Bookmark and Share Email this page Email Print this page Print

Gyro developments

Jan 1, 2003

For most vessels, the choice for a compass boils down to either the classic wet card magnetic unit or a fluxgate electronic device. Larger vessels have the added option of a gyrocompass. Advances in technology, however, have given mariners some new possibilities: a combined fluxgate/gyro model and a fiber optic gyrocompass that has no moving parts.

A standard wet card compass uses magnets to align itself with the horizontal aspect of the Earth's magnetic field. (There is a vertical aspect, too, which causes a compass card to dip downward as it approaches the magnetic pole.) A fluxgate compass also uses the Earth's magnetic field; but rather than using permanent magnets, it uses coils of wire allied with a metal core. A voltage is applied to the windings in the fluxgate and, as the unit moves through Earth's magnetic field, the output voltage is affected. The degree to which the output voltage is affected depends on the orientation of the unit to the field. A big advantage of a fluxgate compass is that the output is in electronic form and it can be further processed, displayed, stored, etc. Both wet card and fluxgate compasses display magnetic direction. To use true direction, this magnetic readout must be corrected for variation and deviation.

A gyrocompass, on the other hand, does provide the mariner with true direction. Gyrocompasses are built around deceptively simple devices called gyroscopes. A gyroscope is composed of a spinning mass, usually in the form of a solid wheel, that is set in gimbals so that it is free to rotate in three axes. When such a wheel is set into motion, it gains stability and will resist outside forces to change its orientation (this is one reason why one is able to ride a bicycle the spinning wheels act like gyroscopes and impart stability).

A gyroscope isn't an example of an immovable object, however; they move when acted upon, but do so in a peculiar way. Any force or torque applied to a gyroscope will result in a motion of the gyro that is 90° to the direction of the torque. This right angle reaction is called precession. (The spinning Earth can be thought of as a giant gyroscope; the moon and sun pull down on the Earth's equator and the result is a 26,000-year precessionary spiral of the Earth's poles.)

A tendency toward precession can be used to transform a gyroscope into a gyrocompass. If we placed a gyroscope at the equator, with its spin axis aligned east/west, it reacts to the rotation of the Earth by dipping its western end and raising its eastern end. Eventually, after 24 hours (one rotation of the Earth), a gyroscope at the equator would rotate through 360° in the vertical plane (like a gymnast doing a somersault). This effect is called its "horizontal Earth rate." We can combine precession and horizontal Earth rate in a way that gives us a north-seeking device. In simplified fashion it works something like this: we make a gyroscope that it is "pendulous" (this is a fancy way of saying that it has a weight on the bottom so that gravity causes it to hang down). After we set it spinning, its horizontal Earth rate causes its spin axis to dip east/west. However, because of the strategically-placed weight, gravity pulls on the gyro in an effort to center the weight. The gyro reacts to this torque by precessing; its spin axis turns 90° and now is aligned north/south. Because the spin axis is no longer aligned east/west, horizontal Earth rate ceases to be a factor and the spin axis remains aligned with the north pole. We've just outlined a very crude, north-seeking gyrocompass.

One of the most important differences between a magnetically-driven compass and a gyrocompass is that the magnetic device points to magnetic north but the gyro points to true north; thus, there is no need to bother with magnetic variation or deviation. With that in mind, a gyrocompass would appear to be the ultimate directional tool for the mariner. This is true if we design our gyrocompass for a specific latitude and if we stay tied to the dock. In that case, our gyro, once properly tweaked, will always show us true north. However, as soon as we get underway, change course, or change latitude, our gyrocompass will start to experience errors. (Since our gyrocompass is designed to react to the torque of gravity, it makes sense that it would also react to the torques imparted on it by acceleration and by course changes. There is also an error imposed by differences in latitude: the closer to the pole, the greater the error.) Additionally, gyrocompasses tend to be intricate electromechanical devices. And, as such, they have numerous moving parts, giving them a high up-front cost and requiring expensive maintenance. So, the gyrocompass isn't quite the perfect solution to determining direction that it might first appear.

But what if we combined some of the benefits of gyroscopes with the good points of a fluxgate compass? This is the approach taken by KVH Industries with their new Digital Gyro Stabilized Sensor System (DGS3 are a fluxgate electronic compass combined with two specialized gyroscopes called rate gyros and an inclinometer. With this system, a user would have an instantaneous readout of magnetic heading that is stabilized for yaw, pitch, and roll. Thus, rather than waiting for a fluxgate sensor to settle out after a turn, accurate magnetic heading would be available throughout a turn. "Gyros drift, but are instantaneous," says Martin Kits van Heynigen, president of KVH. "Fluxgates are slow, but they don't drift. If you combine the two, the two devices stabilize each other."

