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 (V 5.1+) 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 overall electrical system architecture is similar to what was used on the 2011 platform. Its main components are:
- A mainboard that features a NetBurner MOD5234 microcontroller, responsible for processing all sensor inputs except video, and controlling the thrusters used for movement.
- An embedded computer (EC) that uses processed sensor data provided by the mainboard, and video from USB cameras, to make mission-level decisions, e.g. “move to buoy,” “move to surface,” etc., and instruct the mainboard accordingly.
- A power distribution board that steps the 18.5 V lithium-polymer battery output to 12, 5, and 3.3 V to power all electronic components.
- Motor controllers that translate control signals from the mainboard into power signals that operate the thrusters.
- A sonar board that conditions and processes hydrophone input before it’s processed on the mainboard.
Numerous electrical difficulties in the 2011 competition sparked a re-imagining of the implementation of these systems. Poorlyperforming elements have been scrubbed from the system. Upgrades were made with modularity and expandability in mind, since new components are often “piggy-backed” onprevious hardware during testing and design.
The mainboard acts as the control hub for each peripheral board. It houses a NetBurner MOD5234 microcontroller running microLinux, loaded from an on-board SD card, and is responsible for:
- Processing several sensor inputs. Providing PWM and direction signals to the motor controllers.
- Interfacing with the embedded computer via Ethernet.
- Running the mission control software, if required by EC failure.
- Interfacing with a development PC via USB during testing and debugging.
- Handling input from the kill-switch: when the switch is actuated, the motor controllers’ “enable” pin is pulled low and an active-low “RUN” pin on the mainboard is pulled high, resetting the MOD5234.
Without adequate, robust motor control, SubmURSA would literally be dead in the water. This is indeed the lynch-pin of the entire electrical system. In past years, off-the shelf components were used, but these devices often required unnecessary compromises. Commercial devices often lacked required features like feedback, had an excess of features that complicated their integration, and generally larger form factors.
To address these concerns, ARVP has developed a purpose-built motor controller based on the L298 H-bridge integrated circuit. This IC can drive one motor at 2 A or two motors at 4 A in a “bridged” configuration. Two motor control boards, each with three L298 ICs, are used, allowing up to six thrusters to be operated. Each channel accepts a PWM signal for thrust level and a binary direction signal. Power is passed directly from the 18.5 V batteries to the motor controller boards. In addition to these inputs, a “sense” pin feeds an analog voltage, proportional to motor current draw, back to the mainboard, and an active-high “enable” pin disables the load when pulled low, to implement required kill-switch functionality.
Numerous sensors allow SubmURSA to determine its orientation, bearing, and depth, and other mission-specific information:
- An LSM303DLM 3-axis compass and accelerometer to determine orientation and heading.
- An L3G4200D gyroscope to compensate for error and drift in the compass/accelerometer.
- A TDH30 analog pressure transducer to determine depth.
- Four SQ26R1 hydrophones positioned at the corners of the frame, for passive sonar.
- A Microchip MCP9700 Linear Active Thermistor to measure internal hull temperature.
- Two USB video cameras used by the embedded computer for image processing.
Since some of these sensors provide analog outputs, a Maxim MAX1300 analog-digital converter, which communicates with the NetBurner via SPI, is mounted on the mainboard to free up microcontroller pins.
SubmURSA’s passive sonar system enables it to detect to detect the acoustic pinger located at the end of the RoboSub course. Four SQ26R1 hydrophones measure audio from the environment. The signal from the hydrophones passes through a high-pass filter to remove the 60 Hz power signal “buzz.” Since these piezoelectric devices are not externally powered, their output signal is passed though an adjustable-gain preamplifier, with a default gain of -70 V/V. From this point, each signal is split into two paths that allow for two distinct modes of operation. The first mode passes the raw sinusoidal wave through a rectifier that eliminates the negative half of the pinger signal (to avoid damaging other components), leaving the positive half unchanged. The second mode of operation performs additional conditioning on the signal: it is first passed through a zero-crossing detector, which converts the sinusoidal wave to a square wave. The square wave is positive when the sine wave is positive, and vice versa. This signal is rectified to remove negative voltages; the end result is a logical signal that is “high” when the sonar input is positive and “low” when the sonar input is negative. Both signals from all four hydrophones are sampled by an analogdigital converter, and all eight digital signals are passed to the mainboard via SPI for processing and interpretation. The frequency of the hydrophone input can be determined in software by either measuring the time between pulses of the conditioned hydrophone output, or by digitally filtering the raw, rectified sinusoidal signal. The MOD5234 microcontroller logs the instants at which a desired frequency has been detected by each hydrophone. This time-difference-ofarrival (TDOA) data is passed to the embedded computer for interpretation.
In previous years, voltages required to power the different electrical components were provided by separate boards. To simplify SubmURSA’s internal wiring and reduce assembly and troubleshooting times during competition, all of these boards’ functionality has been rolled into a single power University of Alberta RoboSub 2012 Journal Paper Autonomous Robotic Vehicle Project Page 4 of 10 distribution board. A dedicated 18.5 V lithium-polymer battery provides input to a Vicor V24A12E400BG voltage regulator module, which steps the battery voltage down to the 12 V required by the embedded computer. Also connected to the 12 V bus are LM7805 and LM1117-3.3 regulators, which provide 5 and 3.3 V, at up to 1 and 0.8 A, respectively. Each of the 12, 5, and 3.3 V outputs can be connected to up to five loads through Molex Minifit connectors. Standardizing electronics power to use these connectors will improve organization and consistency of electrical wiring, as well as allowing all of the electronics to be powered by an ATX 2.0 compliant PC power supply during testing.
Electronics Power System
In previous years, voltages required to power the different electrical components were provided by separate boards. To simplify SubmURSA’s internal wiring and reduce assembly and troubleshooting times during competition, all of these boards’ functionality has been rolled into a single power distribution board. A dedicated 18.5 V lithium-polymer battery provides input to a Vicor V24A12E400BG voltage regulator module, which steps the battery voltage down to the 12 V required by the embedded computer. Also connected to the 12 V bus are LM7805 and LM1117-3.3 regulators, which provide 5 and 3.3 V, at up to 1 and 0.8 A, respectively. Each of the 12, 5, and 3.3 V outputs can be connected to up to five loads through Molex Minifit connectors. Standardizing electronics power to use these connectors will improve organization and consistency of electrical wiring, as well as allowing all of the electronics to be powered by an ATX 2.0 compliant PC power supply during testing.