Noncontact modal excitation of small structures using ultrasound radiation force

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.