Electronic fluxgate compasses offer some advantages over the mechanical magnetic compass in the areas of accuracy, stability, ease of installation, calibration, and magnetic heading information in readily usable electronic form. At the same time, some on-board instruments that use heading data, like autopilots and satcom antennas, have become more demanding about the accuracy and stability of that information.
Now there is an improved magnetic heading system called the Azimuth Digital Gyro Compass, from KVH Industries, Inc. The Azimuth unit is designed to correct some of the drawbacks of the standard fluxgate electronic compass.
The complex influences that affect a magnetic compass on a ship may be judged in part from the fact that Bowditch devotes an entire chapter, 54 pages, to the need for and process of recognizing, measuring, and correcting for errors in compass information. Fortunately for recreational sailors, in most situations a properly installed and adjusted magnetic compass is fully adequate for navigation. Until fairly recently, serious practical shortcomings were mostly limited to operation at high latitudes, where the horizontal component of the Earth’s magnetic field is minimal, making the very operation of a magnetic compass problematic. However, in the past few years, extremely stable and accurate heading information has become critical to the performance of autopilots and stabilized satellite antennas. Something other than the conventional magnetic compass is needed.
Magnetic heading sensors using flux detectors (fluxgates) offer the convenience of choosing either an analog or digital display; they simplify installation by allowing the sensor to be placed where disturbing magnetic influences are at a minimum; they can automatically correct for deviation errors; and they can readily interface with autopilots and other systems.
However, the output of a fluxgate, like any conventional compass, is adversely affected by magnetic dip and acceleration errors. The effect of these errors on the display is usually reduced by intentional dampening and filtering of the readout. Although dampening reduces the apparent effect of the errors inherent in the sensor system, the instantaneous accuracy of the heading information is unavoidably compromised. While a helmsman may not be aware of the errors, an autopilot can be adversely affected. Unlike the helmsman, who may compare the vessel’s indicated heading with the desired heading a few times per minute, the autopilot is continually evaluating the difference between apparent and commanded heading. In all but the calmest sea conditions, the ability of the autopilot to precisely hold heading will be degraded by short-term errors in magnetic heading information. Introduction of data smoothing or filtering may degrade rather than improve the autopilot’s performance. In difficult conditions, especially in a following sea, the errors may become both large enough and of such a phase relationship to make the autopilot virtually useless.
Other devices, such as stabilized satellite communication antennas, north-up radar, and radar units employing Automatic Radar Plotting Aid (ARPA) also depend upon heading information that is not significantly degraded by vessel motion. Stabilized platforms
The classic way to eliminate errors in heading information caused by platform (vessel) motion has been to actively stabilize the reference platform. Inertial navigation system platforms, such as those used in aircraft, submarines, and missiles, are stabilized held steady in space through the use of accelerometers that sense motion in three axes. This acceleration information is then used to continually reposition the platform, holding it fixed in space while the vehicle in which it is installed moves about. In the past 10 years, “strap-down” systems have been developed that use laser-ring gyros or fiberoptic gyros in conjunction with extensive computer processing to eliminate the need for the mechanically stabilized platform.
The KVH Azimuth Digital Gyro Compass (ADGC) senses the Earth’s magnetic field with an integral magnetic fluxgate. The errors inherent in the fluxgate’s output are compensated for with the use of acceleration and inclination sensors whose data is processed in a group of microprocessors. The net result is accurate, stable magnetic heading information, without the need for complex mechanical servo systems.
The fluxgate in the KVH system operates conventionally, sensing the horizontal component of the Earth’s magnetic field. Like all fluxgates, it is subject to errors induced by vessel position (attitude) and accelerations on the roll, pitch, and yaw axes. It is also sensitive to local distortion of the magnetic field. Eliminating the position- and acceleration-induced errors begins with sensing these conditions.
