Rockwell Automation Publication 750-AT006D-EN-P - January 2022 13
Chapter 1 Background
Figure 9 - Types of Resonances
Each type of resonance is handled uniquely during tuning and is described in the following sections:
• Motor Side Resonances
on page 13
• Load Side Resonances on page 14
Motor Side Resonances
Motor side resonances are resonances that are observable in the feedback signal and are detected by the feedback device on the motor. As
a result, they are suppressed by tuning control loop gains, load observer gains, and torque loop filter parameters. Most mechanical
resonances are motor side and most drive applications exhibit them. Furthermore, all applications ultimately exhibit resonance if gains are
increased enough. Motor side resonances are categorized into three types, based on their frequency range.
Low Frequency (LF) Resonances – LF resonances have resonant frequencies below the Torque Loop Bandwidth. Because they are within
the closed loop bandwidth (in-band), they are automatically suppressed when the load observer is applied with the recommended out-of-box
settings. See Load Observer
on page 29 for more information.
Otherwise, they can cause classical instability that generates an audible low-pitch growling noise. This instability requires detuning the
control loop gains.
High Frequency (HF) Resonances – HF resonances have resonant frequencies above the Torque Loop Bandwidth. They typically generate
an audible high-pitch squealing noise. Because HF resonances are outside the closed loop bandwidth (out-of-band), they are suppressed
using torque loop filters. Adaptive tuning addresses these resonances by automatically configuring these filters. See Adaptive Tuning
on
page 34 for more information.
Otherwise, it is left to the user to identify the audible frequencies and manually configure torque notch filters. If there are more HF
resonances than there are notch filters available, a torque low pass filter is applied to suppress resonances with the highest resonant
frequencies. A last resort is detuning control loop gains and load observer gains until the resonances vanish.
Mid Frequency (MF) Resonances – MF resonances are resonant frequencies near the Torque Loop Bandwidth. Here, torque loop filters are
applied at frequencies close to the closed loop bandwidth. This close proximity allows phase lag generated by these filters to interfere with
the closed loop dynamics and cause instability. Adaptive tuning addresses the issue by automatically narrowing torque notch filter widths as
resonances approach the closed loop bandwidth. If instability occurs, adaptive tuning Gain Stabilization automatically decreases control
loop gains and load observer gains to restore stability. See Adaptive Tuning
on page 34 for more information.
Otherwise, it is left to the user to manually reduce notch filter widths, decrease control loop gains, and decrease load observer gains.
Similarly, torque loop low pass filters impact stability when they are applied at frequencies as low as 3…5 times the closed loop bandwidth
LF Resonances are in-band
Motor Side
Resonances
Load Side
Resonances
HF Resonances are out-of-band
MF Resonances are near CL BW
Load Resonances from end effector vibration
V
M
V
CMD
1
Power
Converter
T
m
PI
LPF
2
+ +
2
+ +
+
2
+ +
V
L
Velocity Loop
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