1 Noncontact Modal Excitation of Small Structures Using Ultrasound Radiation Force

ABSTRACT: Modal analysis of MEMS and other small structures is important for many applications. However, conventional excitation techniques normally require contact, which may not be feasible for small objects. We present a non-contact method that uses interference of ultrasound frequencies in air to produce low-frequency excitation of structures. Objects studied included a MEMS mirror, MEMS gyroscope, hard drive suspensions, and a brass cantilever. The vibration induced by the ultrasound radiation force was varied in a wide range from 0 Hz to over 50 kHz. Object motion was detected using a laser vibrometer; measured frequencies agreed with expected values. Also demonstrated was the unique capability to selectively enhance or suppress modes independently. For example, for a MEMS mirror, the relative amplitude of a torsional mode could be enhanced by a factor of 10 by changing the ultrasound focus spot position. Similarly, the ratio of the vibrational amplitudes of the torsional modes of a MEMS mirror around two axes could be changed from in excess of 20:1 to less than 1:2 by shifting the ultrasound modulation phase 90 degrees. I. NOMENCLATURE c Speed of sound in air dr(r) Drag coefficient on object at location r eΔf(r,t) Instantaneous energy density at frequency Δf at location r at time