The nanoAUV has three actuator systems that enable propulsion, attitude stabilization and diving. These are multiple piezo-electrical thrusters, a mass-shift unit and a buoyancy engine.
Piezo-Electrical Thrusters
- four thrusters at the rear (main propulsion and dynamic attitude control)
- a thruster at the bow (deceleration)
- control via pulse-width modulation
- lower EM interference due to small electric currents
- no torques
- No rotating parts, which can be damaged
- lower losses (induced resistances)
- lower energy consumption
- lower flow resistance in the switched-off state compared to conventional drives
Mass-Shift Unit
- for static position trim (pitch/if necessary roll)
- energy-efficient ascent and descent, as the vehicle has taken a certain „glide angle“
- compensation of dynamic moments during gliding
Buoyancy Engine
- static change of buoyancy (rise/float/sink)
- energy-efficient for longer gliding runs, as the active drive is not required
- important for missions under the ice, since only dynamically diving vehicles have positive buoyancy and would automatically ascend in case of propulsion failure
Guidance and Control
The trajectory following control has the task to keep the nanoAUV on the planned trajectory. In addition to the planned trajecotry, the current state of the nanoAUV estimated by the navigation filter is used for this purpose. The controller calculates signals for the actuators based on the difference between the planned path and the actual state of the nanoAUV.
A key challenge is that the nanoAUV is underactuated. This means, for example, that it cannot be actuated perpendicular to its current orientation in the horizontal plane and moved accordingly. No thrusters are provided perpendicular to the longitudinal axis of the nanoAUV. The overriding goal in the design of the control system is to achieve the lowest possible energy consumption while not exceeding a defined maximum allowable deviation from the planned trajectory, which potentially varies from scenario to scenario.
The current concept provides for a control system consisting of three components. Based on a hydrodynamic model of the nanoAUV, a feedforward control is used to calculate promising actuating variables for the planned trajectory. In order to compensate for deviations of the model from reality and disturbances, such as flow, a closed-loop control with feedback is used in addition. This consists of a superimposed controller that makes specifications for yaw and pitch angles of the nanoAUV depending on the lateral and vertical deviation of the vehicle from the reference trajectory. This superimposed controller is also referred to as guidance. Guidance also ensures that the nanoAUV does not collide with new obstacles detected by environmental perception (collision avoidance). The specifications for pitch and yaw angles as well as the planned speed of the nanoAUV calculated by the guidance are controlled by a subordinate controller – the third component of the controller concept. For this purpose, a so-called linear quadratic controller (LQR) is used.