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计：DFS)，是改进传统的基于模拟反馈系统的宽带地震仪的有效途径之一。DFS似乎使操作更容易，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，并可以在不改变硬件的情况下调整其对观测目标的频率响应。DFS的概念早在几十年前就被提出，但目前还没有被开发出来，也没有在商业基地中生产。本研究通过对数字信号处理器(DSP)构成的电子线路进行数值模拟和实验，检查了实现DFS的难点，评估了实现DFS的效果，并提出了可能的解决方案。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限制了DFS的动态范围。这似乎是最严重的缺陷。然而，即使在选择商用基础上可以获得的现实电子设备的情况下，也可以使用过采样技术和滤波技术来解决这一问题。例如，16位分辨率的SAR型AD和DA转换器以及速度为40MIPS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。在本研究中，我们利用数字信号处理器和质量摆系统开发了一台用于地面运动测量的DFS样机。希望通过对该原型的深入研究，在以后的研究中实现更符合实际的分布式文件系统。

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.