Determining PID Values for Slower-Responding Systems

It's important to realize that the Ziegler-Nichols method will work to determine the PID values for any type of system. For instance, if the system was a much slower responding system, like a heating system with considerably smaller electrical heating elements, the graph would reveal that the system changed from 150°F to 162°F when the output was changed 10%. Keep in mind that the previous system changed 140°F, so this system is slower in response. Since the output only changed 12°F when the 10% change of output was entered, the process gain (K_P) for this system is 1.2 instead of 7. The graph also indicates that the dead time also changes to 2.6 minutes, and the time constant is 1.2 minutes. When these numbers are entered into the Ziegler-Nichols formulas, the gain should be set at 6.4 for a proportional-only controller. If the controller is using proportional and integral (P and I), the gain should be set at 5.77, and the integral should be set at 7.8. If the controller is using proportional, integral, and derivative (PID), the gain should be set at 7.6, the integral should be set at 5.2, and the derivative should be set at 7.3.

PREV: PID Values from Process Gain, Dead Time, and Time Constant

NEXT: Determining PID Values in Automatic (Closed-Loop) Mode