Robust DC Motor Drives Deep-Sea Sensors
Research often means coping with extreme conditions to retrieve the right data. Marine research is a case in point. Underwater sensors that plumb the depths of the world’s ocean have to withstand not only temperature differences but also extreme pressures, yet still deliver
reliable readings. Up until now, measuring devices secured to cables or hawsers have generally been used to record such data. This technique is proven and reliable, but the disadvantage is that the data does not become available until after the devices have been recovered. A different concept used in conjunction with state-of-the-art microdrive systems now allows completely independent recording by free-ranging underwater sensors and virtually online data transmission.
As part of the international Argo project to monitor the oceans of the world, around 2,500 free-ranging floats have been deployed so far. In the final phase of the research project, the aim is to have 3,000 permanently deployable floats. The main job of all the sensors is to measure water temperature and salt levels at different depths. Other readings such as clouding of the water and plankton content can be added at a later stage as required. German sensor manufacturer Optimare Sensorsysteme AG, Bremerhaven, is responsible for building and servicing the NEMO floats contributed by Germany. To make the floats as universally productive as possible, they were designed as independently operating units. This means they are free-ranging, driven only by the currents of the ocean, and capable of recording data below ice as well as from a range of configurable depths. In order to transmit the data via satellite, they come to the surface at pre-deter-mined intervals. To achieve the necessary level of reliability and absolutely maintenance-free depth control, Optimare works with German compact drive systems manufacturer FAULHABER. This is how they managed to come up with a robust, compact system for precise depth control of the underwater sensors.
Putting a biological principle to technical use
Since the beginning of time, many species of fish have used a gas bladder to control their buoyancy in the water. This allows them to float in the water without expending any extra energy, by a simple process of regulating the amount of gas in the bladder. This is also precisely what is required of the monitoring floats if they are to record data for the longest possible periods of time. A hydraulic piston filled with oil is the main flotation device. As oil is practically non-compressible, flotation can be maintained at almost any depth. In order to be able to steer the underwater sensor to the desired depth, the amount of oil in the swim bubble is varied via the piston. Depending on how well the bladder is filled, the overall density of the sensor changes and it sinks, floats or rises to the surface. A control piston serves as the drive element, by varying the gas pressure. To add the necessary muscle to these sensors, a DC micromotor performing at around 26 W drives the piston. Thanks to flange-mounted planetary gears with a reduction ratio of 1526 : 1 and a secondary spindle drive, the torque is sufficient to keep it working even at depths of 2000 meters. The motor produced by the micromotor specialists from Schönaich by its very nature has the best possible credentials for the job.
DC drive still current
Timo Witte, Project Manager for developing the NEMO floats, has this to say about it: “For our purposes, the properties of the DC brush motor are ideal. It starts up even with minimal voltage, and the controls are very simple to install in the on-board electronics as an on/off switch”. This highly reliable standard motor, with compact dimensions of 35 mm in diameter and 57 mm in length teamed with high-performance output, speaks for itself. Its high efficiency – for a DC motor – of around 80% saves on power reserves. Timo Witte cites another key factor in the selection of this drive system: "Particularly crucial for any free-ranging float is trouble-free start-up, even after long rest periods. This float is guaranteed to work for at least 3 years or for up to 150 dive cycles, but a service life of up to 5 years is quite on the cards. This means the motor has to be reliable enough to work for that period of time." As the actual transition time for setting the depth is only brief, brush wear and tear is not an issue, but the advantage of having a simple, reliable drive is huge. Temperature variations of more than 25 °C in the tropics through to subzero levels in the polar region or around 4 °C for long periods in deep-sea settings will not cause this standard motor any bother. The matching step-down gear is just like one of the standard gears in the catalogue, apart from its modified lubricating grease filler. State-of-the-art microdrives are now almost universally applicable to any field. In many cases a standard drive is sufficient; for tougher requirements it is no problem to produce perfectly adapted, customized versions.