1. A subsystem in a feedback control system for a plant having a flexible mode with a modal frequency that is driven by a control signal, the subsystem comprising: an adaptive notch filter configured to substantially suppress the modal frequency from the control signal, the adaptive notch filter being configured such that its notch frequency is governed by a tuning signal; and an online estimator configured to estimate the modal frequency of the flexible mode in the plant in real time based on the dynamic response of the plant to the control signal and to generate the tuning signal based on the estimate of the modal frequency.