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64 Digest | MARIN modular AUV mounted at the rear of the hull, which give an operating speed of 4.5 kph. It’s a limitation chosen for safety reasons, with higher speeds expected in future experiments when needed. All 12 electric thrusters use the T200 model from Blue Robotics, modified to include Hall effect sensors. “This sensor type was included in the thrusters in order to measure their revs,” Cozijn explains. “That gives us the possibility to design a control strategy based on rpm, and by extension on electric currents.” In addition to the modified thrusters, two ballast tanks are installed – one at the bow and the other at the stern – as well as a 5 kg mass that moves back and forth along a rail inside the middle of the mAUV. A piston inside each tank actuates the flow of water entering and leaving them, making the control of submerging and surfacing of the mAUV slightly more energy-efficient than using the thrusters. The 5 kg mass measures 10 cm across and is servo-actuated along a 1 m internal space, further contributing to the control of the vehicle’s tilt by changing the longitudinal CoG. Operations with the mAUV consume a maximum of 2 kW, the lion’s share of which is taken up by the thrusters. To ensure energy efficiency (in terms of the rate and accuracy with which power is allocated and consumed), a comprehensive blank-sheet approach was taken to writing the control algorithms and allocation logic needed for distributing the required forces across all the thrusters – from a mission-overview perspective down to the operational commands to the thrusters. “The control of the mAUV is divided into three layers,” De Kruif explains. “The mission is planned at the top layer, and a set of actions is defined that makes up the mission. Which actions are executed, and the order of them, depends on environmental and mission conditions. “The next layer is the mission execution layer, and the third is the vehicle control layer. The former provides the latter with set points for the craft’s position and velocities, while the latter ensures in real time that the set points are reached. “The onboard INS gives measurements for the real-time [angular] velocities and the absolute pose. Based on the difference between where we want to be and actually are [according to quaternion representations], a required adjustment force is calculated 20 times per second. This force is then divided between the thrusters as appropriate to minimise this difference.” A Windows PC is installed in the mAUV’s central module to run the functions needed for the layers. The code for the control systems was written in Python and C#, and the monitoring and recording of INS data and motor performances are handled by additional embedded software applications, which were developed using C# and C++. Future experiments are aimed at exploring how best to specify high-level mission goals such that the mAUV can perform them amid unforeseen events, through different AI-based approaches established in prior research. Hull design A design geometry known as the Joubert BB2 was chosen for the craft’s hull. This is a well-established generic submarine hull, and gives the mAUV a tubular, torpedo-like shape (with an elliptical bow and a parabolic stern), which is intended to minimise drag while maximising the amount of internal space for electronics. The key exceptions to this design, as indicated above, are the eight thruster tunnels installed near the front and rear, which replace any outward protrusions such as fins or rudders along the hull’s length. The hull’s main structure is made from aluminium, chosen partly because of its strength and durability but also to enable precise machine-cutting of each hull module. That makes it relatively easy (compared with other metals or manufacturing approaches) to accommodate changes in mounting points, dimensions and other variables between section configurations. “For manufacturing these large aluminium parts, including minor modifications to individual hull segments, we have good experience with a nearby subcontractor, which we also worked with in previous submarine testing projects,” says Van Der Schaaf. “Aluminium June/July 2020 | Unmanned Systems Technology Eight tunnel thrusters are used for movement and steering in the vertical and transverse axes