The development of Zero-G class underwater robots: unrestricted attitude control using control moment gyros.
University of Southampton, School of Engineering Sciences,
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.
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