On the horizontal wobbling of an object levitated by near-field acoustic levitation. 
A circular planar object can be levitated with several hundreds of microns by ultrasonic near-field acoustic levitation (NFAL).  However, when both the sound source and the levitated object are circularly shaped and the center of the levitated object does not coincide with the source center, instability problem often occurs.  When this happens, it becomes difficult to pick up or transport the object for the next process.  In this study, when the center of the levitated object was offset from the source center, the moving direction of the levitated object was predicted by using the time averaged potential around the levitated object.  The wobbling frequency of the levitated object was calculated by analyzing the nonlinear wobbling motion of the object.  It was shown that the predicted wobbling frequencies agreed with measured ones well.  Finally, a safe zone was suggested to avoid the unstable movement of an object.