Weekly Update #3: Overview of the Attitude Determination and Control System

Greeting from the Orbital Mechanics and Attitude Control (OMAC) team. Our team consists of students of different faculties. Together we are responsible for the concept and hardware design of the Attitude Control and Determination System (ADCS) of MOVE II.

For many CubeSat missions attitude control and determination is required to ensure proper performance of the satellite systems. The principle task of the ADCS is to stabilize and align the satellite towards a reference orientation.

The task of attitude control is twofold. At first the satellite has to dampen the very high velocity rates after ejecting from a CubeSat deployer right after the launch. Only after this so-called detumbling further mission steps can be initialized. The next step is stabilizing and maintaining a nadir pointing configuration against environmental perturbations.

The ADCS system requires two different types of hardware components: sensors and actuators.

Therefore, magnetorquers shall be implemented for three axis attitude control. They are well established and have been implemented on a variety of CubeSat missions. Compared to other ADCS actuators the advantages of magnetorquers are the relatively light weight, low power consumption and low costs. Furthermore, they have no moving parts and are more effective in LEO, rather than in higher orbits, as the geomagnetic field is decreasing with the altitude. Magnetorquers consists of electromagnetic coils and generate a magnetic moment. Since a magnetic moment experiences a torque in an external magnetic field, a satellite can therefore change its attitude in low earth orbits where the earth’s magnetic field is strong enough. MOVE-II uses coils printed as tracks on multi-layer printed circuit boards (PCB) which makes them robust, easy to produce and light.

Commonly flown sensors on CubeSat missions are sun sensors and magnetometer. They are well established due to their small size and mass, low power needs and low costs. Next to these reference sensors rate gyroscopes as inertial sensors can also be used for CubeSat missions. The only type of gyroscopes, which are currently suitable for CubeSat missions are miniaturized devices based on microelectromechanical system technology (MEMS).

Two ADCS operation modes are considered: Detumbling and nadir pointing. The detumbling controller must be able to slow down the CubeSat after ejection from its deployer. A well-established detumbling controller is the so-called B-dot controller, which has been implemented on several CubeSat missions. An attitude controller complies three axis stabilization using only magnetic actuation. Normally an advanced attitude controller requires full information of the state vector. Therefore, an attitude determination system is required. Sensor data are fused together with reference models in an estimator (e.g. Extended Kalman Filter approaches) to determine attitude.

The full concept of the ADCS is being developed now by our team. Of course the concept has to be verified. Several simulations and hardware tests are going to be done in the future to ensure proper and reliable operation of the ADCS.

Leave a Reply

Your email address will not be published. Required fields are marked *

%d bloggers like this: