How to access raw data with CoreMotion - ios

Question

I'm going to develop an iPhone application based on CoreMotion and Pedestrian Dead Reckoning Algorithm.
I'm experiencing a little strange behaviour reading gyroscope and acceleration data (device: iPhone 5s).
Here is my code:
motionManager = [[CMMotionManager alloc] init];
motionManager.deviceMotionUpdateInterval=0.008;
[motionManager startDeviceMotionUpdatesToQueue:[NSOperationQueue mainQueue] withHandler:^(CMDeviceMotion * motion, NSError *error)
{
NSString *Acc_x = [[NSString alloc] initWithFormat:#"%f ",motion.userAcceleration.x];
NSString *Acc_y = [[NSString alloc] initWithFormat:#"%f ",motion.userAcceleration.y];
NSString *Acc_z = [[NSString alloc] initWithFormat:#"%f ",motion.userAcceleration.z];
NSData *AccData_x = [Acc_x dataUsingEncoding:NSUTF8StringEncoding];
NSData *AccData_y = [Acc_y dataUsingEncoding:NSUTF8StringEncoding];
NSData *AccData_z = [Acc_z dataUsingEncoding:NSUTF8StringEncoding];
NSString *Gyro_x = [[NSString alloc] initWithFormat:#"%f ",motion.rotationRate.x];
NSString *Gyro_y = [[NSString alloc] initWithFormat:#"%f ",motion.rotationRate.y];
NSString *Gyro_z = [[NSString alloc] initWithFormat:#"%f ",motion.rotationRate.z];
NSData *GyroData_x = [Gyro_x dataUsingEncoding:NSUTF8StringEncoding];
NSData *GyroData_y = [Gyro_y dataUsingEncoding:NSUTF8StringEncoding];
NSData *GyroData_z = [Gyro_z dataUsingEncoding:NSUTF8StringEncoding];
[HandleAcc seekToEndOfFile];
[HandleAcc writeData:AccData_x];
[HandleAcc writeData:AccData_y];
[HandleAcc writeData:AccData_z];
[HandleAcc writeData:GyroData_x];
[HandleAcc writeData:GyroData_y];
[HandleAcc writeData:GyroData_z];
NSDate *DataOra = [NSDate date];
NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
[dateFormatter setDateFormat:#"HH:mm:ss\r\n"];
NSString *dataString = [dateFormatter stringFromDate:DataOra];
NSData *data = [dataString dataUsingEncoding:NSUTF8StringEncoding];
[HandleAcc writeData:data];
}
];
And here is my log:
acc_x acc_y acc_z gyro_x gyro_y gyro_z time
0.006040 0.003147 -0.011406 0.001642 0.002079 -0.000864 01:04:40
0.006472 0.000715 -0.008345 -0.003239 -0.001569 -0.000839 01:04:40
After, I tried to compare iPhone's IMU data with other from an Adafruit's 10DOF IMU and I obtain this log:
acc_x acc_y acc_z gyro_x gyro_y gyro_z time
-4.773060 -1.769106 8.427839 -0.030030 0.022133 0.002239 0.008333
-4.789249 -1.741337 8.442565 -0.028949 0.009581 0.000027 0.016667
What am I doing wrong?? Are iPhone data biased? If so, should I acces unbiased data?
Hope someone will help. Thank you, Giacomo.

Answer 1

CMMotionManager has many different things it can measure. I think all but the one you are using are unbiased.
After creating an instance of CMMotionManager, an app can use it to receive four types of motion: raw accelerometer data, raw gyroscope data, raw magnetometer data, and processed device-motion data (which includes accelerometer, rotation-rate, and attitude measurements).
If you want unbiased data, either combine gravity and userAcceleration as documented or measure acceleration directly.