The solid-state compass

Go aboard any tall ship – such as the Coast Guard cutter Eagle – and near the wheel is an impressive magnetic compass. With its floating card marked off in degrees and the traditional fleur-de-lis to indicate magnetic north, many magnetic compasses are functional works of art.
Unlike the showy magnetic compass, electronic compasses are little more than black boxes, often placed in out-of-the-way compartments – all but invisible. For a long time, electronic compasses on voyaging boats were a version of something called a fluxgate compass. Now, the steady advance of technology has produced a new type of solid-state, electronic heading sensor. With no moving parts, these electronic compasses are not only more reliable, they are also free of the errors that can hamper the traditional fluxgate.

Fluxgate technology, which has been in use since the World War II era, uses a sensing core composed of an iron doughnut wrapped with coils of wire. A current is sent flowing down the wire coils. If the sensing core is stationary, then current flowing through the wire remains steady. Should the assembly move, however, an additional current will be induced in the wire as the core moves through the earth’s magnetic field (recall that when a conductor is moved through a magnetic field, a current flow is induced in the conductor – this is how, for example, alternators generate electricity). So the added electricity coming out of the core is caused by the movement of the fluxgate through the earth’s magnetic field. By measuring the amount of this added electricity and its phase, a fluxgate compass senses direction.

To measure the field properly, the fluxgate core needs to sense the field in the horizontal plane. As we know, boats are rarely level. To combat the motion of the boat, fluxgate compass designers will put the sensing core on a pendulum-like gimbal. Another technique is to place the core in damping oil. This slows down the motion of the fluxgate core, making it less likely to jump around with every motion of the boat. Fluxgate engineers can’t use too viscous an oil, however, because then the core can’t react to the pitch and roll of the boat.

One solution to the problem of keeping the core measuring accurately when on a moving platform like a boat was first worked on in airplane avionics. Instead of using a single fluxgate sensor, this approach involves a suite of solid-state sensors, small devices built onto silicon chips. These sensors belong to a class of tiny devices called microelectromechanical systems (MEMs). Aiding the MEMs sensors in an electronic compass are microprocessors and the software to integrate the MEMs output.

A marine electronics company that offers such a “silicon compass�VbCrLf is Maretron. The company’s SSC200 Solid State Compass has no mechanical or electromechanical moving parts – it uses a combination of three sensors to measure the earth’s magnetic field and the motions of the boat. “Several things have come together to make these types of compasses possible,�VbCrLf Rich Gauer, president of Maretron, said. “They have been used in aerospace for a few years now.�VbCrLf

For gauging the earth’s magnetic field the SSC200 has a solid-state, three-axis magnetometer. This unit can detect the geomagnetic field and determine the orientation of the compass in relation to magnetic north. Just like the gimbaled fluxgate, however, the magnetometer needs to measure the horizontal component of the geomagnetic field. But instead of compensating for the tilt and roll of the boat mechanically like a fluxgate, Maretron electronically compensates for vessel motion.

The second component of the SSC200 is tiny devices called accelerometers. These units measure the motion of the boat in its pitch and roll axes. They can sense, for example, that the boat has rolled 10� to starboard and also pitched forward 8�. Using this information, the SSC200 processor can electronically factor out the boat’s motion as if the magnetometer unit were level.

The third MEMs sensing element is a solid-state rate gyro. Unlike a gyrocompass that will show a user his heading related to true north, a rate gyro doesn’t know direction, but only senses change in direction. This unit detects when the SSC200 is turning and can determine the rate of turn. When this data is combined with data from the magnetometer and the accelerometers, the result is heading, pitch, roll and rate of turn data that are put out over a boat’s network in ether NMEA 0183 or NMEA 2000 format.

The quiet revolution in electronics continues inside the closed world of the black boxes. The solid-state silicon chip approach to an electronic compass embodied by the Mareton SSC200 is no doubt the future for electronic compasses on voyaging boats. The result is a better compass for providing heading information to the boat’s systems.

And while the technical advancement of electronic compasses is a great development for voyagers, let’s hope the magnetic card compass, that age-old symbol of marine navigation, is still carried aboard voyaging boats for many years to come.

By Ocean Navigator