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.
These solar cell strings are attached to the solar panel plates like the following figure.
And this plate is attached to the CubeSat.
SYSTEM BLOCK DIAGRAM
Power system concept is as followed.
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.
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 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 and XI-III
About XI-I and XI-II is here
About XI-III is here
XI-IV Power System
System Block Diagram
XI-IV power system succeeded XI-III power system.
XI-IV 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.
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.
Li-ion secondary batteries are selected.
This one has following characteristics.
Since the launch day could be 10 month after the last battery charge,
the battery may be dead when the CubeSat is in orbit.
So the CubeSat select mode when it is started.
One mode is normal one for nominal operation and the other one is
hibernation mode to give high priority for battery recovery.
- Manganate lithium ion secondary battery
- Capacity 780mAhr , 8 batteries are in parallel
- DoD 3%
- XI can be driven for 40 hrs only by batteries
MITSUMI's MM1485 IC is used for the control of Li-ion battery charging.
This IC's functions are
Charging behavior is shown in following graph.
- Constant current, constant voltage charging
- Pre charge battery condition check
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.
A DCDC converter is used to provide power for the transmitter.
Power budget is designed to satisfy following 3 points.
First point means consumed power during eclipse can be charged during the daylight.
So CubeSat can be operated enough long time.
- Total power is balanced.
- CubeSat can live until the first contact only by batteries
- CW transmitter can be directly driven by solar cells
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.
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 component has following characters.
|Permanent Magnet||Hysteresis Damper
|Material ||Alnico5 ||50% Permalloy
|Size ||Phai4mm*25mm ||Phai8mm*0.3mm
|Weight ||2g ||10g
The action items are
- Measure the final characteristics of XI-IV.
- Dairy maintenance until the launch.
- Handling after the shipment.