Noncontact modal testing of hard-drive suspensions using ultrasound radiation force

The head-gimbal assembly suspension is a cantilever-like structure that holds the heads on a hard drive. A noncontact method for modal testing, in air, of suspensions is discussed. This method utilizes the radiation force at the difference frequency generated by two intersecting ultrasound beams. The resulting low-frequency excitations were measured using a scanning vibrometer. This excitation technique has been demonstrated for MEMS and other small devices. There are several unique advantages of the ultrasound radiation force relative to mechanical shakers. Since the ultrasound radiation force is noncontact, a specialized test fixture was not needed; the technique was relatively insensitive to distracting resonances of fixtures and support structures. Another advantage is broadband excitation; a 550-kHz confocal ultrasound transducer excited suspension resonance frequencies from under 1 kHz to at least 50 kHz. Other advantages include the ability to selectively excite different modes. For example, the amplitude of one suspension's 5.0-kHz torsional mode was suppressed by an order of magnitude by shifting the modulation phase between the two ultrasound beams by 90 degrees. In another test, the amplitude of the 6.0-kHz torsional mode was doubled by moving the ultrasound focus point from near the center to near the edge of the suspension.