Bowden cables serve as essential components in various mechanical systems, facilitating power transmission from remote actuators to specific destinations. The pretension of Bowden cables profoundly influences system performance, notably in terms of friction. This study investigates the effects of cable pretension and shape on friction and torque efficiency. A custom self-designed testbed, comprising integrated actuator units, pulleys, and a novel pretension mechanism connected by Bowden cables, is utilized to conduct experimental tests under varying parameters. This work adopts an integrated approach of experimentation, modeling, and validation, offering preliminary insights into the torque transmission characteristics of tendon driven actuator systems. Additionally, the precise model exhibits excellent conformity across a broad range of shapes and provides initial insights into hysteresis modeling attributable to cable material properties.

In this work, an upgraded exoskeleton design is presented with enhanced trajectory tracking and mechanical transparency. Compared to the first version, the design features a 3-DoF actuated shoulder joint and a mechanism to regulate the pretension of Bowden cables. Force/torque sensors are installed to directly measure the interaction forces between the human arm and the exoskeleton at the connecting points. Three control strategies were evaluated to follow a desired trajectory; A PD controller, a PD controller with friction observer, and an adaptive controller based on Radial Basis Function (RBF). These strategies also form the basis for an admittance control, aimed at improving the exoskeleton’s mechanical transparency during interaction with the human arm. Simulations and experimental results demonstrate that the PD control, supported by friction estimation via a momentum observer, achieves superior tracking performance. Moreover, the system’s mechanical transparency is enhanced using the admittance RBF-based controller, showing marginally superior results.