Underwater robots are a valuable asset for search-and-rescue teams, but at some point, the vehicles have to turn around and charge up.
Autonomous underwater vehicles, or AUVs, will eventually start to run low and have to return to a home base to recharge their batteries — an especially problematic limitation when the robots are exploring great distances like the deep sea.
A Purdue University team has an efficient solution in mind: What if the charging station traveled along with the robot?
Researchers, led by Nina Mahmoudian, an associate professor of mechanical engineering, have created a mobile docking system for AUVs, enabling the vehicles to perform longer tasks without the need for human intervention.
"My research focuses on persistent operation of robots in challenging environments," said Mahmoudian . "And there's no more challenging environment than underwater."
The Challenges of Underwater Exploration
Underwater exploration has always been a tough task for a marine robot.
Once an AUV submerges, the vehicle loses the ability to transmit and receive radio signals, including GPS data. The marine robot may use acoustic communication, but this method can be difficult and unreliable, especially for long-range transmissions.
"Typically these robots perform a pre-planned itinerary underwater," Mahmoudian said. "Then, they come to the surface and send out a signal to be retrieved."
Humans then have to go out, retrieve the robot, get the data, recharge the battery, and then send it back out — an inefficient process that limiting the amount of time that the robots can operate.
To solve the range limitations, Mahmoudian and her team created algorithms that, essentially, allow the mobile dock and the robot to coordinate with each other.
Without requiring any human assistance, the robot finds the station, recharges, and uploads data. The Purdue University-developed algorithms maximize trajectories to get the optimum use of the robots.
The researchers validated the method by testing the system on a short mission in Lake Superior. The details of the field experiment appeared in the July edition of IEEE Robotics and Automation Letters .
The key to the system, according to Mahmoudian, is that the docking station is portable.
"It can be deployed in a stationary location, but it can also be deployed on autonomous surface vehicles or even on other autonomous underwater vehicles," said Mahmoudian. "And it's designed to be platform-agnostic, so it can be utilized with any AUV. The hardware and software work hand-in-hand."
In fact, Mahmoudian compares the technology to an autonomous vacuum cleaner that you may be familiar with: The Roomba.
"An autonomous vacuum, like a Roomba, does its vacuum cleaning, and when it runs out of battery, it autonomously returns to its dock to get recharged," said the Purdue professor. "That's exactly what we are doing here, but the environment is much more challenging."
The Purdue team also has published papers on ways to adapt their docking system for AUVs that will explore extraterrestrial lakes, such as those of Jupiter and Saturn's moons.
If the mobile dock can successfully function in a challenging underwater environment, then Mahmoudian sees even greater horizons for this technology.
"This system can be used anywhere," said Mahmoudian.
In a short, edited Q&A with Tech Briefs below, Mahmoudian explains just where she imagines this technology headed, including the Arctic and even space.
Tech Briefs: What does the docking station look like, and what are its components?
Prof. Nina Mahmoudian: In short, the docking station follows a simplified funnel design approach. The station is a rigid frame consisting of several key design features:
1) A simplified flat funnel is mounted at a sweep angle and serve to guide the AUV into the dock along the horizontal plane.
2) A ramp (mounted at angle) pulls the AUV up into the docking station after it has been guided toward the dock.
3) Once the AUV is in the dock, a switchable magnet latches the vehicle into the station.
4) Power and data are transferred wirelessly through an inductive power module.
The unique docking station design has no exposed moving parts to reduce problems caused due to biofouling.
A more detailed figure is available in the patent file .
Tech Briefs: What kinds of strategic deployment of the docking station do you imagine for, say, a search and rescue mission? Do you deploy a bunch of them? Just one?
Prof. Nina Mahmoudian: Long-term long-range search and rescue operations could benefit from deployment of multiple of these docking stations when they are using a fleet of underwater robots. Our approach provides opportunity for both fixed and mobile charging and data transfer since the design is collapsible and portable. Our docking system is adaptable to a wide range of autonomous underwater vehicles.
Tech Briefs: Where did you test this docking station?
Prof. Nina Mahmoudian: The system has been tested in a pool and in open water of Eagle Harbor, Michigan. For the pool, the dock was attached to the pool wall or on the back of a small dinghy. For open water it was attached to a small dinghy.
Tech Briefs: What is the most difficult aspect of making this work as simply as, say, a Roomba?
Prof. Nina Mahmoudian: The biggest limitation with the system (based on published results so far) is that we have only validated the near-range terminal homing and that we have not fully integrated the power electronics into the dock yet. We are making great progress on the mid/long range navigation problem.
Tech Briefs: Is that what you're working on now? What’s next with your research?
Prof. Nina Mahmoudian: Going forward, once the mid/long range navigation problem is solved and power electronics are implemented, then there is nothing fundamentally stopping the docking station from being deployed in a wide range of scenarios for both fixed and mobile cases. We will utilize our mission planning algorithms that optimize the routes of the working robots and charging stations for persistent undersea operation with unmanned systems.
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