PID Performance Indices

When writing the PID Lab program, I need some indices for evaluate the controller.

There are three useful values:                                                    

• Rise Time: time duration from 10% to 90% of final value
• Settling Time: time to approach the final value (within the error band)
• Time Constant: when output value approaches 63.2% of desired value

It seems Settling time is a good index using in this case because it's not only can estimate the performance but be a stop condition.

But I encounter a strange phenomenon that sometimes it soon to complete controlling and sometimes not. (about 4-1) And the first run is the longest and is prone to find steady state error.