Traction properties of the wheels of an underwater crawler on different soils

• Published in: OCEANS 2007 pp. 1 - 7
• Main Authors: Waldmann, C., Richter, L.
• Format: Conference Proceeding
• Language: English
• Published: IEEE 2007
• Subjects: Crawlers; Force measurement; Mars; Oceans; Sea floor; Sea measurements; Soil measurements; Testing; Underwater vehicles; Wheels
• DOI: 10.1109/OCEANS.2007.4449354
• ISSN: 0197-7385

Crawlers are specialized vehicles that allow for underwater intervention by staying in direct contact with the seafloor. These are completely different operating conditions as compared to free flying underwater vehicles. In particular one can expect from this vehicle concept that it offers a very stable platform for conducting delicate measurements at the interface of seafloor and the water column. Additionally, crawlers lend themselves to long term observations with passive phases where the system can go into sleep mode. In space sciences crawlers are already well established platforms ("planetary rovers") and have successfully proven their capabilities in several missions to the Moon and to Mars. For ocean sciences there are still a number of technical problems that have to be solved before a widespread use of these types of platforms can be expected. The attractive properties of crawler systems led to several attempts in the development of wheeled or tracked vehicles. However, it is still an open discussion which type is better suited for underwater applications. The reason why in the case reported here a decision has been made in favor of a wheel driven system mainly goes back to the fact that wheeled system can be designed to be more energy efficient. This is of particular importance for this crawler system as it is supposed to operate autonomously in deep waters, with a fixed energy supply per deployment. As the sediment properties in most parts of the world oceans are unknown a model has to be developed of how to predict the performance of crawler systems under different conditions and adjust the weight in water accordingly or select the right type of wheels. This implies a good knowledge of the traction properties of the individual wheel design. As part of the bi-national funded project MOVE! (Moving Lander) we conducted a number of laboratory tests which were used to firm up the wheel design and predict the performance. But during the last field tests some unforeseen problems with the traction of the wheels showed up. It appeared that the traction has been less than expected in the environment encountered and the wheel had a tendency for digging in although from the sinkage of the wheel and visual observations this should not have been the case. Therefore a set of field tests on different soils in a shallow water environment have been designed and carried out. The objective was to gain more insight into the interaction of an individual wheel with different soil types to be able to come up with an improved model based on a more reliable data set. To have better control on the conditions where the experiment has been carried out a single wheel test set-up has been designed and realised. In particular it is important to control the attitude and the load on the wheel under test. By combining the measurement of the sinkage of the wheel, the pull force, the rotation rate and the translational speed with the electrical parameters of the motors all relevant input parameters for a subsequent modelling activity are at hand. In regard to the soil properties documentation a shear vane device has been used. The set-up proved to be most valuable for the envisaged goal. Employing the wheel performance model that has been developed at DLR in the context of Mars rovers an optimised concept for the operation of the underwater crawler will be presented.