Friday, September 9, 2016

Output Spikes - Sharp as Tacks!

This does not look good!

Here is a warning about derivative gain from a site, highlighted in the Error Browser.

PID Module 1,06,02 Block 214 uses derivative with PV from discontinuous input Module 1,06,02 Block 140

You can see the error marker next to the PID block it concerns.

Dan Reynolds sent me the data from the control loop identified. Here is what it looks like with DBDOC WatchWindow at 250 ms.

Zooming in a bit, we see that Block 280, the output, suffers spikes from its railed value of -5.0%. Block 214 shows a value of +1.7408%, that is, 6.7408 above the -5.0% it should be. The exception reported input Block 140 changes each minute and drives these spikes (not all of which are picked up in the 250ms reporting interval, since they are so brief). You can be sure that, if the control output were in the 0 % to 100% range, large positive and negative spikes would be suffered every minute. Ouch!

Just as clearly, if this PID output were not driven to -5 %, the spikes would be bi-directional and still significant. The effect is purely caused by the value of Kd on S8 for Block 214, only 0.2, but enough to drive the output in a way that is not valid.

By the way, you should notice how any exception report input has steps in its value. The values for Module 1,06,02 Block 140 and Module 1,03,04 Block 1901 show these characteristic steps. See Monitoring Exception Report Performance for details.

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