10:45 AM - 12:15 PM
[SSS12-P06] Effectuality of Silicon Resonant Sensor on Precise Seafloor Pressure Observation
Keywords:Silicon resonant pressure sensor, Seafloor pressure observation, Nankai Trough seafloor network or earthquakes and tsunamis-net, N-net
For observations of tsunamis, precise pressure measurement on seafloor is one of the useful ways. To apply a new silicon resonant pressure sensor to this application, especially to Nankai Trough seafloor network for earthquakes and tsunamis-net (N-net), we conducted various evaluations under assumed conditions in the marine environment.
The silicon resonant pressure sensor developed by Yokogawa has advantage of low power consumption, compact size, high sensitivity, little individual differences, and high stability. Since the sensor chip is made by using Micro Electromechanical Systems (MEMS) technology to integrate sensing parts on the small chip, the sensor has the advantage that it is not easily affected by attitude errors due to movement of the sensor on the seabed and that temperature errors are easy to correct because of small temperature non-uniformity. Moreover, the sensor chips that determine the detection sensitivity are batch-manufactured by the wafer process, so there are little individual differences with the stable quality.
This sensor is an absolute pressure gauge that adopts diaphragm seal type, and pressure is applied to a 3mm square sensor chip through silicon oil separated by a thin metal diaphragm. Two resonators made of single crystal silicon that is an ideal elastic material are built into the same sensor chip. Each resonator is held in a vacuum cell by processes originally developed by Yokogawa to maintains a high Q value in no contact with silicon oil. The vacuum cell also works as a reference pressure cell of absolute pressure. The one of the two resonators is a pressure resonator that mainly detects pressure, and the other is a temperature resonator that mainly detects temperature. The pressure resonator is a both ends fixed beams and has initial tensile stress. When water pressure is applied, the sensor chip shrinks due to hydrostatic pressure, and compressive stress is applied to the pressure resonator. Then the resonant frequency of the pressure resonator changes. Since the ratio of change of resonant frequency versus pressure depends only on the bulk modulus of Si and the frequency of the resonator, the individual differences of pressure sensitivity are very small. On the other hand, the temperature resonator is designed so that compressive stress due to hydrostatic pressure is hardly applied, and the frequency mainly changes due to temperature changes. The temperature-corrected pressure value is obtained by acquiring the frequency outputs of two resonators with a frequency counter and calculating them.
To deploy the sensor at the seabed of 5,000 m corresponding to approximate 50MPa, the pressure sensor functions from atmospheric pressure to 70 MPa with the welded housing. The reproducibility of the sensor was evaluated to confirm whether stable pressure measurement can be realized. As a result, the repeatability at an applied pressure of 70 MPa was excellent with 0.005% of full scale or less.
This pressure sensor has enough resolution to clearly detect water level changes of several centimeters that occur on the sea surface. To investigate other effects, we measured output change due to attitude changes and temperature changes. As a result of evaluation with equipment for changing orientation of the sensor, the pressure change was within 1hPa when the cylinder was rotated 360° around the axis of rotation and was within 2hPa when the sensor was rotated 90° around the center in the longitudinal direction. And, the output change due to the temperature change at around 2℃ was less than 1hPa for a change of 0.1℃ in the evaluation simulating temperature change on the seabed.
The silicon resonant pressure sensor had excellent reproducibility on 70 MPa, small attitude errors, and small measurement errors caused by temperature changes in seawater. Therefore, we concluded that the resonant pressure sensor is appropriate to precise pressure observation at the seabed.
The silicon resonant pressure sensor developed by Yokogawa has advantage of low power consumption, compact size, high sensitivity, little individual differences, and high stability. Since the sensor chip is made by using Micro Electromechanical Systems (MEMS) technology to integrate sensing parts on the small chip, the sensor has the advantage that it is not easily affected by attitude errors due to movement of the sensor on the seabed and that temperature errors are easy to correct because of small temperature non-uniformity. Moreover, the sensor chips that determine the detection sensitivity are batch-manufactured by the wafer process, so there are little individual differences with the stable quality.
This sensor is an absolute pressure gauge that adopts diaphragm seal type, and pressure is applied to a 3mm square sensor chip through silicon oil separated by a thin metal diaphragm. Two resonators made of single crystal silicon that is an ideal elastic material are built into the same sensor chip. Each resonator is held in a vacuum cell by processes originally developed by Yokogawa to maintains a high Q value in no contact with silicon oil. The vacuum cell also works as a reference pressure cell of absolute pressure. The one of the two resonators is a pressure resonator that mainly detects pressure, and the other is a temperature resonator that mainly detects temperature. The pressure resonator is a both ends fixed beams and has initial tensile stress. When water pressure is applied, the sensor chip shrinks due to hydrostatic pressure, and compressive stress is applied to the pressure resonator. Then the resonant frequency of the pressure resonator changes. Since the ratio of change of resonant frequency versus pressure depends only on the bulk modulus of Si and the frequency of the resonator, the individual differences of pressure sensitivity are very small. On the other hand, the temperature resonator is designed so that compressive stress due to hydrostatic pressure is hardly applied, and the frequency mainly changes due to temperature changes. The temperature-corrected pressure value is obtained by acquiring the frequency outputs of two resonators with a frequency counter and calculating them.
To deploy the sensor at the seabed of 5,000 m corresponding to approximate 50MPa, the pressure sensor functions from atmospheric pressure to 70 MPa with the welded housing. The reproducibility of the sensor was evaluated to confirm whether stable pressure measurement can be realized. As a result, the repeatability at an applied pressure of 70 MPa was excellent with 0.005% of full scale or less.
This pressure sensor has enough resolution to clearly detect water level changes of several centimeters that occur on the sea surface. To investigate other effects, we measured output change due to attitude changes and temperature changes. As a result of evaluation with equipment for changing orientation of the sensor, the pressure change was within 1hPa when the cylinder was rotated 360° around the axis of rotation and was within 2hPa when the sensor was rotated 90° around the center in the longitudinal direction. And, the output change due to the temperature change at around 2℃ was less than 1hPa for a change of 0.1℃ in the evaluation simulating temperature change on the seabed.
The silicon resonant pressure sensor had excellent reproducibility on 70 MPa, small attitude errors, and small measurement errors caused by temperature changes in seawater. Therefore, we concluded that the resonant pressure sensor is appropriate to precise pressure observation at the seabed.