Human-Exoskeleton Interaction Portrait

Abstract

Human-robot physical interaction contains crucial information for optimizing user experience, enhancing robot performance, and objectively assessing user adaptation. This study introduces a new method to evaluate human-robot co-adaptation in lower limb exoskeletons by analyzing muscle activity and interaction torque as a two-dimensional random variable. We introduce the Interaction Portrait (IP), which visualizes this variable's distribution in polar coordinates. We applied this metric to compare a recent torque controller (HTC) based on kinematic state feedback and a novel feedforward controller (AMTC) with online learning, proposed herein, against a time-based controller (TBC) during treadmill walking at varying speeds. Compared to TBC, both HTC and AMTC significantly lower users' normalized oxygen uptake, suggesting enhanced user-exoskeleton coordination. IP analysis reveals this improvement stems from two distinct co-adaptation strategies, unidentifiable by traditional muscle activity or interaction torque analyses alone. HTC encourages users to yield control to the exoskeleton, decreasing muscular effort but increasing interaction torque, as the exoskeleton compensates for user dynamics. Conversely, AMTC promotes user engagement through increased muscular effort and reduced interaction torques, aligning it more closely with rehabilitation and gait training applications. IP phase evolution provides insight into each user's interaction strategy development, showcasing IP analysis's potential in comparing and designing novel controllers to optimize human-robot interaction in wearable robots.

Figures (13)

• Figure 1: Regions of Interaction Portrait (IP). Each quadrant of the circle corresponds to different human-exoskeleton interaction modes determined by the variation of the normalized total muscle activation ($_{c_1}^{c_2}\Delta \mu$) with respect to the normalized total interaction torque ($_{c_1}^{c_2}\Delta \tau$) between controllers $c_1$ and $c_2$, respectively. The first quadrant (red) indicates increased disagreement between the user and exoskeleton, resulting in an increase in both muscle effort and the total interaction torque. The second quadrant (green) determines the co-adaptation of the user toward participating in the motion as much as possible and leading the motion. The third quadrant (blue) denotes the decrease in total interaction torque and the total muscle effort, associated with the decrease in human-exoskeleton disagreement. Finally, the fourth quadrant (orange) denotes the condition at which the user yields control of the motion to the exoskeleton and minimally activates their muscles. In this case, muscle activation decreases while the total interaction increases since the exoskeleton has to carry the user's body (passive dynamics) in addition to the exoskeleton dynamics.
• Figure 2: A portion of a typical participant's experimental data; for ease of visualization and interpretation, the interaction torque at the right hip and activation of one of the muscles are illustrated along with the relative oxygen uptake. (A) The mean absolute interaction torque at the right hip at each stride with each controller and speed for Participant #1. (B) Normal muscle activation for the Gastrocnemius Medialis at the right leg. (C) Relative oxygen uptake for each breath for each controller and speed. The oxygen uptake has increased with the increase in treadmill speed.
• Figure 3: The average performance metrics for each treadmill speed and controller across participants. (A) The sum of the relative oxygen uptake across all the strides for each speed in each controller block graphed for each participant. The bars show the average of the sum of the oxygen uptake across all participants. Similarly, the average total absolute value of the human-exoskeleton interaction and total normalized muscle effort are graphed in (B) and (C), respectively.
• Figure 4: Comparing the Average Interaction Portrait for each Pair of Controllers. The average Interaction Portrait (IP) depicted according to the average total muscle effort and the average total human-exoskeleton interaction for each participant computed at each of the ultra-slow, slow, and moderate-speed walking for the TBC$\rightarrow$HTC, TBC$\rightarrow$AMTC, and HTC$\rightarrow$AMTC illustrated in (A), (B), and (C), respectively. The yellow areas denote the area between the 25 and 75 percentiles.
• Figure 5: Comparison of the Interaction Portrait distribution between TBC$\rightarrow$HTC and TBC$\rightarrow$MTBC. Interaction portrait distribution for HTC and AMTC blocks with respect to the average total muscle effort and total interaction torque across all strides during the TBC block graphed for each participant plotted for moderate speed walking. The radius of data points is normalized with respect to the maximum radius computed across all participants’ strides. Participants are arranged increasingly according to their body mass. The polar histograms show the concentration intensity of the depicted points. Each bin of the histogram covers $\pi/6$ rad.