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ELECTRICAL POWER SUBSYSTEM

The function of the power system is to generate power by solar cells and store in batteries. Also provide stable power to other subsystems is a role. For this aim, system configuration power budget and on board components has been designed and selected.

SYSTEM BLOCK DIAGRAM

Power system concept is as followed.

BATTERY

Batteries provide power to satellites during eclipse. Commercial use Manganate Li-ion secondary batteries are selected. Several space environment tests have been done to check the works of the batteries at space.

SOLAR CELL

During daylight, solar cells provide power to other subsystems and charge batteries. Solar cells are body mounted on CubeSat's 6 panels. Silicon solar cells are considered to use.

POWER REGULATION AND CONTROL

Provide regulated dc power to each subsystem. The aim of convert efficiency is 80%.

CHARGE CIRCUIT

Charge circuit monitors and controls energy charge to Li-ion batteries by solar cells. The aim of charge efficiency is 80%.

ATTITUDE COTROL SYSTEM

Power system charges CubeSat attitude control using magnetic force.

XI-I,XI-II

About XI-I and XI-II is here

XI-III Power System

System Block Diagram

XI-III power system succeeded XI-II Gamma power system.

TxDCDC is driven directly by batteries to improve convert efficiency.
XI-III PCB is shown here.

This subsystem's functions are

Power system provides 5V for electronics and communication subsystem
Electronics and communication subsystem monitor consuming current each other and shutdowns power supply if it detects SEL.
Solar cells generate power, drive CubeSat and charge batteries.

Components

Solar Cells

Solar cells are arranged on all 6 surface panels. These cells are mono-crystal silicon cell and generate 1050mW(80 degree typical). The delivered FM solar cells are shown as follows.


+X	-X,+Y,-Y	+Z,-Z

Li-ion Battery

Li-ion secondary batteries are selected. This one has following characteristics.

Manganate Lithium ion secondary battery
Capacity 780mAhr , 8 batteries are in parallel
DoD 3%
XI can be driven for 40 hrs only by batteries

Batter Charger

MITSUMI's MM1485 IC is used for the control of Li-ion battery charging. This IC's functions are

Constant current, constant voltage charging
Pre charge battery condition check

Charging behavior is shown in following graph.

Power Regulation

Switching Regulator:
Switching regulator that driver IC is Linear Technology LT1701 is used to provide 5V for electronics system and communication system. Out put of the regulator is smoothed by voltage stabilizer.
DCDC converter:
A DCDC converter is used to provide power for the transmitter.

Power Budget

Power budget is designed to satisfy following 3 points.

Total power is balanced.
CubeSat can live until the first contact only by batteries
CW transmitter can be directly driven by solar cells

First point means consumed power during eclipse can be charged during the daylight. So CubeSat can be operated enough long time.
Second point mentions that even if all solar cells destroyed by the impact of the launch, CubeSat will live until the fist contact to achieve middle success.
Third point is to assure the minimum activity during the daylight provided all the batteries are lost.
The estimated power profile at the nominal operation is shown here.

Attitude Control

CubeSat's attitude is controlled. The objectives are

Stable power generation
Smoothen thermal input
Point antennae to the ground station

A permanent magnet is used to control. Torque will be generated to align earth magnetic direction and CubeSat's dipole moment. This force make CubeSat rotate twice during one orbit period. Also to stop libration and stable attitude change, a hysteresis damper is used. This dapmer dissipate energy by hysteresis loss and eddy current loss caused by change of the attitude of the damper in the magnetic field. Each components has following characters.

	Permanent Magnet	Hysteresis Damper
Material	Alnico5	50% Permalloy
Size	Phai4mm*25mm	Phai8mm*0.3mm
Weight	2g	10g


Permanent Magnet	Libration Damper

Future Works

Power system is finally examined by one-week continuous operation test.

XI-IV

XI-IV is the flight model. All components including only one set solar cells are equipped.

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