TOWARDS AN AUTOMATED ACTIVE UUV DOCK ON A SLOWLY MOVING SUBMARINE
2. CURRENT DOCKING METHODS 1 STATIONARY DOCKS
Oceanographers are interested in ‘autonomous ocean sampling networks’ in which small UUVs (1 to 2 m long) continuously patrol and sample ocean proper- ties, periodically returning to a stationary dock to download data and recharge their batteries. They use docks that are either fixed or tethered to the ocean floor. Three types of docking capture methods are described: funnels [3,4,5,6,7] which minimize docking hardware on the UUV, a V latch on the nose of the UUV that engages a vertical cable on the dock [8], and an aircraft carrier landing arrangement in which the UUV drops a hook that must catch a transverse guide on a flat dock [9]. Ultrashort baseline acoustic [4,5,8,9], electromagnetic [6], and optical [3,7] posi- tion sensing methods are used, always with the UUV homing towards a passive dock displaying a source.
2.1(a) Sensing
Of the three sensing methods employed, acoustic hom- ing is the most conventional, has by far the longest range (several hundred meters or more), is omnidirec- tional, and is readily implemented with commercial- off-the-shelf components. However, it has low ac- curacy and slow update rates for final docking. In the above references, acoustic homing provided about
±0.5 m accuracy [4] for final docking.
Feezor et al [6] discuss the many advantages of electro- magnetic homing. Their source on the dock was three 64 cm diameter coils requiring 15 W of power each.
Their 2.2 m long UUV used three orthogonal 9 cm di- ameter receiver coils and was able to detect and home in on the dock from 25 to 30 m away with ±0.2 m ac- curacy. The system provides fast update rates and
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Warship 2011: Naval Submarines and UUVs, 29 – 30 June 2011, Bath, UK
the UUV knows both its distance and orientation rel- ative to the dock; indeed, it just follows the magnetic field lines right to the dock. They claim the system is not significantly effected by fouling, bubbles, float- ing organic matter, the surface, the sea floor, or steel vessels not directly in the homing path.
Of the two optical homing methods, Cowen et al [3]
use the simplest approach. They use a light bulb for a source at the apex of a funnel and ‘terminal guidance’
control. This simple method uses a four quadrant photodetector on the UUV which determines the lens quadrant in which the light intensity is greatest. The UUV then adjusts its orientation to keep the light in- tensity equal in each quadrant, which keeps the UUV pointing at the light source. Docking was successful if the light source was acquired in time for the UUV to adjust its approach. Acquisition ranges of 10 to 15 m in turbid harbor water and up to 30 m in clear water were obtained. They achieved positional ac- curacies of ±0.01 m. Park et al [7] used five light sources around the rim of their funnel and a mod- ern charge-coupled device camera, allowing the UUV to estimate distance and orientation from the dock.
Although their tests took place in clear tank water without current or wave disturbance, they had less success docking than did Cowen et al, reflecting the more complicated approaches they were attempting.
In general, acoustic homing provides the best long range sensing and will need to be part of any UUV docking solution that begins with separation distances exceeding about 50 m. For final docking, optical tracking provides the best accuracy (at the expense of range) with excellent update rates for following un- steady trajectories. Electromagnetic sensing overlaps the previous two methods and provides an alternative when, for example, thermoclines or turbidity are an issue (Baiden et al [10] examine the effect of turbidity on underwater optical communication).
2.1(b) Capture
Funnels were the most common capture mechanism for the stationary docks reviewed here, with openings that were approximately a meter in diameter (four UUV hull diameters [5]). This requires less exter- nal hardware on each vehicle than the other methods, which is efficient. However, funnels work only if the UUV is adequately aligned with the funnel axis on entry. Otherwise, the UUV just bounces off the side of the funnel. Park et al [7] put a lot of effort into attitude control. Feezor et al [6] noted that docking failed when the UUV was more than 30 degrees off the dock axis when the source was acquired. Cowen et al [3] attributed their failures to acquiring the source late enough that the tail fins of their Odyssey IIB did not allow the vehicle to turn rapidly enough; if
it could have turned more sharply, perhaps misalign- ment would have been a problem anyway.
Fixed funnels are problematic in the presence of cross- flow caused by wave disturbance, tidal currents if the dock is fixed, or platform motion if the dock is on a vessel. In their ocean tests, Cowen et al [3]
aligned their funnel axis with the current and then hand launched the UUV up-current towards the dock from about 40 m away, so the UUV did not have to deal with a crossflow or do path planning. It is possi- ble to have a moored funnel automatically align with the current in a realistic scenario, but then the dock must communicate its alignment to the UUV in some way. The funnels used in the other experiments were fixed in the ocean, in convenient bays or harbors, ex- cept for Park et al [7] where the experiments took place in a quiescent maneuvering basin.
The moored vertical cable used by Singh et al [8]
was apparently successful but requires a horizontal V catching device and latch on the nose of the UUV.
The dimensions of this device are not given but would need to be as large as the positional error in the ultra- short baseline acoustic homing system they use. In- terestingly, they propose a next generation device in which a side arm/latch extends out from the side of the UUV to catch the cable as the UUV slides by.
Although this could be retractable, one would be re- quired for each UUV using the dock. The big advan- tage to this system is its inherent omni-directionality and insensitivity to vertical positioning error while docking. Crossflow is still a problem but, after sev- eral failed passes at the cable, a UUV should be able to figure out which direction a current, at least, is coming from and adjust its approach accordingly.
