Sans doute du côté de Core Motion ? As-tu regardé dans ce framework dans la documentation Xcode ?
Core Motion reports motion- and environment-related data from the onboard hardware of iOS devices, including from the accelerometers and gyroscopes, and from the pedometer, magnetometer, and barometer. You use this framework to access hardware-generated data so that you can use it in your app. For example, a game might use accelerometer and gyroscope data to control onscreen game behavior.
Many services of this framework let you access both the raw values recorded by the hardware and a processed version of those values. Processed values do not include forms of bias that might adversely affect how you use that data. For example, a processed accelerometer value reflects only the acceleration caused by the user and not the acceleration caused by gravity.