At present, the methods of G measurement can be roughly divided into three categories: geophysical measurement, laboratory measurement and space measurement. Geophysical methods have obvious gravitational effect, but the experimental accuracy is relatively low. Space measurement methods are facing many new technical problems and are still being explored. At present, laboratory measurement is the main means to obtain high-precision G value, and the commonly used tool is precision torsion balance. There are several methods to measure the gravitational constant g by torsion balance: direct tilt method, * * * vibration method and periodic method. Among them, the torsion balance period method is one of the most widely used and the most ideal measurement results. Its basic principle is that after the attractive mass is placed around the torsion balance, the movement period of the torsion balance will change accordingly. It is an arduous and difficult systematic work to measure the gravitational constant g in the laboratory. The improvement of experimental accuracy is mainly restricted by the following four factors: the gravitational interaction is very weak; Gravity cannot be shielded; Absolute measurement of quality, length and time; Independence of gravitational constant g, etc.
In this paper, the gravity constant g is measured absolutely by the periodic method of torsion balance, and the characteristics and systematic errors of torsion balance are systematically studied. At the same time, the background of the experimental environment is monitored synchronously to ensure the accuracy of the experiment. Its innovation lies in the adoption of a long-period and high-q torsion balance, which works in an environment of constant temperature, vibration isolation and relatively small external gravity interference, thus overcoming the influence of torsion wire hysteresis and thermoelasticity on G measurement. Details are as follows:
A. Study on the system error of torsion balance
It is of great significance to understand the error source of the torsion balance system from both theoretical and experimental aspects to improve the experimental accuracy of the torsion balance. We have obtained a series of important results in the research on the error of the torsion balance system: 1) There is an optimal configuration between the inspection quality and the attraction quality of the torsion balance system, which can reduce the nonlinear effect from the attraction quality, make the torsion balance work in a large range, and improve the signal-to-noise ratio of the system (phys. lett.a, 238,1998: 337); 2) Higher experimental accuracy can be obtained by measuring the transient state of the torsion balance instead of measuring the equilibrium state of the torsion balance (phys.lett.a, 238,1998: 341); 3) Theoretical analysis and experimental research show that when the torsion balance works at 10-2 radian, the nonlinear effect of the torsion balance is less than 1 ppm, which can be ignored. This conclusion eliminates people's concern about the nonlinear effect of twisted wire (Phys. Ritter. a，264， 1999: 1 12)。 4) Theoretical analysis and experimental research show that the quality factor Q of the torsion balance system decreases with the increase of its amplitude, which has important guiding significance for reducing the influence of anelasticity on G measurement (Phys. Ritter. A, 268, 2000: 255) (5) Theoretical analysis and experimental research show that the change of environmental temperature has a great influence on the torsion coefficient k of the suspension wire of the torsion scale. For the tungsten wire commonly used in experiments, that is, when the ambient temperature changes, the error brought to the measurement of G will be as high as 165 ppm (Rev. SCI. Instrum.7 1，2000: 1524)。 The research results of this thermoelastic effect of twisted wire show that many previous G measurement results are doubtful, and we can use it to make a reasonable explanation for the inconsistency of G measurement results at present.
B. Research on fundamental frequency fitting method of ultra-long period signal
Generally, the period of the torsion balance ranges from several minutes to more than 1 hour, because the longer the period, the higher the sensitivity. However, it is very difficult to fit the fundamental frequency of long-period torsion balance. The traditional FFT (Fast Fourier Transform) and all-pole (extreme point) methods need n = 10-5 cycles of experimental measurement data to achieve relative fitting accuracy. If the period of the torsion balance is 1 hour and the experimental data length is 15 years, it is obviously unrealistic. At present, the so-called nonlinear fitting is commonly used, such as the least square fitting of sine signal with objective function. The fitting accuracy of this method to frequency depends on the fitting accuracy of amplitude and phase. In order to get the minimum total variance, the variance of the three parameters must be balanced. Because we are only interested in the fitting accuracy of frequency, we can sacrifice the fitting accuracy of other parameters to obtain high-precision frequency fitting. Using this idea, we propose a periodic fitting method. The specific application results of computer simulation and experimental data show that the data fitting accuracy of this method for low-frequency signals with more than ten periods (the period is as long as 1 hour) can reach more than 10-7, thus solving the problem of accurate fitting of the fundamental frequency of long-period torsion balance. This method can be widely used in the fields that need to determine the fundamental frequency of ultra-low frequency signals (Rev. Sci. Instrumental music ，V70， 1999: 44 12)。
C. research on folding pendulum tiltmeter
In order to detect the background of earth tilt tide synchronously in the G-test environment, we applied the horizontal vibration isolation technology used in the laser gravitational wave detection experiment to the study of earth tilt tide, and successfully developed a folding pendulum tiltmeter. The basic idea is to skillfully connect a positive pendulum with an inverted pendulum to reduce the recovery coefficient of the whole pendulum system, so as to obtain an extremely low motion frequency (long period). The period of the folding pendulum developed by us is more than 60 seconds, and the equivalent length of the simple pendulum is more than 1 km. The experimental results of observing the earth tilt tide with a folding pendulum show that the sensitivity of the folding pendulum has reached 3.510-9 radians (physical column. a，256， 1999: 132)。 This result is obviously better than the commonly used water pipe inclinometer and horizontal pendulum inclinometer. In addition, the folding pendulum can also be used as a high-precision seismic pickup to monitor the abnormal signals of earthquakes, especially those before earthquakes. We have detected many earthquakes and their precursor signals by using the folding pendulum. The invention patent application of folding pendulum tiltmeter has been approved by the State Patent Office (patent number: ZL95 1 148222).
D. research on precision temperature sensing system
In the experiment of measuring G-torsion balance, small environmental temperature changes will directly affect the experimental results. In order to monitor the temperature field in the experimental environment synchronously, we developed a set of high-precision micro-temperature change measurement system. Its basic principle is to use the difference of thermal expansion characteristics of two different materials to detect the slight temperature change. The resolution of the temperature monitoring system we developed reaches 0.000 1 oC, which solves the problem of temperature monitoring in the experimental environment. This technology can also be applied to many other fields (Rev. Sci. Instrument. , 68, 1997: 565).
E. research on ultra-low frequency vibration isolation system
Because of the weak gravitational interaction, it is necessary to isolate the interference of external vibration on G measurement experiment. The lower the frequency of the vibration isolation system, the better the vibration isolation effect. The concept of quasi-static reference frame is put forward for the first time, and a new vibration isolation method based on active damping of quasi-static reference frame is realized. The ultra-low frequency vertical torsion bar spring system is designed and manufactured. Its natural period reaches 20 seconds, and the vibration isolation rate of the system exceeds 3 orders of magnitude at 6Hz. Based on the quasi-static reference frame, the active damping of large-scale vibration isolation system is successfully realized, and its vibration isolation performance is better than that of traditional vibration isolation methods (Rev. SCI. Instrum.69， 1998:278 1； Physics, Ritter. a，253， 1999: 1)。
Unique experimental design (long period, high Q value), superior experimental environment (silence, constant temperature and vibration isolation), in-depth and detailed study of the system error of the torsion balance instrument, and synchronous monitoring of the background environment ensure the accuracy of the experiment. We finally measured G as (6.6699 0.0007)10-1m3 kg-1s-2, and its relative accuracy reached 105 ppm. The results were published in phys.rev.d. (Physical Review D) in the United States. This is not only the first high-precision G value in China so far, but also one of the best measured values in the world at present, and it has been adopted by CODATA value recommended by the International Committee on Fundamental Constants of Physics 1998.
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