Center of mass feedback for joint torque control during perturbed standing: a potential generalizable control...

Exoskeletons have the potential to augment balance and decrease fall risk. However, existing balance-augmenting wearable robotic controllers have only been tested in single planes of motion during either standing or walking. Thus, it is unclear whether a single control scheme can generalize across perturbations with varying spatial properties or from standing to walking. Inspired by the nervous system's generalizable balance control strategy across perturbation types and conditions, we propose a novel torque control framework that modulates multi-joint reactive torques based on center of mass (CoM) deviation. We evaluated the generalizability of our delayed CoM feedback controller to predict multi-joint torque responses to perturbations of varying magnitudes, directions, and across movement contexts.

In nine healthy young adults, we tested the ability of a delayed CoM feedback scheme to predict multi-joint torque responses to (1) ramp-and-hold support surface perturbations at three magnitudes in 8 directions, (2) a continuous sinusoidal movement, resulting in a cyclical movement of the CoM with similar periodic features as walking, and (3) a sinusoidal motion with random perturbations superimposed to mimic perturbations during cyclic tasks. We trained the model on single ramp-and-hold conditions and evaluated its ability to generalize across directions, magnitudes, movement contexts, and subjects.

The delayed CoM feedback controller trained on a single ramp-and-hold condition generalized to all ramp-and-hold perturbations for all joints, predicting the joint torques for perturbations of varying directions and magnitudes with high fidelity (average R² > 0.84 and RMSE < 0.08 Nm/kg). However, generalization from standing to cyclic movement only occurred for hip and knee flexion. The CoM feedback parameters from ramp-and-hold perturbations generalized to the continuous sinusoidal movement (cyclic movement) and the sinusoidal movement with superimposed perturbations (unexpected perturbations) for hip flexion and knee flexion (average R²>0.70 and RMSE < 0.13 Nm/kg), but not for ankle plantarflexion and hip adduction (R²>0.20 and RMSE < 0.22 Nm/kg).

Our findings show that a physiologically-inspired CoM feedback controller can robustly predict balance-correcting torques appropriate for driving a hip or knee wearable robotic device during standing and movement, and an ankle device during standing only. The goodness-of-fit of joint torque is comparable to top machine learning algorithms, yet requires orders of magnitude less training data, enabling rapid implementation to reduce fall risk.