Allen-Bradley PID parameters explained

Which direction makes the action stronger

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

Reset is in repeats/second, so a HIGHER 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: repeats/second
• Rate units: seconds
• Algorithm form: parallel independent

Reading the independent gains

In the PIDE independent form the three gains are genuinely separate: changing Kp does not scale the integral or derivative terms. Ki is an integral gain in 1/sec — a rate, not a time — so a bigger number repeats the integral action more often.

The PIDE also has a dependent form, in which the same block behaves like a conventional PID with a single gain scaling all three terms and reset in minutes per repeat. Establish which form is configured before interpreting any number.

The legacy instructions

The PLC-5 and SLC PID instructions express reset as a time in minutes per repeat and rate in minutes, with the proportional term as a gain. A plant running a mix of legacy and PIDE blocks carries both conventions side by side, and a tuning set copied between them needs converting rather than transcribing.

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

Is Allen-Bradley reset in seconds or minutes per repeat?

It depends on the form. The PIDE independent form uses integral gain Ki in 1/sec. The legacy PID instructions and the PIDE dependent form use minutes per repeat. The two move in opposite directions, so confirm which you have first.

Does a higher Ki mean faster integral action?

Yes. Ki is an integral gain in 1/sec, so a bigger number is more integral action per second — the opposite of the minutes-per-repeat convention.

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