[Paper Review] Stabilization of Exoskeletons through Active Ankle Compensation

Thomas Gurriet, Maegan Tucker+3 more|arXiv (Cornell University)|Sep 26, 2019

Prosthetics and Rehabilitation Robotics19 references4 citations

TL;DR

This paper proposes active ankle compensation to enhance sagittal-plane stability in lower-limb exoskeletons, using individual control of stance and swing foot ankles to maintain ground contact, foot parallelism, and pelvis orientation tracking. Experimental results demonstrate improved pelvis pitch tracking and static balance on uneven terrain, with the exoskeleton maintaining stability during crutch-less dynamic walking despite disturbances.

ABSTRACT

This paper presents an active stabilization method for a fully actuated lower-limb exoskeleton. The method was tested on the exoskeleton ATALANTE, which was designed and built by the French start-up company Wandercraft. The main objective of this paper is to present a practical method of realizing more robust walking on hardware through active ankle compensation. The nominal gait was generated through the hybrid zero dynamic framework. The ankles are individually controlled to establish three main directives; (1) keeping the non-stance foot parallel to the ground, (2) maintaining rigid contact between the stance foot and the ground, and (3) closing the loop on pelvis orientation to achieve better tracking. Each individual component of this method was demonstrated separately to show each component's contribution to stability. The results showed that the ankle controller was able to experimentally maintain static balance in the sagittal plane while the exoskeleton was balanced on one leg, even when disturbed. The entire ankle controller was then also demonstrated on crutch-less dynamic walking. During testing, an anatomically correct manikin was placed in the exoskeleton, in lieu of a paraplegic patient. The pitch of the pelvis of the exoskeleton-manikin system was shown to track the gait trajectory better when ankle compensation was used. Overall, active ankle compensation was demonstrated experimentally to improve balance in the sagittal plane of the exoskeleton manikin system and points to an improved practical approach for stable walking.

Motivation & Objective

• To address the critical need for fall prevention in exoskeletons by improving dynamic walking robustness.
• To develop a practical control method that enhances stability without relying on crutches or human patient stabilization.
• To demonstrate that active ankle control can improve tracking of pelvis orientation and reduce instability in the sagittal plane.
• To validate the method experimentally on the ATALANTE exoskeleton using a manikin in lieu of a human subject.
• To isolate and evaluate individual components of the ankle controller to quantify their contribution to overall system stability.

Proposed method

• The nominal gait was generated using the hybrid zero dynamics (HZD) framework with direct collocation to ensure provably stable periodic orbits.
• The ankle controller was decomposed into three components: (1) maintaining the non-stance foot parallel to the ground via inverse kinematics, (2) ensuring rigid contact of the stance foot with the ground through center of pressure (COP) filtering, and (3) closing the loop on pelvis orientation using IMU feedback.
• A PD controller tracked the nominal trajectory, with smooth transitions between swing and stance phases triggered by force sensor thresholds.
• The stance foot controller used a COP filter to distribute commanded torques and maintain the center of pressure near the foot's center.
• The swing foot controller computed ankle joint angles via inverse kinematics to keep the foot horizontal.
• The full controller was implemented on hardware using real-time control on the ATALANTE exoskeleton with a manikin as a surrogate patient.

Experimental results

Research questions

• RQ1 Can active ankle compensation improve sagittal-plane stability in a fully actuated lower-limb exoskeleton during dynamic walking?
• RQ2 How do individual components of the ankle controller—foot parallelism, ground contact, and pelvis orientation feedback—contribute to overall stability?
• RQ3 To what extent can the ankle controller maintain static balance when the stance foot is placed on an inclined platform?
• RQ4 Does active ankle control improve pelvis pitch tracking during dynamic walking compared to baseline control?
• RQ5 Can the controller stabilize the system on uneven terrain despite hardware flexibility and modeling inaccuracies?

Key findings

• The ankle controller successfully maintained static balance on a pivoting platform, keeping the pelvis pitch nearly constant while the stance foot pitch was actively disturbed.
• In simulation, the exoskeleton walked for over 26 steps with active ankle compensation, whereas it fell after only 4 steps without it.
• Hardware experiments showed significant improvement in pelvis pitch tracking when the ankle controller was active, with reduced deviation from the desired trajectory.
• The controller enabled stable, crutch-less dynamic walking on the ATALANTE exoskeleton, demonstrating practical feasibility.
• Despite improvements in the sagittal plane, the system remained unstable in the frontal plane due to physical constraints, indicating a need for additional hip control.
• The individual components of the ankle controller were validated separately, confirming that each contributed meaningfully to overall stability.