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Offline kayakpirateTopic starter
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« on: August 03, 2009, 09:37:12 pm »
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 Could someone explain the difference in single,dual,triple axis magnetic sensors. I've been looking at the Honeywell chips for a while but can't decide which ones to get.
 If you have a differential setup using single axis, do you have to hold the magnetometer a certain way like North/South?

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« Reply #1 on: August 04, 2009, 02:41:55 am »
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Hi
If You thing of a reference system x, y and z, a single axis sensor will only sense in in the x direction, a dual axis sensor has to sensors and will sense to axes perpendicular to eachother i.e. x and y (often used for compassing oriented horizontaly). Triple axis sensor has three sensors and will sense all axes x, y and z (this gives you an opportunity to measure total field and adding tilt correction to your system).
Suggested search terms on the internet;
compass, tilt correction compass

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« Reply #2 on: August 04, 2009, 08:01:56 pm »
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 So for a hand held differential magnetometer all you need is 2 single axis sensors spaced apart from each other?  Would using dual axis have any benefit?

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« Reply #3 on: August 05, 2009, 07:00:10 am »
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You are correct, two single axis sensors along the same axis are sufficient. If using two axis sensors you could achieve superior accuracy and bypass the troublesome work of manual precision alignment of the senors, via software. Two of the sensors would be for sensing the gradient and the two others as a reference for correcting the axial misalignment of the two sensors sensing the gradient. Normally a two axis sensor is used for compassing, but flipping two of those vertically could be a basis for a very well made gradiometer.


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« Reply #4 on: August 07, 2009, 08:22:06 pm »
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 I thought that I would try and build a 2 axis since the sensors are about the same cost as a single axis but it sounds much too difficult. Is there any benefits over a single axis besides the alignment compensation?
 I am comparing the Honeywell HMC 1001 to the HMC 1021. The 1001 sensitivity is 3mV/V/G and the 1021 sensitivity is 1mV/V/G. Does this make the 1001 more sensitive?  I thought that I read somewhere that it doesn't really matter since the sensors are comparing each others signals in a differential type setup.
 If you go to

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http://www.magneticsensors.com/applications/gradiometer.html
  what do you think about that gradiometer.  It says on the last line on the bottom that you can use a HMC1022 2 axis but I am wondering what the main difference would be. Would it detect from the sides better, etc?
 I need a gradiometer to detect airplane engines that are buried. Do you think the Honeywell chips are up to par or should I just build the

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http://www.fatquarterssoftware.com/product.aspx?id=12
    and be done with it?

 Thank you for any information.

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« Reply #5 on: August 08, 2009, 06:47:22 am »
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Hello again.
I?ll try to answer you giving my best opinion. It would be nice to see if others have comments.
If you don?t intend to use 2 axis for alignment it would be a waste to have it. In the application notes for the speake sensors it?s commented how to compensate for a slight misalignment by setting up your search pattern in a consistent  way.
Picking the right sensor is a matter of what you are looking for, what are the engines made of? (steel, cast iron, aluminum  Undecided, ). The sensitivity of the sensor must be sufficient to pick up the gradient which the anomally (ie the engine) is giving in the local earth field and at the depth they are buried. (The gradient fall of at the rate cube root). So even if you set it up in a gradiometer setup, if the individual sensor can?t catch the (sensitivity) influence caused by your anomally (engine) at the expected depth and based on the materials it is built of, you have to go for a more sensitive sensor.


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« Reply #6 on: August 08, 2009, 03:30:06 pm »
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Just checked out the HMC 1001 and it can sense fields with a reolution of 27 nT.

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« Reply #7 on: August 09, 2009, 11:13:39 am »
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 The engines are mostly aluminum but all have steel crankshafts that should give a good signal. Some on land some in water. Haven't got to thinking about the water mag yet. lol
 A few years back, a guy said that it doesn't matter how sensitive the sensor was because they are comparing the magnetic fields to each other (differential setup). Does this make since?
 What is your background? You sound like an engineer. I know just enough about electonics to be dangerous. I went to college to learn how to fix things, but when I graduated it was cheaper just to buy a new one.  Undecided
 I am starting a new post called $30 magnetometer. I hope you can comment on it too.

James

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« Reply #8 on: August 09, 2009, 06:21:27 pm »
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I have to correct myself  Police, The HMC 1001 can sense 27 microGauss (0.000027 Gauss) 1 Gauss equals 100000 nT (the earth field is approx 0.5 Gauss /50000 nT). This should turn out to be a 2,7 nT upwards sensitivity. This should be sufficient for your use, if the engines are within 20 feet or so.
By the way I am a technician in the Norwegian airforce, used to be an EOD tech/ instructor.

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« Reply #9 on: August 09, 2009, 06:46:33 pm »
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About your question; A few years back, a guy said that it doesn't matter how sensitive the sensor was because they are comparing the magnetic fields to each other (differential setup). Does this make since?
It does not make sence! Or we could say it is to oversimplyfying it.
Imagine you are going to measure the differense in windspeed at location A and B. The sensitivity of the windspeed meters is so that they do not give an output for speeds under 1 knot, if the wind is blowing less than 1 knot you have nothing to subtract from eachother, ergo  no output (even though there is a draft in the air). 0 - 0 = 0

Hope I managed to clarify this for you.

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