CM加速计数据始终为负值

我试图检测用户何时将手机向下移动,何时将手机向上移动。手机只能上下移动(没有左右移动)

我在记录加速度数据,但它总是负值,即使我向后移动。这是为什么?

[_motionManager startAccelerometerUpdatesToQueue:[[NSOperationQueue alloc] init]
                                     withHandler:^(CMAccelerometerData *accelerometerData, NSError *error) {
                                         dispatch_async(dispatch_get_main_queue(), ^{
                                             [self performSelectorOnMainThread:@selector(update:) withObject:accelerometerData waitUntilDone:NO];
                                         });

-更新:

- (void)update:(CMAccelerometerData *)accelData {
      NSLog(@"acceldata.z %f",accelData.acceleration.z);
}

示例数据

2014-03-14 02:24:02.973 myApp[16294:60b] acceldata.z -0.863266
2014-03-14 02:24:02.993 myApp[16294:60b] acceldata.z -0.855530
2014-03-14 02:24:03.004 myApp[16294:60b] acceldata.z -0.853592
2014-03-14 02:24:03.024 myApp[16294:60b] acceldata.z -0.793396
2014-03-14 02:24:03.043 myApp[16294:60b] acceldata.z -0.774567
2014-03-14 02:24:03.053 myApp[16294:60b] acceldata.z -0.749664
2014-03-14 02:24:03.072 myApp[16294:60b] acceldata.z -0.716141
2014-03-14 02:24:03.082 myApp[16294:60b] acceldata.z -0.733749
2014-03-14 02:24:03.102 myApp[16294:60b] acceldata.z -0.752518