The development of Zero-G class underwater robots: unrestricted attitude control using control moment gyros

摘要

The ‘Zero-G’ is designated as a new class of underwater robot that is capable of unrestrictedattitude control. A novel control scheme based on internal actuation using Control MomentGyros (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 nonlinearfeedback control law formulated based on energy considerations of the system’s coupleddynamics. Singularities, redundancy and null motion are discussed in the context of CMGsand a mathematical escapability condition is developed based on the differential geometryof null motion. A comprehensive geometric study of the singularities of a CMG pyramid isperformed and together with considerations of the inverse kinematics of attitude control formthe basis of a global steering law that exactly achieves the desired torques, whilst guaranteeingreal-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 CMGsand verify the associated theoretical developments is described. The open-loop dynamics ofthe 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 lawby comparing the response of the robot to that using alternative control laws that neglectthe hydrodynamic interactions of the body and the coupled motion of the CMGs and bodyrespectively. The control law demonstrates a faster response with a smaller overshoot forless overall control activity than the alternative methods. The ability to actively stabilise thepassively unstable translational dynamics of the robot are verified. Next, the unrestrictedattitude control capability is confirmed with the robot demonstrating the necessary rangeof 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 robotperforming vertically pitched diving and surfacing in surge.This is the first time an underwater robot has performed such a manoeuvre. This researchdemonstrates that CMGs are capable of actively stabilising the passively unstable dynamicsof an underwater robot with essentially zero-righting moment and are capable of providingit with unrestricted attitude control. The three-dimensional manoeuvring capabilitiesallow Zero-G Class underwater robots to plan and optimise their missions in a fully threedimensionalmanner, in a way that has not been possible previously. This study concludesthat the application of CMGs for attitude control opens up a path to develop sophisticatedZero-G Class underwater robots and their application to new fields of underwater research.