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CONTROL & COMMUNICATION

Controller computer installed in O-buoy

The O-buoy is a sophisticated autonomous platform that needs to control several different instruments but also to carefully manage power. It does this through the use of a supervisory computer. The Supervisory Computer (SC) was based on a Technologic Systems TS-7260 single board computer (SBC) and additional peripheral components. The SBC has two 16C550 type serial communication ports, two USB 2.0 ports, a 10/100 MBps Ethernet port, an integral SPI interface, 64 MB of RAM memory, 128 MB of Flash memory, an SD card socket, a battery backed-up real time clock, a 16 bit PC-104 expansion interface, an on-board temperature sensor, a user selectable capability for RS-232 or RS485/422 compatibility on its COM 2 serial port, and an ARM9 processor to operate at a clock rate of 200 MHz. The software operating system was the Debian Linux distribution as adapted for the TS-7260 SBC. The SBC was fitted with a four-port 16C550 type serial expansion card and a second Ethernet port (connected via a PC-104 expansion interface). The SBC and its options were specified at the time of purchase for operation to -40oC. Typical power consumption on the buoy was observed to be approximately 1.5W while running a demanding computation benchmark load with all ports operating at high rates.

Upon start-up the computer performed an initial boot from its Flash memory in the YAFFS internal format followed by a “pivot boot” to the full operating system in EXT file format. The full Linux operating system was contained on a 512 MB solid state disk drive. A 16 MB solid-state USB memory was installed to provide an on-board archive of all data that was obtained from every instrument and sensor. In the event of a possible (but unlikely) failure in the satellite communications system, the data complete records from the buoy will be available when the buoy is recovered.

The SC was operated at all times in a watch-dog mode; therefore it represents the baseline power demand of the O-Buoy system. This device was the only subsystem on the buoy that was constantly operating.

Power distribution, monitoring and control were done via a custom built circuit (Figure 5) that was directly managed by the supervisory computer. Power input was from either or both of two possible sources: (A) An Acid Gel Matt (AGM) that was recharged from a solar cell array; or (B) A non-rechargeable lithium-ion (Li) battery bank. The power circuit was based on a negative common design. The input circuit was equipped with a separate current steering diode in series with each positive connection to the AGM bank and the Li bank respectively. Additionally, an electronic switch was located “upstream” of the Li steering diode to allow the Li bank to be positively turned off by the SC under software control or explicit satellite derived command. This single switch plus the two steering diodes creates three possible modes of power input to the buoy system:
1) High Solar Elevation – The Li bank is switched off by automatic software command because the solar cells are sufficient to operate the buoy system and provide sufficient power to fully charge the AGM batteries. The voltage of the AGM bank is monitored by the SC through an external 12-bit digitizer and multiplexer. As long as the AGM voltage cycles between 14.5 volts and 11.5 volts the AGM packs will continue to be the power source. This range is determined by a pulse width modulated (PWM) controller that arbitrates between the unregulated solar array and the AGM battery bank.
2) Low Solar Elevation – The solar cells may not be capable of maintaining the charge on the AGM batteries (AGM voltage level falls to 11.5V). At this point, the watch dog software switches the Li bank on. The lithium ion batteries will exhibit an open circuit voltage above this level (>11.5V) until they are nearly completely exhausted. Our calculations indicate that the Li battery bank will last a minimum of two winter seasons of buoy operation.
3) Intermediate Solar Elevation – Here, both battery banks are put on line and passive diode steering alone apportions the current load. This mode was tested during the deployment in Elson Lagoon from February-May 2009. Despite the sun being not much above the horizon, the solar array provided sufficient power to operate the buoy from the AGM bank by March 2009.

Power distribution was managed by the SC via the power control circuit. Identical electronic switches supplied the unregulated +14.5 volt level to all scientific devices, meteorological instruments and a pre-packaged satellite transceiver system. The power control circuit provided regulated voltage at +3.3 volts for its own analog and digital circuits. This circuit utilized a set of voltage and current sensing amplifiers which were read via a multi-input multiplexer from the SPI port on the SBC. There were sufficient parameters available that the watch dog software could report the distributed voltage level and all significant current loads in the system.

Based on a predetermined scientific observation schedule (which was heavily dependent on the solar elevation angle), individual instruments were sequenced into operation as needed. The objective was to provide a maximum number of scientific observations consistent with the power available (maximum of 16W). The heaviest power consumer on the buoy was the satellite transceiver at 5.5W (up to 7W, for higher latitudes); with a typical data off-load requiring a two hour long satellite network window. Therefore, most scientific functions were scheduled for operation and data acquisition between satellite service intervals. Revised programs and schedules could be loaded to the buoy during an open satellite window. At present, the results of these revisions would have been seen at the next satellite service of the buoy which is 24 hours later.