The development of Zero-G class underwater robots : unrestricted attitude control using control moment gyros
The ‘Zero-G’ is designated as a new class of underwater robot that is capable of unrestricted attitude control. A novel control scheme based on internal actuation using Control Moment Gyros (CMGs) is proposed to provide Zero-G Class Autonomous Underwater Vehicles (AUVs) with this unique freedom in control. The equations of motion for a CMG actuated underwater robot are derived and a nonlinear feedback control law formulated based on energy considerations of the system’s coupled dynamics. Singularities, redundancy and null motion are discussed in the context of CMGs and a mathematical escapability condition is developed based on the differential geometry of null motion. A comprehensive geometric study of the singularities of a CMG pyramid is performed and together with considerations of the inverse kinematics of attitude control form the basis of a global steering law that exactly achieves the desired torques, whilst guaranteeing real-time singularity avoidance within a constrained workspace. The development of the CMG actuated Zero-G Class underwater robot IKURA is described. This is the first Zero-G Class prototype and is the first application of CMGs to underwater robots. A series of experiments to demonstrate the practical application of CMGs and verify the associated theoretical developments is described. The open-loop dynamics of the system and the exactness and real-time applicability of the CMG steering law are verified. Experiments are carried out to assess the performance of the proposed control law by comparing the response of the robot to that using alternative control laws that neglect the hydrodynamic interactions of the body and the coupled motion of the CMGs and body respectively. The control law demonstrates a faster response with a smaller overshoot for less overall control activity than the alternative methods. The ability to actively stabilise the passively unstable translational dynamics of the robot are verified. Next, the unrestricted attitude control capability is confirmed with the robot demonstrating the necessary range of attitude control to adopt and maintain any attitude on the surface of a sphere. Finally, the ability to stabilise any attitude while translating in surge is confirmed with the robot performing vertically pitched diving and surfacing in surge. This is the first time an underwater robot has performed such a manoeuvre. This research demonstrates that CMGs are capable of actively stabilising the passively unstable dynamics of an underwater robot with essentially zero-righting moment and are capable of providing it with unrestricted attitude control. The three-dimensional manoeuvring capabilities allow Zero-G Class underwater robots to plan and optimise their missions in a fully threedimensional manner, in a way that has not been possible previously. This study concludes that the application of CMGs for attitude control opens up a path to develop sophisticated Zero-G Class underwater robots and their application to new fields of underwater research.