Toxic paint is used on the bottoms of large ships to prevent fouling, which is when a biofilm layer develops, decreasing the ship’s efficiency moving through the water. To further complicate the matter, the paint must be blasted off and replaced every 5 to 10 years, at which time literally tons of toxic waste is produced and needs to be disposed of.
A ship that operates with a clean underwater surface free from fouling — even thin biofilm layers — will operate so much more efficiently that potential savings can easily reach over five percent in fuel costs alone. Without the concern for fouling, a ship’s underwater coating can be engineered for corrosion protection and longevity rather than its need to eliminate the potential for biofouling.
To eliminate the requirement for toxic paint and its cleaning waste, there needed to be a method to “groom” the biofilm from the underwater portion of a ship. The idea was to create important changes for the ships being built, as well as for the environment. That’s where the HullBUG (Hull Bioinspired Underwater Grooming) concept originated.
Successful promotion of the HullBUG concept by SeaRobotics attracted the attention of the US Navy and specifically the Office of Naval Research (ONR). A successful proposal was submitted to the ONR for funding, and a project team established. The team currently includes SeaRobotics as system designer and integrator, the Naval Surface Warfare Center Carderock Division (NSWCCD) operating as the team management, and Florida Institute of Technology providing a strong knowledge and research base for understanding how the biofilms affect ship efficiencies and what is necessary to combat them.
“The most important feature of the HullBUG is its small size,” according to SeaRobotics Research Engineer Dr. Kenneth Holappa. “It is only about half a meter in length.” This was a necessary design feature to allow the vehicle to maneuver over the curved surface of the hull while continually maintaining close contact with the surface. An additional benefit of the small size was that it allowed a single operator to deploy the vehicle without the use of additional equipment such as a crane.
Because there are hazards associated with operating such a device underwater and in a harbor environment, occasionally a HullBUG might be lost or destroyed during operation. Keeping the size and cost of the system low definitely helped to eliminate damage as a major obstacle to implementation. So, from the very beginning of the project, small size and low cost have been identified as being critical to the satisfactory implementation of the HullBUG project.
Motion Control Components
The selection of the motors to drive the HullBUG involved a number of critical engineering constraints and compromises. SeaRobotics decided to make two basic models, one with wheels and one with tracks, and offer several options for keeping track of the system’s progress.
Sizing of the motors, for example, required a calculated estimate of the power, speed, and torque characteristics of the manufactured devices. Determining factors included the resistance caused from pushing the grooming tool across the surface of the ship, the hydrodynamic resistance of the vehicle itself as it moved through the water, friction losses in the shaft seals that were used to protect the motors from the saltwater, and track or wheel friction, dependent on which version of the unit was used.
“After extensive component research, we chose to use maxon motors and gearheads,” Dr. Holappa said. “Their motors not only provided a very cost effective solution, but they were highly efficient and extremely simple to implement.”
The company used EC Flat motors with planetary gearheads. Two motors were used on the tracked version of the HullBUG (one for each track), while four flat motors were needed for the wheeled version (one for each wheel). An additional EC Flat motor was implemented in the grooming tool. That motor used a simple spur gear for speed reduction. And a final motor, connected directly, was used for the negative pressure attachment device that held the HullBUG in place.
The EC series brushless motors incorporate ball bearings or ruby bearings that also add to the longevity of the motors — especially needed in such harsh conditions. The flat motors were designed specifically for robotics applications where size and weight are important selection criteria.
The EC45 flat motors selected for the HullBUG weigh only 75 grams. Continuous output power is 30 Watts, while the maximum speed is 10,000 rpm. The important specification for this application was torque. Even under the potentially harsh environments that the HullBUG would be engaged in, the EC45 offered a maximum continuous torque up to 56.2 mNm, depending on the winding chosen by the user. The motors are built to IP54 standards, which was important to the application. Furthermore, the motors were also available in the system voltage that SeaRobotics required for the HullBUG application.
According to Dr. Holappa, “The large load capacity of the shafts of the GP42 gearhead allowed the wheels to directly mount to the gearhead shaft, greatly reducing the complexity of the overall design of the system.”
Given that the HullBUG vehicle was to be completely autonomous, it had to be designed to operate for many hours on batteries (cables would get in the way of the grooming operation). In order to maximize battery life, the biofilm grooming had to be performed in the most efficient manner possible. Navigating in a random pattern may eventually get the job done, but not in a reasonable amount of time. Plus, a typical ship presents a very large underwater surface, often upwards of 3,000 square meters. To keep this amount of area groomed, a user would employ multiple HullBUG vehicles to operate at the same time and, consequently, require sophisticated coordinated navigation.
Navigating the HullBUG
A toolset of navigation modes has been created to allow multiple HullBUGs to efficiently groom a ship by dividing the ship’s underwater surface into regions. Numerous algorithms have been incorporated to accurately groom the ship in steps down to the turn of the bilge. Additional algorithms and associated sensors are used to allow efficient grooming of the flat bottom of the ship.
Miniature acoustic ranging sonar (MARS) is also an option for navigation control of the HullBUG. This is where a very small, close-range, pencil beam sonar was specifically developed to allow the vehicle to “see” an upcoming wall or cliff condition such as bilge keels and bow thrusters.
Another feedback mode uses encoder-based odometry. Hall sensor feedback from the motor is used as an encoder signal to accurately estimate odometry. Hall sensors were used instead of optical encoders due to size and cost, and provide better than 1-mm accuracy in the measurement of odometry with the motor/gearhead combination chosen.
Ongoing Software Development
An autonomous vehicle is often software-heavy in terms of engineering efforts once you’ve selected and implemented the proper motion control system. Getting smooth, reliable navigation maneuvers that result in accurate positioning in a widely varying environment was one of the more difficult challenges for the design team. Multiple layers of software were necessary for handling the number and variety of possible events that can occur during grooming. And, the proper organization of the control logic to allow extensibility of navigation behavior was the most difficult part of this complex system.
As the project moves forward and into the field, there will no doubt be additional issues that will crop up and need to be addressed. Even now, the vehicle must be able to reliably accomplish its task in a hostile environment and in an unmapped terrain. Then, it has to be able to return to the waterline of the ship for retrieval. This operation must be done repeatedly for days, months, and years, and with multiple systems in the water at the same time.
The vehicle is operational and the navigation software is working. The next primary focus will be the structuring of the interface to improve ease-of-use and allow non-engineering personnel to manage operations.
This article was contributed by maxon precision motors, Fall River, MA. For more information, Click Here