SIMATIC PCS 7 PID parameters explained

Which direction makes the action stronger

On your system a HIGHER proportional number means MORE aggressive proportional action.

Reset is in seconds/repeat, so a LOWER number means FASTER integral action.

This is the single most consequential thing to get right. Proportional band and gain move in opposite directions, and so do repeats-per-time and time-per-repeat. An engineer who has moved from one system to another and carried the old habit across will make loops worse while believing they are improving them.

• Proportional term: gain
• Reset units: seconds/repeat
• Rate units: seconds
• Algorithm form: ideal noninteracting

Copying a band-based set onto PCS 7

A gain of one corresponds to a band of 100%. A gain of two is 50%. A gain of one half is 200%. Increasing gain makes proportional action stronger; increasing the band makes it weaker.

Anyone moving between the two conventions needs that inversion in mind on every single change. Convert, then type the gain.

Why the algorithm form matters

The algorithm form matters because the same three numbers behave differently in each. In the series (interacting) form the derivative term multiplies the proportional-plus-integral term, so changing rate also changes the effective gain and reset. In the ideal (non-interacting) form the three terms are independent of each other but all scaled by gain. In the parallel (independent) form even that scaling is absent — gain does not affect integral or derivative action at all. Moving a tuning set between systems without converting for form is one of the commonest ways a loop that worked on one plant fails on another.

Before you change anything

Confirm the units on the actual controller rather than trusting the platform default. Systems get configured in unusual ways, engineering units get changed during a migration, and a controller imported from another site may carry conventions from there.

The fastest check is to make a small deliberate change and watch which way the loop responds. If it gets more aggressive when you expected less, the convention is not what you assumed.

Questions that come up

How do I convert PCS 7 gain to a band-based faceplate?

Divide one hundred by the gain to get the band as a percentage, and divide one hundred by the band to go back. The glossary defines the band; the converter does the arithmetic in both directions.

Are Siemens PID TI and TD in seconds?

On SIMATIC PCS 7 they are. Integral time TI and derivative time TD are both in seconds, with proportional action as gain, not band. Confirm on the actual block before converting a minutes-based set.

Related