Using CMDeviceMotion to track iPhone tilt over time

Question

I'm trying to use CMDeviceMotion to track when the iPhone is tilted backwards, and then do something. I've successfully created the CMMotionManager, I'm getting motion data from the system, and I'm filtering for results above a certain acceleration.

What I want to do though, is detect when the device is being tilted backwards or forwards above a certain speed. How do I do that?

Here's the code I have so far:

UPDATE: I think I solved it. I was looking for the rotationRate property, CMRotationRate. I've paired them together, I really only need the x value, so I'll keep working on it. If anyone has some tips at the below code, it's much appreciated.

    - (void)startMotionManager{
        if (motionManager == nil) {
            motionManager = [[CMMotionManager alloc] init];
        }

        motionManager.deviceMotionUpdateInterval = 1/15.0;
        if (motionManager.deviceMotionAvailable) {

            NSLog(@"Device Motion Available");
            [motionManager startDeviceMotionUpdatesToQueue:[NSOperationQueue currentQueue]
                                               withHandler: ^(CMDeviceMotion *motion, NSError *error){
                                                       //CMAttitude *attitude = motion.attitude;
                                                       //NSLog(@"rotation rate = [%f, %f, %f]", attitude.pitch, attitude.roll, attitude.yaw);
                                                   [self performSelectorOnMainThread:@selector(handleDeviceMotion:) withObject:motion waitUntilDone:YES];

                                               }];
                //[motionManager startDeviceMotionUpdates];


        } else {
            NSLog(@"No device motion on device.");
            [self setMotionManager:nil];
        }
    }

   - (void)handleDeviceMotion:(CMDeviceMotion*)motion{
    CMAttitude *attitude = motion.attitude;

    float accelerationThreshold = 0.2; // or whatever is appropriate - play around with different values
    CMAcceleration userAcceleration = motion.userAcceleration;

    float rotationRateThreshold = 7.0;
    CMRotationRate rotationRate = motion.rotationRate;

    if ((rotationRate.x) > rotationRateThreshold) {
        if (fabs(userAcceleration.x) > accelerationThreshold || fabs(userAcceleration.y) > accelerationThreshold || fabs(userAcceleration.z) > accelerationThreshold) {

            NSLog(@"rotation rate = [Pitch: %f, Roll: %f, Yaw: %f]", attitude.pitch, attitude.roll, attitude.yaw);
            NSLog(@"motion.rotationRate = %f", rotationRate.x);

            [self showMenuAnimated:YES];
        }
    }

    else if ((-rotationRate.x) > rotationRateThreshold) {
        if (fabs(userAcceleration.x) > accelerationThreshold || fabs(userAcceleration.y) > accelerationThreshold || fabs(userAcceleration.z) > accelerationThreshold) {

            NSLog(@"rotation rate = [Pitch: %f, Roll: %f, Yaw: %f]", attitude.pitch, attitude.roll, attitude.yaw);
            NSLog(@"motion.rotationRate = %f", rotationRate.x);

            [self dismissMenuAnimated:YES];
        }
    }
}

Vincent Guerci · Answer 1 · 2011-04-17T16:59:36.220

2

Have you looked at CMAttitude? Sounds like what you need, plus some mathematics I guess.

EDIT: Ok you did.

Quoting Apple documentation, at chapter "Getting the Current Device Orientation" :

If you need to know only the general orientation of the device, and not the exact vector of orientation, you should use the methods of the UIDevice class to retrieve that information. Using the UIDevice interface is simple and does not require that you calculate the orientation vector yourself.

Great, they do the maths for us. Then I guess that tracking acceleration like you do + tracking orientation, as above documentation explains, should do the job, using UIDeviceOrientationFaceUp and UIDeviceOrientationFaceDown ?

If none of the UIDeviceOrientation fit your needs, what you will need is to calculate some spatial angles from two CMAttitude references, which provides a rotationMatrix and a quaternion... these school maths are far away for me... I would then suggest to maybe search/ask a math only question with these.

edited Apr 17 '11 at 16:59

answered Apr 17 '11 at 15:31

Vincent Guerci

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I was more so interested in the acceleration and rotation rate to detect the device 'flipping' backwards'. Think of a cop and his badge when they flip it up (if they still do that). I think I've solved it for the most part. I'll edit my OP. – W Dyson Apr 17 '11 at 16:59
It's the math I have problems with. I can grasp the basics, I can read it, but doing my own is way above me. I think I'll try and stick with what i know and ask other math questions.... – W Dyson Apr 17 '11 at 17:02
matricial calculation... brrr... I wouldn't like to dig "back" in this. Ask a math only question with mathematica / matricial / quaternion tags... I'm sure someone might help there. Good luck! – Vincent Guerci Apr 17 '11 at 17:05

Using CMDeviceMotion to track iPhone tilt over time

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