G measurement – uncorrect values

[Sten-Helge]

Hi there,
According to the datasheet and your tutorials – when the accelerometer is horizontal and at rest, the value for Z should hover around 1 G.
When using a sensitive level of ±8g, the raw value for Z should hover around 4,475.
My readings from the below code snippet gives;

….
Loop 20 Xraw = -588 Yraw = -34 Zraw = 5304
Loop 20 Xraw = -520 Yraw = -67 Zraw = 5374
Loop 20 Xraw = -599 Yraw = -84 Zraw = 5460
Loop 20 Xraw = -549 Yraw = -70 Zraw = 5441
Loop 20 Xraw = -549 Yraw = -92 Zraw = 5341
Loop 20 Xraw = -517 Yraw = -91 Zraw = 5406
Loop 20 Xraw = -527 Yraw = -70 Zraw = 5281
Loop 20 Xraw = -538 Yraw = -73 Zraw = 5382
….

which gives – converted to G;

….
Loop 20 X = -0.139 G Y = -0.029 G Z = 1.184 G
Loop 20 X = -0.128 G Y = -0.020 G Z = 1.197 G
Loop 20 X = -0.136 G Y = -0.027 G Z = 1.228 G
Loop 20 X = -0.137 G Y = -0.028 G Z = 1.225 G
Loop 20 X = -0.118 G Y = -0.025 G Z = 1.190 G
Loop 20 X = -0.126 G Y = -0.024 G Z = 1.214 G
Loop 20 X = -0.137 G Y = -0.026 G Z = 1.210 G
Loop 20 X = -0.128 G Y = -0.024 G Z = 1.216 G
….

See below the used code for this test;

int mymillis()
{
	struct timeval tv;
	gettimeofday(&tv, NULL);
	return (tv.tv_sec) * 1000 + (tv.tv_usec)/1000;
}
void  readBlock(uint8_t command, uint8_t size, uint8_t *data)
{
    int result = i2c_smbus_read_i2c_block_data(file, command, size, data);
    if (result != size)
    {
        printf("Failed to read block from I2C.");
        exit(1);
    }
}
void selectDevice(int file, int addr)
{
        if (ioctl(file, I2C_SLAVE, addr) < 0) {
		 printf("Failed to select I2C device.");
        }
}
void writeAccReg(uint8_t reg, uint8_t value)
{
    selectDevice(file,ACC_ADDRESS);
  int result = i2c_smbus_write_byte_data(file, reg, value);
    if (result == -1)
    {
        printf ("Failed to write byte to I2C Acc.");
        exit(1);
    }
}
void enableIMU()
{
        char filename[20];
        sprintf(filename, "/dev/i2c-%d", 1);
        file = open(filename, O_RDWR);
        if (file<0) {
			printf("Unable to open I2C bus!");
                exit(1);
        }
        // Enable accelerometer.
        writeAccReg(CTRL_REG1_XM, 0b01100111); //  z,y,x axis enabled, continuos update,  100Hz data rate
		//writeAccReg(CTRL_REG2_XM, 0b00100000); // +/- 16G full scale
		writeAccReg(CTRL_REG2_XM, 0b00010000); // +/- 8G full scale
        //writeAccReg(CTRL_REG2_XM, 0b00000000); //+/- 2G full scale
}
void readACC(int  *a)
{
        uint8_t block[6];
        selectDevice(file,ACC_ADDRESS);
        readBlock(0x80 | OUT_X_L_A, sizeof(block), block);
        *a = (int16_t)(block[0] | block[1] << 8);
        *(a+1) = (int16_t)(block[2] | block[3] << 8);
        *(a+2) = (int16_t)(block[4] | block[5] << 8);
}
int main(int argc, char *argv[])
{
	int  accRaw[3];
	int startInt  = mymillis();
	struct  timeval tvBegin;
    signal(SIGINT, INThandler);
	enableIMU();
	gettimeofday(&tvBegin, NULL);
	while(1)
	{
	startInt = mymillis();
	//read ACC and GYR data
	readACC(accRaw);
	
