Fixing Measured Frequency Response Near Underdamped Dynamics for Loaded-Motor Feedback Design

System dynamics in the vicinity of a resonance or an antiresonance having a low damping factor can be approximated with sufficient accuracy for designing controllers with practical margins without knowing the damping factor. This is particularly relevant for isolated resonance or antiresonance, where the damping factor may be low and, therefore, neglected due to the normalized phase–amplitude relation. Using the proposed algorithm, the loss of accuracy in frequency-domain identification near resonance or antiresonance is easily compensated. We also discuss a noniterative algorithm that calculates the uncertainty region for the damping factor and the resonance or antiresonance frequencies, where the required closed-loop specifications are met by the designed controller.