In reference to the patent: US8726746
Does this 2002 journal article, "An integrated piezo-acoustic shear-force distance sensor with nanometer resolution for a micropipette tool" (pdf), demonstrate prior art? The authors document a sensing system comprised of two piezo-electric elements coupled to a pipette:
"One of the piezos excites the pipette at the mechanical resonance of its tapered tip while the other piezo is used for detection of the vibration."
The inventors of the '746 patent claim a method comprising:
"coupling an ultrasonic signal into the wall of the suction tube, wherein said ultrasonic signal is generated by a piezo actuator disposed between said wall of said suction tube and an additional mass, said piezo actuator mounted on said wall of said suction tube, said piezo actuator in contact with said additional mass on a side facing away from said suction tube;"
- Note that the inventors claim the use of only one piezo element (which they suggest can be used to both generate vibrations and sense them), whereas the prior work used two piezo elements;
- the inventors claim their invention is useful for determining the volume of fluid inside the pipette tip, while the prior work documented using the piezo system for sensing the distance of the tip from a fluid sample;
- the inventors claim the use of a "plurality of frequencies" although the prior work also involved a plurality of frequencies.
"The outer electrode of one of the piezos is connected to a frequency generator (model 8550, Tabor Electronics Inc., USA) and excites the pipette at its mechanical resonance in the frequency range of 10± 150 kHz and at amplitudes between 50 mVpp and 5 Vpp. The second piezo-element serves as piezo-acoustic detector and is connected to a lock-in amplifier (model 5302, EG&G Inc., USA) to measure magnitude and phase shift of the system response."
"measuring the frequency-dependent damping of the ultrasonic signal in the wall of the suction tube in a predefined frequency range comprising a plurality of frequencies as a function of the frequency;"