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COMPASS CONSIDERATIONS FOR STEERING AND AUTOPILOTS
By Paul Wagner
The autopilot compass is perhaps the most critical component in an autopilot system. No matter how good the other components may be, no autopilot can steer better than the heading information provided by its compass. Most modern autopilots use an electronic Flux Gate compass, or equivalent, which directly senses the earth's magnetic field. These direct sensing compasses are frequently claimed to be far superior to "old fashioned" fluid compasses. In fact, electronic compasses have been in use for over 70 years and their limitations are well known to compass experts. They are in common use today mainly because they are less expensive to manufacture than the conventional fluid-filled compass with its floated card, magnets, pivots, jewels and sealing system. The flux gate consists of field sensor, usually an inductor, mounted to a gimbaled platform which is intended to sense the horizontal component of the earth's magnetic field. The earth's field has two components: the horizontal field, which gives directional information, and the vertical field, which provides no useful heading information (see figure 1).
At mid latitudes in the U.S., one degree of sensor tilt off horizontal will have an apparent two-degree shift in indicated heading, even though the vessel is still on course. In higher latitudes where the horizontal field decreases in strength and the vertical field increases, one degree of tilt can cause over 10 degrees compass error. The Great Lakes and Eastern parts of Alaska are particularly bad areas in this regard (see figure 2).
A few electronic compass manufacturers fill their compass sensor with a heavy oil to dampen the gimbal action and minimize these vertical field errors. Others resort to electronic damping, which either increases the compass deadband (lowers its sensitivity) or averages the heading (delays the availability of current heading information). Some designs use more sophisticated signal processing, but the end result is roughly the same. Any delay in autopilot response to heading changes, especially in quartering seas, results in excessive yaw and the need for excessive rudder corrections. Some electronic compass manufacturers recognize this deficiency by offering a rate gyro which provides a more current short-term heading reference than their sluggish and over damped electronic compass is capable of.
Another electronic compass manufacturer has a "turn" button on the compass display. They recommend that this button be activated when a change of course is made. This button simply changes the compass damping to minimum and is a tacit admission that the normal amount of damping, which is required to provide a steady display, causes such a delay in heading indication that the helmsman would overshoot a course change. Clearly, any autopilot using this heading information would have great difficulty steering in quartering seas where immediate correction of course changes is essential.
Over 50 years ago, during World War II, direct sensing electronic compasses using flux gates were used on aircraft and even on vessels, due to the ability to have the compass sensor remotely mounted away from strong magnetic interference and to provide multiple repeaters (see figure 3). However, due to high acceleration such as turning and banking in an aircraft and rolling and pitching on vessels, it was determined that simple pendulous support of the sensor resulted in unacceptable course instability. To resolve this problem on aircraft, a vertical gyro was integrated into the gimbaled sensor so the sensor always remained horizontal. On ships, a directional gyro was used for heading reference, but since these would slowly drift away from North, they were slaved to a flux gate sensor. This sensor would be unstable for the reasons previously stated, but when averaged over about one minute would provide a reasonably stable reference for the drifting gyro and keep the heading smooth and reasonably accurate. The cost to produce these systems with the older technology precluded their use on small commercial vessels or yachts.
A more sophisticated and more modern approach to a flux gate controlling the directional gyro is now produced by a leading compass manufacturer, who uses multi-axis gyros to provide enhanced stability in the face of heavy acceleration from roll and pitch, etc. in addition, this newer design provides heading output to the NMEA 0183 standard, making it usable with a wide range of equipment, including autopilots, which require better compass stability than that available from simple, non-fluid damped flux gate sensors. While considerably more expensive than simple flux gate compasses, it provides an economical alternative to conventional north-seeking marine gyrocompasses.
All electronic compasses only correct for this horizontal field error. On non-ferrous vessels, this is generally acceptable; however, on vessels with large amounts of ferrous metal on board, the earth's vertical field induces a changing horizontal field in the magnetic deviation, as the vessel rolls or heels; hence the name, vertical heeling error. As a vessel rolls, the compass error varies, causing oscillation in the indicated course, even though the vessel may still be on the same heading. Not only does this make hand steering difficult, but it also causes autopilots to wander and cause unnecessary steering corrections. On sailing vessels, which may remain heeled for 30 degrees or more for some time, large fixed errors occur, and variable errors due to rolling are superimposed on this. Under these conditions, an uncompensated vertical heeling error can have serious consequences.
The flux gate stabilized gyrocompass may give acceptable performance where the roll period of the vessel is well below the averaging time of the unstable flux gate North reference, but if the vessel has a long rolling period, this can begin to degrade the North reference stability. On a heeled sailboat, within minutes of heeling, large errors can develop, since the roll filtering is no longer effective. The only way to correct this is to use the tried and true technique of installing a permanent magnet directly above or below the sensor and through its center, while adjusting the distance and polarity for minimum healing error.
Professional compass adjusters use a "vertical force instrument" which measures the vertical field errors and allows precise compensation. Vessel owners may perform an approximate compensation by adjusting the magnetic in a similar manner by monitoring for maximum compass stability (minimum autopilot activity) while rolling in a seaway. On commercial vessels or any vessel going offshore and where vertical healing error is considered to be a possibility, the services of a qualified compass adjuster should be contemplated.
There are some compass adjusters whose experience may be limited and do not correct for or even understand what vertical healing error is, and if they express any hesitation about making this correction or don't seem to understand what you are talking about, find another compass adjuster! Incidentally, this vertical heeling error correction is only valid for the magnetic latitude at which the compensation was made. If the vessel is expected to go on long ocean voyages where the latitude will change by more than approximately 5 degrees, an additional correction made with "Flinders Bars" should be carried out. Just as horizontal compensation requires quadrantal spheres (soft iron balls) to be placed around the compass, vertical compensation also requires a vertical soft iron corrector to be mounted and operated in conjunction with the vertical heeling error magnet to maintain compensation over wide latitude changes.
The ultimate in compass stability is achieved with a true North seeking gyrocompass. While very expensive, the stability is unmatched. Traditional technology uses a spinning inertia wheel which possesses high directional stability despite vessel roll or pitch. Through mechanical or electronic means this gyro wheel is controlled to point to the earth's geographic North pole and transmitting devices send this information to various repeater stations. The more modern designs provide this to NMEA 0183 format. Unfortunately, the cost of the gyrocompass precludes its use on smaller yachts and workboats.
In summary, a simple air-suspended flux gate or equivalent electronic direct sensing compass is only suitable for calm seas and/or low latitudes, and a fluid-filled sensor is acceptable for heavier seas and higher latitudes. An improvement in performance, especially on steel vessels. may be achieved with a flux gate-aided directional or rate gyro, and for the most demanding applications, a North-seeking gyro compass is the preferred choice. With the above increasing performance, increasing costs may be expected.
While there are many factors that could be discussed concerning the design and construction of autopilots, it may be seen that the compass and its stability are of prime importance. No autopilot can steer better than its heading reference.
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