Smooth sailing with magnetic sensors

Dec. 1, 2006
Determining speed is easy on land, where drivers can look outside their cars (or at their speedometer) and estimate their speed by how quickly the scenery

Determining speed is easy on land, where drivers can look outside their cars (or at their speedometer) and estimate their speed by how quickly the scenery is passing by. Oceangoing vessels don't have this easy visual gauge, however, as the flat horizon does not change. The surest way to determine speed on fixed propeller marine vessels is to measure shaft speed, since the shaft is direct coupled to the engine and has the propeller affixed to it. Using custom magnets has helped International Marine Systems (IMS), Schriever, La., measure such speed without imparting stresses onto the shaft.

Andreas Gottschalk, engineering manager of IMS, says the company wraps magnetic tape around the circumference of the shaft. A magnetic pick-up sensor is then installed, so that when the shaft is spinning, the sensor picks up the alternating (north/south) domains.

“It creates a sine wave,” says Gottschalk. “When it spins faster the frequency of the wave changes. Our electronics can take the frequency of the wave generated and determine how fast the shaft is moving.”

The tape is a custom-designed magnetic sheet with an adhesive back that is produced by The Electrodyne Co., Cincinnati. The sheet has high-energy magnetic fields or stripes within it that have alternating north and south poles to allow a magnetic sensor to register the poles as they pass by.

When the shaft is idle, no poles pass the sensor. As the shaft begins to turn, the time delay between each passing of the poles is measured and indicated. Relatively long time periods between poles equate to slow shaft (and propeller) speed. As the shaft speed increases, the time between each passing of poles decreases, indicating faster shaft (and propeller) speeds.

The stripes are spaced ¼ in. apart for continuous speed-sensing data. They use high-energy Plastalloy magnetic material that is easily read by the sensors — even at high rates of speed and despite their close proximity to each other. Skewing of the stripes allows the sensors to determine the direction of the rotation or whether the vessel is going forward or in reverse.

Alternatives sink to the bottom

Most importantly, the magnetic tape imparts no stress onto the vessel's shaft. Alternate methods of determining shaft speed require attaching targets to the shaft that can be counted as they pass by. One method is to weld the targets onto the shaft and monitor them with a proximity sensor. This system works but imparts stress onto the shaft. Any flaws or defects can be aggravated when the shaft is stressed under load, producing disastrous effects. Using fewer targets lightens this stress but also reduces the effectiveness of speed sensing, because there are less targets to count. One can also bolt a collar onto the shaft, which is less damaging than welding, but still imparts stresses onto the shaft and provides poor speed calculations.

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