Part3.1 - Head Impact Detection [ADXL377]

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

If you could elaborate a little bit more what your values mean this would help. The ADXL377 outputs a voltage of about 6.5mV/g, with a bias of about 1.5V (the sensitivity can be between 5.8 and 7.2 mV/g, and the zero-G bias 1.4-1.6V).

I don't even know what your numbers mean. What is 675? What is the unit of your numbers? What is your ADC reference voltage? When did you convert the ADC reading to mV? Where do you correct for the zero-G bias voltage?

To correct for zero-G, add a function to your code that you run when your system is in a zero-G situation. Then you can do some ADC readings, store their results and use them to correct further readings.

Btw: an accelerometer is not really suited to measure rotational forces, a gyroscope would be much better.

My code is here :  https://www.dropbox.com/s/ivdus3843ga02lq/code.txt?dl=0

As the tutorial here says, https://learn.sparkfun.com/tutorials/adxl337-and-adxl377-accelerometer-hookup-guide?_ga=1.48641120.711058906.1436800294

The ADXL377 are 3.3V devices, so  analog to digital (ADC) readings will vary depending on whether we’re using a 5V or 3.3V micro! As i am using Arduino Mega and hence a 5Volt microcontroller, In the code 675 came from the fact that a 5V Arduino will measure 3.3V as 675, while a 3.3V Arduino will measure 3.3V as 1023.  (As mentioned in the tutorial)

Now we are converting to g values using this mapf function;

float mapf(float x, float in_min, float in_max, float out_min, float out_max)

{

  return (x - in_min) * (out_max - out_min) / (in_max - in_min) + out_min;

}

In the output: I am getting the Raw vaues which are directly being read from Analog pins of arduino Mega  A0, A1, A2 as

X: 367

Y: 367

Z: 369

and after converting to g values , i am getting here

X: 8.74 g

Y: 8.74 g

Z: 9.33 g

Yes, you are right, I am also using a Gyroscope , but for a sensor fusion using kalman filter i would need this Accelerometer as well. But this is not showing correect reading at all at the stationary state even?

Should i just substarct these offset values to get 0,0,1 g in the stationary state? Or i need to change somethingelse in the code?

Do you have some snnippet of your calibration method, May be it will be helpful to understand the process. Thank you

Hello Nayak,

I am not that much familiar with the Arduino library, sorry .

However, I can give my opinion from the point of common ADC measuring technique.

As you said that you are using a 5v supply for the microcontroller, I assume that your ADC  reference is AVCC ~= 5v. In that case the output you are getting is fairly close enough. But there are some small issues I think.

First of all, your are right that the ADC value can be upto 675 for about 3.3v output from the sensor [assuming that you have exact 5v ADC reference].

     Now, the sensor produces around Vs/2 at 0g and varies up to Vs and down to 0v respectively for positive and negative variation of acceleration value. It means, for 3.3v supply the 0g value would be around 1.65v .

     For 5v ADC reference, 1.65v will give an ADC value of 338. What you are getting is about 367, which is close enough to prove that your sensor is functioning properly. You can be sure by measuring the output voltage of the sensor using a digital multimeter to check whether it is around 1.65v or not.

     Now, if you look at my ADC data for 0g, it varies from 510 to 515, where the typical value should be 512.  The deviation is not that high. So, your ADC values are not close enough to precise output. Why is it? I don't know.... But what I can assume is that, you are not having exact 5v at your AVCC [may be you are powering your board with USB]. So, may be your ADC reference is less than 5v and that's why you are getting higher values for 0g.[I am not sure though]

If it is so, then what you can do is to -

     1. ensure that you have exact 5v at your ADC reference or

     2. measure the reference voltage using a multimeter [at AVCC pin] and change your code according to that [in my calculation, I think you have an ADC reference voltage of about 4.6v instead of 5v. So, 3.3v span for 4.6v ADC reference will produce 734 instead of 675] or

     3. you can use internal reference 2.56v as ADC reference. But in that case, ADC output [ 3.3v > sensor output >2.56v ] will not change for extreme high +ve g values.

Secondly, for calibration, you just need to set the values you are getting at 0g as reference and then measure the difference. remember, sensor output voltage will change typically +/- 6.5mv for every single change in +/- g.

