The acronym AOCS stands for the full name of the system – Attitude and Orbit Control System. Such a system is needed to perform orbital maneuvers. It is an extension of the more common ADCS (Attitude Determination and Control System). Its main purpose is to determine and adjust the orientation (angular position) of the satellite.
Attitude and Orbit Control System
The satellite orientation is determined by using appropriate sensors such as star trackers, horizon sensors, sun sensors, magnetometers, gyroscopes. In the case of PW-Sat3 AOCS, sun sensors and magnetometers will be used, as well as gyroscopes as an auxiliary. Our algorithms perform validation and fusion of the measured data to determine the state of the satellite (its orientation and angular velocity).
Position of the reaction wheels in the PW-Sat3 model
Based on the measurements, appropriate algorithms prepared by us determine the control signals. The executive elements (actuators) are, in the case of AOCS PW-Sat3, reaction wheels and magnetorquers. Their principle of operation is quite simple. Reaction wheels use the angular momentum conservation principle. This allows the satellite’s structure angular velocity to be impacted by increasing the angular velocity of the reaction wheel. Reaction wheels are able to rotate the satellite around its center of gravity but are not capable of transmitting forces. So, if we want to change the orbit of a satellite, we have to use propulsion.
Control algorithms should take into account gyroscopic effects. It is related to the rotation of the reaction wheels relative to the satellite structure. As a result, the change in the angular velocity of the wheel does not directly affect the structure. It is influenced by the resultant torque of the reaction wheel, the angular velocity of which is changed, and the moment of force resulting from the rotation of the axis of the remaining reaction wheels.
Animation showing how reaction wheels work
Unfortunately, the principle of conservation of angular momentum has a flaw that forces the use of another alternative source of angular momentum. For this purpose, we use magnetorquers. They allow interaction with the Earth’s magnetic field. However, they are not a sufficient solution in the case of PW-Sat3, hence the need to use reaction wheels, which allow the satellite to be controlled in 3 axes.
The AOCS system is a new thing for the PW-Sat project; our predecessors used very simplified systems based only on magnetorquers and magnetometers placed on a COTS board. Due to the main goal of the mission (demonstration of satellite propulsion technology), there was a need to use an orientation control system with much greater capabilities. We made a bold decision to make such a subsystem ourselves based on COTS sensors and actuators. Our solutions include navigation, failure detection, and correction, control algorithms, as well as low-earth orbit simulation environments. We are also developing the flight software of the control system, and in the future, we will integrate the system in a clean room.