Акселлерометр айфона: движение телефона вверх/вниз. Как реализовать?

-(void) start{
    double __block firstY, mediumY, lastY=0;
    int __block i=0;
    [self addObserver:self forKeyPath:@"count" options:NSKeyValueObservingOptionNew context:nil];
    self.motionManager = [[CMMotionManager alloc]init];
    self.motionManager.deviceMotionUpdateInterval = 0.1;
    [self.motionManager startDeviceMotionUpdatesToQueue:[[NSOperationQueue alloc]init] withHandler:^(CMDeviceMotion *motion, NSError *error) {
        CMAcceleration acceleration = motion.userAcceleration;
        [self changeFilter:[LowpassFilter class]];
        [filter addAcceleration:&acceleration];
        if([UIDevice currentDevice].orientation!=1){
            [self.messageText performSelectorOnMainThread:@selector(setText:) withObject:@"Держите телефон вертикально!" waitUntilDone:NO];
        } else {
            [self.messageText performSelectorOnMainThread:@selector(setText:) withObject:@"" waitUntilDone:NO];
            double y = filter.y>0.5?filter.y:0;
            if (i < 3){
                if(i%3==0){
                    firstY = y;
                }
                if(i%3==1){
                    mediumY = y;
                }
                if(i%3==2){
                    lastY = y;
                }
            }else{
                firstY = mediumY;
                mediumY = lastY;
                lastY = y;
            }
        
        if(mediumY>firstY){
            if(mediumY>lastY){
                self.count++;
                AudioServicesPlayAlertSound(kSystemSoundID_Vibrate);

            }
        }
        i++;
        }
    }];
    self.trained = YES;
}