Accelerometer 101

What follows is the 30,000 foot introduction to working with the accelerometer on the iPhone.

There is one instance of the UIAccelerometer object that is shared throughout your application. To start receiving input from the accelerometer, set the update interval for the frequency of updates and set the delegate to receive the callbacks. If often looks like this:

// Start receiving callbacks, every 40 milliseconds
[[UIAccelerometer sharedAccelerometer] setUpdateInterval:1/25];
 
// Set the delegate to this object
[[UIAccelerometer sharedAccelerometer] setDelegate: self];

To get this to work as expected the class that is referenced by self must implement the UIAccelerometerDelegate, so your class definition might look as follows:

@interface ShootWeaponViewController : UIViewController <UIAccelerometerDelegate>
{
  ...
  UIAccelerationValue accelerationArray[3];
  ...
}

The code above creates an array with three entries, each of the type UIAccelerationValue (a floating point number) to hold the accelerometer info for each of the three axis (x, y and z), more on this in a moment.

The accelerometer will deliver data to the delegate method accelerometer:didAccelerate:. The values returned are between -1.0 and 1.0. The figure below shows the reference of x,y and z as it relates to the device.

A common filter that is applied to the incoming data is one that weeds out the effect of gravity on the device, often times referred to as a high-pass filter. Once the data has been filtered, we can begin to look for the specific movements of the device:

// High-pass filter constant
#define HIGHPASS_FILTER 0.1
 
// Axis
#define X_DIR  0
#define Y_DIR  1
#define Z_DIR  2
 
- (void)accelerometer:(UIAccelerometer *)accelerometer didAccelerate:(UIAcceleration *)acceleration 
{
  // High pass filter
  accelerationArray[X_DIR] = acceleration.x - ((acceleration.x * HIGHPASS_FILTER) + (accelerationArray[X_DIR] * (1.0 - HIGHPASS_FILTER)));
  accelerationArray[Y_DIR] = acceleration.y - ((acceleration.y * HIGHPASS_FILTER) + (accelerationArray[Y_DIR] * (1.0 - HIGHPASS_FILTER)));
  accelerationArray[Z_DIR] = acceleration.z - ((acceleration.z * HIGHPASS_FILTER) + (accelerationArray[Z_DIR] * (1.0 - HIGHPASS_FILTER)));
 
  // Was the device moved away or toward you...
  if (z - accelerationArray[Z_DIR] > 0.5 || z - accelerationArray[Z_DIR] < -0.5)
  {
    ...
  }
  // Was the device moved up or down
  else if (y - accelerationArray[Y_DIR] > 0.5 || y - accelerationArray[Y_DIR] < -0.5)
  {
    ...
  }
  // Was the device moved left or right
  else if (x - accelerationArray[X_DIR] > 0.5 || X - accelerationArray[X_DIR] < -0.5)
  {
    ...
  }
}

You’ll find that tinkering with the values shown above (0.5 and -0.5) will allow you to adjust how much force in movement is required to detect the events you are after.

Stop Receiving Data:
To stop receiving data from the accelerometer, set the delegate method to nil, which will notify the device it can turn off the accelerometer hardware.

[[UIAccelerometer sharedAccelerometer] setDelegate:nil];