Modeling and parametric optimization of 3D tendon-sheath actuator system for upper limb soft exosuit

This paper presents an analysis of parametric characterization of a motor driven tendon-sheath actuator system for use in upper limb augmentation for applications such as rehabilitation, therapy, and industrial automation. The double tendon sheath system, which uses two sets of cables (agonist and antagonist side) guided through a sheath, is considered to produce smooth and natural-looking movements of the arm. The exoskeleton is equipped with a single motor capable of controlling both the flexion and extension motions. One of the key challenges in the implementation of a double tendon sheath system is the possibility of slack in the tendon, which can impact the overall performance of the system. To address this issue, a robust mathematical model is developed and a comprehensive parametric study is carried out to determine the most effective strategies for overcoming the problem of slack and improving the transmission. The study suggests that incorporating a series spring into the system's tendon leads to a universally applicable design, eliminating the need for individual customization. The results also show that the slack in the tendon can be effectively controlled by changing the pretension, spring constant, and size and geometry of spool mounted on the axle of motor

Elongation Modeling and Compensation for the Flexible Tendon-Sheath System

In tendon-driven systems, the elongation of the tendon would result in inaccuracy in the position control of the system. This becomes a critical challenge for those applications, such as surgical robots, which require the tendon-sheath system with flexible and even time-varying configurations but lack of corresponding sensory feedback at the distal end due to spatial restrictions. In this paper, we endeavor to address this problem by modeling the tendon elongation in a flexible tendon-sheath system. Targeting at flexibility in practical scenarios, we first derived a model describing the relationship between the overall tendon elongation and the input tension with arbitrary route configurations. It is shown that changes in the route configuration would significantly affect the tendon elongation. We also proposed a remedy to enhance the system tolerance against potential unmodeled perturbations along the transmission route during operation. A scaling factor S was introduced as a design guideline to determine the scaling effect. A dedicated platform that was able to measure the tensions at both ends and the overall tendon elongation was designed and set up to validate the new findings. Discussions were made on the performance and the future implementation of the proposed models and remedy.published_or_final_versio

Experimental characterization of Bowden cable friction and compliance

textThis thesis presents a systematic method for experimental characterization of Bowden cable friction and compliance. A novel tension and elongation measurement method using a motion capture system and a spring is introduced. With the measurement method, the effects of nine variables on friction and cable compliance are investigated through a comprehensive set of experiments under 144 different cases. We have generated specific guidelines for Bowden cable configurations and design parameters to achieve optimal performance which may help robotics researchers in choosing and configuring Bowden cables, and designing control systems for actuation.Mechanical Engineerin

Modeling tendon-sheath mechanism with flexible configurations for robot control

Surgical and search/rescue robots often work in environments with very strict spatial constraints. The tendon-sheath mechanism is a promising candidate for driving such systems, allowing power sources and actuation motors placed outside to transmit force and energy to the robot at the distal end through the constrained environment. Having both compactness and high force capability makes it very attractive for manipulation devices. On the other hand, the friction attenuation of tendon tension is nonlinear and configuration-dependent due to tendon/sheath interactions throughout the transmission path. This is a major obstacle for the tendon-sheath mechanism to be widely adopted. Here, we focus on the friction analysis for flexible and time-varying tendon-sheath configurations: the most challenging but yet commonly encountered case for real-world applications. Existing results on fixed-path configurations are reviewed, revisited, and extended to flexible and time-varying cases. The effect of tendon length to friction attenuation is modeled. While focusing on tension transmission, tendon elongation is also discussed with the length effect applied. In the end, two-dimensional results are extended to three-dimensional tendon-sheath configurations. All propositions and theorems are validated on a dedicated experimental platform. © 2013 Cambridge University Press.link_to_subscribed_fulltex