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45 also landing and marking with the accuracy desired,” Yeshurun explains. Picking the right GNSS-IMU is therefore critical to Civdrone, and it is still trialling at least five different GNSS suppliers’ navigation systems to be sure they use one that will consistently provide RTK-GNSS accuracy at the same level that workers on the ground normally use. Carrying out a marking operation starts with inputting a list of coordinates. Civdrone’s software then creates a flight plan for landing at those coordinates. The marking payload incorporates a three-axis gantry, to compensate for landings slightly ‘off’ from the desired marking GNSS point by mechanically aiming the marker stake above the ground before driving it into the soil. The UAV then takes off and flies to the next coordinate, repeating the process until a battery swap or stake refill is needed. The marking payload has been in development for just over a year, with the design, fabrication and assembly performed in-house at Civdrone’s labs using additive and CNC manufacturing. The current payload weighs 8 kg and carries 20 stakes, but a 10 kg version carrying 40 stakes is in development. “At the moment our system can fly and mark 20 points in 25 minutes – 12 minutes of flight time and 13 minutes of markings,” Yeshurun says. “We expect to achieve 40 markings in 40 minutes with the newer model and still land with more than 20% energy in our battery pack.” Autonomous excavation Since 2016, US company Built Robotics has been developing autonomous earth moving vehicles for various construction projects. The most recent involved working with US construction company Mortenson, using Built’s autonomous excavator to dig the foundations for wind turbines. “That involved cutting an inverted frustum [a portion of a cone or pyramid whose upper part has been cut off] to finished grade, about 9 ft deep and 30 ft wide, and managing the dirt piles, or spoil,” says Erol Ahmed, creative director at Built Robotics. “Mortenson have been investing heavily in augmenting human work with new tools. They have a technology division that scouts for opportunities for that, and they saw our tech as being a good fit.” Most construction sites work off detailed topographic site plans prepared for a given area. Built Robotics typically starts with these plans, loading the information into its software to use as a ‘task model’. Once on site, Built’s team sets a GPS point to add its own layer of geo-referencing data. The client provides the team with the target numbers needed for the excavation – how wide and how deep to dig, the location based on their GPS, and any additional metrics. That data is then paired with Built Robotics’ internal models of how to effectively excavate a given piece of land, and Built’s team calculates the custom process for the excavator to perform the task. “There are dozens of variables that can be tweaked according to the type of soil, the foundation style and needs of the specific project,” Ahmed says. “We use a combination of on-site cameras, and GPS and Lidar on the vehicle to create a spatial terrain map of the actual ground, which is used in the task model as well. “The excavator is always reporting Construction | Insight There are dozens of variables that can be tweaked according to the type of soil, the foundation style and the specific project’s needs Unmanned Systems Technology | December/January 2020 This Built Robotics excavator uses several IMUs as well as vision and Lidar to navigate autonomously and track the amount of earth it has moved (Courtesy of Built Robotics)