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On low-level control strategies of lower extremity exoskeletons with power augmentation

Biomechanical studies prove that the human can undergo hard stresses throughout his/her body especially the low region of the back while lifting/carrying loads. Design of a wearable robot (an exoskeleton) can support and amplify the user power. Three possible points of view are considered for the design of control architecture: (1) minimization of interaction force wrench if possible, (2) modification of reference trajectory of the coupled user-exoskeleton system for compensation of undesirable interaction force wrench, and (3) adding the power assist rate required for empowering the user. To achieve this, three possible levels of control architectures are used: high-level control for capturing the user intention, mid-level control for determining the switching periods of the walking phases, and low-level control for stabilization of the user locomotion. Accordingly, this paper introduces a systematic overview of low-level control strategies adopted for lower extremity exoskeletons with power augmentation. They can be summarized as indirect force control, direct force control, and observer-based control. The features and limitations of each control strategy are described considering some state-of-art powered exoskeleton prototypes.