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Old January 26th, 2018, 07:46 AM   #1

aand74 is offline
Join Date: Dec 2005
Location: Deinze
Posts: 83
Fb41 cont_c : Pid-blok

We have to migrate a project from S7 300 to S7 1500 where FB41 CONT_C is used.
It is called in OB (every cycle), I see that the parameter 'Cycle' is 500 ms.
I don't know what the average OB1 cycle time is.
I know this was not implemented as it should.
But I should want when implemented in S7 1500 that the pid parameters don't have to be retuned.
Does the calling frequency has an influence on the effect of the proportional parameter?
What about the I-parameter?
In the original program, if the OB1 cycle time would fi be 40ms, the 'Cycle' parameter of FB41 is at 500 ms, what will be the influence? If I would like to implement it in a cyclic OB, what cycle time should I choose?
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Old January 26th, 2018, 12:08 PM   #2

Thomas_v2 is offline
Join Date: Apr 2009
Location: Ger
Posts: 386
All time-relevant parameters must be adjusted. These are TI for a PI-Controller, and TD and TM_LAG if you use derivative action (D_SEL). If you have proportional control only, you don't have to recalculate any parameters.

You must multiply the time values by the called cycle time divided by the time at parameter CYCLE.
For your example, if the block is called inside OB1 cycle and you have a real average OB1 cycle time of 50ms, and CYCLE has a 500ms, then you have to multiply TN with 50/500 = 0.1.

If in your S7-300 the PI-controller runs fine with TI = T#100s and calling cycle every T#50ms but CYCLE parameter wrong at T#500ms, then you'll need a TI = T#10s when CYCLE parameter matches the real calling cycle.
You can use the same calculation for TD and TM_LAG.

Have you checked if there is no conditional call of the FB41 block, or timer-based jump or block-end?
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