POWER MANAGEMENT SYSTEM
A dedicated Power Management System is developed to support the onboard electronics and sensor payload. A Battery Management System is developed for optimal power distribution among various boards such as the onboard CPU, thrusters and the microcontroller board. A Battery Management and Protection board is custom designed to provide even discharge of Lithium Polymer (Li-Po) batteries. A visual feedback system to provide battery level information for thrusters and electronic peripherals is developed. Special care has been taken to ensure water leakage detection and overheating. Each component is protected with resettable fuses. Alpheus is powered by two 14.8V (4S), 10Ah Lithium Polymer batteries in parallel.
A custom board has been designed to monitor the power level of each battery which is also provided with a Hall Effect current sensor to continuously measure the current. A point contact temperature sensor is placed on each battery to continuously measure the temperature. A graphic LCD displays the status of the batteries, power lines and hull temperature. LED strip lighting provides visual feedback for software debugging.
A M4-ATX (250W) power supply unit provides power to the mainboard computer which is equipped with features like programmable voltage output and time out auto shutdown features. A DC-DC boost converter receives the raw voltage from batteries and converts it to different levels of voltage (5v, 12v, 18v) required by microcontrollers, actuators and sensor payloads. These channels are monitored and displayed on the LCD and protected in case of an overcurrent or overvoltage.
Computer design for Alphues is governed by the vehicle’s need to perform complex computer vision and machine learning in real time in spite of restrictive space requirements. The software system is powered by an Intel Haswell CPU Core i7-4785T quad core processor with a maximum Thermal Dissipation Power (TDP) of 35W on a Gigabyte GA-Z97N-WIFi motherboard along with a 256 GB SATA Solid State Drive (SSD). The Motherboard requires a non-fluctuating and uninterrupted DC power supply to deliver optimum performance, and it is provided by M4-ATX (250W) PSU. A USB hub interfaces the embedded sensors and actuators as well as other serial devices, i.e. Battery Management System (BMS), AHRS-8 and cameras. The main purpose of the Arduino board is interfacing Alphues’s various sensors and thruster.
Alpheus is equipped with a suite of sensors used for sensing the environment and providing orientation feedback as well as odometry information. Sensors for current, temperature, inertia, angular velocity, pressure and leakage are used in Alpheus. Two vision cameras are provided for driving the image processing software stack. The sensor suite provides 6 degrees of freedom state space solution. A brief description of the sensors is given below:
1. PRESSURE SENSOR
The vehicle uses UltraStable™ US300 Series and SWITZER 717-V series submersible pressure transducer to obtain analog pressure data. The sensor returns the pressure exerted by the mass of water above the vehicle. Using Pascal’s Law, the depth of the vehicle is extrapolated.
2. INERTIAL MEASURMENT UNIT (IMU)
Alphues is equipped with a MEMS based Sparton AHRS-8 system It is fully temperature compensated and uses Advance sensing technology (3-axis magnetic, 3-axis MEMS acceleration, and 3-axis MEMS gyro) to compute yaw, pitch and roll measurements. It provides critical inertial data at a rapid rate of 100 Hz. The IMU is used to provide vehicle angular velocities and linear acceleration that is used to compute the pose of the vehicle.
Alpheus uses two Microsoft LifeCam cinema cameras, one forward and other at bottom. Cameras are used to drive the vision system of the vehicle and are housed in custom fabricated external enclosures that provide a clear field of view to the camera lenses.
4. CURRENT SENSOR
A low noise producing current sensor is used in Alpheus. Hall Effect current sensors (ACS 709) are used by the power board to get a feedback of current being consumed from the batteries, It continuously monitors the current going in and out of the battery.
5. TEMPERATURE SENSOR
Alphues utilizes a LM35 digital thermometer temperature sensor. The digital thermometer has the capability of deriving power directly from the data line, thus eliminating the need for an external power supply. The sensor monitors the temperature within the hull in areas where higher temperatures might be a cause of concern.
6. LEAK SENSORS
Alphues has integrated leak sensors to detect possible water leaks. It consists of an array of wires. When these wires become wet, an electrical short occurs which is transmitted by a binary signal to the microcontroller and it is processed and desired action is taken. In addition, LED strips are integrated as state indicators. These indicators are especially useful during autonomous runs for understanding the vehicle’s current state.
Lithium polymer batteries built with Li-Po Nano-technology substrate complex are used for providing power to Alphues. The advantage of using these batteries is that there is less voltage sag and a higher discharge rate. The batteries are connected to a Battery Management System, which efficiently supplies power to the thrusters, microcontroller carrier board, CPU and other components used in the AUV. It can power the AUV for 120 minutes continuously.
8. KILL SWITCH
Alphues is provided with a kill switch which is used to shut down the entire AUV system, in an emergency. When the kill switch is activated, it stops power supply to electronic components completely and disables the thrusters. The kill switch minimizes the risk of the AUV getting damaged when an emergency is detected. Emergency situations include water leakage which may cause short circuiting, attacks caused by marine animals and destructive human activities which may inflict severe damage on the AUV.
9. DROPPING & TORPEDO CIRCUITRY
The vehicle uses a pneumatic cylinder assembly for dropping markers. The piston is actuated using a double acting cylinder, which is connected to a solenoid valve allowing the marker to fall into the bin. This design was chosen due to its low offset and high accuracy.
Alpheus uses 8 Blue Robotics T200 series thrusters systemized in three main groups: Two horizontal thrusters for surge, four vertical thrusters for heave and two side thrusters for heading and sway control. Each of these thrusters are controlled using an independent motor driver. This enables uniform and accurate propulsion, since it allows for individual control of each thruster’s rotation speed. The depth rating of the thrusters is 150 meters in fresh water.
The Acoustics System enables real-time detection and estimation of the Direction of Arrival (DoA) of underwater impulsive audio signals produced by the pinger. The main objective is to compute the angle and elevation of the source of signal.
National Instrument and a 3-dimensional array of four Sparton PHOD-1 hydrophones are used for the acquisition and real-time processing of the signals. Once the event (impulsive signal) is detected, its DoA is estimated using Generalized Cross Correlation (GCC) with Phase Transform weights (PHAT) to measure the Time Difference of Arrival (TDoA) between pairs of hydrophones.
Parameterized predictions of TDoA’s are compared to actually measured TDoA’s such that the parameter can be obtained by a Least-Squares minimization. Using real-time techniques, there is no loss of information from the environment for the processes of signal detection and DoA estimation occur in parallel.