Open-loop operation mode is often used to control the Brushless DC Motors (BLDCMs) without rotor position sensors when the back electromotive force (EMF) is too weak due to the very low rotor velocity. The rotor position information is not necessary in this mode and the stator windings are supplied with voltages under a certain ratio of the amplitude to the frequency. However, the rotor synchronization will be destroyed once if the commutation instant is inappropriate. In order to improve the reliability of the open-loop operation mode, a dynamic equation is established to represent the synchronization error between the rotor and the stator. Thereafter, the stability of the open-loop control mode is analyzed by using Lyapunov indirect method. Theoretical analysis indicates that the open-loop control mode is asymptotically stable only when the commutation instant of the stator current lags behind the ideal one suitably. Finally, theoretical analysis is verified through the experimental results of a certain BLDCM.

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In order to obtain accurate angular position and velocity from resolver signals, Resolver-to-Digital Conversion (RDC) is necessary. However, there are inevitable harmonics in resolver signals, which will deteriorate RDC accuracy seriously. Although the harmonics of resolver signals can be suppressed by using low-pass filters (LPFs), the phase lag of LPFs will result in additional errors in RDC, especially for the suppression of lower-order harmonics. In this paper, a novel filtering strategy is proposed for resolver signals by combining two complementary filters (CFs) with a frequency locked loop (FLL). Firstly, CFs are designed for the sinusoidal and cosinusoidal channels by using the natural orthogonality in the resolver signals. Each CF consists of two LPFs assisted by the estimated frequency from FLL with a frequency discriminator and a second-order observer. Secondly, a frequency discriminator is designed to detect the frequency error between the resolver signals before and after CFs. Thirdly, a second-order observer is designed to estimate the signal frequency by regulating the frequency error. Compared with conventional LPFs, FLL based CFs can suppress the low-frequency harmonics without phase lags and can improve RDC accuracy. Simulation and experimental results demonstrate the effectiveness of the proposed strategy.INDEX TERMS Resolver-to-digital conversion (RDC), low-pass filter (LPF), frequency locked loop (FLL), complementary filter (CF).

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