| The ground tilt mainly observes the angle between the ground plane and the horizontal plane(the angle between the normal line of the ground plane and the plumb line)and its change with time.Tilt observation is an important part of crustal deformation observation in China.It is one of the important means to study crustal deformation and earth tide.It plays an important role in earthquake precursor observation,volcanic precursor observation and earth tide geodynamics observation.Quartz horizontal pendulum tiltmeter is a kind of precursory seismic observation instrument for measuring ground tilt variation.Among them,V-M quartz horizontal pendulum tiltmeter is the first short baseline tiltmeter used for ground tilt measurement.In China,sq-70 d quartz horizontal pendulum tiltmeter for photographic recording was developed in 1970 s,and ssq-1 digital tape recording quartz horizontal pendulum tiltmeter was developed by digital transformation in 1990 s.The quartz horizontal pendulum tiltmeter has high sensitivity and good stability,and has been widely used in the earthquake precursor network of China.At present,the operation is generally good and good observation data are obtained.However,there are still some shortcomings,such as unknown transfer function and low observation frequency band.In order to further improve the technical performance of the tiltmeter,Li Jihong et al.Introduced the electrostatic force feedback technology.The output voltage of the tiltmeter was transmitted to the central plate of the capacitive transducer through the feedback circuit.The static feedback force was formed by cooperating with the fixed DC voltage of the two plates,so as to improve the dynamic range,observation frequency band and distortion of the quartz horizontal pendulum tiltmeter.For quartz horizontal pendulum tiltmeter,there are two commonly used calibration methods.One is to calibrate the quartz horizontal pendulum tiltmeter by using the precise tilt platform in the laboratory,which is cumbersome to operate and can not realize the field calibration at the station;the other is to use the expansion box mercury cup device,that is,by raising or lowering the mercury cup to make the expansion box produce elastic deformation,and observe the output of the instrument under the small deformation The grid value / sensitivity of the instrument is calculated and the static calibration is realized.This static calibration can only test the sensitivity,and can not give / measure the transfer function of the instrument.In order to adapt to the unattended observation mode and ensure the quality of observation data,it is imperative to carry out the on-line detection(calibration)method and technology of observation instruments.In this paper,a method of voltage balance is proposed based on the voltage balance signal of quartz wave.We introduced the method of introducing calibration signal,and established the feedback theoretical model of force balance feedback quartz horizontal pendulum tiltmeter after introducing calibration signal;according to the theoretical model,the transfer function and output mathematical expression of force balance feedback quartz horizontal pendulum tiltmeter were deduced when step voltage signal input and sine wave voltage signal input.Then,we have carried out the test in the laboratory,input a group of equal amplitude sine waves of discrete frequency points to the force balance feedback quartz horizontal pendulum tiltmeter,record the output of the instrument,and obtain the sinusoidal calibration output response data;input a group of relatively long period square wave signals to obtain the step calibration output response data.For the calibration response data of sine wave,the response amplitudes of each frequency point are extracted from time domain and frequency domain respectively,and the normalized amplitude response curve of the instrument is obtained.Then,using the derived transfer function and output mathematical expression of the sine wave voltage input signal of the quartz horizontal pendulum tiltmeter derived from the force balance feedback,the nonlinear fitting of the output response of the sinusoidal calibration is carried out,and the period and damping of the instrument calculated from the sine calibration are obtained.For the step calibration response data,after preprocessing such as drift correction and normalization,the transfer function and output mathematical expression of the step voltage signal derived from the instrument are used to nonlinear fit the output response of the step calibration after preprocessing,and the period and damping of the instrument calculated from the step calibration are obtained.The results of laboratory test and data processing show that: 1)the amplitude of sine wave calibration response obtained based on frequency domain is more accurate than that of time domain,and the fitting result is also better;2)the period and damping of the instrument obtained by sine wave calibration test are close to that of the instrument obtained by step calibration test,which verifies the effectiveness and effectiveness of the proposed test method uniformity.According to the period and damping obtained from the test,the transfer function of the instrument can be given,which plays an important role in describing the observation frequency band of the tiltmeter and its quantitative transformation relationship,which is conducive to the realization of the on-line monitoring of the working state of the tiltmeter and ensuring the quality of the observation data of the tiltmeter. |
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