Comparing values from accelerometer over time

[nick_beaumont]

Hi I am new to Arduino and I am trying to use an accelerometer to detect motion.

So far I have got the accelerometer to send values to my Arduino Particle Argon board but I am struggling with implementing some logic that can compare the change in values over time.

To be clear I am trying to detect a significant change in the values which would imply the accelerometer is moving.

#include "Wire.h" // This library allows you to communicate with I2C devices.

const int MPU_ADDR = 0x68; // I2C address of the MPU-6050

int16_t acc_x, acc_y, acc_z;

char tmp_str[7]; // temporary variable used in convert function

char* convert_int16_to_str(int16_t i) { // converts int16 to string. Moreover, resulting strings will have the same length in the debug monitor.
  sprintf(tmp_str, "%6d", i);
  return tmp_str;
}

void setup() {
    
  Serial.begin(9600);
  Wire.begin();
  Wire.beginTransmission(MPU_ADDR); // Begins a transmission to the I2C slave (GY-521 board)
  Wire.write(0x6B); // PWR_MGMT_1 register
  Wire.write(0); // set to zero (wakes up the MPU-6050)
  Wire.endTransmission(true);
  
}

void loop() {

    
  Wire.beginTransmission(MPU_ADDR);
  Wire.write(0x3B); // starting with register 0x3B (ACCEL_XOUT_H) [MPU-6000 and MPU-6050 Register Map and Descriptions Revision 4.2, p.40]
  Wire.endTransmission(false); // the parameter indicates that the Arduino will send a restart. As a result, the connection is kept active.
  Wire.requestFrom(MPU_ADDR, 7*2, true); // request a total of 7*2=14 registers
  
  // "Wire.read()<<8 | Wire.read();" means two registers are read and stored in the same variable
  acc_x = Wire.read()<<8 | Wire.read(); // reading registers: 0x3B (ACCEL_XOUT_H) and 0x3C (ACCEL_XOUT_L)
  acc_y = Wire.read()<<8 | Wire.read(); // reading registers: 0x3D (ACCEL_YOUT_H) and 0x3E (ACCEL_YOUT_L)
  acc_z = Wire.read()<<8 | Wire.read(); // reading registers: 0x3F (ACCEL_ZOUT_H) and 0x40 (ACCEL_ZOUT_L)
  

}

any help in this would be appreciated thank you

[nick_beaumont]

So I need to compare the "acc_x" value at time_1 to "acc_x" at time_2 and if the difference between the two values is significant then produce some action

[UKHeliBob]

Your post was MOVED to its current location as it is more suitable.

[Railroader]

Accelerometers can't tell if there is motion or not, only that there was a change in motion.
Moving at constant speed the acceleration is 0.

[nick_beaumont]

yes I know, that's the whole point of the question. I want to detect when the accelerometer changes from rest to motion, which would be represented as a significant change in values.

ill try explain it like code:

x = read accelerometer
y = read accelerometer 1 second later
threshold = some value

delta = y - x

if (delta > threshold)

then ("motion detected")

[TomGeorge]

Hi,
Wouldn't it be easier to get data out of the MPU-6050 if you used a library.

newaccelreading = read accel
diffaccel = newaccelreading - oldaccelreading
if diffaccel > threshold then change in accel was up
if diffaccel < -threshold then change in accel was down
oldaccelreading = newaccelreading
delay()
loop back.

Do that for your 3 axes.

Tom...