A PHYSICALLY INSIGHTFUL APPROACH TO THE DESIGN AND ACCURACY ASSESSMENT OF FLUX OBSERVERS FOR FIELD-ORIENTED INDUCTION MACHINE DRIVES

Rotor flux observers can provide an attractive means for achieving dir ect field oriented control of induction machines. This paper presents a physics-based design methodology and uses it to evaluate open-loop o bservers and to develop a new closed-loop flux observer. It is shown t hat the new flux observer is a straightforward structure with properti es that combine the best features of known methods. A distinction is m ade between observers, which use only integration and feedback summati on operations, and those estimation methods requiring approximate diff erentiation which are, in essence, ''cancellation'' methods. Furthermo re, a clear distinction is made between accuracy and dynamic robustnes s of the observer. This distinction is important because the accuracy of flux observers for induction machines is inherently parameter sensi tive. Whereas robustness of observers, in a controls sense, is not par ameter sensitive. Moreover, it is shown how flux observers can provide robust field oriented control because the flux angle is substantially more correct than the flux magnitude. A distinctive form of frequency response function (FRF) analysis similar to that used in classical co ntrol engineering is demonstrated to be a useful and insightful tool e ven though flux observers are multiple-input, multiple-output systems. Finally, the limits of such flux observers are experimentally evaluat ed.