Optimal Time before AOCS Wheels Slow Down
- Paper number
IAC-05-C1.7.06
- Author
Prof. Magne Aarset, Statcon as, Norway
- Year
2005
- Abstract
Abstract – When monitoring and controlling a system, maintenance is of vital interest. When the system is a satellite in orbit, though, the opportunity for maintenance is slim, but still, probability models regarding optimal replacement times may be utilized to establish procedures for controlling the spacecraft. It is common to make assumptions regarding the models to ensure analytical solutions. Here, a more general maintenance model is proposed and a numerical solution calculated. 1. INTRODUCTION Here, we shall discuss the Attitude and Orbit determination and Control Subsystem (AOCS) of a satellite. Spacecraft attitude refers to the angular orientation of a defined body fixed coordinate frame with respect to a separately defined external frame. As in a boat, roll is the motion tilting the deck over to the port and starboard. Pitch is the motion of the boat up and down. In yaw, the boat moves to starboard as the stern moves to port, and vice versa. For the purpose of our discussion, we can ignore the speed of the spacecraft and its trajectory (its path through space). Attitude determination refers to the process of measuring spacecraft orientation. Attitude control implies a process, usually occurring more or less continuously, of returning the spacecraft to a desired orientation, given that the measurement reveals a discrepancy. One common method of three-axis stabilization uses electrically driven reaction wheels (momentum wheels), mounted in orthogonal axes. To rotate the whole spacecraft in one direction, we spin the proper momentum wheel in the opposite direction. To rotate the vehicle back, we just slow down the same wheel. There are constant small forces acting from outside to rotate a spacecraft, such as the slight pressure from sunlight and the gravity gradient. All these factors cause excess momentum to build up, and the momentum wheels end up moving faster and faster. So the spacecraft must occasionally use its rocket thrusters or its magnetic torques to slow the momentum wheels down. Utilizing the concept of preventive maintenance, we shall now discuss when is the optimal time to slow the momentum wheels down.
- Abstract document