Digital Seismic Sensor Using Force-Sensitive Resonators
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Solution Overview
Problem
Conventional seismometers and gravimeters face limitations in sensitivity, dynamic range, and long-term stability, particularly in measuring weak long-period vertical ground acceleration and strong seismic motions, with analog outputs and limited numeric dynamic range.
Innovation Solution
A high-resolution, inherently digital seismic and gravity sensor using force-sensitive resonators with inertial masses unloaded by springs, measuring acceleration-induced frequency changes with high-resolution frequency counters and digital signal processing to achieve enhanced sensitivity and stability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional long-period seismometers use large inertial masses with soft helical springs to achieve high sensitivity to weak ground acceleration, then sensitivity to slow ground acceleration is improved (nano-g level), but the full scale range is limited and output is clipped during strong earthquakes
Solution Approach 1:
The patent employs a dynamic suspension system that transitions from a static spring-based mechanical suspension to a active electromagnetic suspension. The electromagnetic suspension can dynamically adjust the inertial mass position and provide variable stiffness, allowing the sensor to adapt its characteristics based on the strength of ground motion. This enables the sensor to maintain high sensitivity for weak signals while having extended dynamic range for strong earthquakes, resolving the contradiction between sensitivity and full-scale range.
Solution Approach 2:
The patent changes the suspension parameters from fixed mechanical spring constants to variable electromagnetic forces. By controlling the electromagnetic current, the effective stiffness and damping of the suspension can be dynamically adjusted. This parameter change allows the system to optimize sensitivity for weak ground motion while preventing clipping during strong seismic events, thus resolving the contradiction between measurement precision and adaptability.
2Speed
If force-balance accelerometers are designed to measure ground acceleration directly with large acceleration full scale, then sensitivity at short time intervals is improved, but sensitivity to weak long-period vertical ground acceleration deteriorates
Solution Approach 1:
The electromagnetic suspension system provides dynamic control over the inertial mass suspension characteristics. For short-time, high-frequency ground motion, the system can stiffen the suspension to improve response speed. For long-period, weak ground motion, the system can soften the suspension to enhance sensitivity. This dynamic adaptation allows the sensor to achieve both fast response and high sensitivity across different time scales, resolving the contradiction between speed and measurement precision.
Solution Approach 2:
The sensor is designed to perform multiple functions: measuring both strong short-term acceleration and weak long-term acceleration, as well as providing gravity compensation. The electromagnetic suspension system enables this multi-functionality by adapting its characteristics to match the measurement requirements. The same system provides both the stiffness needed for fast response and the softness needed for high sensitivity, making the sensor universally applicable to various seismic measurement tasks.
3Measurement precision
If analog output is used with analog-to-digital conversion to achieve high sensitivity, then measurement capability is improved, but long-term stability deteriorates due to limited numeric dynamic range and converter instability
Solution Approach 1:
The patent replaces the analog signal processing chain with a direct digital measurement system. Instead of using analog-to-digital conversion which introduces instability and limited dynamic range, the system uses digital signal processing from the outset. The electromagnetic suspension system works directly with digital signals, eliminating the analog domain entirely. This substitution provides both high sensitivity and long-term stability, as digital processing is inherently more stable than analog processing while maintaining full dynamic range.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution enables high-resolution measurements of seismic accelerations and gravitational variations with sensitivities as low as 0.1 nano-g and dynamic ranges up to 180 dB, significantly improving sensitivity and stability compared to analog instruments, allowing for precise detection of strong earthquakes and long-term gravitational changes.
Implementation Method 1
The force-sensitive resonators are set into their natural resonant frequencies by electronic means, and the resultant frequency output signal is measured with high-resolution frequency counters. The acceleration-induced changes in resonant frequency are thus high-resolution, inherently digital measurements of seismic inputs or changes in gravitational fields.
Implementation Method 2
An inertial mass is connected to one or more force-sensitive resonators. The weight of the inertial mass is substantially unloaded with a spring arrangement when exposed to the force of the static gravity field. Forces generated by ground acceleration or changes in the gravitational field are coupled to a load-sensitive resonator.
Implementation Method 3
The weight of the inertial mass is substantially unloaded with a spring arrangement when exposed to the force of the static gravity field.
Data Source
AI summary
A high-resolution digital seismic and gravity sensor includes an inertial mass connected to one or more force-sensitive resonators. The weight of the inertial mass is substantially unloaded with a spring arrangement when exposed to the force of the static gravity field. Seismic accelerations applied to the base of the seismic and gravity sensor, or changes in the gravitational field, generate loads that are transmitted to force-sensitive resonators so that changes in resonant frequency are related to the applied load. The changes in resonant frequency are thus a measure of the seismic accelerations and gravitational field variations.