Robustness is a common term for various aspects of a control system’s tolerance of model errors with respect to stability [30]. Specifying a phase margin of at least 60◦, as we have done, is a good way of ensuring robustness. Not only it is unlikely that system stability will be jeopardized but also it is likely that robust performance will be obtained. That is, reasonable model errors will not result in severely degraded performance.
A. Unmodeled Dynamics In the second-order (including the PI controller) system model considered, some modes remain unmodeled, including the dynamics of the following:
1) current and flux (the latter for DCMs and IMs in the field-weakening region and for IMs under imperfect fieldorientation conditions);
2) measurement and antialiasing filters (for a sensored drive);
3) sensorless estimation algorithm (for a sensorless drive, particularly at low speeds);
4) time delays resulting from analog/digital conversion and computation.