Abstract
Kinematic control approaches for exoskeletons replicate normative joint kinematics associated with one specific task and user at a time, which makes it difficult to adjust to continuously-varying activities during gait training. These approaches also overly constrain individuals who have partial or full volitional control of their limbs, preventing these individuals from choosing their own preferred gait patterns. To address these issues, we proposed a matching framework for underactuated total energy shaping (i.e., shaping both the potential and kinetic energies) with human and environmental interaction to provide task-invariant, energetic assistance. In our prior work, we designed assistive strategies to compensate for lower-limb inertia in the actuated part of the mass matrix while leaving mass related terms unshaped. While these strategies have demonstrated potential gait benefits, shaping mass related terms in addition to lower-limb inertia can produce greater benefits as they are more dominant in determining human dynamics during locomotion. Moreover, previous definitions of closed-loop mass matrix with reduced inertial parameters cannot guarantee its positive definiteness. Having a non-positive definite mass matrix in the closed loop can render chaotic behaviors such as unbounded exoskeleton torques that cause danger to human users. In this paper, we generalize our prior work to shape all inertial terms in the actuated part of the mass matrix while ensuring its positive definiteness in the closed loop. In addition, given a positive-definite, closed-loop mass matrix, we prove passivity from human input to joint velocity and highlight two Lyapunov stability results based on common assumptions of human joint control policies. We then show benefits of the proposed approach and its advantages over conventional exoskeleton control methods with simulations on a human-like model. We also show that the corresponding assistive torques closely match the human torques of an able-bodied subject.
Highlights
Powered lower-limb exoskeletons are external mechanical structures equipped with actuators that support and assist human users during locomotion
STATEMENT OF CONTRIBUTIONS The specific contributions of this paper can be summarized from the following four aspects: 1) Without making assumptions on the human input mapping matrix, we prove that the matching condition for human inputs are satisfied as a consequence of shaping the bottom-right part of the mass matrix; 2) We generalize our prior work in [25] to shape all inertial terms in the shapeable part of the mass matrix while ensuring its positive definiteness in the closed loop; 3) Given a positive-definite, closed-loop mass matrix, we show inputoutput passivity from human inputs to joint velocity with total closed-loop energy as the storage function
SIMULATION RESULTS AND DISCUSSION For studying the effects of the proposed shaping strategies during simulated walking, we consider the coupled dynamics of the two legs shown in Fig. 1, which is termed as the full biped model and is modeled as a kinematic chain with respect to the IRF defined at the stance heel
Summary
Powered lower-limb exoskeletons are external mechanical structures equipped with actuators that support and assist human users during locomotion. STATEMENT OF CONTRIBUTIONS The specific contributions of this paper can be summarized from the following four aspects: 1) Without making assumptions on the human input mapping matrix, we prove that the matching condition for human inputs are satisfied as a consequence of shaping the bottom-right part of the mass matrix; 2) We generalize our prior work in [25] to shape all inertial terms in the shapeable part of the mass matrix while ensuring its positive definiteness in the closed loop; 3) Given a positive-definite, closed-loop mass matrix, we show inputoutput passivity from human inputs to joint velocity with total closed-loop energy as the storage function. Using common assumptions of human joint control policies, we show Lyapunov stability of the closed-loop human-exoskeleton system; 4) We show extensive results of the proposed assistive strategies such as reduced metabolic cost and human torques in simulation. We summarize the limitation of the proposed study and provide possible future research directions
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