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Subject: Fwd: Marine Electronics Issue #18: Fluxgate Compasses
From: Aubrey O'Callaghan (ocallaghan@XXX.XXX)
Date: Thu Sep 12 2002 - 07:15:25 EDT
Here goes in entirity, I was going to edit it but perhaps the other website
may be of interest.
There have recently been discussions on COLREGS.
It's an interesting subscription.
Aubrey.
>Date: Tue, 10 Sep 2002 17:00:20 -0400 (EDT)
>From: Ocean Navigator Online <newsletters@XXX.XXX>
>Subject: Marine Electronics Issue #18: Fluxgate Compasses
>Message-id: <3278348.1031691619908.JavaMail.javaproc@XXX.XXX10>
>
>OCEAN NAVIGATOR EMAIL NEWSLETTER
>
>Topic: Marine Electronics Issue No.: 18 Date: 09/10/0002
>Title: Fluxgate Compasses
>
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>TITLE: Fluxgate Compasses
>
>By: Larry McKenna
>
>First off, thanks to all of you who emailed greetings. I greatly appreciate
>all of the "welcome onboard" messages I received. Also enjoyable, in a weird
>sort of way, were the many harrowing stories about single-platform
>navigation. The clearest exposition of that thread was by Peter Savage, who
>wrote, "All this (electronic) stuff is no good unless you use it and use it
>correctly."
>
>Using "it" correctly is what navigation is all about, and the feedback I
>received indicates that our readership consists primarily of navigators who
>want to navigate correctly. To me, this means being able to use any and
>every tool you have to establish and maintain a position estimate at all
>times. Using a tool is different than operating one. Anyone can operate a
>many?buttoned black box -- using one takes intelligence and judgment. At the
>core of judgment is understanding how something works, and thus we arrive at
>today's topic: fluxgate compasses.
>
>Every boat going to sea should have at least two compasses. One MUST be an
>analog, or mechanical, unit -- one that requires no batteries or electrical
>connections, most often this is a steering compass, mounted within easy view
>of the helm. The second compass MUST be a hand-bearing compass of some kind,
>Either a mechanical or electronic compass will do. Given that this is a
>marine electronics column, I thought we'd start with a review of how
>electronic, or fluxgate, compasses work.
>
>For many of us, the Autohelm Personal Compass was our introduction to
>electronic compasses. It was (and mine is, if I can remember where in the
>chart table I put it) a great, easy-to-use, ergonomically pleasant machine
>with one great drawback -- it only worked if you held it absolutely flat. On
>a rolling deck, you became a human gimbal. If you didn't keep the thing
>exactly level, your bearing would be off, perhaps even way off. Alas, there
>was no way you could tell this unless you took multiple bearings of the same
>target. All fluxgate compasses suffer from this problem, though few as
>severely as the Autohelm.
>
>The reasons for this have to do with how fluxgate compasses are constructed.
>According to Dr. Bill Lee, of AlphaLab Inc., the core of every fluxgate
>magnetometer is a loop of iron nickel foil. Approximately 10,000 times per
>second, this loop is magnetized, demagnetized and then magnetized in the
>opposite polarity by an excitation coil wrapped around the loop. As the AC
>current in the coil increases, the loop of foil becomes increasingly
>magnetized. At some point (fixed by the system's construction), the loop's
>magnetic field saturates and fails to keep pace with the excitation coil.
>The loop's field remains saturated even as the current in the excitation
>coil begins to decrease. Eventually, the loop's field decreases, goes to
>zero and then strengthens in the opposite polarity as the excitation coil
>goes through the remainder of its AC cycle. Each episode of saturation or
>desaturation of the loop produces a brief pulse of current in a pickup coil
>surrounding the foil loop and excitation coil. If the earth had no magnetic
>field, the foil loop would always saturate at exactly the same point in the
>excitation coil's AC cycle, and the pulses would be recorded at exactly the
>same phase of the AC cycle.
