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, visit http://info.hotims.com/40436-324.