This paper considers the problem of controlling flexible systems, such as rotating shafts and gantry cranes, whose dynamics are described by the wave equation. In previous works, infinite dimension transfer functions with a special structure and an Absolute Vibration Suppression (AVS) linear controller were derived. The controller eliminates the infinite spectrum and results in a closed loop transfer function that has a low order rational part plus delay. The robustness properties of the AVS controller are investigated by using the Nyquist stability criterion. It is shown that in some cases it can be very sensitive to feedback, i.e. measurement and computation delays. A Robust AVS controller (RAVS), which is robust to feedback delays, is derived. The absolute suppression in AVS is only approximated in RAVS, but its performance compares favorably with that of other controllers.

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