5:15 PM - 7:15 PM
[STT41-P05] Enhanced Sensitivity of Quartz FM Transducer Using Double-Ended Tuning Fork for Digital Microtremor Measurement
Keywords:Microtremor Measurement, Ground Motion, Broadband Measurement, Quartz-MEMS Transducer, Double-Ended Tuning Fork Quartz Resonator, Digital Transducer IP
In recent years, microtremor measurement has become increasingly important for maintaining aging social infrastructure and monitoring the health of ground and structures. Conventional microtremor measurements primarily utilize expensive velocity-type seismometers, requiring large-scale systems for multi-point measurements and continuous monitoring. While MEMS sensors are compact and cost-effective, they lack the sensitivity, resolution, and stability required for detecting minute vibrations. Particularly, accurate determination of ground predominant periods and structural natural frequencies requires stable measurements from 0.1 Hz to 50 Hz, which has been challenging for conventional MEMS sensors. This research presents the development of a high-sensitivity acceleration transducer that addresses these challenges.
The developed sensor employs a quartz crystal resonator as a transducer, measuring physical quantities by comparing a resonant frequency with a reference frequency (Todorokihara et al., Proc. DGON ISS, 2018). This approach fundamentally resolves the linearity degradation issues associated with conventional capacitive MEMS sensors while eliminating the trade-off between input range and resolution through simple counter bit width adjustment.
The sensor element utilizes a thin, narrow Double-Ended Tuning Fork (DETF) quartz resonator to achieve high Q-factor, high sensitivity, stable scale factor, and bias drift suppression (RD Andajani et al., IEEE Sensors, 2024). To significantly enhance sensitivity from the previous generation, we replaced phosphor bronze weights with tungsten weights, increasing the mass nearly fourfold. Furthermore, we adopted a 3-channel differential configuration using six transducers to improve stability against temperature variations and external disturbances, enabling first-order cancellation of temperature gradient and mechanical stress effects.
The DETF is fixed to a cantilever using low-melting-point glass, with tungsten weights attached using epoxy adhesive. Both DETF and cantilever are made of quartz to match thermal expansion coefficients. The sensor element is fabricated from a 40×40 mm² quartz wafer using high aspect ratio wet etching with approximately 1 µm accuracy. The DETF dimensions (80 µm thickness, 50 µm width) were optimized through finite element analysis, with manufacturing parameters precisely controlled to achieve high-precision vibrating arm width control.
The sensor element, packaged in a vacuum ceramic housing (9×7×3 mm³), features a DETF resonant frequency of approximately 124 kHz (Q-factor ~15,000) and a cantilever resonant frequency of 450 Hz. Through differential configuration and enhanced sensitivity, we achieved an acceleration sensitivity of approximately 550 Hz/g, enabling detection of minute accelerations required for microtremor measurements.
The readout circuit incorporates a digital measurement IP (Todorokihara, IEEE Sensors, 2021) inspired by moiré pattern phase sensitivity, implementing six channels in FPGA. Unlike conventional reciprocal frequency measurement requiring analog components like mixers and phase detectors, our fully digital approach eliminates nonlinearity issues while achieving fast and precise measurements.
The prototype 3-axis accelerometer module (48×24×16 mm³, 30 g) operates at 3.3 V with power consumption under 40 mA. Performance evaluation demonstrated bias stability below 0.5 mg and scale factor stability within 500×10-6 over temperatures from -30°C to +85°C. Additionally, it achieved excellent noise performance of 0.020 µg/√Hz in the 1-10 Hz band (Fig. 1), sufficient for microtremor detection.
The presentation will include actual microtremor measurement results, demonstrating performance verification in the 0.1 Hz to 50 Hz band through comparison with conventional microtremor meters. We will also introduce applications for multi-point measurements leveraging its compact size and cost-effectiveness.
The developed sensor employs a quartz crystal resonator as a transducer, measuring physical quantities by comparing a resonant frequency with a reference frequency (Todorokihara et al., Proc. DGON ISS, 2018). This approach fundamentally resolves the linearity degradation issues associated with conventional capacitive MEMS sensors while eliminating the trade-off between input range and resolution through simple counter bit width adjustment.
The sensor element utilizes a thin, narrow Double-Ended Tuning Fork (DETF) quartz resonator to achieve high Q-factor, high sensitivity, stable scale factor, and bias drift suppression (RD Andajani et al., IEEE Sensors, 2024). To significantly enhance sensitivity from the previous generation, we replaced phosphor bronze weights with tungsten weights, increasing the mass nearly fourfold. Furthermore, we adopted a 3-channel differential configuration using six transducers to improve stability against temperature variations and external disturbances, enabling first-order cancellation of temperature gradient and mechanical stress effects.
The DETF is fixed to a cantilever using low-melting-point glass, with tungsten weights attached using epoxy adhesive. Both DETF and cantilever are made of quartz to match thermal expansion coefficients. The sensor element is fabricated from a 40×40 mm² quartz wafer using high aspect ratio wet etching with approximately 1 µm accuracy. The DETF dimensions (80 µm thickness, 50 µm width) were optimized through finite element analysis, with manufacturing parameters precisely controlled to achieve high-precision vibrating arm width control.
The sensor element, packaged in a vacuum ceramic housing (9×7×3 mm³), features a DETF resonant frequency of approximately 124 kHz (Q-factor ~15,000) and a cantilever resonant frequency of 450 Hz. Through differential configuration and enhanced sensitivity, we achieved an acceleration sensitivity of approximately 550 Hz/g, enabling detection of minute accelerations required for microtremor measurements.
The readout circuit incorporates a digital measurement IP (Todorokihara, IEEE Sensors, 2021) inspired by moiré pattern phase sensitivity, implementing six channels in FPGA. Unlike conventional reciprocal frequency measurement requiring analog components like mixers and phase detectors, our fully digital approach eliminates nonlinearity issues while achieving fast and precise measurements.
The prototype 3-axis accelerometer module (48×24×16 mm³, 30 g) operates at 3.3 V with power consumption under 40 mA. Performance evaluation demonstrated bias stability below 0.5 mg and scale factor stability within 500×10-6 over temperatures from -30°C to +85°C. Additionally, it achieved excellent noise performance of 0.020 µg/√Hz in the 1-10 Hz band (Fig. 1), sufficient for microtremor detection.
The presentation will include actual microtremor measurement results, demonstrating performance verification in the 0.1 Hz to 50 Hz band through comparison with conventional microtremor meters. We will also introduce applications for multi-point measurements leveraging its compact size and cost-effectiveness.