Introduction
The Electrical Power Supply Subsystem is one of the fundamental subsystems in our Nano-Satellite. It is responsible for a variety of operations which can be categorized in the following sectors:
- Solar Energy Harvesting
- Energy Transformation and Distribution
- Energy Storage in battery cells
- Grounding
- Spacecraft deployment and electrical commissioning
- Over-current protection for subsystems and components
Solar Energy Harvesting
With the usage of Triple Junction Gallium Arsenide (GaAs) surface-mount Solar Arrays we are harvesting the solar energy directly from the sun, transforms it to electrical and then use it to power on our Nano Satellite.
These arrays, manufactured by EnduroSat, have an efficiency of 29.5%, high radiation resistance, can reach a maximum out power of 8.4W with a nominal voltage of 16.8V and a nominal current of 504mA. Each Solar Array is made of 7 cells with very low mass and is equipped with a temperature and a coarse sun sensor.
Furthermore, these arrays are used with a configuration of:
- 7-cell panels on the 2 fully available sides
- 6-cell panels on the sides equipped with the patch antenna and the Fine Sun Sensor correspondingly
- A 2-cell panel on the -Z face of the AcubeSAT
Electrical Energy Storage
Power regulation and management aboard AcubeSAT are being undertaken by the EPS power supply system. All generated electrical power is stored in a 4-cell battery pack, which consists of Li-ion 18650 battery cells, connected in a 2 series, 2 parallel configuration, totalling 46 Wh. The battery pack is hosted on its own PC/104 PCB, right beneath the main board.
Electrical Energy Distribution
The COTS board incorporates two low drop voltage diodes in its four MPPTs, which are wired in order to increase their efficiency during the mission. The power form the MPPTs is used to charge the battery cells and provide the average 3W of power to the satellite subsystems.
The configuration of the rails is chosen in such a way as to minimize the total load on the individual rails and to have independent control on each channel.
Inhibits
To facilitate cessation of all electrical activity on the satellite, a combination of inhibits is used to achieve the desired result with required redundancy. The currently chosen panels include an RBF pin which cuts the electrical connections, shutting down the satellite on the ground. Also the frame is equipped with 2 kill or deployment switches at the -Z face. Last but not least, the deployment timer is handled by the OBC to accommodate for the delay during commissioning mode.
Protection Mechanisms
Since EPS is a mission critical subsystem, safety is considered the primary concern. The board utilizes battery over-voltage, under-voltage and over-current protections, assuring the levels stay within the rated range of the battery cells.
During ground operations the supplied charger is also equipped with a voltage protection not allowing the voltage to exceed 8.4V. Each channel of the board has a hardware and a software over-current protection, with the hardware protection having the highest priority.
Apart from the per channel over-current protection, the EPS board is equipped with output converter over-current protection, transitioning to a constant current mode by lowering the voltage output. Avoiding complete battery depletion is crucial for the mission, thus, when the battery voltage is critically low, the board switches to EPS Safe Mode, which also corresponds to our system level Safe Mode.
