Question

I am trying to grab the device acceleration using CoreMotion. I have this code:

for setting

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;

for reading

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);

All I have here are zeros!!!!!!!!

The reading method repeats at 30 Hz.

this is taken from Apple sample and should not be giving me zeros!
any clues?

thanks.

Answer 1

What do you see if you drop the low-pass filter and just log the raw acceleration? (I bet I know the answer 🙂