Bryce Bjork, C.J. Hunt
Ultrasonic Excitation of Cantilevers
Physics, Dr. Huber
About Our Research:
Sound waves can be used to vibrate solid objects, provided they are powerful enough and focused at specific points. An ultrasound transducer emits two signals equidistant from, but on opposite sides of, a central carrier frequency. The distance between each signal and the carrier is the modulation frequency. The frequency that physically vibrates the target object is the beat frequency between these two signals, or twice the modulation frequency. Two focused, high-frequency transducers were aimed at the upper corner of a 0.07x0.75x2.64 cm brass macrocantilever. The modulation frequency was swept through a range of 1400-Hz to 1700-Hz and the cantilever’s vibrations were monitored. The cantilever was shown to vibrate in a transverse mode at 1665-Hz and in a torsional mode at 1685-Hz. Each of these modes can be individually excited and the other suppressed by varying the phase difference between the two transducers’ signals. This process is called selective excitation. When the signals are in phase, the transverse mode is excited and when out of phase, the torsional mode becomes more prominent. In each case, the mode that is not excited is significantly suppressed.
In the student's own words :
"The chance to learn about other research going on around campus was also a benefit of working here. The weekly meetings to present research gave a feel for what a real research facility was like, as far as presenting results and ensuring understanding for those outside of the field. It was a great immersion into the research world."
The ultrasound radiation force can be used as a noncontact method for exciting vibrational modes. An audio-range modulation signal is combined with a 440-kHz carrier to form a double-sideband suppressed-carrier (DSSC) waveform that is emitted from an ultrasound transducer. The frequency of the ultrasound radiation force is at the beat frequency, or twice the modulation frequency. Two focused high-frequency ultrasound transducers emitting such a signal were aimed at a large rectangular brass cantilever with dimensions: 0.07x0.75x2.64 cm. These transducers were aimed at the upper right and upper left corners of the cantilever and were swept through a frequency range encompassing both a transverse (1665 Hz) and a torsional mode (1685 Hz). Both modes are separated by about 20 Hz and can be seen to overlap somewhat through a modulation frequency sweep. We describe a method for selective excitation of each mode in which the phase between the two transducers is varied. When the two transducers are in phase, the transverse mode is at a maximum and the torsional mode is at a minimum. When the transducers are out of phase, the torsional mode is at a maximum and the transverse mode is at a minimum.
More student thoughts:
"This was my first real research experience and it was a great help to already be familiar with the facilities and people involved. I didn’t have to go through the hassle of learning my way around a new campus or building. It took a lot of initial stress off. In addition to this, and applicable to any student, the facilities at Gustavus are wonderful. Dr. Huber and even other professors would always be willing to help with any equipment troubles and we were always able to find what we needed."