How do you implement a Highpass filter for the IPhone accelerometer?

Question

I remember seeing the code for a Highpass filter a few days back somewhere in the samples, however I can't find it anywhere now! Could someone remember me where the Highpass filter implementation code was?

Or better yet post the algorithm?

Thanks!

Answer 1

From the idevkit.com forums:

#define kFilteringFactor 0.1
static UIAccelerationValue rollingX=0, rollingY=0, rollingZ=0;


- (void)accelerometer:(UIAccelerometer *)accelerometer didAccelerate:(UIAcceleration *)acceleration {

    // Calculate low pass values

    rollingX = (acceleration.x * kFilteringFactor) + (rollingX * (1.0 - kFilteringFactor));
    rollingY = (acceleration.y * kFilteringFactor) + (rollingY * (1.0 - kFilteringFactor));
    rollingZ = (acceleration.z * kFilteringFactor) + (rollingZ * (1.0 - kFilteringFactor));

    // Subtract the low-pass value from the current value to get a simplified high-pass filter

    float accelX = acceleration.x - rollingX;
    float accelY = acceleration.y - rollingY;
    float accelZ = acceleration.z - rollingZ;

    // Use the acceleration data.

}

Answer 2

Just in case someone wants to know, the highpass filter can be found in the Accelerometer Graph sample.