1/26/2018 3:43 PM | |
Joined: 9/27/2006 Last visit: 10/14/2024 Posts: 12287 Rating: (2685)
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Hello David; The PID calculations in the S7-300 must have been unreliable with the programming of FB 41 in OB1 and the output must have been unstable and difficult to tune. The basic principle of the FB 41/42 closed-loop control blocks is that the Cycle parameter must be a multiple of the calling OB cycle, and OB1 is not called regularly. The only way to ensure a regular OB cycle is to use a cyclic interrupt OB, such as OB 35. if OB 35 is set at 100ms and the PID Cycle parameter at 500ms for example, then the loop calculations will be reliable, This is an extract of Section 27 of the System and Standard Functions manual for s7-300, describing SFB 41 (exactly similar to FB 41, Cont_C): System Software for S7-300 and S7-400 System and Standard The calculation of the values in the control blocks is only correct if the block is called at regular intervals. For this reason, you should call the control blocks in a cyclic interrupt OB (OB30 to OB38). Enter the sampling time in the CYCLE parameter. The selected value of the loop parameters ishould be independant of the calling cycle of the FB 41. If the block had been configured and called correctly, the output oof the loop would have been correct and stable. However, since the calculations were varyng constantly because of the OB1 irregular cycle, we cannot say if the programmers had to change parameter values to force the loop output to a more stable response. It would seem advisable to select tuning parameter values for the S7-1500 PID controller without taking the S7-300 values into consideration.Hope this helps, Daniel Chartier
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Last edited by: dchartier at: 1/26/2018 3:50:02 PMLast edited by: dchartier at: 1/26/2018 5:12:01 PMLast edited by: Jen_Moderator at: 1/29/2018 10:40:39 AMInserted right link. |
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