I have a project that involves moving the larger object that my Arduino is in in two degrees of freedom parallel to some sort of flat surface, but it could be a horizontal or vertical surface. The actual device doesn't ever touch the surface, so I can't use some sort of wheel with encoding. I am wondering if there is a good type of sensor and general method to use for determining how fast the Arduino is being moved. I don't need direction, only absolute velocity, and it would be being moved on the scale of cm/sec.
If you want a better idea of what I am talking about, imagine taking your phone and slowly passing it about an 2-3cm over your keyboard. I want to know how fast it was moving during that time, and then make LEDs flash or a buzzer beep, if you are moving too quickly.