ADXL345 bad data

[austinlambert82]

I purchased an accelerometer to try to measure vehicle movements for tuning purposes. The problem is I am not getting consistent readings.

When I set it on the desk the numbers are relatively constant although not at zero (I believe it to be a calibration issue) then I move it around the desk and it again gives me approximately the numbers I should be seeing. Then I strap it to the vehicle and the numbers are all over the place. I thought it might be an over sampling problem, so I backed the samples down from 100 per second to 10.

I thought it could be an issue with vibrations transfer so we packaged the whole thing in a box full of wash cloths to dampen vibrations. While it may not get them all it should get most of them. But that is not what I see. I know that this go kart is rated to between 3 and 4Gs of lateral acceleration so there is no reason I should ever see a number above that let alone the 15G plus spikes I see.

The code is simple in purpose, it grabs the x and y accel and writes it to an sd card shield. The code and the output is a csv file in the attachment. If anyone has any advice it would be greatly appreciated.



#include <SD.h>
#include <Wire.h>
#include <Adafruit_Sensor.h>
#include <Adafruit_ADXL345_U.h>

Adafruit_ADXL345_Unified accel = Adafruit_ADXL345_Unified(12345);

File myFile;
int lineNumber = 1;


void setup()
{
Serial.begin(9600);
while (!Serial) {
}


Serial.print("Initializing SD card...");
pinMode(10, OUTPUT);

if (!SD.begin(10)) {
Serial.println("initialization failed!");
return;
}
Serial.println("initialization done.");


/* Initialise the sensor */
accel.begin();

accel.setRange(ADXL345_RANGE_16_G);

SD.remove("TEST.TXT");

}

void loop()
{
lineNumber += 1;

/* Get a new sensor event */
sensors_event_t event;
accel.getEvent(&event);

/* Display the results (acceleration is measured in m/s^2) */
Serial.print(lineNumber); Serial.print(",");
Serial.print(event.acceleration.x); Serial.print(",");
Serial.print(event.acceleration.y); Serial.println(" ");



// nothing happens after setup

// open the file. note that only one file can be open at a time,
// so you have to close this one before opening another.
myFile = SD.open("test.txt", FILE_WRITE);

// if the file opened okay, write to it:
if (myFile) {
myFile.print(lineNumber); myFile.print(",");
myFile.print(event.acceleration.x); myFile.print(",");
myFile.print(event.acceleration.y); myFile.println(" ");
// close the file:
myFile.close();
} else {
// if the file didn't open, print an error:
Serial.println("error opening test.txt");
}

delay(100);

}

[adafruit_support_mike]

All accelerometers will measure the force of gravity when they're being supported. The only time you get a true zero reading is when they're in free-fall.

WRT the data you posted, it does seem to show reasonable clustering if you take a running average of terms:

Code: Select all

    k = 3;
    avg_x = (( avg_x * ( k - 1 )) + x ) / k;
    avg_y = (( avg_y * ( k - 1 )) + y ) / k;

Higher values of k provide more smoothing, but are slower to respond to actual changes in the input.