Delfi-n3Xt Attitude Determination & Control Subsystem
The Attitude Determination and Control Subsystem (ADCS) consists of a set of sensors, actuators and a redundant microcontroller with the algorithms. The objective is to demonstrate the following capabilities:
- Detumbling of the satellite fromorbit injection from initial rates up to 10 °/s to less than 0.2 °/s.
- Align the satellite with an accuracy of 3° to the sun vector, the velocity vector, the magnetic field and nadir.
- Slewing manoeuvre for ground station tracking of the S-band antenna with 5° accuracy.
TU Delft has developed its own compact sun sensors based on a quadrant photodiode and a window in PEEK material. It is a cost effective solution with decent performance. The end-to-end noise levels are comparable with high end sun sensors by integrated the bias circuit and read-out electronics directly at the back of the sun sensor. The bias consists of a static placement error and a dynamic bias caused by temperature variations. Future research will be performed to reduce the end-to-end bias by looking at deterministic effects. The total sun sensor is so small that it can be integrated in the side margins of the CubeSats.
For measuring the magnetic field, a commercial integrated thriple-axis digital compass is chosen. This micro sensor provides accuracy for measuring the magnetic field in LEO of about 3° once static bias is filtered out successfully. As the magnetometer is placed inside the satellite near the actuators, a smart method is implemented to account for the local magnetic pertubation.
Microcontrollers & Algorithms
The primary controller is an ARM9 400 MHz processor. This runs a b-dot controller for detumbling, a Kalman filter for advanced attitude determination and a PID controller with smart reaction wheel unloading algorithm for the actuation. The backup controller is an X-MEGA 32 MHz microcontroller and only runs the b-dot controller as this function is critical.
TU Delft has developed its own reaction wheels which provide direct control over each of the principle axes of the satellite. They are designed to be able to counteract the highest possible disturbance torque and store the equivalent momentum produced in half an orbit. They have been intensively tested for launch vibrations acceptance.
The magnetorquers are designed to dump momentum of the spacecraft and the reaction wheels. It consists of a vertical coil which encloses the reaction wheels and electronics and two coils on permeable rods in the horizontal plane. This design allows a very small overall ADCS volume.