iphone-Coremotion加速度始终为零!

用于设置

if (motionManager == nil) 
        motionManager = [[CMMotionManager alloc] init];

motionManager.deviceMotionUpdateInterval = framesPorKeyframes / 60.0; //60 Hz
motionManager.accelerometerUpdateInterval =  framesPorKeyframes / 60.0; //60 Hz

[motionManager startDeviceMotionUpdates];
referenceAttitude = nil;

供阅读

vec3f_t translation = {0.0f, 0.0f, 0.0f};
CMRotationMatrix rotation;
CMAcceleration userAcceleration;


CMDeviceMotion *deviceMotion = motionManager.deviceMotion;      
attitude = deviceMotion.attitude;

rotation = attitude.rotationMatrix;

userAcceleration = deviceMotion.userAcceleration;
[userAccelerationLpf addAcceleration:userAcceleration withTimestamp:deviceMotion.timestamp];

// The user acceleration we want to use is the one computed by userAccelerationLpf
userAcceleration.x = userAccelerationLpf.x;
userAcceleration.y = userAccelerationLpf.y;
userAcceleration.z = userAccelerationLpf.z; 

NSLog(@"acc = %f, %f, %f", userAcceleration.x, userAcceleration.y, userAcceleration.z);

读取方法在30赫兹重复。

这是从苹果的样品,不应该给我零! 有线索吗?