Design and Evaluation of a Mobile Ankle Exoskeleton With Switchable Actuation Configurations

Lower limb exoskeletons with proper assistance can improve the mobility of people with leg impairments. Prior studies have shown that tethered ankle exoskeletons can reduce the metabolic cost during walking and running, but few mobile ankle exoskeletons can achieve similar performance. The additional weight of the actuation system and compromised control performance decrease the assistance efficiency. Here, we developed a mobile exoskeleton that can provide plantarflexion torque assistance at the ankle. The actuation system is switchable between a single- and dual-motor ones to adapt to different users and tasks. The single-motor system can achieve lighter weight and better transparency while the dual-motor one can provide higher assistance. The single-motor exoskeleton system has a 0.95 kg weight worn on the leg with another 1.15 kg power belt mounted on the waist. The dual-motor system can provide up to 100 N$\cdot$m assistive torque and reduce an average of 32.5% of soleus muscle activity in preliminary tests. A real-time controller was implemented in an integrated embedded system and achieved high torque control performance. The closed-loop bandwidths of the single- and the dual-motor systems were 13.0 and 15.3 Hz, respectively. The average root-mean-squared torque tracking error was 2.6% of peak torque during walking and running. The mobile ankle exoskeleton can assist for 3 h of continuous active walking or running with an integrated 4500 mAh battery, making it promising in investigating assistance strategies off the treadmill.