Exo-muscle: A semi-rigid assistive device for the knee

TL;DR

Exo-Muscle tackles knee exoskeleton misalignment by introducing a semi-rigid chain tendon-routing mechanism that creates a deterministic moment arm, aided by a series elastic element and a strap routing system for comfort. The control framework combines online knee torque estimation with a force-control loop that converts knee torque into tendon tension via a moment-arm–dependent relationship, using feedforward and PI feedback to track tendon displacement. Key contributions include the semi-rigid routing profile with a fitted $L_a(\theta_{kf})$ and a nonlinear elastic model $Y(F)$ for real-time actuation, achieving up to $38\,\mathrm{Nm}$ of assistive torque and high transmission efficiency in practice. Validation on a mechanical knee joint testbed and human experiments demonstrates the device’s ability to reproduce the designed torque profile and maintain stable force tracking, highlighting potential benefits for rehabilitation and industrial augmentation. Future work aims to enable full mobility with a foot-worn GRF sensor and to quantify metabolic cost and EMG responses to assess user effort reductions.

Abstract

In this work, we introduce the principle, design and mechatronics of Exo-Muscle, a novel assistive device for the knee joint. Different from the existing systems based on rigid exoskeleton structures or soft-tendon driven approaches, the proposed device leverages a new semi-rigid principle that explores the benefits of both rigid and soft systems. The use of a novel semi-rigid chain mechanism around the knee joint eliminates the presence of misalignment between the device and the knee joint center of rotation, while at the same time, it forms a well-defined route for the tendon. This results in more deterministic load compensation functionality compared to the fully soft systems. The proposed device can provide up to 38Nm assistive torque to the knee joint. In the experiment section, the device was successfully validated through a series of experiments demonstrating the capacity of the device to provide the target assistive functionality in the knee joint.

Figures (10)

• Figure 1: The Exo-Muscle device worn by a human subject in different knee poses. The main components of the device are indicated.
• Figure 2: The semi-rigid chain concept at two knee flexion angles (left at $60^\circ$ and right at $100^\circ$).
• Figure 3: (a) Visualization of simulation in MATLAB. (b) The change of torsion arm during the increase of knee flexion angle.
• Figure 4: (a) Human body link mass model. (b) Indication of a posture used in the simulation (ADA: $-30^\circ$, KFA: $90^\circ$, HFA is regulated to move the COP from heel limit to toe limit.).
• Figure 5: Tensile test results for three types of rubber belts (type1: $27.5 \times 4.15mm$, type2: $24 \times 4mm$ and type3: $13 \times 4.5mm$ respectively) and tensile curves of the bungee cords provided by Shandow Technic.