Evaluation of controllers for augmentative hip exoskeletons and their effects on metabolic cost of walking: explicit versus implicit synchronization

Abstract

Background: Efficient gait assistance by augmentative exoskeletons depends on reliable control strategies. While numerous control methods and their effects on the metabolic cost of walking have been explored in the literature, the use of different exoskeletons and dissimilar protocols limit direct comparisons. In this article, we present and compare two controllers for hip exoskeletons with different synchronization paradigms.Methods: The implicit-synchronization-based approach, termed the Simple Reflex Controller (SRC), determines the assistance as a function of the relative loading of the feet, resulting in an emerging torque profile continuously assisting extension during stance and flexion during swing. On the other hand, the Hip-Phase-based Torque profile controller (HPT) uses explicit synchronization and estimates the gait cycle percentage based on the hip angle, applying a predefined torque profile consisting of two shorter bursts of assistance during stance and swing. We tested the controllers with 23 naïve healthy participants walking on a treadmill at 4 km ⋅ h−1, without any substantial familiarization.Results: Both controllers significantly reduced the metabolic rate compared to walking with the exoskeleton in passive mode, by 18.0% (SRC, p < 0.001) and 11.6% (HPT, p < 0.001). However, only the SRC led to a significant reduction compared to walking without the exoskeleton (8.8%, p = 0.004). The SRC also provided more mechanical power and led to bigger changes in the hip joint kinematics and walking cadence. Our analysis of mechanical powers based on a whole-body analysis suggested a reduce in ankle push-off under this controller. There was a strong correlation (Pearson’s r = 0.778, p < 0.001) between the metabolic savings achieved by each participant with the two controllers.Conclusion: The extended assistance duration provided by the implicitly synchronized SRC enabled greater metabolic reductions compared to the more targeted assistance of the explicitly synchronized HPT. Despite the different assistance profiles and metabolic outcomes, the correlation between the metabolic reductions with the two controllers suggests a difference in individual responsiveness to assistance, prompting more investigations to explore the person-specific factors affecting assistance receptivity.

Adaptive Hip Exoskeleton Control using Heart Rate Feedback Reduces Oxygen Cost during Ecological Locomotion

“…In the case of Indoor flat, which served as a warm-up period, the reduction was most likely limited due to the insufficient initial re-acclimation of the participants to assistance. Notably, the fixed-amplitude variant of this strategy yielded an average metabolic rate reduction of 8.8% in level-ground walking with lower assistive torques in our previous study 26 . The re-acclimation hypothesis is further supported by the larger increases in stride length and hip RoM (Figs.…”

“…The basic structure of the SRC has been described in previous work 26 . Here, we will briefly present the structure again for completeness.…”

“…In a previous study 26 , we presented a control strategy for hip exoskeletons based on the simple principle of assisting hip extension during stance and hip flexion during swing. A mapping from weights borne by the feet to the assistive torques was proposed to define the shape and timing of assistance, and the amplitude was determined by a scaling parameter.…”