Research On Cryogenic And Low-frequency Vibration Measurement Of Geophone Based On Pole-zero Compensation Method

Cryogenic environment is one of the effective tools to suppress many kinds of noise for precise physical instruments.Thus,the refrigeration system based on 4.2K cryocooler has been extensively used.However,if the mechanical vibration resulted from the cryocooler is ignored and the coldhead is mechanically coupled to the measuring sensitive element,the precise measurement process will be seriously disturbed and the fundamental noise will be remarkably raised.Therefore,the measures of vibration isolation or reduction are frequently taken to suppress the vibration of cryocooler for different refrigeration systems.However,it is difficult to accurately evaluate the residual vibration and noise level of the cryocooler after vibration isolation.On the one hand,ordinary vibration sensors cannot work at 4.2 K.On the other hand,limited by many factors,such as accuracy,bandwidth and installation conditions,these sensors are also not suitable for refrigeration systems.Therefore,it is particularly important to develop a vibration measurement sensor working at 4.2 K that has high accuracy and meets low-frequency operating conditions to evaluate the residual vibration and noise level of the cryocooler.In this paper,a cryogenic and low-frequency vibration measurement method of geophone based on pole-zero compensation method is proposed.Firstly,the theoretical modeling of the speed-type vibration sensor geophone is established.Then,the characteristic parameters of geophone at different temperatures are calibrated by the sine frequency sweep method.Secondly,the zero-pole compensation circuit is designed and manufactured according to the transfer function of Geophone.And the noise levels of each link have been analyzed in detail and also verified by Pspice simulation software.Besides,the output noise of the zero-pole compensation circuit has been tested by grounding the input terminal.The experimental results are in good agreement with the theoretical results.Finally,the room temperature and cryogenic(4.2K)vibration test platforms have been built respectively,and technical indicators of the instrument have been evaluated by comparing with the micro-vibration instrument.The experimental results show that the sensitivity of the instrument is increased by 101 times with the pole-zero compensation circuit for the GS-ONE geophone.The lowfrequency cut-off frequency is extended from 4.5Hz to 0.4Hz,and the noise level can reach20nm/s/√Hz@1Hz at room temperature or cryogenic environment,limited by the ground vibration noise level.