Small-size MEMS accelerometer encapsulated in vacuum using Sigma-Delta modulation

A vacuum encapsulated MEMS accelerometer using Sigma-Delta modulation is here presented. Three different modulation orders (second, third, and fourth) were implemented in a field-programable gate array (FPGA), enabling flexibility for tuning the loop parameters in real-time. Three devices were measured, and the results are in good agreement with simulations performed in Simulink. A noise figure of 123 μg/√Hz for a bandwidth of 400 Hz and a range of at least ±1 g was experimentally measured. A figure of merit considering device size and bandwidth is proposed, highlighting the relevance of the results for the current state of the art.FCT - Fundação para a Ciência e a Tecnologia (PDE/BDE/114563/2016

Influence of mechanical stress in a packaged frequency-modulated MEMS accelerometer

Frequency modulated accelerometers composed of two double-ended tuning fork (DETF) resonators on a differential configuration were characterized for their sensitivity to force applied to their package. Commonly, differential architectures are employed to cancel common mode errors, such as the mechanical stress or temperature dependency. The device dependence to mechanical stress was experimentally measured for forces up to 15 N and a reduction of about 5.6 times was obtained on the differential measurement. Additionally, the silicon dies were glued to chip-carriers using two different glues with distinct properties, and their sensitivity to stress was compared. The effectiveness of a viscoelastic glue over an epoxy-based glue for stress decoupling was tested. Long-term measurements under constant force were experimentally performed and for a time period of approximately 100 min, the stress relaxation and creeping of the viscoelastic glue enabled the recovery to the initial output of the sensor.The first author is supported by FCT - Fundacao para a Ciencia e Tecnologia through the grant PDE/BDE/114564/2016. This work is supported by FCT with the reference project UID/EEA/04436/2019

Low-pressure small MEMS accelerometer using Sigma-Delta modulation

Encapsulation of MEMS accelerometers in vacuum is advantageous, since it allows integration with other sensors, leading to size and cost reduction. One technique to operate MEMS accelerometers in vacuum is the use of closed-loop Sigma-Delta modulation, which has many advantages. In this paper, the design of a vacuum encapsulated small size MEMS accelerometer (0.2 mm2) and the preliminary measured results are presented. Experimental devices were fabricated and operated in 2nd and 3rd order Sigma-Delta modulators, achieving a noise figure of 389 µg/√Hz for a bandwidth of 200 Hz and a measurement range of at least ±1 g.The first author is supported by FCT– Fundação para a Ciência e Tecnologia through the grant PDE/BDE/114563/2016. This work is supported by FCT with the reference project UID/EEA/04436/2013, COMPETE 2020 with the code POCI-01-0145-FEDER-006941.info:eu-repo/semantics/publishedVersio

Resonant accelerometer based on double-ended tuning fork and a force amplification mechanism

Resonant accelerometers are an alternative to amplitude modulated devices due to their higher integration capabilities, since they are encapsulated in vacuum and are stable at low pressures. Vacuum is required for some sensors (i.e., gyroscopes) but amplitude modulated accelerometers tend to be unstable under such conditions and therefore cannot be integrated in the same package. Herewith, a device composed by double-ended tuning fork resonators (DETF) and a force amplification mechanism for sensitivity enhancement is presented. Characterization of the fabricated devices was performed, and the design was successfully validated. A sensitivity close to 80 Hz/g was experimentally measured and the DETF characterization for different driving (AC) and bias voltages (DC) is also presented.The first author is supported by FCT– Fundação para a Ciência e Tecnologia through the grant PDE/BDE/114564/2016. This work is supported by FCT with the reference project UID/EEA/04436/2013, COMPETE 2020 with the code POCI-01-0145-FEDER-006941.info:eu-repo/semantics/publishedVersio

Impact of ultraviolet radiation on interactions between plants and herbivorous insects: a chemo-ecological perspective

Kuhlmann F, Müller C. Impact of ultraviolet radiation on interactions between plants and herbivorous insects: a chemo-ecological perspective. In: Lüttge UE, Beyschlag W, Büdel B, Francis D, eds. Progress in Botany 72. Berlin: Springer Science + Business Media; 2011: 305-347

Highly sensitive MEMS frequency modulated accelerometer with small footprint

A single-axis resonant MEMS accelerometer is presented here. The goal is to achieve the maximum sensitivity on a set of predefined constraints: small footprint of 500 μm × 500 μm, vacuum operation under 150 Pa (requirement for a single-chip IMU) and fabrication using a Bosch silicon surface micromachining process. The sensor is composed by double-ended tuning fork resonators in differential architecture and a force amplification mechanism to increase its sensitivity. A complete characterization of the device was performed including closed-loop operation. A proportional-integral-derivative closed-loop controller architecture updates in real-time the excitation frequency to half the resonance frequency of the resonators. A scale-factor of 170 Hz/g and a non-linearity of 0.63 %FS (operation range of ±1 g) were experimentally measured. The relative sensitivities of 0.08 %Hz/g/nkg and 0.48 %Hz/g are among the highest reported for DETF-based devices. Long-term (700 μg/√Hz noise floor measured), dynamic and thermal drift measurements are also reported. The differential operation improved the thermal performance by 77 %.The authors would like to offer special thanks to the author Luis A. Rocha, who, although no longer with us, continues to inspire by his example and dedication to the students and collaborators he served over the course of his career. The first author is supported by FCT– Fundação para a Ciência e Tecnologia through the grant PDE/BDE/114564/2016. This work is supported by FCT with the reference project UID/EEA/04436/2019