have to provide the right cycle time for the block call to obtain good results. Another example - you have the same process as you already had before and you did a good PID tuning before so you have the right PID constants. If you use them now again but provide bad cycle parameter, the PID algorithm will behave differently. Regards Sydney

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4/15/2014 1:56 PM	Rate (0)
BCNC Posts: 7 Rating: (0)	Thanks all for the fast replies. Really appreciate it. So if i understand correctly: It's best to match the cycle time and scan time But in fact it all depends on the one who has programmed the PID-action? So if i have correct PID-values and someone changes the cycle time it can get wrong results. But when you start to change the PID-values until it is quite right, it is also good?
BCNC Posts: 7 Rating: (0)
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4/15/2014 6:57 PM	Rate (0)
Ton Verra Gold Member Joined: 4/20/2007 Last visit: 9/1/2025 Posts: 731 Rating: (43)	You are correct, it will work, but that doesn't mean it is right. If you know what I mean.

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6/11/2018 10:13 AM	Rate (0)
Wit_Moderator Moderator Joined: 8/21/2014 Last visit: 12/9/2025 Posts: 1091 Rating: (15)	New question published by AutoGen_499970 is split to a separate thread with the subject Throughput difference between EtherCAT and Profinet?. Best regards The Helping Hand
	My Forum is the place to personalize your profile, track your topics, and maintain content that interests you. Best regards Wit_Moderator
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