Fault-tolerant ripple-free torque control of brushless motors

Fault tolerance is critical for servomotors used in high-risk applications, such as aerospace, robots, and military. These motors should be capable of continued functional operation even if insulation failure or open-circuit of a winding occur. This paper presents a fault-tolerant (FT) ripple-free torque controller for brushless DC (BLDC) motors that can maintain accurate torque production with minimum power dissipation even if one of its phases fails. The distinct feature of the FT controller is that it is applicable to BLDC motors with any back-EMF waveform. Firstly, an observer estimates the phase voltages from a model based on Fourier coefficients of the motor waveform. The faulty phases are detected from the covariance of the estimation error. Subsequently, the phase currents of the remaining phases are optimally reshaped so that the motor accurately generates torque as requested while minimizing power loss subject to maximum current limitation of the current amplifiers. Experimental results illustrate the capability of the FT controller to achieve ripple-free torque performance during a phase failure at the expenses of increasing the mean and maximum power loss by 28% and 68% and decreasing the maximum motor torque by 49%.