Controller synthesis and clinical exploration of wearable gyroscopic actuators to support human balance

Abstract

Gyroscopic actuators are appealing for wearable applications due to their ability to provide overground balance support without obstructing the legs. Multiple wearable robots using this actuation principle have been proposed, but none has yet been evaluated with humans. Here we use the GyBAR, a backpack-like prototype portable robot, to investigate the hypothesis that the balance of both healthy and chronic stroke subjects can be augmented through moments applied to the upper body. We quantified balance performance in terms of each participant's ability to walk or remain standing on a narrow support surface oriented to challenge stability in either the frontal or the sagittal plane. By comparing candidate balance controllers, it was found that effective assistance did not require regulation to a reference posture. A rotational viscous field increased the distance healthy participants could walk along a 30mm-wide beam by a factor of 2.0, compared to when the GyBAR was worn but inactive. The same controller enabled individuals with chronic stroke to remain standing for a factor of 2.5 longer on a narrow block. Due to its wearability and versatility of control, the GyBAR could enable new therapy interventions for training and rehabilitation.

Figure 1

GyBAR prototype wearable gyroscopic actuator. (a) Model wearing the prototype GyBAR used in the experiments. (b) Simplified gyroscopic moment, $\mathit{\tau }\approx -\tau {\stackrel{\mathit{ˆ}}{\mathit{g}}}_t$, is proportional to the angular momentum, $\mathit{H}=H{\hat{\mathit{g}}}_s$, of a spinning mass and the angular rate, $\dot{\mathit{\gamma }}=\dot{\gamma }{\stackrel{\mathit{ˆ}}{\mathit{g}}}_g$, at which the motorized frame is gimballed (Eq. 2). (c) The GyBAR-fixed coordinate frame $({\hat{\mathit{g}}}_s,{\hat{\mathit{g}}}_t,{\hat{\mathit{g}}}_g)$ is initially aligned with the human-fixed frame $(\hat{\mathit{x}},\hat{\mathit{y}},\hat{\mathit{z}})$ prior to rotation by the gimbal angle, γ, about the axis ${\stackrel{\mathit{ˆ}}{\mathit{g}}}_g\Vert \stackrel{\mathit{ˆ}}{\mathit{z}}$. This angle is controlled to orient τ in the direction of interest: the blue region for assistance in the sagittal plane, or the yellow region for assistance in the frontal plane. (d) Example use of a moment to restore upright posture in the sagittal plane.

Figure 2

Description and main results of Experiment 1. (a) Schematic of balance control feedback loop. (b) Schematic of assistive controllers ‘spring-damper’ (S-D), ‘damper’ (D) and, ‘spring’ (S). (c) Subject wearing the GyBAR while traversing the beam of width 30 mm and length 4 m. (d) Example primary outcome measures (distance walked) and time series data for subject C13. Shown are the trunk roll angle ϕ, angular impulse $\mathrm{\Delta }{H}_{ML}$, and exerted gyroscopic moment ${\tau }_{\mathrm{ML}}$ for baseline condition ‘inactive’ (IN) and assistive conditions S-D, D, and S. (e) Distance walked by all (n = 10) healthy subjects under all testing conditions, normalized to condition IN and displayed in logarithmic scale. For clarity, pairwise significance brackets (p < 0.05) are shown only for comparisons of controllers S-D, S, and D and conditions ‘free’ (FR, no device) and IN. Circled numbers are the perceived ‘helpfulness’ rankings, from best (1) to worst (5). (f) Centroidal frequency of the trunk roll angle ϕ for all subjects.

Figure 3

Description and main results of Experiment 2. (a) Illustration of ‘damper’ (D) balance controller. (b) Balancing tasks with reduced AP or ML bases of support. (c) Individual with chronic stroke wearing the GyBAR during AP balancing task over reduced BoS (100 mm). (d) Example primary outcomes (stance duration) and time series data for the individual with chronic stroke who exhibited the median degree of improvement with controller D, subject S1 (□). Shown are the trunk pitch angle $\theta$, angular impulse $\mathrm{\Delta }{H}_{\mathrm{AP}}$, and exerted gyroscopic moment ${\tau }_{\mathrm{AP}}$. (e) Duration standing for all healthy controls (n = 5) and individuals with chronic stroke (n = 5), normalized to condition ‘inactive’ (IN) and displayed in logarithmic scale; shown are condition ‘free’ (FR) and IN and assistive controller D. Subjects H1 (+), H4 (○), and S4 (∇) all reached the maximum score in condition D. (f) Centroidal frequency of the trunk pitch angle $\theta$ for all subjects. The dashed line represents the median value for healthy participants in condition IN with a full BoS.