Posted on September 24, 2017 at 17:56 Hello
khaledaljehani99
,
Khaled Aljehani wrote:
The original equation is:
magnetic_intensity (in gauss) = raw_data / sensitivity
So, for this equation the sensitivity is a value from the table in the datasheet(6842 or 2..etc)
Am I correct here?
Yes, you are.
Now, regarding LSB/gauss. What I understand is
6842 LSB = 1 gauss,
6842 LSB = 1000 mgauss
So, sensitivity = 6842/1000 = 6.8
My question is why did you calculate it in that way? (1000 mgauss / 6842 LSB = 0.14)
Last thing, in the cube library, they use this equation: mgauss = raw data * sensitivity.
Should not we use this one (magnetic_intensity (in gauss) = raw_data / sensitivity)?
There are at least two ways of calculating the output value, if you know that the sensitivity is 6 842 LSB per 1 gauss (=1 000 mgauss) and the sensor gives you output of
–32,768 to 32,7
1)
magnetic_intensity = raw_data / sensitivity
with the following units:
[gauss] = [LSB] / [LSB/gauss]
so sensitivity is represented here as[LSB/gauss]
This relates to the values printed in the datasheet and discussed in the linked discussion.
2)
magnetic_intensity = raw_data * sensitivity
with the following units:
[gauss] = [LSB] * [gauss/LSB]
so sensitivity is represented here as
[gauss/LSB] (which is an inverted value) The fact whether you count with gauss or miligauss doesn't affect the equation, just the sensitivity coefficients (and/or the result).
This relates to the values in the Cube library.
0.14 < Sensitivity value for 4 gauss full scale [mgauss/LSB] (= 0,000 14 gauss/ LSB)
0.29 < Sensitivity value for 8 gauss full scale [mgauss/LSB] (= 0,000 29 gauss / LSB)
0.43 < Sensitivity value for 12 gauss full scale [mgauss/LSB] (= 0,000 43 gauss / LSB)
0.58 < Sensitivity value for 16 gauss full scale [mgauss/LSB]
(= 0,000 58 gauss / LSB)
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ST chose version 2, however, I like version 1 more. Version 1 can be faster and use less resources, moreover, with a higher precision!
If the sensitivity was a round number in both cases, it would perform the same, but it is not in this case.
Example 1:
If the sensor gives you an output of 6 842, method 1 will tell you that the output is 1 gauss. Method 2 will tell you that the output is 957 mgauss (about 4 % difference). (This also comes from the fact that
0,146156 mgauss / LSB was rounded to 0,14, not 0,15 as I would expect).
You might say that method 1 output only an integer value in gauss and that you want output in miligauss. There is no problem with that, just multiply the raw_data with a 1000 and you get a 1000 x bigger result (which is what you need, because 1 gauss = 1000 mgauss).
Example 2, raw data = 10 000:
Method 1:
magnetic_intensity [mgauss] = ( raw_data * 1 000) / sensitivity
magnetic intensity [mgauss] = 10 000 000 / 6 842 = 1 461 mgauss (=1,461 gauss)
Method 2:
magnetic_intensity [mgauss] = raw_data * sensitivity
magnetic_intensity [mgauss] = 10 000 * 0,14 = 1 400 mgauss ( = 1,400 gauss) =again, about 4 % error
However, method 1 has a downside - if you multiply an output value (by a 1 000 in this example), make sure your data type will handle this.
In this case sensor raw_data is –32,768 to 32,767 , multiplied by 1 000 gives us -32 768 000 to 32 767 000 and this easily fits into a signed int32 with –2,147,483,648 to 2,147,483,647 range.
You can the code of method 1 on your own, just call the method below with raw_data as signed int16 as a parameter.
#define DECIMAL_3DIGIT_FACTOR 1000u // 3decimal points multiplication
#define COUNT_TO_GAUSS_4G_SCALE 6842; // measured data count per 1 Gauss, full scale +/- 4 g
#define COUNT_TO_GAUSS_8G_SCALE 3421; // measured data count per 1 Gauss, full scale +/- 8 g
#define COUNT_TO_GAUSS_12G_SCALE 2281; // measured data count per 1 Gauss, full scale +/- 12 g
#define COUNT_TO_GAUSS_16G_SCALE 1711; // measured data count per 1 Gauss, full scale +/- 16 g
// converts magnetometer data (hex) to Gauss unit
int32_t MM_Convert_To_Gauss (int16_t magnetometerData) {
int32_t mgaussData;
mgaussData = (magnetometerData * DECIMAL_3DIGIT_FACTOR);
mgaussData /= COUNT_TO_GAUSS_4G_SCALE;
return gaussData;
}
