Development of bio-inspired autonomous underwater robots
Description of design
Unmanned Underwater Vehicles (UUV) have many applications, such as underwater inspection (oil platforms and pipelines), seafloor mapping, water quality measurement, harbour security and mine detection. We are developing a small, low-cost, efficient and flexible UUV. This underwater robot, named Galatea after a sea-nymph from Greek mythology, has a unique bio-inspired propulsion system.
As a basis for the shape of the body a modified Wortmann FX 71-L-150/20 airfoil was chosen, a laminar profile designed to have 20 percent of the chord length available for control surfaces. The body (size 1.2 x 0.8 x 0.2 m) is made of glass fibre reinforced plastic.
The propulsion system, based on undulating fin propulsion as used by rays and squids, is more silent and stable with higher manoeuvrability than classical propellers. A single fin, controlled by servomotors, is placed on each side of the robot. The side fins are for generating the propulsive force and for turning the robot around its vertical axis (yaw). Pitch and roll of the robot is accomplished by two ailerons at the back, both controlled by a single servomotor.
The heart of the robot consists of an augmented microcontroller, providing all the required communication ports to interface with the onboard communication, navigation and propulsion subsystems. The Galatea prototype robot is remotely controlled from shore using an underwater radio modem. The robot has a miniature, gyro-enhanced attitude, heading and reference system for 3D motion tracking.
Nine battery packs with a total capacity of 540 Wh supplies energy to all subsystems.
Due to its flexible and modular design, the Galatea platform can easily be equipped with various off-the-shelf (COTS) sensors, e.g. a simple camera for underwater inspection.
Klick here to download the Galatea poster.
Design approach
The design tasks can be thought of as consisting of the following three main parts:
Hull design:
Design of the hull and its interior is a trade-off between optimal hydrodynamic behaviour and minimum internal space required for payload and subsystems. In addition external pressure, buoyancy and construction are important design issues.
As cavitation does not occur for the assumed operating conditions, the low speed wind tunnel of AE has been used for determining the drag and stability at the various swim conditions. Subsequently, simple Reynolds scaling can be applied and the vehicle dynamics in water can be calculated.
Propulsion:
As there is not much theory available on undulating fin propulsion, a special test facility has been constructed in order to be able to study the amount of thrust that can be generated by a particular fin configuration. Many fin parameters were tested, including undulating frequencies, amplitudes and wave numbers. For this study it was needed that each of the 17 fin rays is controlled independently by a servomotor.
Control:
The company Xsens Technologies B.V., specialist in small and highly accurate 3D motion tracking products based upon miniature MEMS inertial sensor technology, supplies a miniature, gyro-enhanced Attitude and Heading Reference System.
A possibility for long-distance control from shore is through LF radio waves. The performance of a specially developed radio modem is currently investigated.
A first full prototype of the Galatea robot has been built and various swimming manoeuvres were tested. The current maximum speed amounts to 0.4 m/s, already sufficient for many applications.
Innovativeness
- The most innovative aspect of the Galatea UUV is its bio-mechanical propulsion system, being very stable, highly manoeuvrable and silent. In addition, stationary operation is possible with this propulsion, either in the water column by hovering or at the seafloor since landing and take off from the bottom is feasible without causing damage.
- The propulsion system is highly efficient and, combined with the lowest possible drag due to its airfoil shape, energy consumption is minimized.
- The bio-inspired propulsion generates less noise than the classical propellers, which makes Galatea a very silent platform to do acoustic measurements.
- Its flexible design makes the Galatea underwater robot very versatile with respect to all subsystems including the sensor package, which can easily be tailored to the customer’s application.
Example applications
- Algae growth in lakes prohibits their use for recreational purposes. Detailed 3D measurements with an autonomous Galatea equipped with the right chemical sensors can significantly contribute to understanding and modelling of these processes and thereby to the development of corresponding mitigation measures or even the prevention of algae growth.
- The so-called benthic zone is the ecological region consisting of the lowest part of the water column and the sediment surface. Organisms living in the benthic zone generally live in close relation with the superficial layer of the sediment. Therefore marine biologists study the benthic zone extensively. Efficient mapping of the benthic habitats can be accomplished with UUV’s equipped with an optical camera or a simple sonar system. Here, the Galatea platform is of particular interest because of its hovering and bottom dwelling capability.
- It is known that the increased noise levels in the underwater environment due to offshore wind farms at sea have a detrimental impact on marine life, especially marine mammals. The 4D mapping of the underwater noise level (3D space plus time) can easily be carried out with the silent Galatea platform when equipped with a hydrophone. Based on these measurements noise mitigation measures to protect marine life can be developed.
- Mapping properties of the water column, such as temperature, conductivity, current and turbidity, can easily be accomplished with autonomous Galatea platforms equipped with a relevant sensor suite. The 4D measurement of these parameters are crucial for pollution or water quality models of rivers, lakes, harbours and on the shallow part of the continental shelf.
For more information see: www.galatea-project.nl
