rise time and settling time

Rise time and settling time are two stopwatch numbers that describe how quickly a controlled thing reacts when you ask it to change. Imagine you nudge a thermostat from 20 to 25 degrees. Rise time answers "how fast does it get close?" — it measures the time the temperature takes to climb from low to nearly its new target (commonly from 10 percent to 90 percent of the jump). Settling time answers a different, more patient question: "how long until it stops wobbling and stays put?" — the time until the value gets inside a small tolerance band around the target, say within 2 percent, and never strays back out.

The two numbers can pull in opposite directions, and that tension is the heart of tuning a controller. A system can rise very fast by charging hard at the target, but a hard charge tends to overshoot and then bounce above and below before calming down, which makes settling time long. A gentler approach rises slowly but slides smoothly into place with little wobble, so it settles sooner relative to how quickly it arrived. Engineers pick a balance based on what the machine is for: a camera autofocus wants a quick rise even with a tiny wobble, while a crane lowering a load wants a calm settle even if it takes its time getting there.

Both are measured against a step input — a sudden, clean jump in the command — because that is the harshest, most revealing test of how a system handles change. Reporting rise time and settling time together gives a fair picture: one tells you the speed of the first move, the other tells you the cost of that speed in lingering wobble.