Bio-inspired Tensegrity Flexural Joints Erik Jung 1,2 , Victoria Ly 1,2 , Nicholas Cessna 1,2 , Lawrence Ngo 1,2 , Dennis Castro 1,2 , Vytas SunSpiral and Mircea Teodorescu 1,2 Abstract— Most robotics literature model the human’s knee and hip as a revolute joint with limited range of rotation. Although somehow close to reality, this approach neglects a critical aspect of these joints, which is their internal flexibility. This paper presents a prototype tensegrity flexural ma- nipulator whose kinematic behavior is inspired by human leg’s gait. This prototype, which considers a hybrid (flexible- rigid) structure of the knee and hip would be able to better approximate real behavior and hopefully lead to a better design of artificial (prosthetic) knees and hips. The behavior of the proposed tensegrity manipulator was firstly predicted using OpenSim simulation environment. The paper reports the comparisons between the simulations, physi- cal prototypes and human leg behavior for a variety of ranges of motions and tension analysis. I. I NTRODUCTION Flexibility and structural compliance allow biological sys- tems to deform under load, whilst maintaining their structural integrity. Solid skeletal systems work in concert with mus- cles and tendons to distribute load throughout the entirety of the system. Therefore, the anatomical shape adapts to the external applied forces and internal stresses distributed within the structure, enabling it to operate in unpredictable environments [1]. In contrast, most robotic systems (e.g., robotic industrial manipulators) are relatively stiff with little to no structural flexibility. These systems typically consist of rigid links and sliding or revolute joints that will compress beyond intention to the point of failure when excessive load is applied. [2]. These could be classified as “powered systems” [3][4], “pas- sive systems”, which do not have any electrical actuation, and use weight re-distribution, energy re-capturing, dampening and locking mechanisms (e.g., springs or shock absorbers) to alleviate strain [5][6]. The advantage of stiff linkages is that under normal operat- ing conditions, the behavior of these hard robotic systems is fully predictable leading to an easy (ideally analytical closed form) solution for its kinematics and dynamics [3]. Structurally soft robotic systems embody much of the opposite characteristics to those of their rigid counterparts. [7] Soft robots distribute strain and load throughout the system and therefore adapt better to unpredictable conditions (e.g., uneven terrain, unexpected impacts). However, the lack of a rigid support structure lowers the load carrying capacity 1 University of California, Santa Cruz, Santa Cruz, CA 95064, USA eajung@ucsc.edu, vily@ucsc.edu, ncessna@ucsc.edu, malngo@ucsc.edu, dacastro12@gmail.com, vytas@sunspiral.org, mteodore@ucsc.edu 2 Dynamics Autonomous Navigation Surface Engineering and Robotics (DANSER) Lab at University of California Santa Cruz (a) OpenSim Simulation (b) Physical Model Fig. 1: Tensegrity manipulator consisting of three compression elements (“Tibia”, “Femour” and “Pelvis”) and two flexural joints (“Knee” and “Hip”) controlled by three active tensile elements (a) Hip: 3D CAD (b) Hip: Prototype (c) Knee: 3D CAD (d) Knee: Prototype Fig. 2: Prototype Design for the Tensegrity Hip and Knee Flexural Joints.