A system for operating a memory device includes a memory array having a number of memory cells and a set of dynamic reference cells coupled to the memory cells in word lines. The eccentric weights are mounted on the interior of the tubular shaft, thereby simplifying manufacture and providing a smoother operation. Power to each wing can be controlled separately by varying the amplitude of the wing flapping, the frequency of the wing flapping, or both. On the outer surface 6a, a first mark 7 and a second mark 8 are provided.