A Note on ISO AMB Stability Margin

Stability is a primary robustness requirement. The stability margin is therefore a measure of the amount of system deviation from some nominal, stable configuration that can be tolerated before the system becomes unstable. The notion of ‘system deviation’ can imply many forms and mechanisms: engineers must, perforce, focus on those most likely to afflict a given system in order to obtain a meaningful assessment of relative stability. This note discusses the stability margins of rotor-AMB sys-tems. Particular attention is focused on the output sensitivity, whose peak amplitude is a common measure of sensitivity and is adopted by the new ISO standards for AMB systems. One objective is to illustrate that many common sources of instability in rotordynamic systems may be completely missed by this measure and, further, that controller design targeting reduction in output sensitivity may, in fact, lead to very poor robustness to these mechanisms. Several examples are developed to show that, while limiting the sensitivity peak is a necessary condition for rotor-AMB system stability, it may not be sufficient to ensure commercially viable robustness to common destabilizing mechanisms such as aerodynamic cross-coupling or hysteretic damping in the rotor. The primary observation is that, if the AMB is the destabilizing mechanism, then the output sensitivity will reveal it, but if the destabilizing mechanism is endemic to the rotor and the relevant feedback is not collocated with the AMB connectivity, then the output sensitivity may not assess its effect.