Development of a vibration detector using a magnetic spring and an astable rotational-pendulum with a plunger permanent magnet inserted in to the uniform magnetic field

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In this study, we have developed a seismometer more sensitive than present one using a new principle. It's an astable rotational pendulum that a plunger permanent magnet is inserted in to the uniform magnetic field generated by a window frame type permanent magnet and attached to a rotational axis outside of the bore of the magnet. The results obtained in this study were as follows. 1. The prototype of a seismometer employing a rotational pendulum and a magnetic spring has been built and been evaluated by experiments. The result of the experiments showed that the natural period of the pendulum which could be an index of seismometer's performance was more than ten seconds. This period is almost comparable to that of usual highly sensitive/wide band seismometers. 2. The prototype with a feedback circuit, a feedback coil and a capacitance displacement sensor worked as a servo type seismometer. However feedback parameters were not so optimal at present stage and so should be tuned to the best. 3. The natural period of the prototype could be changed with the magnetic spring. These values of the period are consistent with theoretical calculations. 4. The magnetic field uniformity of the window frame magnet is important for stable operation of the prototype. The further uniformity is necessary for more stable operation, through we could get the value of 10^<-3>. 5. To reject the drift of a spring constant caused by a temperature change, we have procured stable elastic modulus metal. The leaf springs using the metal will be installed as x-hinges for the rotational axis of the prototype. From the above mentioned results, the methods to improve the operation stability of the prototype were figured out as well as it was confirmed that the rotational pendulum could be worked in order to a theory.