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Scientific Reports on Micro and Nanosystems
edited by Christofer Hierold
Vol. 29
Michelle
Müller
Micromechanical broadband vibration
amplitude-amplifier for microseismic
and acoustic emission detection
1st Edition 2019. XVIII, 168 pages.
€ 64,00.
ISBN 978-3-86628-627-6
Abstract
In Switzerland, villages and infrastructure are often
built close to steep rock slopes, where rock falls are not uncommon. A microseismic/acoustic emission monitoring system is
desirable, which detects precursory activity in the rock slopes and can give an
advance warning. Commercial microseismic/acoustic
emission monitoring systems are expensive, generate a large amount of data and
consume considerable energy. MEMS potentially offer a more cost-effective
solution and also highly integrated systems. For example, power hungry
electrical preamplifiers may be replaced with a passive micromechanical amplitudeamplifier by shifting the amplification from the
electrical to the mechanical domain.
In this thesis, a passive micromechanical broadband
amplitude-amplifier for ultra-low-power detection of microseismic/acoustic
emission signals is explored. It is based on a coupled mass-spring system.
Masses and spring constants decrease towards the end of the coupled mass-spring
chain and weak vibrations exciting the first resonator are amplified while
traveling towards the last resonator, if they are within the allowed frequency
band.
Three different device designs and fabrication
processes based on front- and backside dry etching of a silicon wafer,
respectively silicon-on-insulator wafer, are presented. Out-of-plane and
in-plane moving structures were fabricated and a two-level fabrication process
for multiscale device fabrication is shown. It enables the coupling of MEMS
resonators with two different thicknesses (e.g. a weight ratio of 26’244) and
reduces die size due to resonator stacking. Steadystate
and transient responses of the fabricated devices were optically determined and
compared with a 1D lumped element model.
Proof-of-concept devices showed an increased amplification
and bandwidth by coupling more resonators. While a minimum (average)
amplification of 3.4 (16.7) and a bandwidth of 0.93 x f0, N4 was achieved with 4 coupled resonators, a system with
8 resonators reached a minimum (average) amplification of 10.2 (58.4) over a
bandwidth of 1.08 x f0, N8 (with resonance frequencies of single resonators: f0, N4 = 16.2 kHz, f0, N8 = 5.2 kHz).
Amplification and bandwidth tuning by design was also demonstrated.
Coupling more resonators leads to a higher amplification over a broader
bandwidth, while increasing the ratio at which the masses and spring constants
decrease leads to a higher amplification but smaller bandwidth (e.g. 24.3
minimum (54.3 average) amplification over a bandwidth of 0.43 x f0, N3 versus 7.9 minimum (20.6 average) amplification over a
bandwidth of 1.06 x f0, N7, with f0, N3 = 9.7 kHz, f0, N7 = 10.4 kHz).
Finally, a multiscale device with ten resonators
coupled in series, which decrease in mass by a factor of three each, is
presented. The first ten Eigenmodes of the device are
in-plane and unidirectional. A minimum (average) amplification of 63 (295) over
a bandwidth of 4.4 - 15.1 kHz (1.00 x f0, N10A, with f0, N10A = 10.7kHz) can be achieved in less than 1 ms.
A second device with the same design but from another fabrication run showed
similar characteristics (100 minimum (343 average) amplification over a
bandwidth of 0.95 x f0, N10B, with f0, N10B = 10.1 kHz). The short response time below 1 ms is crucial to detect vibrational bursts in e.g.
steep rock slopes or for any other structural health monitoring applications.
Keywords:
Scientific Reports on Micro and Nanosystems
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