Feedforward Control of Lower Limb Exoskeletons: Which Torque Profile Should We Use?

Despite the increased use of lower limb exoskeletons as gait training and mobility assistive devices, their controllers often lack the ability to synchronize and adapt to meet individual users' needs. This letter investigates two control approaches for lower limb exoskeletons: a real-time kinematic state-dependent estimation of desired torques with an inverse dynamics model and a data-driven component in the first approach, and a pre-defined torque control based on gait speed and phase in the second approach. These controllers are linearly combined to shift the controller behavior between pure kinematic state-dependent and pure gait phase-dependent control. These combinations were tested during overground and treadmill walking with nine able-bodied participants. The linearly combined controller with a greater emphasis on kinematic state-dependent control produced a more natural gait in terms of spatiotemporal metrics. This is reflected by 0.1 m/s increases in overground walking speed and 5% decrease in percent stance compared to walking with a passive exoskeleton. This controller also decreases the overall activity of lower limb muscles by up to 25% and thigh co-contractions by up to 40%. Participant feedback through a questionnaire, in terms of perceived effort, walking naturalness, and stability, also favored the aforementioned controller.