62 items such as tyres or pipes, but in principle the map serves as a reference point on which to overlay other data. This other data comes from the Matrice 210 and the Mini Tortuga. The former has a power and data tether to the USV for persistent flight as well as to receive navigation commands for sweeping overhead the USV. Its altitude enables wide visual scanning of the water, which (when clear enough) allows the UAV to spot groups of litter and assign them RTK-GNSS coordinates. Regardless of water condition, it can also look for surface obstacles to aid the USV’s collision avoidance. The litter’s RTK-GNSS coordinates are transmitted to the Mini Tortuga via the SeaCAT, so that the ROV can travel to them (the SeaCAT tracks its own GNSS position, while the Mini Tortuga localises itself relative to the USV using its BluePrint SeaTrac USBL). The ROV uses its 2D sonar – either a BluePrint Oculus or a Teledyne BlueView M900 – to pinpoint the location, orientation, size and shape of litter. That data is added to the reference map and transmitted to the Tortuga, which navigates to the litter’s location via USBL (BluePrint’s SeaTrac or Applied Acoustics’ Easytrak) and its DVL (a WaterLinked A50 or Teledyne WayFinder). It grabs the litter using a customised suction-based gripper, avoiding issues with litter that standard manipulators might struggle with. The Tortuga lifts up to 10 kg of litter at a time, which is deposited into a customdesigned basket that the USV lowers by winch to the seafloor. The system has the capacity and speed to remove up to 300 kg a day. The basket’s opening has a gate made from organic fibres to prevent litter floating or falling out, and also features acoustic beacons to help the Tortuga localise and approach it. “Most of the machine vision for spotting, classifying and grabbing litter, spotting obstacles and so on have been developed by university partners,” Chardard explains. “The SeaCAT’s own autonomous navigation and comms software was developed by us, with litter identification and categorisation mostly by the University of Delft. Meanwhile, the automatic flagging of litter on the reference maps came from the Technical University of Cluj-Napoca in Romania, intelligent coordination between the USV and the ROVs was written by Fraunhofer, and algorithms such as automatic landing for the UAV were provided by the University of Dubrovnik.” As well as housing all this intelligent software for commands and control, the SeaCAT supplies up to 10 kVA of power for the Tortuga and up to 3 kW for the Mini Tortuga, along with 1 kW for the UAV, although each robot can be stowed when not needed, to avoid overburdening the generator. The SeaCAT also moves very slowly (if at all) when its robot subordinates are sighting and collecting refuse, which helps to balance out conflicting power needs. On top of that, Chardard explains that the onboard CPU and GPU are quite low-duty systems, and the SeaCAT itself requires no power-hungry electronics, as GNSS, AIS and the UAV’s mapping provide for all its navigation and collision avoidance needs. The operators meanwhile monitor the mission via the tablet and a 5 GHz wi-fi link to the SeaCAT, which has dualredundant wi-fi antennas with a range of up to 5 km (although Chardard notes that these missions rarely need to go more than 1 km from the shoreline to find the vast majority of ocean litter). The tablet has joysticks if operators feel the need to take remote control of the USV or any of its subordinate robots. Diesel-electric power As mentioned, the SeaCAT runs on a diesel generator, which produces up to 12 kW. That is enough for two unidirectional Fischer Panda Aziprop thrusters and speeds of up to 6 knots, as well as for all control, navigation, comms and other electronics. Two tanks containing a total of 200 litres of fuel provide the USV’s endurance. “It’s quite a simple powertrain, and the electric propulsion is a good choice for us in control terms, as it’s much easier to control an electric thruster than a gas outboard or similar marine engine,” Chardard says. “With this being an environmental mission, we would of course have loved to use a totally zero-carbon power system, but for the project to demonstrate autonomy as a viable long-term solution for clearing up coastline litter, we needed a much longer endurance than batteries alone could give.” Alternative zero or low-carbon options were found wanting. For instance, designs and simulations of a hydrogen fuel cell powertrain were more complicated and expensive than the current power configuration. “If the SeaClear project is to offer a solution that any coastal authority can afford, we can’t drive up the price and then cut the endurance by, say, twothirds,” Chardard says. The company has also considered October/November 2023 | Uncrewed Systems Technology The Mini Tortuga is deployed from the front of the SeaCAT, where it finds and confirms the location of litter, with the USV using that data to direct the larger Tortuga for collection
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