Feasibility study on Digital Feedback Seismometer

将数字反馈系统应用于宽带地震仪（以下简称数字反馈地震仪：DFS）是对传统的基于模拟反馈系统的宽带地震仪进行改进的一种有希望的方法。DFS似乎使操作更简单，测量更精确，观测更灵活，因为它可以补偿测量中的误差和非线性，并且可以在不改变硬件的情况下根据观测目标调整其频率响应。DFS的概念早在几十年前就被提出，但目前还没有得到开发，也没有在商业基地生产。这是因为DFS上的技术问题目前还无法解决。本研究通过对DSP（数字信号处理器）组成的电子电路的数值模拟和实验，检查了实现DFS的难点，评估了它们的影响，并揭示了可能的解决方案。AD和DA转换器的量子误差、反馈计算和AD-DA转换引起的延迟时间的影响导致了DFS动态范围的限制。这似乎是最严重的缺陷。然而，我们可以使用过采样技术和滤波技术来解决这个问题，即使我们选择的是商业基地可以获得的现实电子设备。例如，采用16位分辨率的SAR型AD和DA转换器和速度为40 MIPS的数字信号处理器，实现了动态范围为140dB的DFS，几乎与传统的宽带地震仪兼容。这说明了DFS的开发是可行的，并鼓励我们实现高性能的DFS。在这项研究中，我们开发了一个使用DSP和质量摆系统来测量地面运动的DFS原型。我想利用这个原型在未来的研究中进行更深入的研究，实现更真实的DFS。

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.