Unlike a mechanical gyrocompass, which tends to be on the bulky side, a DGS fits into two small boxes. One of the reasons for this small size is the design of the rate gyros. (A rate gyro is a gyroscope built to react to torques along one axis.) Rather than using a spinning wheel inertial mass allied with mechanical linkages, these rate gyros measure torques electronically. A thin disk, similar to but thinner than a floppy disk, is spun by an electric motor. Separated from the disk by a narrow airspace is a circuit board that carries a voltage. Thus, the disk forms one side of a capacitor. When a rate gyro experiences a torque, the thin spinning disk flexes slightly. This deformation changes the capacitance between the disk and the circuit board. The change can then be interpreted as either a yaw, a pitch, or a roll, depending on the orientation of the rate gyro and on information derived from the inclinometer. Since the rate gyros offer instantaneous data, they can correct the slow and confused data from the inclinometer and the fluxgate whenever a vessel is maneuvering. And on a long-term basis, the fluxgate can provide stable data that supersedes the less stable information from the rate gyros. In addition to being used as a steering device, the DGS can also be used as a heading reference for an autopilot, for stabilizing a magnetic north up radar display, and in a dish antenna stabilization platform. The basic DGS is expected to sell for $2,495.

Meanwhile, there is another type of gyrocompass that does away with the spinning mass and other mechanical aspects of gyroscopes and uses pulses of light moving through optical fibers for determining direction. Called a fiber optic gyrocompass (FOG), this device is manufactured by Litef Corporation, a German company owned by Litton Corp. Litton plans to sell these devices in the marine market through its C. Plath North America subsidiary. "This product has been in production for seven years and is used on more than 2,000 aircraft," says Craig Wilson, electronics sales manager for C. Plath. "We will be offering a fully marinized version next year."

While a traditional gyrocompass uses a spinning inertial mass to point to north, a fiber optic gyrocompass determines heading changes, relates those changes to an initial heading reference, and then calculates a new heading. (Called a "strapdown gyrocompass," it is a type of inertial navigation device.) Thus, a fiber optic gyro is neither a true north-seeking machine like a gyrocompass nor a magnetic north-seeking unit like a fluxgate.

What makes a fiber optic gyro impressive is its use of light beams to determine if a vessel has rotated on its yaw, pitch, or roll axes. The principle behind a fiber optic gyro (and its close relative the ring laser gyro) is something called interferometry. This technique takes a light source and splits it into two separate beams or wave trains. These waves are then combined to form an interference pattern. The interference pattern formed reveals important properties of the light. (This technique is also used to measure continental drift in a technique called very long baseline interferometry. Two antennas, separated by the diameter of the Earth, measure the signals coming from a distant quasar. The difference in phase between the two received signals can be converted into a distance separating the stations along the baseline. Measurements are made over several years and any change represents movement of the continents on which the antennas are riding.)

Inside a fiber optic gyro, two pulses of light are sent in opposite directions along a ring of optical fiber. Located yaw, pitch, or roll axes. The principle behind a fiber optic gyro (and its close relative the ring laser gyro) is something called interferometry. This technique takes a light source and splits it into two separate beams or wave trains. These waves are then combined to form an interference pattern. The interference pattern formed reveals important properties of the light. (This technique is also used to measure continental drift in a technique called very long baseline interferometry. Two antennas, separated by the diameter of the Earth, measure the signals coming from a distant quasar. The difference in phase between the two received signals can be converted into a distance separating the stations along the baseline. Measurements are made over several years and any change represents movement of the continents on which the antennas are riding.)

Inside a fiber optic gyro, two pulses of light are sent in opposite directions along a ring of optical fiber. Located inside the ring is a light sensor. If the ring of fiber is stationary, the two pulses will each complete a half circle and then meet to form an interference pattern as the two wave trains collide. If, however, the optic fiber is rotated, one light pulse will have a shorter distance to travel and the other light pulse will have a longer way to go. The sensor will detect this, and the result can be converted into a rotation one way or the other. The longer the path a light pulse is allowed to travel, the more accurate the measurement; so, in practice, the pulses are allowed to make several complete circuits before being measured.

When a vessel is at the dock, its heading can be determined from a magnetic or gyrocompass and then used to initialize the fiber optic gyro. From then on, all movements of the vessel will be detected by the fiber optic unit and the new heading of the vessel calculated. Like any inertial navigation scheme, however, errors creep in and the heading information becomes less accurate over time. Eventually, the unit would need to be reinitialized. The projected price for the marine version of the FOG is $35,000. While this is a substantial figure, a fiber optic unit won't have the maintenance cycle of a traditional gyrocompass. "Most gyrocompasses require an overhaul every year to year and a half," says Russ Williamson, president of Gyrotech, a gyrocompass service company. "A fiber optic unit requires very little maintenance."

These products are currently in use for tank navigation, for stabilizing tank guns so tank crews can shoot on the move, and for fin stabilization tasks on modern high-speed ferries.

Even a few years ago, the idea of using inclinometers, rate gyros and fiber optic gyros in marine applications would have seemed wildly optimistic. These devices were very expensive and offered strictly for military or aerospace applications. It is a measure of how fast technology is changing that now they are being marketed to the top end of the voyaging vessel market. Can 40-foot sailboats be far behind?