Fei Hou, Jiabiao Li, Xinke Zhu, Weiwei Ding, Zhiteng Yu. A new fixed-depth suspension control algorithm for mobile marine seismometer and its testing results[J]. Acta Oceanologica Sinica.
Citation:
Fei Hou, Jiabiao Li, Xinke Zhu, Weiwei Ding, Zhiteng Yu. A new fixed-depth suspension control algorithm for mobile marine seismometer and its testing results[J]. Acta Oceanologica Sinica.
Fei Hou, Jiabiao Li, Xinke Zhu, Weiwei Ding, Zhiteng Yu. A new fixed-depth suspension control algorithm for mobile marine seismometer and its testing results[J]. Acta Oceanologica Sinica.
Citation:
Fei Hou, Jiabiao Li, Xinke Zhu, Weiwei Ding, Zhiteng Yu. A new fixed-depth suspension control algorithm for mobile marine seismometer and its testing results[J]. Acta Oceanologica Sinica.
Hubei Key Laboratory of Marine Geological Resources, China University of Geosciences, Wuhan 430074, China
2.
Key Laboratory of Submarine Geosciences & Second Institute of Oceanography, Ministry of Natural Resources, Hangzhou 31002, China
Funds:
National Key Research and Development Program of China under contract Nos. 2021YFC3101401 and 2022YFC3003802; Deep Blue Fund under contract No.SL2103; Zhejiang Provincial Key Research and Development Program under contract No.2021C03186; Zhejiang Provincial Natural Science Foundation of China under Grant No. LDQ24D060001
A mobile marine seismometer (MMS) is a vertical underwater vehicle that detects ocean seismic waves. One of the critical operational requirements for an MMS is that it remains suspended at a desired depth. This article aimed to propose a fixed-depth suspension control for the MMS with a limited onboard energy supply. The research team established a kinematic model to analyze fluctuations in the vertical motion of the MMS and the delayed response of the system. We ascertained a direct one-to-one correlation between the displacement volume of the mobile ocean seismic instrument and the depth at which it reaches a state of neutral buoyancy (commonly referred to as the hover depth). A fixed-depth control algorithm was introduced, allowing a gradual approach to the necessary displacement volume to reach the desired suspension depth. The study optimized the boundary conditions to reduce unnecessary adjustments and mitigate the time delay caused by the instrument’s inertia, thereby significantly minimizing energy consumption. This method does not require calculating the hydrodynamic parameters or transfer functions of the MMS, thereby considerably reducing the implementation complexity. In the three-month sea trial in the South China Sea, the seismic instrument was set to hover at 800 meters, with a permissible fluctuation of ±100 meters, operating on a seven-day cycle. The experimental results show that the seismic instrument has an average hover error of 34.6 meters, with a vertical drift depth of 29.6 meters per cycle, and the buoyancy adjustment system made six adjustments, indicating that our proposed control method performs satisfactorily. In addition, this method provides new insights for the fixed-depth control of other ocean observation devices that rely on buoyancy adjustment.
Figure 1. Schematic diagram of the operation of an MMS
Figure 2. (a) Seismometer structure (b) seismometer force analysis.
Figure 3. Relationship between displacement volume and buoyancy for a mobile marine seismometer
Figure 4. MMS movement trends. (a) The initial state of the MMS moves downward over the upper boundary but eventually hovers above it. (b) The initial state of the MMS moves down over the lower boundary but eventually hovers below it. (c) MMS attains fixed-depth hovering.
Figure 5. Principles of buoyancy adjustment. The seismometer gradually reduces its displacement until it reaches the required volume for hovering.
Figure 6. Fixed-depth control process
Figure 7. Seawater density data for the South China Sea
Figure 8. Number of adjustments of the fixed-depth algorithm concerning oil volume. Here, X represents the depth ($ m $) at which hovering is possible after the initial buoyancy adjustment, Y represents the adjustment step size of the displacement volume ($ ml $) for each iteration, and Z represents the number of adjustments required to achieve the neutral buoyancy necessary for hovering.
Figure 9. The relationship between the variation in displacement volume (with the displacement volume at 1000 m hover depth as the reference)
Figure 10. Trial area and MMS drift tracks
Figure 11. The dive depth curve for the MMS N0-9 and the two red dashed lines represent the upper and lower boundaries of the set constant depth interval, respectively.
Figure 12. The dive depth curve for MMSs, where MMS No-12 (purple dotted line) and No-16 (light blue) were set to a fixed depth of 500m, and the rest of the MMSs were set to a fixed depth of 1000m.
Figure 13. MMS operating status. (a) the operational time of MMS (h) of MMS duty cycles (b) time in suspension at a fixed depth as a percentage of dive time for the MMS.
Figure 14. (a) Number of buoyancy adjustments needed to achieve fixed-depth suspension at different points along the drift tracks of the MMS No-12. (b) Average suspension depth and error at different points along the drift tracks of the MMS No-12.
Figure 15. (a) Number of buoyancy adjustments needed to achieve fixed-depth suspension at different points along the drift tracks of the MMS No-13. (b) Average suspension depth and error at various points along the drift tracks of the MMS No-13.
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