SOLAR-POWERED AUTONOMOUS UNDERWATER VEHICLE DEVELOPMENT

To meet the rapidly expanding requirements for Autonomous Underwater Vehicles (AUVs), Falmouth Scientific, Inc. (FSI) is working in cooperation with the Autonomous Undersea Systems Institute (AUSI) and Technology Systems Inc. (TSI) to develop a vehicle capable of long-term deployment and station-keeping duties. It has long been considered that AUV platforms, in-principle, could provide an effective solution for surveillance (security and anti-terrorist), environmental monitoring and data portal (to sub-sea instruments) requirements, but limitations in battery life have limited AUV usefulness in such applications. The concept of a vehicle that would allow on-station recharging of batteries, using solar cells, has been presented as a means to significantly enhance the effectiveness of AUV platforms where long-term or ongoing deployment is required. The Solar Powered AUV (SAUV) is designed for continuous deployment (weeks to months) without requirement for recovery for service, maintenance or recharging. The SAUV under development is designed as a multi-mission platform to allow payload configuration by the end-user to optimize the SAUV for coastal/harbor monitoring, data portal (to moored sub-surface instruments) applications, or any other application where long-term deployment is required. The SAUV is designed to reside on the surface while recharging batteries and then to execute its programmed mission. While on the surface the SAUV is designed to communicate via Iridium® satellite or RF communications link to upload collected data and to allow reprogramming of mission profiles. Development of the SAUV has generated numerous engineering challenges in design of the solar recharge system, design of a propulsion/direction control system capable of handling the unique shape requirement, design of the telemetry system, and development of mission control algorithms that include surfacing and battery recharge requirements. This paper discusses the details of unique SAUV design requirements, specific engineering solutions for hull, panel, battery, communication, charge control, navigation, mission control, and propulsion systems. SAUV II System Description The SAUV II is a solar powered AUV designed for long endurance missions that require monitoring, surveillance, or station keeping, with real time bi-directional communications to shore. The vehicle can be pre-programmed to submerge to depths down to 500 meters, to transit to designated waypoints, or to operate on the surface during conditions suitable for battery charging via solar energy input. The SAUV II system functional requirements include: • Operate autonomously at sea for extended periods of time from weeks to months. Typical missions require operation at night and solar energy charging of batteries during daytime. • Communicate with a remote operator on a daily basis via Satellite phone, RF radio, or acoustic telemetry. • Recharge batteries daily using solar panels to convert solar energy to electrical energy. • Operate at depths to 500 meters. • Operate at speed up to about 3 knots when needed and cruise at speed of about 1 knot. • Battery system is to provide a total capacity of about 1500 whrs. • Acoustic altimeter capable of 100 meter altitude tracking and depth sensor to 500 meters. • Capability to acquire GPS updates when on the ocean surface. Capability to compute SAUV position at all times using GPS when on surface and Dead Reckoning when submerged. • Capability to maintain fixed depth and fixed altitude and to smoothly vary depth or altitude profile. • Capability of navigating between waypoints (latitude and longitude). • Capability to log and upload all sensor data correlated in time and SAUV geodetic position. • Provide sufficient volume, power, interfaces, and software hooks for future payload sensors. • Allow user to program missions easily using a Laptop PC. Allow user to checkout basic operation of SAUV system in the lab or aboard ship using the Laptop PC. • Provide for graphical display of mission and payload sensor data on Laptop PC. A block diagram of the system is shown in Figure 1. The vehicle sensors measure: pressure depth, acoustic altitude, speed, compass heading, pitch, roll, thruster current and various thruster actuator parameters. The main processor is a PC104 that communicates with and controls the various subsystems including the Energy Management Subsystem (EMS), the Navigation subsystem (NAV), the Propulsion and Motion Control Subsystems (PMC), and the Communication systems (COMMS) which include the FreeWave radio, the Iridium satellite phone and the acoustic modem. These will be discussed below. Figure 1. SAUV II Block Diagram Solid models of the vehicle’s major mechanical structures are shown in Figure 2. The wing section is a fiberglass shell filled with syntactic foam capable of 500 meter depths. The pressure tube is 8 inch ID and 46 inches long and houses the batteries and all dry electronics. The vectored thruster section attaches to the wing and includes the thrusters motor as well as the vertical and horizontal actuators. The fuselage is a flooded fiberglass shell that covers the pressure tube and also provides space for wet components such as acoustic altimeter, speed sensor, and depth sensor and payload sections. Figure 2. Solid Models of Major Structures Two BP 585 solar panels are mounted atop the wing section. These are each 85 watt highefficiency monocrystalline photovoltaic modules with the following specifications: BP585 Solar Panel Specifications Maximum power (Pmax): 85W Voltage at Pmax (Vmp): 18.0V Current at Pmax (lmp): 4.72A Warranted minimum Pmax: 80.8 Short-circuit current (Isc): 5.0A Open-circuit voltage (Voc): 22.1V Panel weight: 13.4 pounds The Current-voltage (I-V) curves for the solar panels is shown in Figure 3. Figure 3. I V Curve for BP 585 Solar Panels The overall SAUV II System Specifications are : Total Vehicle: Length = 78" Width = 47” Pressure tube: Length = 46” ID = 8” OD = 9” Weight in air: 370 lbs Buoyancy (net) = 2 lbs Max operating depth = 500 meters Speed: 0.75 to 3 knots Endurance: week to months Energy source: Li-Ion battery (2kWh) with Solar Panel charging at sea and Gas Gauge monitoring of battery system. Communications: Iridium Satellite phone FreeWave radio modem Acoustic modem Navigation: GPS with Dead Reckoning Thruster: Vectored thruster AUV sensors: Pressure Depth Acoustic Altitude Speed sensor Compass Pitch Roll thruster parameters Payloads: TBD Figures 4, 5, and 6 are photographs of the SAUV II at Buzzards Bay, Massachusetts during initial in water testing of vehicle systems. Figure 4. SAUV II on Trailer Figure 5. SAUV II Initial Trim & Balance Check Figure 6. SAUV II Enroute to Test Area Vectored Thruster Description A vectored thruster design was selected over a fin and thruster design for (1) better motion control when the vehicle is on the surface, (2) smoother transition from surface to underwater operation and (3) simplify the system. The technical features of the vectored thrusters include: • 12 bit absolute position sensors for angle feedback • High efficiency stainless steel ball screw actuators • Propeller shroud for safety and to deflect debris from propeller • High aspect ratio adjustable pitch propeller blades • High efficiency brushless dc thruster motor • +-20degree actuation capability • Single underwater cable to thruster The vectored thruster implemented on the SAUV II has the following components: