Tortuga II Electronics
The electronics are the like the nervous and circulatory system of the vehicle. The electronics carry all sensor inputs and commands outputs between the components, as well as distributing the power through the vehicle. As you can see in figure 6 below the components of the electrical system are distributed between the pressure hull and camera housing.
Figure 6. View of internal rack with all major components labeled.
Backplane
A backplane is a board which connects multiple daughter boards together. The vehicle's backplane is composed of a four layer printed circuit board (PCB) with six rows of connectors. The six rows of connectors allow six daughter boards to be plugged into it. The connectors carry 26 volts, 12 Volts, 5 volts, and the inter-board communications bus (ICB) to each daughter board. The backplane reduces the number of wires running between the individual electronics boards in the vehicle.
Power System
A power system distributes and regulates power. The vehicle's internal power system also balances load on the vehicle's batteries to prevent damaging or destruction of the batteries. The power system itself is composed of three separate PCBs. Each board preforms one the power system's main functions: battery load balancing, power regulation, power distribution. The load balancing board draws more power from the batteries with the most charge, keeping all batteries at about the same charge level. It then provides the 26 volts they provide to the backplane. The power regulation board draw 26 volts off the backplane and use switching DC-DC converters to regulate it down to 12 volts and 5 volts. Those voltages are then supplied to the backplane. The power distribution board takes the difference voltage sources off the backplane and distributes then throughout the vehicle. It provides power to vehicle's motor controllers, Carnetix power supply for the computer, and cooling fans.
Figure 7. Power flow overview.
Motor Controllers
Motor controllers supervise the power going to a thruster to produce the desired amount of thrust. The vehicle has six, one for each thruster, Solutions3 Motion Mind motor controllers. They receive commands from the sensor board over RS-232 (Recommended Standard 232, a method for sending serial data signals between devices) and respond by adjusting power going to the attached thruster.
Sensor System
The sensor system gathers information about the outside world and sends it to the vehicle's computer for processing. It is composed of a variety of devices: a depth sensor, two firewire cameras, an inertial measurement unit (IMU), SONAR, and a sensor board. The sensor board gathers from most of the other components in the sensor system and sends it to the main computer over USB.
Sensor Board
The sensor board gathers data from the depth sensor and other electronics and relays that information to the vehicle's computer. It is composed of a PCB with a network of dsPIC microprocessors. The microprocessors use the ICB to talk between each other, and through the backplane to other dsPIC's mounted on the power system's boards. These microprocessors gather information from the depth sensor, current monitors, and voltages monitors and send it over USB to the main computer. Due to lack of serial ports on the main computer, the sensor board also forwards motor commands to motor controllers.
Figure 8. Sensor Data Flow.
Depth Sensor
A depth sensor determines the how far below the surface a submersible is. The vehicle's depth sensor is a small cylinder three inches long and approximately ½ inch in diameter which screws into the rear end cap. It outputs a voltage representing the absolute pressure on the outside of the vehicle. The voltage is read by the sensor board and using a predetermined calibration the voltage is turned into the depth of the vehicle in feet and then passed sent to the main computer.
Inertial Measurement Unit
An inertial measurement unit (IMU) provides the orientation, acceleration, and rotation rate information. The MEMsense nIMU is 1.7 by 0.53 by 0.9 inches. Inside is set of microelectromechanical systems (MEMS) which provide a three axis gyro (senses rotation rate), three accelerometer (senses acceleration), and three axis magnetometer (senses magnetic field). The data from the IMU allow the vehicle to determine its orientation, how fast it is rotating, and how fast it is accelerating. This information is relayed to the computer using a USB adapter supplied with the nIMU. The nIMU is located near the front end cap above the electronics. This places it away from harmful interference and as close to the vehicle's center of rotation as possible.
Cameras
A camera turns images of environment outside the vehicle into digital data. The vehicle has two Unibrain Fire-I OEM board cameras. Each camera is a PCB approximately two inches squares with an open lens mount. A 1.9 mm wide angle lens with 107 degree by 78 degree field of view is mounted on the lens mount. The cameras captures images at thirty frames per second and send them over firewire to the computer for analysis. Both cameras are mounted in the camera housing, with one facing forward and the other downward.
SONAR
The vehicle's sound navigation and ranging (SONAR) system determines location of underwater sounds. The SONAR system is composed of eight PCB Piezotronics 106-series hydrophones and a processing board. The processing board utilizes a Microchip dsPIC33FJ256GP710 processor running at 40 MIPS. When a pulse of sound goes over the vehicle, the signal is picked up by the hydrophones. That signal is filtered and digitized by the processing board. From the differences between the arrival times of each signal the processing board calculates the position where the sound originated. The hydrophones are mounted in two arrays of four on the front of the vehicle's frame. The internal sonar processing board is mounted in a slot on the backplane.
Computer
The computer runs the program which controls the vehicle. The vehicle's is an Apple Mac Mini with an Intel Core Duo processor running at 1.83 Ghz with 2 GB of RAM. It is 6.5 by 6.5 by 2 inches and sites at the read of the main pressure hull.
