There is something I don't understand about accelerometers. I know it's possible to make an Inertial Measurement Unit (IMU) by using "three" accelerometers. So with that, I could calculate the $x$, $y$, $z$ coordinates of something in 3-D space.
The point that I don't understand is, how could it sense constant-speed movements? When the velocity is constant, acceleration is zero. So how does it work?