The electrical system for SubmURSA was divided into multiple separate boards. The purpose of this was to make the design more modular and to try and multiple layers of redundancy to the system. The team utilized CadSoft Eagle for the design of the electrical schematics and for the PCB layout. All of the team's PCBs were fabricated by the wonderful people at Alberta Printed Circuits out of Calgary, Alberta, Canada.
The main board is responsible for communications between all external subsystems except for the vision component which is handled by the embedded computer. The mainboard features a MOD5270 from Netburner which is running microLinux. The micro controller can load the kernel (version 2.6.34-uc0) and the applications from the onboard SD card. The microcontroller communicates with the motor controllers using TTL and with the IMU using RS232. In addition, a 6 pin male header allows for a TTL-USB cable to be directly connected to the main board which allows for data to be written to a terminal program for debugging purposes. Power is supplied to the main board from the electronics power board mentioned further down. LEDs indicate if power is being supplied to the board and is also protected by a fuse. The main board has a 3.3V regulator that can supply up to 800mA. The main board supplies power for the sonar subsystem, the IMU, the digital compass, and the depth sensor. A 5V supply line is provided by the motor controllers, negating the need to step up the 3.3V supply line with additional circuitry. The kill switch is implemented on the mainboard; triggering the external kill switch will pull the second data line of each of the motor controllers to ground,shutting down the motor controllers and all propulsion.
SubmURSA utilizes four Sabertooth dual 12A motor controllers and one dual 10A motor controller. The 12A units are used to power the pumps while the single 10A unit controls the two thrusters. The motor controllers receive their instructions via a packetized serial protocol from the microcontroller. This allows up to 8 separate units to be placed on the same serial line, decreasing the number of serial ports required and simplifying the software. The motor controllers for the pumps are powered using two custom propulsion power boards while the motor controller connected to the thrusters is powered directly by a single 18.5V lithium-polymer battery.
In order to achieve a proper bearing, the electrical team chose Ocean Servers OS5500, a combination of an inertial measurement unit (IMU) and digital compass. The OS5500 also allows the incorporation of a pressure transducer as a means of obtaining the depth of SubmURSA. The OS5500 has connectors for both USB and RS232 communication and offers an ASCII interface to allow programming with the microcontrollers. SubmURSA is capable of taking internal temperature values from multiple sources and can bemeasured by the motor controllers and the IMU.
The passive sonar system uses four SQ26R1 hydrophones with built in pre-amplifiers to ‘hear’ the acoustic pinger. The captured signal from the pinger first passes through a high-pass filter to remove the 60Hz power signal. The signal then passes through a variable pre-amplifier to boost the signal as it was determined that the built in pre-amplifier did not posses a sufficient gain to process the signal. After the preamplifier, the signal is split along two different paths. One path passes the signal through a zero-crossing detector which converts the sinusoidal wave into a square wave where the positive sections of the sinusoidal wave correspond to the positive sections of the square wave and the negative sections of the sinusoidal wave correspond to the negative sections of the square wave. The second path leaves the signal untouched. Both sections of the signal are then passed through their own rectifier to eliminate the negative portions of each signal. Finally, the two signals from each of the four hydrophones are sampled by an analog to digital converter.
Once the eight signals have been discretized, they are passed via an SPI connection to the microcontroller for processing. The frequency of each signal is calculated by measuring the distance between the square waves from the zero-crossing detectors or by digitally filtering the raw rectified signal. The time at which the desired frequency is detected by one of the hydrophones is recorded. The length of time between the moment when each hydrophone picks up the desired frequency is calculated as the time-distance-of-arrival (TDOA). The TDOA data is then sent via UART-USB converter to the embedded computer.
For SubmURSA, it was decided to completely separate the power for the propulsion and the electronics. This was done primarily because of the increased power requirements of the new propulsion system, but it also allows for more redundancy and power savings.
The Propulsion Power Board is based around the VICOR V24A12C400BL voltage regulator. It draws power from up to two separate 5Ah 18.5V lithium polymer batteries and distributes it to the 8 LVM Congo 111 centrifugal pumps which run at 12V and draw approximately 6A each at full power. As the voltage regulators can only source up to 400W at 12V, each board is only responsible for 2 motor controllers. Other features include a 3.3V line for battery sensing, additional resistor ports for stepping up/down voltage and a “parallel-out” port for running multiple VICOR V24A12C400BL voltage regulators in a Master/Slave parallel configuration. For the competition this year, the team chose to use sockets for the voltage regulators because prolonged heat exposure during the soldering process can easily damage or even destroy the regulators.
Electronics Power System
The Electronics Power Board is essentially a bus-board for distributing power from the 3.7V lithium-polymer batteries to any electronic board that operates at 3.3V. It incorporates three battery input headers and three fused power output headers connected in parallel. Since discharging lithium-polymer batteries below 3V can damage the cells and therefore prevent them from accepting a full charge, a simple voltage monitoring circuit activates a red LED to indicate if the voltage has fallen below a preset threshold. This voltage threshold is adjusted using a trim pot on the board, and is typically set at approximately 3.3V.