	// +/- 2G full scale
	//printf("X = %7.3f G\t" , ((accRaw[0] * 0.061)/1000));
	//printf(" Y =   %7.3f G\t" , ((accRaw[1] * 0.061)/1000));
	//printf(" Z =  %7.3f G\n" , ((accRaw[2] * 0.061)/1000));
	
	// +/- 8G full scale
	printf("X = %7.3f G\t" , ((accRaw[0] * 0.224)/1000));
	printf(" Y =   %7.3f G\t" , ((accRaw[1] * 0.224)/1000));
	printf(" Z =  %7.3f G\n" , ((accRaw[2] * 0.224)/1000));	
	
	//printf("Xraw = %d \t" , accRaw[0]);
	//printf(" Yraw =   %d \t" , accRaw[1]);
	//printf(" Zraw =  %d \n" , accRaw[2]);			
	//Loop time
        while(mymillis() - startInt < (DT*1000))
        {
            usleep(100);
            //printf(".");
        }
	printf("Loop %d\t", mymillis()- startInt);
    }
}

May I kindly ask for your advice on how to proceed in order to get the proper values.

Thanks

[Sten-Helge]

According to the datasheet the settings for CTRL_REG2_XM register should be as follows for the different scales;

....
        // Enable accelerometer.
        writeAccReg(CTRL_REG1_XM, 0b01100111); //  z,y,x axis enabled, continuos update,  100Hz data rate
		
        //writeAccReg(CTRL_REG2_XM, 0b00000000); //+/- 2G full scale
        //writeAccReg(CTRL_REG2_XM, 0b00001000); //+/- 4G full scale
        //writeAccReg(CTRL_REG2_XM, 0b00010000); // +/- 6G full scale
        //writeAccReg(CTRL_REG2_XM, 0b00011000); // +/- 8G full scale
        writeAccReg(CTRL_REG2_XM, 0b00100000); // +/- 16G full scale
....
....
	//read ACC values
	readACC(accRaw);
	
	// +/- 2G full scale
	//printf("X = %7.2f G\t" , ((accRaw[0] * 0.061)/1000));
	//printf(" Y =   %7.2f G\t" , ((accRaw[1] * 0.061)/1000));
	//printf(" Z =  %7.2f G\n" , ((accRaw[2] * 0.061)/1000));
	
	// +/- 4G full scale
	//printf("X = %7.2f G\t" , ((accRaw[0] * 0.122)/1000));
	//printf(" Y =   %7.2f G\t" , ((accRaw[1] * 0.122)/1000));
	//printf(" Z =  %7.2f G\n" , ((accRaw[2] * 0.122)/1000));
	
	// +/- 6G full scale
	//printf("X = %7.2f G\t" , ((accRaw[0] * 0.183)/1000));
	//printf(" Y =   %7.2f G\t" , ((accRaw[1] * 0.183)/1000));
	//printf(" Z =  %7.2f G\n" , ((accRaw[2] * 0.183)/1000));	
	
	// +/- 8G full scale
	//printf("X = %7.3f G\t" , ((accRaw[0] * 0.244)/1000));
	//printf(" Y =   %7.3f G\t" , ((accRaw[1] * 0.244)/1000));
	//printf(" Z =  %7.3f G\n" , ((accRaw[2] * 0.244)/1000));	
	
	// +/- 12G full scale
	printf("X = %7.2f G\t" , ((accRaw[0] * 0.732)/1000));
	printf(" Y =   %7.2f G\t" , ((accRaw[1] * 0.732)/1000));
	printf(" Z =  %7.2f G\n" , ((accRaw[2] * 0.732)/1000));	
	
	//printf("Xraw = %d \t" , accRaw[0]);
	//printf(" Yraw =   %d \t" , accRaw[1]);
	//printf(" Zraw =  %d \n" , accRaw[2]);		
....

Please advice if not correct.

Thanks

[Mark Williams]

You are correct.. there is a typo in the code.

This;
writeACC(CTRL_REG2_XM, 0b00010000) #+/- 8G full scale
should be
writeACC(CTRL_REG2_XM, 0b00011000) #+/- 8G full scale

I have updated the page.

And your scales in CTRL_REG2_XM above are correct.

Mark --OzzMaker.com --

[Author]

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