The three gains
- Proportional Gain: reacts to how far RPM is from target right now. A big gap gets a big correction. More P makes the idle react faster, but too much and it overshoots and swings.
- Integral Gain: builds correction up over time while any error remains. It's what closes the last small gap that P alone leaves behind. Too much and the idle slowly surges.
- Derivative Gain: looks at how quickly the error is changing and pushes back as RPM rushes toward target. It's the brake that stops an aggressive P from overshooting.
Each gain is a table, so you can give the controller different behavior in different conditions, for example softer when cold.
How we tune them
- Get the closed loop base table close first, so the controller only fixes small errors.
- Bring P up until the idle recovers quickly from a load without swinging.
- Add I until the idle settles on target instead of sitting just off it.
- Use D only if you need it to calm overshoot.
Our PID tuning guide covers the method in more depth, and the Idle Control article explains the rest of the system.
