Development and Optimisation of 3D Printed Compliant Joint Mechanisms for Hypermobile Robots

Hypermobile robots are an area of robotics that are often used as exploratory robots, but have facets that feature in other areas of the field. Hypermobile robots are robots that feature multiple body segments or modules, with joints between each. These robots are often used for exploratory purposes due to being able to maintain contact with the ground due to their flexible bodies. Wormbot was a hypermobile robot developed at the University of Leeds, which used a locomotion gait based on that of a Caenorhabditis elegans nematode worm, otherwise known as C.elegans. This movement pattern is reliant on compliance; a mechanism where the joints are slightly sprung and comply to the environment. The next iteration of Wormbot needs to be reduced in size, which would also require a new actuation and compliance system. This thesis describes the process of investigating a method of compliance to be used in the next version of Wormbot, while utilising the multi-material 3D printing capabilities available at the University. 3D printing provides quick manufacturing, allowing for fast changes to made to prototype components if required. During the process of this research, two 3D printed compliant actuation systems were produced; a pneumatic bellow and a Series Elastic Element (SEE) to be used in tandem with a servo motor. Both methods were tested to analyse their performance. The bellow was produced to utilise the capabilities of multi-material printing to strengthening suspected weak areas of the actuator. However, the performance of the bellow was unsatisfactory, failing twice in two actuation tests tests due to the device breaking. The SEE on the other hand, designed with two stiffer plates and a rubber-like spring element in the middle, initially proved to be reliable and repeatable in performance, with potential to behave linearly to a set spring constant. These results were acquired by performing rotational step response tests and fitting a spring-damper model to the results. However, issues with the plastic material were discovered when it was found to deform much more than anticipated, behaving in a similar manner to an additional spring element, complicating the model. Simulation work to explore the potential for using different spring constants of joint compliance in varying environments was also explored. This involved testing a virtual Wormbot in a range of environments while altering joint compliance. These simulations revealed that softer joints allow for favourable performance in constricting environments, while stiffer joints lend themselves more to quicker movement

Safe and effective physical human-robot interaction: Approaches to variable compliance via soft joints and soft grippers

The work described in this thesis focusses on designing and building two novel physical devices in a robotic arm structure. The arm is intended for human-robot interaction in the domestic assistive robotics area. The first device aims at helping to ensure the safety of the human user. It acts as a mechanical fuse and disconnects the robotic arm link from its motor in case of collision. The device behaves in a rigid manner in normal operational times and in a compliant manner in case of potentially harmful collisions: it relies on a variable compliance. The second device is the end-effector of the robotic arm. It is a novel grasping device that aims at accommodating varying object shapes. This is achieved by the structure of the grasping device that is a soft structure with a compliant and a rigid phase. Its completely soft structure is able to mould to the object's shape in the compliant phase, while the rigid phase allows holding the object in a stable way.In this study, variable compliance is defined as a physical structure's change from a compliant to a rigid behaviour and vice versa. Due to its versatility and effectiveness, variable compliance has become the founding block of the design of the two devices in the robot arm physical structure. The novelty of the employment of variable compliance in this thesis resides in its use in both rigid and soft devices in order to help ensure both safety and adaptable grasping in one integrated physical structure, the robot arm.The safety device has been designed, modelled, produced, tested and physically embedded in the robot arm system. Compared to previous work in this field, the feature described in this thesis' work has a major advantage: its torque threshold can be actively regulated depending on the operational situation. The threshold torque is best described by an exponential curve in the mathematical model while it is best fit by a second order equation in the experimental data. The mismatch is more considerable for high values of threshold torque. However, both curves reflect that threshold torque magnitude increases by increasing the setting of the device. Testing of both the passive decoupling and active threshold torque regulation show that both are successfully obtained. The second novel feature of the robot arm is the soft grasping device inspired by hydrostatic skeletons. Its ability to passively adapts to complex shapes objects, reduces the complexity of the grasping action control. This gripper is low-cost, soft, cable-driven and it features no stiff sections. Its versatility, variable compliance and stable grasp are shown in several experiments. A model of the forward kinematics of the system is derived from observation of its bending behaviour.Variable compliance has shown to be a very relevant principle for the design and implementation of a robotic arm aimed at safely interacting with human users and that can reduce grasp control complexity by passively adapting to the object's shape