Hi Dale,
It turned out the problem was caused by using the default factory
setting. After calibration, it works fine. Thank you for your time
helping me out.
Best,
Weiguang
On 22/01/2015 10:50 AM, Dale Roberts wrote:
Great, I'd be curious to hear what the problem is!
dale
On 1/22/2015 9:21 AM, Weiguang guan wrote:
Dale,
Thank you again for the instant helpful response.
Regarding the 3 possibilities. 1) I actually used ShimmerConnect to
show the streamed Magnetometers on the fly, instead of plotting the
data in Matlab after data acquisition. 2) I will chose another
location to run the test again. 3) I will confirm this possibility by
a test. Thank you again for pointing out these likely causes of the
problem. I'll share my investigation once I have a conclusion.
Best,
Weiguang
On 21/01/2015 3:51 PM, Dale Roberts wrote:
... Sorry, I meant to go back and correct #1 below. The RAW values
are in fact signed. So, if you were expecting them to be unsigned,
then maybe you are subtracting an offset when you should not be.
dale
________________________________________
From: [email protected]
<[email protected]> on behalf of Dale Roberts
<[email protected]>
Sent: Wednesday, January 21, 2015 3:47 PM
To: Weiguang guan; [email protected]
Subject: Re: [Shimmer-users] Magnetometer puzzle
You are correct in your expectation that if you rotate the Shimmer
360 degrees, you should generally see both positive and negative
numbers on the X and Y axes due to the Earth's magnetic field. There
are a few possibilities I can think of.
1) Maybe you are making an error in handling the data in Matlab. Run
ShimmerConnect, enable the Magnetometers, and see what numbers you
get there on the screen. I just did that, and Z stays relatively
stable, and X and Y RAW values both go above and below zero. The raw
sensors may be "unsigned", in which case you may have to subtract an
offset to get properly "signed" numbers.
2) If you do indeed have a large magnet very nearby (like w/in a few
cm of a neodymium magnet, or you are near an MRI machine!), the
sensors can saturate and the sign may appear to change erroneously.
3) Your calibration is messed up. Collect a CSV file with
ShimmerConnect. Open in Excel or some other spreadsheet. Look at
both the RAW and CAL mag data. If the RAW X and Y signals are going
positive and negative, and the CAL is not, then your calibration is
no good.
dale
________________________________________
From: [email protected]
<[email protected]> on behalf of Weiguang guan
<[email protected]>
Sent: Wednesday, January 21, 2015 2:47 PM
To: [email protected]
Subject: [Shimmer-users] Magnetometer puzzle
Hi Shimmer users,
This morning I played with a single Shimmer3 sensor and watched the
plots of magnetometer x, y, z (all CAL) on the fly. I placed the sensor
on a table, and slowly rotated it while keeping one of its face in full
contact with the table surface. I found one of the axes (which is
Cal Z)
remained roughly constant so I believe the Z axis points up. What
confused me is that the other two components are mostly negative during
the rotation, especially one of them never had a positive value no
matter how much I rotated it.
How to explain this? Even if some electrical device in the room had
impact on the local magnetic field, the combined field (with earth
magnetic field) should point to certain direction. I would very much
appreciate it if someone could shed a light on the confusion. Thank you
very much.
Best,
Weiguang
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