The design of an affordable fault-tolerant control system of the brushless DC motor for an active waist exoskeleton

The brushless DC (BLDC) motor is a widely used method of powering various active exoskeletons such as waist exoskeleton devices. In this study, an affordable three-phase BLDC motor was designed using three 120° Hall-effect sensors to actuate an active waist exoskeleton. The fault in a Hall-effect sensor may cause the system failure. Thus, taking safety measures for the operating BLDC is a very important aspect for the device. This paper presents a model-based single-phase fault-tolerant control as a safety measure that is able to estimate speed and signal delay for Hall-effect sensors of BLDC motors used in the active waist exoskeleton. Because of motor inertia that resists changing rotational speed, the exoskeleton controller can estimate the time interval of the signal edge between the fault Hall-effect sensor and its adjacent sensor based on the sampled values of the average speed at the previous motor status. The signal delay can be used to reconstruct the faulty Hall signals. Then, the rotor position and velocity information can be corrected in time to restore the motor operation. The BLDC motor along with the controller was modeled, and a simulation was conducted to evaluate the effectiveness of the control strategy. The result showed that the fault-tolerant control could rapidly reconstruct the Hall signal required for motor rotation in the case of a single Hall-effect sensor failure, and ensured the stable operation for the BLDC motor on the exoskeleton.