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Fwd: Marine Electronics Issue #18: Fluxgate Compasses
From: Aubrey O?Callaghan
Date: 2002 Sep 12, 07:15 -0400
From: Aubrey O?Callaghan
Date: 2002 Sep 12, 07:15 -0400
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>Subject: Marine Electronics Issue #18: Fluxgate Compasses >Message-id: <3278348.1031691619908.JavaMail.javaproc@10.76.10.10> > >OCEAN NAVIGATOR EMAIL NEWSLETTER > >Topic: Marine Electronics Issue No.: 18 Date: 09/10/0002 >Title: Fluxgate Compasses > >This newsletter is a free bi-weekly resource published by Ocean Navigator. >You have received this newsletter as a registered newsletter subscriber. If >you wish to or feel you have received this in error, please see >the information at the end of the newsletter. > >***************************** ADVERTISEMENT ***************************** > >Subscribe to Ocean Navigator magazine and receive essential, need-to-know >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 > >***************************** ADVERTISEMENT ***************************** > >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@oceannavigator.com. 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@oceannavigator.com > >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 ***************************** > >Subscribe to Ocean Navigator magazine and receive essential, need-to-know >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 > >***************************** ADVERTISEMENT ***************************** > >To from this newsletter or change your email address, please go >to http://www.oceannavigator.com/members/subscription/newsletterUpdate.jsp > >To subscribe to additional topically-focused free email newsletters from >Ocean Navigator, including Weather and Offshore Safety/Medicine, please go >to http://www.oceannavigator.com/members/subscription/newsletterUpdate.jsp > >If you have subscription questions, please email >mailto:subscriptions@oceannavigator.com > >If you would like to comment on this newsletter, please email >mailto:lmckenna@oceannavigator.com > >If you would like to provide editorial suggestions, please email >mailto:jsnyder@oceannavigator.com > >For information on advertising in this newsletter, please email >mailto:advert@oceannavigator.com > >Ocean Navigator is committed to protecting your privacy. 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