This thesis presents a novel actuation system and morphologies for tensegrity robots. The research aim is to design a structural deformation method utilising adjustable-length strut elements (SEs) in cooperation with passive-retractable joints with cable locking mechanisms (PRMs). This structural deformation method is employed for robot configurations for locomotion tasks such as crawling and climbing motions.
A tensegrity structure consists of compressive elements and tensile elements. Each compressive element, or strut element, is connected by the tensile elements, or cables; the structure operates like a cable network structure without any rigid joints. Tensegrity structures thus have unique properties in terms of their weight, force distribution, modularity, and fault tolerance. These unique properties are proving useful in the field of exploration and inspection robotics. Compliant and lightweight tensegrity robots can, for example, withstand external impact forces to protect payloads and their mechanisms due to their force distribution properties. Tensegrity robots can operate even when some cables have failed. Unlike conventional modular robots, which have rigid joints for locomotion mechanisms, tensegrity robots are flexible; their compliant structure allows them to access diverse geological and dangerous areas inaccessible to humans. Both strut and cables can be assembled and reassembled easily due to tensegrity robots’ modularity. Tensegrity robots can be constructed in various geometries providing a wide range of locomotion types such as rolling, crawling, climbing, and jumping depending on the application requirements.
However, several limitations must be addressed. Firstly, the structural configurations for existing tensegrity robots do not change quickly. Furthermore, the existing mechanisms for spooling cables are time-consuming; the mechanisms require high torque together with low speed to sufficiently actuate the cables for robot configuration. Secondly, the power use for locomotion is high, causing a short-operative time when the robots are powered by batteries. Current actuated cable mechanisms use high-torque and active actuators to spool cables for structural configurations; actuated-cable mechanisms require constant energy while actuating cables, resulting in high-energy consumption. Thirdly, existing individual element mechanisms that relate to actuation methods need to be reduced because they require high-torque actuators to pull cables for configuration, and the high-torque actuator, in turn, has high power demands.
A new structural deformation or actuation method that can overcome these limitations is needed. This method would have to enable quick changes in robot configurations, allowing for swapping between soft and hard structures while using less energy and staying lightweight. This new method can be achieved via the proposed cooperation between adjustable-length strut elements (SEs) and a set of passive-retractable joints with a cable locking mechanism (PRMs). Fundamentally, an SE can change its length to exert pushing and pulling forces actively, whereas PRMs can passively exert a small rotational force and produce a braking force to restrict cable lengths to perform robot configurations. 3-DOF and 5-DOF tensegrity structures were developed and successfully tested in end-effector position and angle controls by applying the locking and unlocking stage procedures of the PRMs and changing SE-slider lengths. The forward kinematics equations were derived, analysed, and physically demonstrated to identify the end-effector position and angle of the given cable lengths of the PRMs. PRM and SE control procedures were produced by new control rules to achieve a closed-loop control system. After demonstrating the proposed concept, the speed for changing robot structure was evaluated by developing and demonstrating different tensegrity robot motions that can extend and retract sizes and swap between soft and hard structures.
The experimental results demonstrate that the proposed deformation concept was successful in demonstrating the position and angle controls of the 3- and 5-DOF systems. This method could be used to develop different reconfigurable tensegrity robots capable of performing locomotion tasks by swapping between soft and hard structures quickly thanks to cooperation between the SEs and PRMs. Due to the PRM requires less power to restrict and release a cable during changing SE lengths for structural configurations, a development of a tensegrity robot using low energy consumption may be possible.
