Fault-tolerant control of robot servomotors

The servomotors used in the joints of a manipulator or wheels of a rover in high risk applications, such as space applications, 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) torque con troller for brushless motors that can maintain accurate torque production with minimum power dissipation subject to current limitations even if one of motor phases fails. The controller is developed based on the Fourier series model of the motor back-EMF waveform and the cogging torque. The faulty phase is detected from the covariance of the estimation error of a voltage observer. 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. A closed-form solution to the optimization programming problem is found by using the Kuhn-Tucker theorem; rendering the control approach suitable for real-time implementation. 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 power loss by 28%.