# Algorithm for optimal control on space ship using accelerometer input data

Does someone have a good algorithm for controlling a space ship in a vertical shooter game using acceleration data?

I have done a simple algorithm, but works very badly. I save an initial acceleration value (used to calibrate the movement according to the user's initial position) and I do subtract it from the current acceleration so I get a "calibrated" value. The problem is that basing the movement solely on relative acceleration has an effect of loss of sensitivity: certain movements are independent from the initial position.

Would anyone be able to share a a better solution? I am wondering if I should use/integrate also inputs from gyroscope hardware.

Here is my sample of code for a Cocos2d iOS game:

- (void) accelerometer:(UIAccelerometer *)accelerometer didAccelerate:(UIAcceleration *)acceleration {
if (calibrationLayer.visible){
[self evaluateCalibration:acceleration];
initialAccelleration=acceleration;
return;
}

if([self evaluatePause]){
return;
}

ShooterScene * shooterScene = (ShooterScene *) [self parent];
ShipEntity *playerSprite = [shooterScene playerShip];

float accellerationtSensitivity = 0.5f;

UIAccelerationValue xAccelleration = acceleration.x - initialAccelleration.x;
UIAccelerationValue yAccelleration = acceleration.y - initialAccelleration.y;

if(xAccelleration > 0.05 || xAccelleration < -0.05) {
[playerSprite setPosition:CGPointMake(playerSprite.position.x +  xAccelleration * 80, playerSprite.position.y + yAccelleration * 80)];
}
else if(yAccelleration > 0.05 || yAccelleration < -0.05)
{
[playerSprite setPosition:CGPointMake(playerSprite.position.x +  xAccelleration * 80, playerSprite.position.y + yAccelleration * 80)];
}

}

• I'm not sure what the issue is. You mention a loss in sensitivity, but that should not be caused by calibration. Can you give a concrete example of a problem you've run into with this method? Jun 13, 2013 at 13:09

## 1 Answer

I think some of your problems might be caused by a naive calibration like:

calibratedAcceleration = currentAcceleration - initialAcceleration


When you're rotating your phone and not moving it then the acceleration returned will lie on a sphere with Earth G radius.

For example let's say your phone lies almost flat on the table - your acceleration X and Y values will be small and your method should work, but when you turn your phone 90 deg your acceleration X or Y will get its maximum value and your method will break. In practice, for rotations just around 15-30 deg you should start noticing some strange effects, like non-symmetrical steering in front-back directions.

To solve it you could store entire initial rotation, computed from initial acceleration vector and during the game compute actual rotation and a X/Y differences between them.