Surf Zone Robotic Platform

Editor's note: Reprinted with permission from the Naval Postgraduate School's CRUSER News.
By Frederick E. Gaghan, Director of Program Development, Applied Research Associates 

The near-shore environment is one of the most dynamic and technically challenging for both man and machine. Significant research efforts have been conducted to investigate sea-floor crawling robots, but they usually involve “water-proofing” a standard ground robot and attempting to operate it underwater. These designs often experience difficulties in maintaining positional accuracy or operability due to the water flow and wave action.

Over the course of several months, ARA studied two key engineering concepts for the Strategic Environmental Research and Development Program (SERDP) that directly affect the ability of a
robotic system to operate in the surf-zone (SZ); 1) platform hull shape and, 2) propulsion.

To address platform shape a study was completed of a horseshoe crab’s carapace as a biomimetic representation for the hull shape of a robotic system (Figure 1). It was hypothesized that a hull shape based on a horseshoe crab would provide the appropriate balance between lift and drag, and allow hydrodynamic forces in the Very Shallow Water (VSW)/SZ to assist in the ability to station keep and maneuver without the need for excessive weight or a complex propulsion system to achieve platform stability and traction. The study focused on answering the following questions:

• Can a biomimetic hull design provide better stability in the dynamic wave conditions found in the VSW/SZ?
• Is the required scale of this hull design sufficient for carrying a usable payload and other system components?
• What are the maximum flow vectors for which the biomimetic hull can remain effective?
• What are the resultant forces from those maximum flow vectors?
Figure 1 (baseline model)                    Figure 2 (Archimedes Screw)
Several biomimetic hulls were modeled and underwent simulated and empirical testing in a water channel. The empirical testing was used to validate the data obtained from the Computational
Fluid Dynamics simulations used to identify a more effective hull shape.

To address locomotive factors a separate study was completed using an Archimedes screw drive as the mode of propulsion to assess platform traction and mobility (Figure 2). An Archimedes screw was chosen because of its ability to operate in various mediums with varying flow rates. It was hypothesized that an Archimedes screw with optimal geometry could provide the tractive force to propel a robotic system. Archimedes screw drives have been successfully used on larger underwater robots, such as those found in the deep sea mining industry, but it has not been widely applied to a robotic system in the near shore environment.

A test bed was designed to measure the speed, forces, and displacements created by an Archimedes screw interacting with various mediums. Several drive designs with different barrel diameters, flange heights, and flange depths were empirically tested to record efficacies in a range of mediums, including water, sand, and pebbles to answer the following questions:
Figure 3
• Can the Archimedes screw drive be scaled appropriately for a small to medium robotic system?
• What performance characteristics (speed, efficiency, tractive force) would this system provide?
• How will the system perform across a variety of medium?

Results of the two studies confirmed that is possible to design a biomimetic hull shape to improve stability and an optimized geometry for an Archimedes screw that would provide good tractive force on the aquatic floor in the dynamic wave conditions found in the VSW/SZ (Figure 3).


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