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Topic: Calibrating ADXL345 as per Sparkfun and Adafruit Guides - Calculations assist? (Read 385 times) previous topic - next topic



I am trying to calibrate a ADXL345 accelerator using the sparkfun and adafruit guides as per:


I used both as 1 was better in one area, and the other in another area, but regardless I am not sure how I go from the result to the calculated offsets and gain to modify the raw data.

As per the sparkfun guide linked above:


The offset values and gain factors are calculated with the following equations as stated in the Application Note from Analog Devices (page 8: equations 17 and 18).

In the DEFINED VARIABLES section of the code is where we will place the new calculated values for offset and gain.

Code: [Select]
/************** DEFINED VARIABLES **************/
/*                                             */
#define offsetX     0    // OFFSET values
#define offsetY     0
#define offsetZ     0

#define gainX       1     // GAIN factors
#define gainY       1
#define gainZ       1
I am sorry if I am just being slow or if I am not clear on this, but I performed the calibration steps and the results for the 6 tests were:

Code: [Select]
Accel Minimums: -4  0  0
Accel Maximums: 0  3  276

Accel Minimums: -4  0  0
Accel Maximums: 12  258  276

Accel Minimums: -250  0  -6
Accel Maximums: 12  258  276

Accel Minimums: -250  -259  -6
Accel Maximums: 12  258  276

Accel Minimums: -250  -259  -6
Accel Maximums: 264  258  276

Accel Minimums: -250  -259  -230
Accel Maximums: 264  258  276

What I am missing is how do I go from those calibration results to the offset and gain values as per their formula?


I did a similar calibration using the SparkFun guide.

I made a little excel spreadsheet with the values (such as the ones at the bottom of your serial port's readout):
   +1g   -1g   Gain      Offset
X   246   -268   257       -5.5
Y   243   -264   253.5    -5.25
Z   238   -265   251.5    -6.75

The gain and offset values are from the two equations you posted:
For example: Gain for x = 0.5*(236-(-268)) and Offset for x = 0.5*(236+(-268)).

How did it work out for you? I'm still not getting the results I was expecting exactly (with the default increments of 62.5 mg per increment my z-axis is reading ~30 which looks like ~1.8g).

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