imu--加速度计

Accelerometers measure specific force (proper acceleration), which is the acceleration relative to free-fall,[1] not the "standard" acceleration that is relative to a coordinate system.

 

意思就是呢，如果我们是静止，或者匀速直线运动的，那么相对于自由落体的物体，就是有个向上的加速度的。