The city of Amsterdam envisions a future where fleets of autonomous boats cruise its canals to transport goods and people, collect trash, or self-assemble into floating stages and bridges. To further that vision, researchers have given new capabilities to their fleet of robotic boats that let them target and clasp onto each other and keep trying if they fail.
The Roboat autonomous robotic boats — rectangular hulls equipped with sensors, thrusters, microcontrollers, GPS modules, cameras, and other hardware — provide intelligent mobility on water to relieve congestion in the city’s busy streets. One of the project’s objectives is to create roboat units that provide on-demand transportation on waterways. Another objective is using the roboat units to automatically form “pop-up” structures such as foot bridges, performance stages, or even food markets. The structures could then automatically disassemble at set times and reform into target structures for different activities. Additionally, the roboat units could be used as agile sensors to gather data on the city’s infrastructure and air and water quality, among other things.
The roboats can move forward, backward, and laterally along a preprogrammed path and come equipped with advanced trajectory-tracking algorithms that enable them to identify and connect to docking stations. Control algorithms guide the roboats to the target where they automatically connect to a customized latching mechanism with millimeter precision. The roboat notices if it has missed the connection, backs up, and tries again.
Each roboat is equipped with latching mechanisms including ball and socket components on its front, back, and sides. The ball component resembles a badminton shuttlecock — a cone-shaped, rubber body with a metal ball at the end. The socket component is a wide funnel that guides the ball component into a receptor. Inside the funnel, a laser beam acts like a security system that detects when the ball crosses into the receptor. That activates a mechanism with three arms that closes around and captures the ball, while also sending a feedback signal to both roboats that the connection is complete.
On the software side, the roboats run on custom computer vision and control techniques. Each roboat has a LiDAR system and camera, so they can autonomously move from point to point. Each docking station — typically, an unmoving roboat — has a sheet of paper imprinted with an augmented reality tag, called an AprilTag, that resembles a simplified QR code. Commonly used for robotic applications, AprilTags enable robots to detect and compute their precise 3D position and orientation relative to the tag.
Both the AprilTags and cameras are located in the same locations in the center of the roboats. When a traveling roboat is roughly one or two meters away from the stationary AprilTag, the roboat calculates its position and orientation to the tag. Typically, this would generate a 3D map for boat motion including roll, pitch, and yaw (left and right). But an algorithm strips away everything except yaw. This produces an easy-to-compute 2D plane that measures the roboat camera’s distance away and distance left and right of the tag. Using that information, the roboat steers itself toward the tag. By keeping the camera and tag perfectly aligned, the roboat is able to precisely connect.
The funnel compensates for any misalignment in the roboat’s pitch (rocking up and down) and heave (vertical up and down), as canal waves are relatively small. If, however, the roboat goes beyond its calculated distance and doesn’t receive a feedback signal from the laser beam, it knows it has missed.
The researchers are now designing roboat units roughly four times the size of the current iterations to be more stable on water. An update to the funnel will include tentacle-like rubber grippers that tighten around the pin like a squid grasping its prey. That could help give the roboat units more control when they’re towing platforms or other roboats.
A system is being developed that displays the AprilTags on an LCD monitor that changes codes to signal multiple roboat units to assemble in a given order. At first, all roboat units will be given a code to stay exactly a meter apart. Then, the code changes to direct the first roboat to latch. After, the screen switches codes to order the next roboat to latch, and so on.
This type of autonomous docking could also be used beyond aquatic/naval systems including in-flight refueling, space docking, cargo container handling, and robot in-house recharging.