BackgroundLower extremity exoskeletons are mainly used to provide stepping support, while balancing is left to the user. Designing balance controllers is one of the biggest challenges in the development of exoskeletons. The goal of this study was to design and evaluate a balance controller for a powered ankle-foot orthosis and assess its effect on the standing balance of healthy subjects.MethodsWe designed and implemented a balance controller based on the subject’s body sway. This controller was compared to a simple virtual-ankle stiffness and a zero impedance controller. Ten healthy subjects wearing a powered ankle-foot orthosis had to maintain standing balance without stepping while receiving anteroposterior pushes. Center of mass kinematics, ankle torques and muscle activity of the lower legs were analyzed to assess the balance performance of the user and exoskeleton.ResultsThe different controllers did not significantly affect the center of mass responses. However, the body sway based controller resulted in a decrease of 29% in the biological ankle torque compared to the zero impedance controller and a decrease of 32% compared to the virtual-ankle stiffness. Furthermore, the soleus muscle activity of the left and right leg decreased on average with 8%, while the tibialis anterior muscle activity increased with 47% compared to zero impedance.ConclusionThe body sway based controller generated human-like torque profiles, whereas the virtual-ankle stiffness did not. As a result, the powered ankle-foot orthosis with the body sway based controller was effective in assisting the healthy subjects in maintaining balance, although the improvements were not seen in the body sway response, but in the subjects’ decreased biological ankle torques to counteract the perturbations. This decrease was a combined effect of decreased soleus muscle activity and increased tibialis anterior muscle activity.

Human-like balance controllers are desired for wearable exoskeletons in order to enhance human-robot interaction. Momentum-based controllers (MBC) have been successfully applied in bipeds, however, it is unknown to what degree they are able to mimic human balance responses. In this paper, we investigated the ability of an MBC to generate humanlike balance recovery strategies during stance, and compared the results to those obtained with a linear full-state feedback (FSF) law. We used experimental data consisting of balance recovery responses of nine healthy subjects to anteroposterior platform translations of three different amplitudes. The MBC was not able to mimic the combination of trunk, thigh and shank angle trajectories that humans generated to recover from a perturbation. Compared to the FSF, the MBC was better at tracking thigh angles and worse at tracking trunk angles, whereas both controllers performed similarly in tracking shank angles. Although the MBC predicted stable balance responses, the human-likeness of the simulated responses generally decreased with an increased perturbation magnitude. Specifically, the shifts from ankle to hip strategy generated by the MBC were not similar to the ones observed in the human data. Although the MBC was not superior to the FSF in predicting human-like balance, we consider the MBC to be more suitable for implementation in exoskeletons, because of its ability to handle constraints (e.g. ankle torque limits). Additionally, more research into the control of angular momentum and the implementation of constraints could eventually result in the generation of more human-like balance recovery strategies by the MBC.

In this study, our goal was to improve the standing balance of people with a spinal cord injury by using a wearable exoskeleton that has ankle and knee actuation in the sagittal plane.Three test-pilots that have an incomplete spinal cord injury wore the exoskeleton and tried to maintain standing balance without stepping while receiving anteroposterior pushes. Two balance controllers were tested: one providing assistance based on the subject's body sway and one based on the whole body momentum. For both controllers, the balance performances of the test-pilots wearing the exoskeleton were assessed based on the center of mass kinematics and compared to the condition in which the device did not provide any assistance.One of the test-pilots was not able to maintain balance without assistance, but could withstand small pushes when any of the balance controllers was implemented. For this test-pilot the recovery time and sway amplitude hardly varied with the type of balance controller that was used. For the other two testpilots the recovery time and the sway amplitude were smallest using the body sway controller.In conclusion, the wearable exoskeleton with balance controller was able to improve the balance performance of the test-pilots by reducing the recovery time after a perturbation and by enabling one of the test-pilots to maintain balance, who could not maintain balance by himself.

In this study, our goal was to improve the standing balance of people with a Spinal Cord Injury (SCI) by using a powered Ankle-Foot orthosis acting in the sagittal plane. We tested four different controllers on two SCI subjects that have a lesion at a low level. In the experiments the subjects repeatedly had to recover from pelvis perturbations, while receiving ankle assistive torques from the orthosis. We found that the controllers that use centroidal dynamics as input parameters were able to provide proper support to the subjects after a perturbation had been applied, even though they worked against the subjects after they had recovered from the perturbation. These preliminary results show the potential of balancing controllers that operate in Center of Mass-space.SYMBITRON is supported by EU research program FP7-ICT-2013-10 (contract #611626). SYMBITRON is coordinated by University of Twente.

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