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Original question:

fluxgate sensor for nT measurements

fluxgate sensor for nT measurements

Eduardo Rey17
Prodigy 230 points
Other Parts Discussed in Thread: DRV425

Hello,

I am trying to measure very small field changes in the earth's magnetic field at a distance of about 2ft to detect ferromagnetic objects a distance, so I am using the DRV425 in high sensitivity mode to see if I can achieve the desired level of sensitivity with this part.  However, when I start increasing my Rshunt value from 100 OHMS (which is the default value in the eval board) to say 330, I get an increase in noise in Vout.  In relation to this observation, the response to this question mentions "If you would like to shied the earth's magnetic fields or any other stray magnetic fields you will need to place a material that is magnetic around it" and furthermore, section 8.2.1.2 (pg 25) of the datasheet states "The high sensitivity of the DRV425 may require shielding of the sensing area to avoid influence of undesired magnetic field sources (such as the earth magnetic field).  Alternatively, an additional DRV425 can be used to perform difference measurement to cancel the influence of a static magnetic field source, as shown in Figure 68."

So really two methods are recommended to deal with the elimination of any static field sources that would contribute an error to low field changes

1) Shield using magnetic material, I'm assuming some type of mu-metal or soft ferromagnetic material
2) Use two DRV425 in differential mode to capture the difference between their respective outputs

Now the questions...

Question 1: What type of materials have you used or can recommend to shield the DRV425?

Question 2: The geometry of the shield has large influence on the effectiveness of being able to divert lines of flux.  Can you recommend a shield geometry that is effective at shielding against interfering static field while still allowing measurements to be made

Question 3: You mention using the simulation software FEMM to calculate the fields.  Do you have an example of this that you can share since I am having difficulty setting this up in this software

Thanks in advance,


E

  • 0
    TI__Mastermind 26475 points

    Hello Eduardo,

    First,  thank you for using the forum to answer your questions.    I will start off with some statements first before getting to your questions.

    I am not sure if your signal to noise will ever be enough with a 2 ft distance of your object also you are using the changes of the external field to determine its presence if you shield it you will no longer see the changes.  The noise for the DRV425 will limit the range you can measure.  If you give me your bandwidth that you are limiting to I can give you the peak to peak noise you can expected derived from the Noise density plot provided in the datasheet.  If you are using the full 47KHz bandwidth you can expect about 1.5uT Peak to Peak noise.  This does not include noise other sources.

    Q1)If you still need to shied then you would use material higher in permeability than air.  The question here is how much field are you going to need to divert away from your measurement.  As permeability increases you will divert more field but you can also saturate the material.  The thickness of this material is also a factor as the larger the material the more field it can hold before becoming saturated.

    Q2)  This is all based on the exact measurement you are making.  Normally shielding requires a full enclosed.  At lease in the orientation of the field being measured.  This would also depend on what you are actually trying to shield.  As stated above I am not sure how you can make your change in field if you are going to shield it.

    Q3)  This is a two dimensional simulation tool.  I have a bus bar script that does through and draws something but this is for a bus bar.  A drawing and I could draw something up.  I believe simulation for you application is critical as this will help determine if your application can be done.