Abstract
Generation of control torque at low cost for highly agile satellite missions is generally achieved with momentum exchanged devices, such as momentum or reaction wheels and control moment gyros (CMGs) with high slew manoeuvrability. However, the generation of a high control torque from the respective actuators requires high power, large mass and high cost. The main objective of this research is to introduce a novel type of attitude control actuator that generates control torques about all three principal axes of a rigid satellite using only a spinning wheel and tilt mechanisms. The tilt mechanism changes the spin axis of the spinning wheel about the tilt plane axes thereby generating control torque about an axis in the plane that is orthogonal to both the tilt axes and the spinning wheel axis. To complete the 3 DoF torque generation, torque is generated about the spinning wheel axis by varying the spinning wheel speed. Few literature sources describe the concept of inertial actuator with 3 DoF torque capabilities. But in this research, a novel equation of motion is developed for the proposed inertial actuator from the fundamental laws of physics that does not require the popular pseudo-inversion as obtained with CMG systems. An extended LQR control law named HPB (High Performance Bounded) control that uses gain-scheduling and bounded torque control to provide better attitude performance than classical LQR with the advantage of incorporating a maximum torque constraints, was adapted to control a mathematical model of a rigid satellite. A prototype of the proposed actuator was built using commercial off the shelf components (COTS). The entire hardware design process is described and is accompanied with extended hardware and software simulations developed using CAD and MatLab/Simulink software. The newly proposed actuator has several distinct advantages compared to other existing inertial actuators. This includes the ability to generate active control torque in all principal axes of a rigid satellite compared to having conventional reaction wheels aligned to each of the three principal axes of the satellite or a cluster of CMGs. This translates to the lower mass, lower power requirement and low cost that are the critical driving factors in the design of any small satellite ACS. This new concept presents advantages for earth observation missions where the required slew angle is limited and for small satellites where accommodating multiple actuators reduces the size that can be allocated to payloads. Academically, significant contributions have been made to the field including: development of a new set of dynamic equations of motion for the inertial actuator, extending the conventional LQR control logic for a more time efficient control, 3 DoF testbed development, systematic design and build of the proposed actuator using commercial of the shelf components, and 3 DoF torque capability experimentation.