The ADGC detects a vessel’s long-term attitude (for example, a sailboat’s sustained heeling angle when sailing) using a viscous fluid inclinometer, which measures long-term deviations in both roll and pitch. Accelerations on the roll, pitch, and yaw axes are sensed by a pair of innovative rate gyros. Each of the gyros consists of a small-diameter, very thin and compliant, electrically conductive disk, similar to the material used in the familiar floppy disk, which is spun rapidly by a brushless DC motor. As with any other rotating mass, the rapidly rotating disk exhibits a gyroscopic effect, resisting any force that would tend to displace it from its plane of rotation. Two rate gyros
One of the rate gyros is mounted on a motor shaft aligned with the longitudinal, fore and aft axis of the vessel. The disk therefore rotates in an athwartships vertical plane. A yawing (turning) motion of the vessel will impart a force on the disk, tending to deflect the disk at the 3 and 9 o’clock positions. Due to gyroscopic precession, this force will result in a deflection of the edge of the disk at the 6 and 12 o’clock positions. A sensor at either the 6 or 12 o’clock position can measure the deflection of the disk, providing yaw acceleration data.
A second gyro, with its motor shaft vertical, senses both roll and pitch accelerations. The system’s software combines the data from the attitude and acceleration sensors and removes the motion effects from the magnetic heading data supplied by the fluxgate. The correction for local magnetic deviation is applied conventionally, with a display of the quality of the correction shown at system start-up.
My evaluation of the ADGC was somewhat limited by lack of time for interfacing the output of the ADGC with the 1980-vintage autopilot on my 46-foot ketch. Therefore, heading stability was judged by visual comparison with the data from the autopilot’s conventional fluxgate heading sensor and with the ship’s compass. The claimed performance limits of up to plus or minus 45ï¿½ in roll and pitch were evaluated by intentionally moving the sensor unit about each axis, up to the specified angular limits. The angular velocity limit of more than 45ï¿½ per second far exceeds the realistic acceleration limits of any vessel.
As with any magnetic sensing system, installation in the most magnetically benign environment is desirable. Unlike other flux detector systems, the ADGC emits a low buzzing sound from its rate gyro motors.
After installation, the system was switched to its calibration mode, and the vessel was steered slowly through a 720ï¿½ turn. The automatic compensation process was verified by reference to the calibration score, which appears on the digital display each time ADGC is turned on. Calibration performance is displayed with three digits. The first indicates the quality of the calibration, 1 through 9, with higher numbers being better. The second digit indicates the relative quality of the magnetic environment in which the sensor is installed (this reading reflects the magnitude of the corrections that were generated by the computer in its compensation effort). The third digit shows the total number of calibrations that have been performed. Configuration possibilities
The system can interface with a PC, using software supplied with the unit. This permits the user to reconfigure the outputs to match virtually any interface requirement. Examples of configuration possibilities include various NMEA sentences and baud rates as well as KVH and Cetrek type sentences. It is also possible to correct for angular misalignment between the fore-and-aft axis of the sensor box and the vessel’s centerline. Local magnetic variation can also be entered, yielding a display of true instead of magnetic heading information. An interesting option is to input heading information from a GPS. When the vessel is underway, the GPS can provide true-north information for the system, making true-north data available for all connected systems. An output interface unit is available that converts the NMEA 0183 digital heading signal to an analog sine/cosine output signal for use with autopilots or other devices that require this type of input. The system package is composed of three basic elements: a digital gyrocompass, a digital gyro inclinometer, and a junction box/power supply. Analog or digital heading displays are available. The system components are available housed in a very impressive, gasketed, 17-pound, 12.6-by-12.6-by-4.7-inch aluminum enclosure, or separately for installations where the single, large enclosure may be difficult to install. The system will accept a DC supply voltage of between 12 and 32 volts, consuming 330 milliamps, or eight amp-hours, per day. A backup power supply, perhaps from a continuously float-charged battery, would be a worthwhile addition.
Getting good value for the money must be the final consideration for all equipment placed on board a vessel. Regardless of how clever the device may be, it must earn its keep. Compared with a conventional marine gyrocompass, which can cost more than $20,000, the $3,000 ADGC may seem a great bargain. It appears to provide roughly equivalent performance in many areas, with superior performance in not requiring a period for stabilization prior to use and in needing far less electrical energy. In the event of a power interruption, the ADGC will be fully operational after only a few minutes.
On vessels that have systems that can benefit from really stable heading information, the investment for the ADGC can be very worthwhile. If the primary use of magnetic heading information will be guidance for the helmsman, a conventional fluxgate compass system and a high-quality magnetic steering compass is a more appropriate investment.
Contributing editor Chuck Husick is a sailor, pilot, and Ocean Navigator staff instructor.