This paper presents innovative mechanisms capable of advantageously providing attitude control for spacecrafts. These new mechanisms, which we have named flexure wheels, are the dynamic equivalent of a rotating wheel and can be entirely implemented with flexures.A reaction wheel is a well known device for controlling the orientation of spacecrafts. It consists in a motorised fly-wheel which is placed within the spacecraft. To set the wheel into angular rotation, a torque is applied to the wheel which in response applies the opposite torque back to the spacecraft, according to Newton's third law. This reaction torque is how the spacecraft rotates in order to control its orientation. In order to enable this wheel to rotate around a fixed axis, several methods have been implemented such as ball bearings, which suffer from frictional losses and imperfections which lead to vibrations and failure, as well as magnetic bearings which do not suffer from these issues but have an increased power consumption and complexity.The subject of this paper is to introduce alternative mechanisms that are able to produce the same constant angular momentum as a rotating wheel, but which do not suffer from the above defects.In order to reach this goal, our inventions use flexure mechanisms to produce the required constant angular momentum. Note that the term flexure mechanism is exactly equivalent to compliant mechanism. The difficulty in this task is that flexures only have a limited stroke making it virtually impossible for a flexure bearing wheel to rotate around a fixed axis with constant angular momentum. We therefore found alternate methods for generating angular momentum by using flexure mechanisms.Two methods are presented in this paper. The first consists of a rigid body whose centre of mass has a circular trajectory around a fixed point, but the body does not rotate around its centre of mass. The body moves in translation and acts dynamically as a point mass, and thereby generates angular momentum in a constant direction. The second consists of two bodies rotating around their centres of mass, but whose total angular momentum lies in a fixed direction. The first method was successfully exploited in the IsoSpring project whose goal was to introduce new two degree of freedom oscillators in mechanical clocks and watches, in order to remove their traditional escapement mechanism. The second mechanism is also inspired from the IsoSpring project where a sphere oscillating around its centre of mass provided a two degree of freedom oscillator less sensitive to the direction of gravity.The paper presents flexure wheel designs along with their implementations. Moreover, methods to control the uniform circular motion are presented, among which a novel flexure bearing which restricts the motion of a body to translation on a circular orbit. Two prototypes were successfully built and tested. Finally, qualitative results from this proof of concept are presented.
Spacecraft Attitude Control Using Approximate Receding-Horizon Model-Error Control Synthesis