Thomas M. Huber

Noncontact modal analysis of a pipe organ reed using airborne ultrasound stimulated vibrometry

The goal of this study was to excite and measure, in a noncontact manner, the vibrational modes of the reed from a reed organ pipe. To perform ultrasound stimulated excitation, the audio-range difference frequency between a pair of ultrasound beams produced a radiation force that induced vibrations. The resulting vibrational deflection shapes were measured with a scanning laser vibrometer. The resonances of any relatively small object can be studied in air using this technique. For a 36 mm x 6 mm brass reed, displacements and velocities in excess of 5 microm and 4 mm/s could be imparted at the fundamental frequency of 145 Hz. Using the same ultrasound transducer, excitation across the entire range of audio frequencies was obtained. Since the beam was focused on the reed, ultrasound stimulated excitation eliminated background effects observed during mechanical shaker excitation, such as vibrations of clamps and supports. The results obtained using single, dual and confocal ultrasound transducers in AM and two-beam modes, along with results obtained using a mechanical shaker and audio excitation using a speaker are discussed.

Magnetoelastic resonance sensor for remote strain measurements

A low cost passive wireless strain sensor is proposed. The basis of the sensor is formed by two softmagnetic magnetostrictive ribbons. The first magnetostrictive ribbon transforms mechanical stress into a stress dependent magnetic field. The second ribbon senses this field by magnetoacoustic oscillations. The resonance frequency directly depends on the applied mechanical stress. For the proposed sensor, a gauge factor Gf, which is defined as the relative change of the resonance frequency divided by the strain ɛ, of Gf = 380 is obtained. This is significantly higher than the gauge factor of standard metal foil strain gages.

Calculation of coercivity of magnetic nanostructures at finite temperatures

We report a finite temperature micromagnetic method (FTM) that allows for the calculation of the coercive field of arbitrary shaped magnetic nanostructures at time scales of nanoseconds to years. Instead of directly solving the Landau-Lifshitz-Gilbert equation, the coercive field is obtained without any free parameter by solving a non linear equation, which arises from the transition state theory. The method is applicable to magnetic structures where coercivity is determined by one thermally activated reversal or nucleation process. The method shows excellent agreement with experimentally obtained coercive fields of magnetic nanostructures and provides a deeper understanding of the mechanism of coercivity.

Mode-selective noncontact excitation of microcantilevers and microcantilever arrays in air using the ultrasound radiation force

We demonstrate the use of focused ultrasound waves to excite eigenmodes of microfabricated structures such as atomic force microscopy microcantilevers and coupled microcantilever arrays, among the smallest objects that have been excited in air using ultrasound radiation force. The method is based on the radiation force produced by a double-sideband suppressed carrier ultrasound waveform, centered at 500 kHz. The difference frequency between the sidebands, ranging from 10 to 200 kHz, excited resonances of these structures. Frequency response curves and deflection shapes were consistent with conventional base excitation, demonstrating the feasibility of noncontact excitation for a variety of microscale devices.

Noncontact Modal Excitation of a Classical Guitar Using Ultrasound Radiation Force

Previous studies have demonstrated that it is possible to use the ultrasound radiation force in air for modal excitation of objects ranging in size from microcantilevers that are a few hundred microns in length to hard drive suspensions and other cantilevers a few centimeters long. The current study demonstrates that the ultrasound radiation force excitation technique can also be used for modal excitation of significantly larger objects, in this case an acoustic guitar. It was demonstrated that the noncontact combination of ultrasound radiation force excitation and a scanning vibrometer allowed measurements of both the frequency response and operating deflection shapes of a Cordoba 45R classical guitar in the range from 70 to 800 Hz. The resonance frequencies and deflection shapes are similar to those measured using a conventional mechanical shaker. By using a pair of ultrasound transducers and adjusting their relative phase difference, it was possible to selectively enhance or suppress different resonances. This is a substantial extension over previous studies because the guitar is several orders of magnitude larger than devices used previously.

Elimination of standing wave effects in ultrasound radiation force excitation in air using random carrier frequency packets.

The ultrasound radiation force has been used for noncontact excitation of devices ranging from microcantilevers to acoustic guitars. For ultrasound radiation force excitation, one challenge is formation of standing waves between the ultrasound transducer and the device under test. Standing waves result in constructive/destructive interference causing significant variations in the intensity of the ultrasound field. The standing-wave induced intensity variations in the radiation force can result from minor changes in the transducer position, carrier frequency, or changes in the speed of sound due to changes in ambient temperature. The current study demonstrates that by randomly varying the ultrasound carrier frequency in packets, it is possible to eliminate the negative consequences resulting from the formation of standing waves. A converging ultrasound transducer with a central frequency of 550 kHz was focused onto a brass cantilever. The 267 Hz resonance was excited with the ultrasound radiation force with a carrier frequency that randomly varied between 525 kHz to 575 kHz in packets of 10 cycles. Because each packet had a different carrier frequency, the amplitude of standing wave artifacts was reduced by a factor of 20 compared to a constant frequency excitation of 550 kHz.

Non‐contact mode excitation of small structures in air using ultrasound radiation force

With the advent of MEMS, modal analysis of small structures is increasingly important. 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 hard‐drive HGA suspensions and MEMS devices. The vibration induced by the ultrasound radiation force was varied in a wide range from 0 Hz to 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, the ratio of the vibrational amplitudes of the 175 Hz first‐bending and 1.33 kHz torsional modes of a small cantilever could be changed from in excess of 10:1 to less than 1:10 by shifting the ultrasound modulation phase 90 degrees. Similar changes were obtained for a 3 mm square MEMS mir...

Wireless and passive temperature indicator utilizing the large hysteresis of magnetic shape memory alloys

An ultra-low cost, wireless magnetoelastic temperature indicator is presented. It comprises a magnetostrictive amorphous ribbon, a Ni-Mn-Sn-Co magnetic shape memory alloy with a highly tunable transformation temperature, and a bias magnet. It allows to remotely detect irreversible changes due to transgressions of upper or lower temperature thresholds. Therefore, the proposed temperature indicator is particularly suitable for monitoring the temperature-controlled supply chain of, e.g., deep frozen and chilled food or pharmaceuticals.

Reactivable passive radio-frequency identification temperature indicator

A low cost, passive radio-frequency identification (RFID) temperature indicator with (re-) activation at any point of time is presented. The capability to detect a temperature excursion is realized by magnets and a solution with a melting point at the critical temperature. As the critical temperature is exceeded, a magnetic indicator switches to non-reversible and this can be monitored via a giant magnetoresistance sensor connected to a RFID tag. Depending on the solutions or metal alloys, detection of critical temperatures in a wide range from below 0 °C and up to more than 100 °C is possible. The information if a threshold temperature was exceeded (indicator state) as well as the identification number, current temperature, and user defined data can be obtained via RFID.

Three-dimensional magneto-resistive random access memory devices based on resonant spin-polarized alternating currents

Selective switching of a magneto-resistive random access memory (MRAM) multilayer stack is demonstrated using resonant spin-polarized alternating currents (AC) superimposed on spin-polarized direct currents. Finite element micromagnetic simulations show that the use of frequency triggered AC allows one to maximize the transferred spin transfer torque selectively in order to merely reverse the magnetization of a single storage layer in a stack. Using layers with different resonance frequencies, which are realized by altering the anisotropy constants, allows one to address them by tuning the AC frequency. A rapid increase of the storage density of MRAM devices is shown by using three-dimensional sandwich structures.