The Sapling spacecraft is currently configured for attitude determination, detumbling, and coarse attitude control. The attitude determination system includes six (6) light sensors mounted on the exterior faces of the CubeSat, a 3-axis IMU (within approximately 20° accuracy) and a Skytraq GPS with dual antennas for orbit determination. Sapling is equipped with air-coil PCB magnetorquers for detumbling and coarse attitude control.
The goal of the Sapling magnetorquer design was to produce a solution that is comparable in strength to commercially available torque rods, but easily integrated into the 1U form factor and lower in cost.
The magnetorquers are PCB embedded and attach directly to the existing spacecraft exterior side panels. Most ADCS systems are a significant contributor to spacecraft mass and volume. This solution allows them to be easily exterior mounted. The PCBs were designed using a Python automation script that draws coil traces into KiCad (an open source electric design software) as shown in the figure below. This Python script automatically leaves space for existing structural or electrical elements on the PCB. In our case, the coils were automatically drawn to accommodate computer access ports, camera, and radio mounts on the existing side panel designs. This script could be reused for future CubeSat missions to streamline the process of integrating magnetorquers into most any CubeSat mission. To accomplish the magnetic moment goal (1 Am2) within the constraints of typical board manufacturers, four (4) to five (5) dedicated coil boards (each of which has six (6) layers) are used on each side of the CubeSat in series.
ADCS System Requirements
|Attitude determination calculations
|The onboard computer shall accurately and efficiently integrate spacecraft position and orientation.
|The spacecraft shall calculate a solar reference vector.
|Vector calculated based on light intensity at the ambient light sensors.
|The spacecraft shall calculate a magnetic reference vector.
|Vector calculated based on comparison of three magnetic axis measurements to a magnetic vector map.
|The spacecraft shall possess a GPS for attitude determination.
|ACS-101, SYS-102, SYS-402
|> 1 Am2
|The magnetorquer coils shall meet or exceed the magnetic moment of commercially available torque rods.
|< 30 microteslas
|The spacecraft shall sense the Earth's magnetic field with accuracy.
|The spacecraft shall track its acceleration in 3 axes.
|The spacecraft shall track its rotation in 3 axes
|The spacecraft shall detect sunlight conditions with ambient light sensors on 5 exterior faces.
|The ADCS system shall fit within the CubeSat form factor.
|Magnetorquer coils shall not interfere with existing spacecraft structure.
|The ADCS shall not operate when the system is in low power mode.