Oceanic probes float free — thanks to a brush-dc motor

Dec. 13, 2007
With the growing interest in brushless and permanent-magnet synchronous motors, one might get the feeling that the venerable brush-dc motor is on the decline.

Not so for a high-tech research probe that relies on brush-dc motors to keep it afloat as it drifts along in the sea.

Scientists involved with the Argo Project have deployed over 2,900 of the projected 3,000 freeranging Nemo probes, or floats, that measure water temperature and salinity at varying depths in oceans around the world. The selfcontained floats are driven only by ocean currents and can record data when below ice as well as in open waters. The floats surface at preset intervals to transmit the data via satellite.

Until now, oceanic measuring devices with data-collection systems were secured with cables or hawsers. The technique has proven reliable, but limits data accessibility. Collected information simply wasn’t available until after the devices were recovered and data was only from a single sample point. In contrast, Nemo floats use a different concept to control sampling depth during dives. A dc microdrive coupled to a novel flotation control mechanism handles float buoyancy using an idea borrowed from nature.

Many species of fish use a gas bladder to control their buoyancy. The simple process of regulating the amount of gas in this bladder lets them float at any given depth without expending energy. The floating ocean monitors needed a similar technique if they were to record data for the longest possible time.

Optimare Sensorsysteme AG, the German manufacturer of the Nemo floats, uses a hydraulic bladder filled with oil in place of the gas bladder. An oil-filled piston acts as reservoir and pump to drive oil into the bladder at the bottom of the probe. The oil expands the bladder, displacing heavier sea water. This reduces the density of the float so it can rise. Removing oil shrinks the bladder and the float sinks. Maintaining any specific depth is as easy as controlling the amount of oil in the bladder. As oil is practically noncompressible, flotation control can be maintained over any depth.

The piston pump in the Optimare float is handled by a 26-W dc micromotor and compact drive from Faulhaber Corp. The motor drives the piston through a flange-mounted planetary gear reduction of 1,526:1. Torque is sufficient to keep the piston pump working to depths of 2 km where water pressure is several thousand psi.

A brush-dc motor was chosen for its reliability over an operating temperature range from subzero to 25°C with prolonged periods at approximately 4°C. To maintain reliable probe operation, the motor must start even after extended rest periods. The brushdc motor starts at minimum voltage with only simple on-off and reversing switches in the control electronics. The 80% efficiency of the motor helps conserve battery power, an important factor for the life of a probe designed to operate for at least three years or 150 dive cycles. It’s possible the floating probes might reach five years of actual service. Because setting the float depth is quick, brush wear and tear was not considered a problem.

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