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LIS3MDL: unexpected readings

Question asked by Dragos Ionescu on Jun 5, 2017
Latest reply on Sep 26, 2017 by Khaled Aljehani

Hi,

 

We're using LIS3MDL to detect changes in the earth magnetic field due to various metallic objects flowing in the sensor proximity. So, we are not interested in the correctness of the absolute values in order to properly detect the magnitude of the field or the North direction, we only need to compare the magnitude of the field between consecutive measurements. That being said, I don't see any reason to calibrate the readings.

 

The problem we are facing is that for unknown reason, the readings (mostly on Z axis, but I've seen also for Y) on some of our sensors are sometimes evolve from initial "right after boot & install", normal values to really unexpected and different values. The external conditions are the same, the magnetic field is the normal earth field.

 

Please have a look on this table, it's basically a log with recordings (x1, y1, z1 is the first measurement, x2, y2, z2 - second measurement, typically after two minutes):

 

Id                     Timestamp                              x1        x2          y1         y2          z1           z2

00000094 | 2017-05-18 02:32:08.625 | 0    | -53  | -89  | -173 | 496   | 438   | 000007d2
00000094 | 2017-05-18 02:34:16.625 | -60  | -41  | -162 | -160 | 456   | 504   | 000007d2
00000094 | 2017-05-18 02:36:39     | -84  | -15  | -177 | -85  | 448   | 519   | 000007d2
00000094 | 2017-05-18 02:36:55.625 | -14  | -4   | -90  | -114 | 538   | 508   | 000007d2
00000094 | 2017-05-18 02:37:51.625 | -6   | 15   | -112 | -114 | 502   | 530   | 000007d2
00000094 | 2017-05-18 03:04:49.75  | 10   | -27  | -111 | -198 | 486   | 432   | 000007d2
00000094 | 2017-05-18 03:13:48.75  | -49  | -31  | -205 | -212 | 471   | 509   | 000007d2
00000094 | 2017-05-18 03:18:19.75  | -40  | 0    | -206 | -134 | 462   | 544   | 000007d2
00000094 | 2017-05-18 03:18:58.687 | -2   | 52   | -136 | -187 | 544   | 425   | 000007d2
00000094 | 2017-05-18 03:24:35.687 | 101  | 75   | -169 | -156 | 452   | 397   | 000007d2
00000094 | 2017-05-18 03:37:30.187 | 58   | -43  | -120 | -160 | 395   | 437   | 000007d2
00000094 | 2017-05-18 04:09:08.812 | -69  | -4   | -197 | -112 | 471   | 552   | 000007d2
00000094 | 2017-05-18 04:10:04.75  | -6   | 18   | -112 | -119 | 538   | 486   | 000007d2
00000094 | 2017-05-18 04:22:32.75  | 1    | 19   | -132 | -130 | 521   | 562   | 000007d2
00000094 | 2017-05-18 04:38:05.687 | 11   | 26   | -88  | -87  | 474   | 508   | 000007d2
00000094 | 2017-05-18 04:49:14.562 | 64   | 131  | -63  | 88   | 511   | 243   | 000007d2
00000094 | 2017-05-18 04:53:23.562 | 131  | 151  | 102  | 103  | 235   | 282   | 000007d2
00000094 | 2017-05-18 05:15:09.187 | 112  | -670 | 115  | 739  | 193   | -2581 | 000007d2
00000094 | 2017-05-18 05:17:39.187 | -675 | -692 | 757  | 821  | -2587 | -2698 | 000007d2
00000094 | 2017-05-18 05:33:11.187 | -671 | -546 | 713  | 868  | -2369 | -2268 | 000007d2
00000094 | 2017-05-18 05:33:57.687 | -547 | -546 | 869  | 916  | -2286 | -2309 | 000007d2
00000094 | 2017-05-18 05:42:30.187 | -555 | -560 | 910  | 864  | -2299 | -2261 | 000007d2
00000094 | 2017-05-18 05:43:49.187 | -559 | -555 | 871  | 906  | -2262 | -2302 | 000007d2
00000094 | 2017-05-18 05:50:51.187 | -533 | -545 | 874  | 826  | -2307 | -2270 | 000007d2

...

00000094 | 2017-05-23 07:48:58.187 | -486 | -414 | 539   | 534   | -1976 | -2196 | 000007d2
00000094 | 2017-05-23 07:49:38.187 | -415 | -489 | 539   | 544   | -2185 | -1990 | 000007d2
00000094 | 2017-05-23 08:09:22.687 | -491 | -524 | 540   | 418   | -1994 | -2097 | 000007d2
00000094 | 2017-05-23 08:24:54.375 | -532 | -501 | 399   | 523   | -2075 | -1946 | 000007d2

...
00000094 | 2017-05-28 10:57:12.562 | 126  | -52  | -4770 | -4770 | -4788 | -4752 | 000007d2
00000094 | 2017-05-28 10:57:13.062 | -56  | 352  | -4770 | -4770 | -4752 | -4788 | 000007d2
00000094 | 2017-05-28 11:47:07.5   | 342  | -156 | -4770 | -4770 | -4788 | -4752 | 000007d2
00000094 | 2017-05-28 12:05:15.562 | -134 | -344 | -4770 | -4770 | -4752 | -4753 | 000007d2
00000094 | 2017-05-29 01:27:41.312 | -325 | -181 | -4770 | -4770 | -4753 | -4752 | 000007d2
00000094 | 2017-05-29 01:34:39.062 | -194 | -336 | -4770 | -4770 | -4752 | -4753 | 000007d2
00000094 | 2017-05-29 01:45:55.062 | -313 | -384 | -4770 | -4770 | -4753 | -4753 | 000007d2

 

 

In bold with red are rows where things went wrong in my opinion, values shifted significantly without any external influence since the sensors where in a controlled area, on ground.

 

Basically we are stuck here, I can't imagine what could trigger such a shift in the values.

 

Some other facts that might help:

 

1. The sensors are potted in a resin which is certified for electronics encapsulation.

2. Temperature variations are likely to occur, first during the potting process (max. 60 C), next due to exposure to the sun light (again, max 60 C by estimation)

3. We brought the sensor from we've taken the log from the field in the lab and we did a reset on the battery. We were surprised to see that the values came to normal, right after reset!

3. We performed some tests with sensors without encapsulation and the shift did NOT occurred. Those tests happened about one year ago.

4. Batteries. We are using Lithium SoCl2 batteries which are quite old and they passivate but we have managed to brought them alive. Theoretically it could happen to have some voltage drops from time to time... but we also have a radio transmitter on sensor which works well with no sign of interruption due to a voltage drop.

5. Some sensors (in fact more than a half) does not visibly manifest the issue but we are suspecting that the issue is still present in small amounts. 

 

It seems that the encapsulation somehow triggers the issue but we don't know what could be the real reason. So far we've tested the temperature variation in lab, the sensors behavior is normal - a small drift which is acceptable in our case.

 

Does anyone have an idea what could be wrong? We are not able to reproduce the behavior in a controlled environment.

 

Thanks,

Dragos

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