How to calculate back and forth motion of phone using coremotion

Question

I would like to know if there is a way or how one would calculate the back and forth motion of a device.

For instance, if you lay a phone on a flat surface and push it back and forth on the y axis it give readings for the y axis.. if you rotate the phone or turn it it might then start giving reading on the x axis etc.

I am trying to make a stroke calculator for a rower where they just attach their device to any part of the boat, what I want to know is how to handle the fact the device could be put at any angle or rotation...

Answer 1

You do it by using core motion's device motion.

@property (strong,nonatomic,readonly) CMMotionManager *motionManager;



// Motion Manager setup
    if(!_motionManager)
    {
        _motionManager = [[CMMotionManager alloc] init];
        _motionManager.deviceMotionUpdateInterval = 1.0/70.0;

        [_motionManager startDeviceMotionUpdatesUsingReferenceFrame:CMAttitudeReferenceFrameXTrueNorthZVertical];

        NSLog(@"AppDelegate: Motion Manager created and started.");
    }

Notice how i set a reference frame.

Then you can get the readings by doing

_motionManager.deviceMotion.userAcceleration

I dont remember if these readings were in device coordinates or global coordinates, but if they are in global coordinates you can use the attitute (thats the device orientation at a given point in reference to the true magnetic north specified earlier that matches whatever acceleration there was at that same point) to translate device coordinates to world coordinates.