Autonomous Underwater Vehicle (AUV) - Propulsion
Autonomous Underwater Vehicles (AUVs) generally carry their own onboard power source such as batteries or fuel cells, further differentiating them from ROVs. The onboard power source must power both the vehicle's propulsion as well as the onboard instrumentation. To minimize the power used for propulsion and maximize battery life and mission duration almost all AUVs take the shape of a torpedo. This torpedo shape minimizes the vehicle's drag in the water.
The torpedo design conserves power, allowing for longer missions given a fixed amount of power. AUVs of this design are typically capable of two-day long missions, although advances in battery and fuel cell technology promise longer duration missions in the future. Of these torpedo-like AUVs, there are some generally shared features among all of them.
For example, propulsion derives from a variable speed propeller driven by an electric motor near or at the rear of the vehicle. Flight surfaces, e.g., an elevator and rudder are generally mounted just forward of the propeller and may be replicated in whole or in part farther forward on the vehicle, depending on the mission. In some cases, flight surfaces may be absent and the main propeller or propulsion system is articulated so as to direct thrust as needed for steering. While there do exist AUVs with thrusters, these are generally smaller, lighter vehicles for shallow water applications such as, e.g., hull inspection. Typical torpedo-like AUVs do not have thrusters but rather depend on hydrostatic forces of water moving over their adjustable flight surfaces (wings) to adjust their heading and/or depth.
Future UUVs will have to carry or harvest all the fuel required to power the vehicle’s propulsion, payloads, sensors, and onboard computers while maintaining continuous, unassisted, fully submerged, and very quiet operation.
This is an enormous challenge, considering that future UUVs will need to operate continuously for more than 60 days without refueling; by comparison, using today’s best batteries, such vehicles only would be able to operate for fewer than five days before requiring a recharge. In addition, the engines or motors used to push the vehicles through the water will need to be upward of three times more efficient than today’s best automobile engines, all while being much more reliable.
The technology solutions considered to address these challenges focus on fuel cells, including solid oxide fuel cells (SOFC) and polymer electrolyte membrane (PEM) cells. Fuel cells and hybridized approaches with batteries and/or capacitors have the potential to meet both the near-term and far-term UUV energy requirements — which will be in excess of 500 Wh/l (energy density) and 500 Wh/kg (specific energy), including the necessary fuel and oxidizer sources, all while being inherently safe so that they will not impose undue hazards to personnel during handling and storage.
Energy management and efficient propulsion remains a fundamental limitation of UUVs. As more stress is placed on autonomy requiring more power intense sensors and computing, not having to compromise range and duration will necessitate the most efficient use of power for propulsion. What is performed currently to design a UUV propulsion system is a market survey and piecing together the adequate components. This methodology might provide a propulsion system for the UUV, but it is often far from optimized for the UUVs structure, mission, and size, weight, and power (SWaP) requirements. Due to this increased Navy need, the Navy seeks the development of a propulsion system design toolkit that is parametrically validated through prototype evaluation and resulting in a Fleet-delivered design.
The intention of the design tool and ultimate propulsion system design is to optimize and increase the overall propulsion efficiency and reduce the noise signature of underwater vehicles. The new propulsion system design tool will ensure scalable performance when applied to different UUVs sizes, from micro-UUVs to Large Diameter Unmanned Underwater Vehicles (LDUUVs). The model will integrate the following components into a single simulated system: electrical energy storage (batteries or equivalent) system, transformation and distribution (electrical/electronic components) systems, conversion into mechanical energy (harmonic drive), including any energy transfer losses, and final conversion into effective thrust and vehicle operation. It is expected that the model will provide a multi-objective optimization algorithm that will iteratively act on the physical and geometrical parameters of each virtual prototype component converging onto the optimum characteristics of the propulsion plant as a whole.
This type of approach is fundamentally different from a traditional design approach, where each component is designed and optimized individually, but when assembled as a system, it does not provide the most efficient and lowest radiated noise approach. Additionally, this traditional approach methodology ignores important interaction effects that may prevent the convergence on the best overall performance of the system. The proposed integrated co-simulation approach is the key to enable the evaluation of several non-linear interactions, such as dynamic effects due to transients, which are typically neglected in traditional design approaches based on steady-state performance characteristics.
The use of widely recognized, open-source, high-level interface protocols will ensure the best compatibility and interfacing capability of the numerical propulsion system simulator with existing and future modules/components. The system development capability will be based upon high-fidelity physics-based dynamic simulation models of the whole propulsion chain, starting from the propulsion power supply, and continuing to, and including, the propeller. In using the modeling capability, it is expected that it will facilitate the investigation of new propulsor technology using unconventional blade designs, including either open or ducted propellers. Some possible propeller solutions include those with high rake and tip skew, tip loaded propellers, and newer unconventional blade sections with reverse camber. The basis for the decision for which propeller design is included in the system will be based on optimum and efficient performance in transitional flow.
Further, it is expected that the model will also facilitate the investigation of prime movers offering high torque at low speed, and ensuring high efficiency and silent operation, including those prime movers (i.e. motors) custom designed for the particular applications. Some examples of the applicable recipient systems include the current Knifefish vehicle being used for mine detection, localization, and identification; and the Large Diameter UUV, which is 48” in diameter and offers a payload capacity that lends the vehicle to multiple missions.
|
NEWSLETTER
|
| Join the GlobalSecurity.org mailing list |
|
|
|
