Feasibility study on Digital Feedback Seismometer

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An application of digital feedback system to a broadband seismometer (we call it digital a feedback seismometer : DFS, hereafter) is one of hopeful ways to improve the conventional broadband seismometers that is based on analog feedback system. The DFS seems to make the operation easier, measurement more precisely and observations more flexible, because it can compensate the error and non-linearity in the measurements and can adjust its frequency response to the target of the observation without any change in its hardware. The concept of the DFS has been proposed a few tens years ago firstly, but it has not been developed and has not been produced in commercial base in present. It is because that the technical problems on the DFS can not be solved now.In this study, we checked difficult points for realizing the DFS, evaluated their effects and revealed a possible solution on it, using the numerical simulation and experiments on the electronic circuits composed of DSP (digital signal processor). The effect of quantum errors of AD and DA converters, delay time caused by feedback calculation and AD-DA conversion give the limitation of the dynamic range in the DFS. It seems to be the most serious defect. However, we can solve this problem using the over-sampling techniques and filtering ones even in the case that we choose the realistic electronic devices that can be got by commercial base. For example, SAR type AD and DA converter with 16bits resolutions and digital signal processor with the speed of 40 MIPS realize the DFS with dynamic range of 140dB, which is almost compatible with the conventional broadband seismometers. It shows that the DFS is feasible to develop, and it encourages us to realize the DFS with high performance.In this study, we developed a prototype of a DFS using a DSP and mass-pendulum system to measure ground motion. I would like to carry out the advanced research to realize more realistic DFS in future study using this prototype.