Ziegler-Nichols Closed Loop Tuning

The Ziegler-Nichols Closed Loop method is one of the more common methods used to tune control loops. It was first introduced in a paper published in 1942 by J.G. Ziegler and N.B. Nichols, both of whom at the time worked for the Taylor Instrument Companies of Rochester, NY.

The open loop method is useful for most process control loops. To use the method the loop is tested with the controller in automatic. The Closed Loop method determines the gain at which a loop with proportional only control will oscillate, and then derives the controller gain, reset, and derivative values from the gain at which the oscillations are sustained and the period of oscillation at that gain.

The ZN Closed Loop method should produce tuning parameters with will obtain quarter wave decay. This is considered good tuning but is not necessarily optimum tuning.

Steps

• Ensure that the process is "lined out" with the loop to be tuned in automatic with a gain low enough to prevent oscillation.
• Increase the gain in steps of one-half the previous gain. After each increase, if there is no oscillation change the setpoint slightly in order to trigger any oscillation.
• Adjust the gain so that the oscillation is sustained, that is, continues at the same amplitude. If the oscillation is increasing, decrease the gain slightly. If it is decreasing, increase the gain slightly.
• Make note of the gain which causes sustained oscillations and the period of oscillation. These are the "Ultimate Gain" (GU) and the "Ultimate Period" (PU) respectively.
• Calculate the tuning for the following set of equations. Use the set which corresponds with the desired configuration: P only, PI, or PID.

  Tuning Equations

• P Only: Gain = 0.5 GU
• PI: Gain=0.45 GU, Reset=1.2/PU
• PID:Gain=0.6GU, Reset=2/PU, Derivative=PU/8

Updated January 9, 1996.
Provided by John Shaw.
Process Control Solutions