The ‘aircraft carrier landing’ method described by Ka- wasaki et al [9] not only requires specialized hardware on the UUV but also involves dangling a hook, which can catch on anything.
2.1(c) Success Rates
Docking success is not always reported clearly since these are research trials in which new equipment and techniques are often being tried for the first time.
The Woods Hole Oceanographic Institution studies conscientiously reported success and failure. After ini- tial trials and adopting various performance improve- ments using a REMUS 100 UUV, acoustic homing, and a 1 m diameter funnel, Stokey et al [4] achieved about a 62% success rate per docking attempt, or an 88% success rate per mission, where each mission is defined as five docking attempts. Allen et al [5], who used an updated version of this system, including a slightly smaller rectangular funnel, report a decreased
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Warship 2011: Naval Submarines and UUVs, 29 – 30 June 2011, Bath, UK
mission success rate of 60%. The tests by Stokey et al and Allen et al each spanned several days in different locations.
Feezor et al [6] used a 2.2 m long SeaGrant Odyssey IIB UUV, electromagnetic homing, and a 1 m diam- eter funnel to dock successfully for five out of eight docking attempts. This was done in one location over a two week period. The UUV speed varied from 1.5 to 2 m/s and docking took place in the presence of prevailing cross currents as large as 0.3 m/s. Docking failed when the UUV was misaligned with the dock axis by more than 30 degrees when the source was first acquired.
Cowen et al [3] used a 2.2 m long SeaGrant Odyssey IIB UUV, optical terminal guidance, and a funnel of undisclosed size in a simplified scenario to achieve suc- cessful docking providing the optical source was ac- quired soon enough.
Park et al [7] used a 1.2 m long UUV, a 1 m diame- ter funnel, and optical vision guidance in a quiescent maneuvering basin. They provide a frank discussion of the issues they faced but give no final feel for their success rate.
These success rates are not high enough to justify routinely risking the million dollar UUVs naval ves- sels would employ. Success rates close to 100% are required, and they are required in more challenging environments than the above tests experienced.
One wonders how much more successful the trials us- ing funnels would have been had the docks had the ability to keep the funnel axes pointing at the oncom- ing UUV. This would require additional complexity and infrastructure, but on the dock rather than on every UUV that is deployed.
2.2 DOCKING WITH NAVAL PLATFORMS Naval capabilities are not discussed as freely in the open literature as the oceanographic research de- scribed above. Seizer [11] notes that there are cur- rently no autonomous systems for surface ship launch and recovery of UUVs. The same is likely true for submarines. The Director of Innovation in the US Office of Naval Research recently identified:
• sea state,
• operational tempo,
• autonomy,
• motion prediction, and
• UUV maneuvering and control authority
as challenging objectives for UUV recovery by Naval vessels [12].
Surface ships currently recover UUVs, when the sea state allows it, using ramps, slings, and/or cranes with man-in-the-loop control to anticipate and correct for large relative motion between the UUV and docking apparatus. We are aware of three proprietary sys- tems that attempt to recover UUVs as autonomously as possible to surface ships. These are under devel- opment and/or are unproven. They use a reasonably maneuverable surface ship to close with the UUV and deploy a towed body over the stern. Seizer [11] de- scribes the Advanced Technology & Research Corpo- ration’s method in some detail. Using tow bodies with a submarine is risky, but possible if the tow is short.
Man-in-the-loop submarine UUV recovery methods are available or under development [13,14]. They de- ploy a remotely operated vehicle (ROV) from one tor- pedo tube which attaches to and maneuvers the UUV into a second tube. This takes two torpedo tubes out of action and restricts the UUV to 21 inches in di- ameter in current submarines, which restricts UUV endurance. The Saab system [13] requires both the UUV and submarine to sit on the bottom during dock- ing, so it is neither a deep water capability nor one that would be reliable amongst the fjords of Canada’s west coast. The US Naval Underwater Warfare Cen- ter (Newport) system [14] is similar but is intended for use in deep water while the vehicles are underway or hovering. A secondary benefit of these systems is that a deployable ROV provides the submarine with many other capabilities.
The US is pursuing other UUV docking options for submarines. The ‘long-term mine reconnaissance sys- tem’ (LMRS) is a torpedo tube launch and recov- ery UUV requiring, again, the use of two torpedo tubes. UUV recovery involves a telescoping robotic arm extending from one tube, docking with the UUV, and then inserting the UUV back into a second tube.
Homing and docking with this system was success- fully demonstrated in January 2006 [15]. However, the project has been discontinued, perhaps because of cost and the limited endurance provided by the torpedo tube sized UUVs. Schuette [12] discusses the ‘universal launch & recovery module’ (ULRM) which is intended to deploy UUVs from large diame- ter SSGN or Virginia class SSN missile tubes, which will allow for UUVs with greater endurance. An ini- tial demonstration is scheduled for 2012. No mention is made of how docking will take place.
The ROV docking solution may be the best current solution for submarines if it can be made functional while the submarine is underway, especially since it allows for far-field docking which avoids the flow nonuniformities, wakes, and vortices close to the sub- marine hull [16]. However, it lacks automation and will have difficulty with environmental disturbance.
c 2011 Defence Research and Development Canada
Warship 2011: Naval Submarines and UUVs, 29 – 30 June 2011, Bath, UK