Part3.1 - Head Impact Detection [ADXL377]

RE: Part3.1 - Head Impact Detection [ADXL377]

Former Member — Mon, 02 Nov 2015 23:39:00 GMT

If I Use your method:
have raw values = (352,352,353)
1)Now say, CalX = 0 , CalY = 0, CalZ = 0 and

2)1024 Values are (352,352,353)

3) Dividing by 1024 we get (0.350,0.349,0.351)
4)Now , CalX = 0.350 , CalY = 0.349, CalZ = 0.351
5)Now Next reading Substract Calx,CalY, CalZ from new averaged raw datas(i mean after dividing by 1024)
6)for each change of one LSB , we add or substract 0.75g , so for each 0.1 change in ADC datas in step 5, we add or substract 0.075.

Is it correct? I have a doubt , If i get nest raw datas averaged as (0.350,0.349,0.351) and after substracting CalX, calY, CalZ, we get (0,0,0)G, instead of (0,0,1) G.

Should i manupulate the reading so as to get (0,0,1)G here?

RE: Part3.1 - Head Impact Detection [ADXL377]

hlipka — Mon, 02 Nov 2015 18:58:03 GMT

Regarding the calibration: I wrote it for the PSoC4 BLE, and used a ADXL375. So maybe its not really useful to paste my code. The logic I used was:

set the calibration values to 0
read 1024 values for each axis and add them
divide each result by 1024 (shift-left 10 bits) to get the average
store the result in the calibration values
afterwards, when reading values, substract the calibration values from the result

RE: Part3.1 - Head Impact Detection [ADXL377]

hlipka — Mon, 02 Nov 2015 18:52:47 GMT

After doing some more math, I think you should use an external ADC with higher resolution. In your setup one LSB of the ADC result corresponds to 4.88mV. And the ADXL377 gives you 6.5mV/g, so that each single bit by which the output changes (and this easily in the noise here given that your 5V power rail is probably not that stable and clean) changes your result by 0.75g,.

Regarding the measured voltages - what is the supply voltage for the ADXL377? Its outputs are ratiometric, meaning that the zero-G output is always half of the supply voltage (+-0.5V according to the data sheet). If your reference is actually 5V, your result of 367 would mean 1.79V which is a little bit high.

RE: Part3.1 - Head Impact Detection [ADXL377]

Former Member — Mon, 02 Nov 2015 15:22:04 GMT

Hello,
I am using this ADXL377 to detect some high rotational motion. I used the following code from Sparkfun
https://github.com/sparkfun/ADXL377_Breakout/blob/master/firmware/ADXL377_example/ADXL377_example.ino

However my OUTPUT in the normal scenario (Sensor resting on a table flat on surface) is as follows

X: 367

Y: 367

Z: 369

X: 8.74 g

Y: 8.74 g

Z: 9.33 g

X: 367

Y: 367

Z: 369

X: 8.74 g

Y: 8.74 g

Z: 9.33 g

I am using Arduino Mega and selected boolean micro_is_5V = true;

scaledX = mapf(rawX, 0, 675, -scale, scale); // 3.3/5 * 1023 =~ 675

scaledY = mapf(rawY, 0, 675, -scale, scale);

scaledZ = mapf(rawZ, 0, 675, -scale, scale);

Can you please tell, What is going wrong here? How can i calibrate it to get nearly (0.0.1)G values in standstill condition?

Thanks

RE: Part3.1 - Head Impact Detection [ADXL377]

DAB — Tue, 05 May 2015 19:36:13 GMT

Good post.

I look forward to seeing your sensor in action.

DAB