Mitsubishi MELSEC PID parameters explained

Which direction makes the action stronger

On your system a LOWER proportional number means MORE aggressive proportional action. Proportional band is the inverse of gain.

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: proportional band
• Reset units: seconds/repeat
• Rate units: seconds
• Algorithm form: ideal noninteracting

PLC instructions versus process controllers

The MELSEC range spans PLC PID instructions and dedicated process-control blocks, and the two do not always present the tuning the same way. Instructions may expose gain directly while the process blocks work in proportional band.

That is why the faceplate matters more than the brand: the same plant can carry both conventions, and a number only means something once its units are known.

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

Does Mitsubishi use gain or proportional band?

Both appear across the range. Process-control configurations commonly use proportional band with integral time in seconds, while some PLC PID instructions expose gain directly. Read the units off the block before interpreting a number.

Is a higher proportional band more aggressive?

No. Band is the inverse of gain, so a higher band is weaker proportional action. Lower is more aggressive.

Related