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At 06:33 PM 1/29/02, George H wrote:

>>... What exactly
>>is a bolt-down sensor? And what errors does it "abolish"?
>>
>>As I understand it, a fluxgate compass, like any other compass, needs to
>>establish the direction of the horizontal component of the Earth's field,
>>with respect to the direction of the vessel. For this to happen, somehow
>>the fluxgate compass, just like any other compass, has to be aware of the
>>direction of the horizontal. ...
>>George Huxtable.
>
>  I'll...look at what the commercial fluxgate offerings provide.
>
>It may be that for a 2-axis fluxgate, a pendulous mount might be adopted.
>This would retain the pendulous errors associated with a compass card
>  of the conventional kind. I'll get back to the list, if I find useful
>engineering input.
>
>A three axis fluxgate can be mounted rigidly. This is sometimes called a
>'bolt-down' sensor.


I have looked at what is on offer as net resources. I paid particular
attention to
the compass comments offered by Capt. Schenker (USN Retd) and the
compass notes offered by the Single Handers Society.

Schenker writes [2-axis fluxgate compasses] "are often less stable than a
good quality magnetic compass in rough sea conditions."

Here's a note intended for Transpac sailors, where self steering gear and
fluxgate autopilots are de rigeur:




I see that the fluxgate offerings from KVH, Simrad, and Ratheon Marine are
all two axis floated or gimballed sensors which have much the same
characteristics as orthodox magnetic compasses, except on the one hand,
they require electrical power, and on the other, they can deploy deviation
correction automatically. They are advertized as being able to maintain
accuracy up to around 30 degrees of  pitch or roll.


Here's a two axis spec from George's neck of the woods:


[sample text]

35� FLUXGATE SENSOR COIL (AR35S) SPECIFICATION:
Offset Error+/- 3�
Non LinearityWill not exceed 1� in any interval of 9�. By putting a
straight line through the end point of this interval, the intermediate
values will not exceed a value of 0.3� from this line.
Tilt ErrorsTilt by 0-25� - <1�
25-30�-<1.5�
30-35�-<4�
Settling TimeConstant after 1 second after a sudden vertical change

This narrative from Coursemaster is readable, if not necessarily rigorously
accurate on every detail:



Almost all autopilots use a fluxgate compass as the primary heading sensor.
The fluxgate has been proved over many years to give cost-effective and
reliable service. But it does have two minor problems.
1.      To provide an accurate reading, the fluxgate compass must operate
in a horizontal plane, so it has an internal gimble or floating suspension.
This is a gravity-based level, so if the boat or ship accelerates,the
vertical gravitational force is temporarily disturbed. This commonly
happens during a fast turn or in a seaway. If the local magnetic field has
a significant dip angle -- particularly in higher latitudes (above 30
degrees north or south) -- the acceleration tilt of the fluxgate shows up
as a heading change. The result can be inaccurate steering when the vessel
is heading north in the northern hemisphere orsouth in the southern hemisphere.
2.      On steel vessels, the magnetic field can change when the boat
rolls, which effects the fluxgate compass and causes a heading change.

Here is another popular description of a 2 axis fluxgate:


LIFE WITH THE MAPSTAR/CRITERION FLUXGATE COMPASS The digital fluxgate
compass in your MapStar/Criterion measures the earth's magnetic field in
much the same way as a magnetic hand compass. In other words, it measures
horizontal angles (azimuths) relative to the magnetic poles. The sensor has
a ring that floats freely in fluid inside a cylindrical housing. The
floating ring keeps the sensing element horizontal, which allows you to
take accurate azimuth readings even when the instrument is tilted as much
as 15 degrees from the horizontal plane.


Here are some do it yourself article references:

Black, "Build This Digi-Compass" Radio-Electronics, November 1989, pp.
43-51, 82
Noble, "Electronic Fluxgate Compass," Electronics and Wireless World,
January 1992, pp. 14-18
Ramsden, "Measuring Magnetic Fields with Fluxgate Sensors," Sensors
Magazine, September 1994, pp. 87-90


KVH has some amusing caveats to offer in their FAQs (frequently asked
questions)
in answer to the question:
What is a strap-Down Fluxgate?

They assert that a 3 axis flux gate compass must be held rigidly horizontal (!)

(KVH)


Though the 2 axis fluxgates evidently predominate the current market,, the
3 axis device has its commercial offerings, and I expect this offering from
Crossbow at San Jose will be the wave of the future:


[sample text]
The CXM544 sensor detects the Earth's magnetic field using a 3-axis
magnetometer. The sensor computes a continuous measure of orientation using
the 3-axis accelerometer as a gravitational reference field. The product
uses the SoftSensor� architecture to compensate for temperature drift,
alignment, and other errors. The CXM544 finds applications in old well
logging, marine systems, and magnetic compassing. ...
Download CXM544, Micro Orientation Sensor, Datasheet (PDF)


Towed Sonar Heading Sensor (HS)
The CHS100 Sensor is a specialized product designed specifically for towed
sonar array applications. It is small, low power and designed to replace
traditional compass solutions. Unlike compass solutions, there is no
floating gimbal in the CHS100. The unit has been tested in latitude as high
as 85 Degrees.
Download CHS100, Towed Sonar Heading Sensor, Datasheet (PDF)

  I hope you found this review helpful in evaluating fluxgates.

Sincerely


Brian Whatcott
   Altus OK                      Eureka!

   

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