The search for alien life is often associated with distant planets. However, there are areas right on our doorstep about whose inhabitants we know less than about the surface of Mars. For example, the biological and chemical processes that take place in deep-sea trenches are still largely unknown. The 'Hades-ERC' research project therefore aims to provide new insights into these oceanic abysses.

The so-called hadal zone begins 6000 m below the water surface and extends to the sea floor. Its deepest point is in the western Pacific in the Mariana Trench, around 11 km below sea level. The ambient pressure, which increases by one bar for every 10 m of depth under water, is accordingly 600 to around 1070 bar at the deepest point.

Figure 2: The DC micromotors used in the submersible robot passed the practical tests for use in the extreme conditions of the hadal zone without any problems.

© Faulhaber

Manned dives to depths of several kilometers are possible in principle and have already been carried out, but they are complex, expensive and dangerous. Instead, an autonomous robot is used to take samples of the bottom sediment and bring them to the surface for examination. However, without further precautions, the collected microbes, which are adapted to the high pressure, would burst during the ascent due to the drop in pressure, making the sediment samples worthless.

The robot itself must also function reliably in every pressure range from the surface to the seabed so that it can successfully descend, collect the samples under pressure, preserve them with fixative and finally rise back to the surface together with the samples. The drive, the sensors and the tools that touch the sediment also have to perform various movements during this process and work autonomously under water for many hours.

After a practical test, the team decided to use just one particularly robust, compact and powerful motor type from Faulhaber with a suitable encoder and planetary gearbox. The motor used, from the 2342 ... CR series (Fig. 2), has a diameter of just 23 mm and a length of 42 mm and offers a continuous torque of up to 19 mNm as well as a high level of efficiency, meaning that the robot can dive autonomously for a long time on just one battery charge.

Engines working under high pressure

A normal motor would stop working far above the hadal zone - either because seawater ingress would destroy it from the inside or - even if it is sealed - the prevailing ambient pressure would crush it from the outside. Accordingly, there are two ways in which a DC-micromotor can fulfill its function under these conditions. One solution is to house the motor and other pressure-sensitive components in a pressure-resistant housing. The researchers at Hades-ERC were able to do this for many components with a pressure-resistant titanium cylinder. However, some components such as the motor and gearbox can only perform their work in contact with the environment being researched. This is why another solution is used here: it makes use of the fact that the force acting on the components is not determined by the total pressure per se, but by the pressure difference between the inside of the components and the external ambient pressure. The motor and gearbox are therefore located in a further cylinder filled with an inert liquid which, unlike salt water, does not restrict their function. This cylinder can adapt the internal pressure to the ambient pressure via a flexible membrane. The pressure equalization ensures that the high pressure prevails both inside and outside. This means that no force is exerted on the components.

With these highly reliable and pressure-resistant drives, the Hades ERC robot is ready to explore the ocean depths. The project is scheduled to run for five years and the first dives in three Pacific trenches - the Japan, Atacama and Kermandec trenches - will begin in fall 2018.

Authors:
Andreas Seegen is Head of Marketing at Faulhaber;
Marco Antoni is an employee of the Stutensee editorial office.