Download 1 - dept.aoe.vt.edu

USUSAT Software
1.1
Operating Software for Power
The computer will serve two purposes for the power system. First, it will track current
conditions for the purposes of rough sun sensing as well as to aid in the power
management of the system. The second purpose that the computer will serve will be to
act as a backup to the analog charge control circuitry.
The computer to aid in attitude determination will read the five current sensors on the
solar panels. The frequency of these reads is entirely at the discretion of the attitude
determination and control group. The current in the shunt branch will also be measured.
This current as well as the measured bus voltage will be used to determine the amount of
available shunted power. This number can then be used by the mission planning software
to activate any devices that may have been switched off to conserve power or that may
only be used during periods of sufficient power generation. The battery current will also
be measured to assure that no excessive current conditions are placed on the battery
system.
The secondary role of the computer in charge control will be accomplished by tracking
the charge state of the battery and sending the override signals FSN and FSF as needed.
When the spacecraft is initially powered up (or after a power cycling) the analog circuit
will be allowed to detect the first initial full charge of the battery. When this condition is
detected (as indicated to the computer via the SST signal and the shunt current
measurement) the coulomb counter is initialized at the battery full state. The coulomb
counter then begins to integrate the current in and out of the battery to determine the
charge state of the battery. During charging the coulomb counting software must take
into account the inefficiency of charging. In testing this inefficiency has be shown to be
that only about 90% of the current flowing into the battery during charging is effective in
building battery capacity.
When an overcharge condition (more than 110%) is detected the signal FSN is sent to the
power board. This signal will override the analog charge monitor and force the shunt
system to null out the current. This signal is held on until the shunt current goes to zero
and the battery enters a discharge state (more than 100mA).
This system will also be used to detect a malfunction in the power system. If the
coulomb counter indicates a battery capacity of less than 75% and the SST signal
indicates that the shunt system is active the computer can then send the signal FSF to
force the shunt off. This signal has higher priority than FSN and should override both the
internal shunt enable system of the power board and the FSN signal of the power system.
This signal shouldn’t ever need to be used because if it is the power board has failed to
regulate the battery charging properly. Once this signal has been activated the coulomb
counter takes over from the analog circuitry. The signal is released when the counter
indicates full charge. If for some reason the analog control system suffered only a
temporary glitch (i.e. a bit error in the monitor circuit’s memory cell) the analog system
USUSAT Software
should be able to take over its responsibilities again without any further computer
intervention.
In general the power system will have a total of nine analog lines that will be read via the
i/o board and three direct digital lines that connect it with the computer via the back
plane. The analog lines are the five current monitors on the solar panels, a current
monitor on the shunt system, a current monitor on the battery, the bus voltage
measurement, and the input signal from the thermistors to the battery monitor circuit.
The digital lines are the FSN (force shunt on), FSF (force shunt off), and SST (shunt
status) signals. The two former are outputs from the computer while the latter is an input
logic line to the computer.
Other power software requirements involve the management of power in general on
board the spacecraft. The measurements from the bus voltage and all the current
monitors, as well as the known efficiencies of the DC-DC converters, will be used to
determine what systems can be operated during specific times during the mission. More
specifically the firing of the deployables (and of the lightband) should only allowed when
the battery has reached full charge (or at least a reasonable charge state) and should only
occur while the spacecraft is not eclipsed. The transmitter should also be inactive when
the deployables are extended. The coulomb counter will also need to be calibrated once
in a while. This might be after a certain number of hours have elapsed and be completely
autonomous, or it could be dependent of a ground command.