Let me know, if you have any query.

Hallo Md. Kamrul,

Thank you very much. I have now a clear Idea of this operating voltage and would try to play with changing the code and give a try as per your suggestion. Yes, I am using Mega which is operating at 5 Volt although it is getting 4.7 (you are right) volt as i just checked. The arduino has two pins for 5V and 3.3 V and i have connected sensor it to 3.3 Volt.

The ADXL377 breakout board (Sparkfun) I am using is operating at 3.3 Volts. Yes, I am using the Arduino from USB Port of my Laptop (USB2). I also checked with the Multimeter at the following point and the datas i got are as follows; (Stationary case)

1) VCC = 3.332 Volt
2) X      = 1.659 Volt
3) Y      = 1.631 Volt
4) Z      = 2.012 Volt

so i calculates for 4,7 volt it should be approximately 718 instead of 675. After changing it i get the following raw datas

X: 352

Y: 352

Z: 354

X: 352

Y: 351

Z: 353

X: 351

Y: 351

Z: 353

X: 352

Y: 352

Z: 353

If I take Offset_X = 352 , Offset_Y = 352 , Offset_Z = 353  then i get 0, 0, 0. I am not sure if you mean I should substract the 352 from each and then it is 0, 0, 1 and afterwards for each 6.5 mv i should change 1g ?

Can you please tell me Which offset would be substracted at Z axis? How we convert it to g values?

I am relaxed to know this sensor is working correctly .  Thank you very much for the support. I am really glad that this post has been very very helpful to me.

Hallo Md. Kamrul,

Thank you very much. I have now a clear Idea of this operating voltage and would try to play with changing the code and give a try as per your suggestion. Yes, I am using Mega which is operating at 5 Volt although it is getting 4.7 (you are right) volt as i just checked. The arduino has two pins for 5V and 3.3 V and i have connected sensor it to 3.3 Volt.

The ADXL377 breakout board (Sparkfun) I am using is operating at 3.3 Volts. Yes, I am using the Arduino from USB Port of my Laptop (USB2). I also checked with the Multimeter at the following point and the datas i got are as follows; (Stationary case)

1) VCC = 3.332 Volt
2) X      = 1.659 Volt
3) Y      = 1.631 Volt
4) Z      = 2.012 Volt

so i calculates for 4,7 volt it should be approximately 718 instead of 675. After changing it i get the following raw datas

X: 352

Y: 352

Z: 354

X: 352

Y: 351

Z: 353

X: 351

Y: 351

Z: 353

X: 352

Y: 352

Z: 353

If I take Offset_X = 352 , Offset_Y = 352 , Offset_Z = 353  then i get 0, 0, 0. I am not sure if you mean I should substract the 352 from each and then it is 0, 0, 1 and afterwards for each 6.5 mv i should change 1g ?

Can you please tell me Which offset would be substracted at Z axis? How we convert it to g values?

I am relaxed to know this sensor is working correctly .  Thank you very much for the support. I am really glad that this post has been very very helpful to me.

Something in you numbers is wrong. The Z voltage is way off with 2V 8this would correspond to about 60g. And then you say the ADC result is the same as the others.

Regarding the calibration: the sum of 1024 values of 352 is not 352, its 360448, so the result of your calibration reading should be around that mark. Take the sum of 1024 values! Then divide by 1024 to get the average.

(But even here your numbers are off: when the values are {352, 352, 353} then a division by 1024 cannot result in {0.350, 0.349, 0.351} - the first two numbers must be equal. Something in your code is wrong.) Whether you want to calibrate so that you readings end up with 1g for the z axis depends on what you are doing - in my case I was interested only in the resulting acceleration, so I removed the Z axis force of 1g out of my data (this also meant that my system is not depending on its orientation).

You calculate the resulting G force like this: (assuming that you are using calibrated values, so they should be in the range of about -350 to +350): ADC_result*(4.7/1024)/0.0065. The part 4.7/1024 calculates how many mV one LSB represents, multiplying this with the ADC result gives you the reading in mV. Then you divide this by 6.5 to convert mV into g, as per the ADXL377 data sheet.