>
>But earth does have a magnetic field, about half as strong as the one
>produced in the foil loop. Earth's field penetrates the loop, inducing a
>magnetic field in it even before the excitation coil starts its cycle. Once
>the AC cycle starts, the foil loop saturates at an earlier phase in the AC
>cycle than it should, an event indicated by the emission of saturation
>pulses, duly picked up by the pickup coils. On the second half of the AC
>cycle, earth's magnetic field acts to delay the onset of saturation.
>Combining the two readings gives the strength of the magnetic field in one
>direction.
>
>To get direction, we need to measure the field strength in at least two
>perpendicular directions. Modern units do this by using two different pickup
>coils perpendicular to one another. Comparison of these two-axis
>measurements provides the orientation of earth's magnetic field and hence
>the magnetic bearing. Typical units run at 10,000 cycles per second;
>approximately 1,000 cycles are averaged to give 10 readings per second. Some
>manufacturers use software to select an update rate, but rest assured that
>any fluxgate compass you have is going through at least 1,000 measurements
>every time it shows you a single reading.
>
>The problem with the Autohelm was that the foil loop was fixed in the
>compass' housing. Tilting the compass changed the apparent strength of the
>magnetic field hitting the loop and hence the apparent direction. The
>sensitivity at North American (magnetic) latitudes is pretty high -- as much
>as 3? bearing error per degree of tilt! Newer fluxgate compasses solve this
>in a number of ways. For example, according to Chris Watson at KVH
>Industries, their Datascope uses a foil loop that floats in a light oil
>bath. To first order, the ring is self-leveling. The Datascope can be tilted
>up to 20? from horizontal and still work within the 0.5? accuracy limits.
>Beyond this angle, however, beware of any readings you take. One might want
>to measure the horizontal angle between two marks by holding the Datascope
>sideways, and that's great -- just don't take a bearing at the same time!
>
>For those of you with the interest, you can easily see this behavior
>yourselves. I know this technique works with Autohelms, and Chris Watson
>thought it might work with the Datascope. Take the compass to the least
>magnetically disturbed area you can find. Find magnetic north, and then
>slowly tilt the compass down while observing the bearing. Eventually you
>will find a point where the inaccuracy of the reading is maximized. The
>compass is now pointing exactly parallel to earth's magnetic field. The
>angle of the compass from the horizontal is the magnetic inclination, or
>magnetic latitude. It was sometimes used in days of yore as a line of
>position. I'd appreciate hearing from any readers who try this. Send your
>name, lat/long, the inclination and the type of compass you have to
>lmckenna@XXX.XXX For those of you interested in comparing your
>readings to the real thing, see http://geomag.usgs.gov/usimages.html#us_i.
>
>Thanks again for all your emails -- I haven't come close to answering all of
>them yet, but keep them coming. Practical Sailor recently rated various
>hand-held bearing compasses, find them at www.practical-sailor.com.
>
>
>
>-- Larry McKenna
>
>lmckenna@XXX.XXX
>
>Use of trade names or manufacturers in this article isn't meant as an
>endorsement.
>
>Related Articles:
>Title: Four hundred degree compass explained
>http://www.oceannavigator.com/public/action/ArticleAccess?doc=cbghxzsr
>Title: GPS compass
>http://www.oceannavigator.com/public/action/ArticleAccess?doc=epghvuob
>Title: Trickle-down technology
>http://www.oceannavigator.com/public/action/ArticleAccess?doc=cpjrhssr
>
>Author Bio:
>
>
>Larry McKenna is a former Ocean Star instructor and frequent contributor to
>Ocean Navigator, as well as other publications. Raised in New England, he
>sails Restless (and any other boat he can get aboard) along the mid-coast of
>Maine.
>
>
>
>
>
>***************************** ADVERTISEMENT *****************************
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>information that will put you in control of your boat and make bluewater
>voyaging more enjoyable. Navigation, Weather, Electronics, Sail Handling,
>Provisioning, Route Planning, Personal Safety - it's all in Ocean Navigator.
>Start your subscription today at:
>http://www.oceannavigator.com/public/subscription/subscriptionLanding.jsp
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