Rockwell Automation Publication 2198-UM004D-EN-P - December 2022 229
Chapter 9 Tuning
Guidelines for Gain/Phase Margin
• Gain and phase margin must be positive values.
• Phase margin range between 30…60 is considered acceptable. A value
lower than 30 will lead to instability. This means you must lower gains to
raise the Phase margin, which means you lower the performance
(Bandwidth). A value greater than 45 means you have a stable system,
and you can increase the gains further, which means you raise the
performance (Bandwidth).
• Gain margin range between 5…10 dB is considered acceptable. This
should be positive, if its negative, the system is unstable.
• If the response curves never cross 0 dB or -180 degrees, the system has
low performance, but is likely stable.
• The gain and phase margin values vary by machine (mechanism) type
and by the inertia reflected to the motor shaft.
Figure 96 - System Analysis Tool - Bode plot
Method for Using the System Analysis Tool
You can use the System Analysis tool to develop a bode plot; and ultimately
characterize your system. This type of information is useful with complex
mechanisms that are difficult to tune.
Before any tuning is attempted, a baseline System Analysis test should be
performed so you can quantify the improvements from any tuning changes
(default, autotune, or manual tuning). Once tuning changes are made, the
System Analysis can be used again to verify the response and see if it has
improved.
The System Analysis can also show resonances that exist with your system. If
you are not able to use an FFT (Fast Fourier Transform) tool, you can use the
System Analysis tool.
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