Collaboration with the EPFL and DigiSens
A new generation of resonant force sensors is studied and developed. The new sensor should reduce power consumption, and find place in a smaller housing, as compared to the present sensor. In addition, it should cover a wider applications range.
DIGI SENS is the leading company in oscillating wire technology (OWT) for load and force measurement. In order to keep the lead over competing technologies, it is sought to develop a new generation of sensors by changing the excitation and read-out mechanism from electromagnetic to piezoelectric. The expected advantages are reduced power consumption, reduced sensor size, and a higher competitiveness regarding production costs. These are favorable tendencies for combining OWT with wireless operation and thus meeting requirements in the frame of “IOT” or Industry 4.0. Wireless operation will widen up the application range, and thus the market volume. The key element of the innovation is to realize excitation and detection by a piezoelectric transducer in the form of a piezoelectric thin film integrated onto the wire. The chosen film material Al1-xScxN is very promising for this purpose, because it shows a factor 2 to 3 larger piezoelectric effect than AlN, and can be deposited at moderate temperatures that do not degrade the wire material.
The major challenge of the project is to integrate such piezoelectric thin films onto the wires. The latter are cut out from brass sheets, which currently exhibit quite rough surfaces. However, the growth surfaces for high quality piezoelectric thin films must be smooth on the nanometer scale. An important task is thus devoted to polish the brass sheets by chemical mechanical polishing (CMP). A first feasibility study has come to the conclusion that this should be possible. On the polished surface, suitable electrodes will be grown first, followed by AlScN thin films, all of them deposited by reactive magnetron sputtering. Top electrode films will be added and patterned, and the sheets will be cut into wires. The latter two operations are non-standard ones when using a brass sheet. Optimal solutions have to be identified first. The wire will be fixed in a new type of frame imposing a stretching force bias. Finally, the fabricated wires will be characterized on a test bench. The obtained response will be compared to results of finite element modeling and analytical calculations. Signal-to-noise and signal drift will be investigated. The electrical characterization will serve to define an integrated circuit (IC) to drive and read-out the sensor. This IC is not part of this proposal. The industrial partner will undertake a new project for this purpose, which could already start during the second half of this project.