1. A falling body optical center mass center measuring device comprises a machine shell and a torsion testing unit, wherein a falling body torsion swing mechanism is arranged in the machine shell, and the falling body optical center mass center measuring device is characterized in that the machine shell is a vacuum cavity, and the torsion testing unit comprises a quadrature interferometer and a photoelectric autocollimator; the side wall of the vacuum cavity is provided with a window for the light of the torsion testing unit to enter and exit; the window of the photoelectric autocollimator is arranged at a position corresponding to the reflector on the surface of the falling body; the window of the orthogonal interferometer is arranged on the inner wall of the vacuum cavity and corresponds to the pyramid prism in the falling body, laser emitted by the orthogonal interferometer is divided into at least two paths of reflection signals with 180-degree difference through a light path, and the reflection signals are output to the orthogonal interferometer; and the photoelectric autocollimator outputs the torsional pendulum data of the falling body.

2. The drop optical center centroid measuring device according to claim 1, wherein said drop wiggling mechanism comprises a vacuum guide, a damping system suspension wire, a damping system, a drop suspension wire and a drop; the vacuum guide is arranged at the top of the vacuum cavity; the upper end of the damping system suspension wire is connected with the bottom end of the vacuum guide, and the lower end of the damping system suspension wire is connected with the upper end of the movable part of the damping system; the upper end of the falling body suspension wire is connected with the lower end of the movable part of the damping system; the lower end is connected with the top end of the falling body; the fixed part of the damping system is fixedly connected with the inner wall of the vacuum cavity through a bracket; the damping system suspension wire is a tungsten wire; the falling body suspension wire is quartz wire.

3. The drop optical center mass center measuring device according to claim 2, wherein a torsion amplitude control unit for controlling the torsion amplitude of the drop is further arranged in the shell; and the electromagnetic coils of the amplitude-twisting control unit are symmetrically arranged on the vertical plane where the falling body central shaft is located when the electromagnetic coils are in the static position.

4. The drop optical center centroid measuring device according to claim 1, wherein the optical path of the quadrature interferometer is a four-path signal single-frequency laser interference optical path.

5. The drop optical center centroid measuring device according to claim 1, wherein the fixed portion of the damping system employs an electromagnet as a damping magnet.

6. The drop optical center centroid measuring device according to claim 1, wherein the mirror of the photoelectric autocollimator is disposed on a top end face of the nut adjusting knob.

7. The method of using the drop optical center centroid measuring device according to claim 1, comprising the steps of:

1) the falling body torsion swing mechanism is used for exciting the falling body to generate torsion swings with angles larger than a specified angle;

2) obtaining torsional pendulum data of the falling body through the torsion testing unit;

3) the torsion swing angle of the falling body is reduced to the designated angle, and the torsion angle of the falling body is recorded through the photoelectric autocollimator; simultaneously, recording the displacement of the falling body through the orthogonal interferometer;

4) the orthogonal interferometer records the amplitude frequency spectrum obtained by Fourier transform of the displacement signal of the falling body, and extracts the fundamental frequency peak value A of the amplitude frequency spectrum₁(ii) a Amplitude frequency spectrum obtained by Fourier transformation of falling body torsion angle signal is extracted to obtain fundamental frequency peak value B thereof₁(ii) a Calculating to obtain the offset delta of the optical center of the pyramid prism and the center of mass of the falling body along the X-axis direction_xThe formula is as follows:

5) fitting the torsion angle waveform of the falling body recorded by the photoelectric autocollimator to obtain theta₀Omega and beta, and correcting the relation between the falling body torsion angle and other parameters through a correction function formula (7);

θ₀the initial angle of torsion, beta is a damping coefficient, omega is the frequency of the torsion angle, and t is time;

6) let equation (7) beExtracting the frequency spectrum graph obtained after Fourier transform to obtain the double frequency peak B₂Substituting into the calculation of formula (8); obtaining the offset delta of the optical center of the pyramid prism and the mass center of the falling body along the Y-axis direction_y：

Wherein A is₂Measuring the double frequency amplitude, B, of the displacement signal for the quadrature interferometer₂The correction function curve is a frequency doubling peak value obtained by Fourier transform;

7) calculating formula (4):

d_yis the displacement of the optical center of the pyramid prism along the direction of the Y axis.

8. The use method as claimed in claim 7, wherein in the step 2), the torsion swing angle of the falling body is reduced to the designated angle through the torsion amplitude control unit; the specified angle is 1.6-2.4 degrees.

9. The use method as claimed in claim 7, wherein in the step 2), the fixed part of the damping system adopts an electromagnet as a damping magnet; and when the simple pendulum movement and the shaking of the falling body are less than 0.1 degree, the electromagnet is powered off.

Background

A large measurement error of the absolute gravity measurement is derived from the rotation error of the falling body. Because the moment that the falling body releases, its strong point is not released simultaneously, and the barycenter and the optical center of falling body can not guarantee complete coincidence, and the optical center is rotatory around the barycenter and can produce an additional acceleration, because the acceleration of gravity acts on the falling body barycenter, and the interferometer measurement be the displacement of prism optical center in the falling body, consequently can produce rotation error, this item can be expressed as by approximation:

△g＝ω²δ_z (1)

wherein: omega is the falling body angular velocity, delta_zIs the projection of the optical center and the mass center of the falling body in the gravity meter at the initial position along the gravity direction.

The rotational speed of the falling body in an absolute gravimeter during falling is normally less than 0.01rad/s, but as the time of use of the gravimeter increases, the rotational speed of the falling body is as high as 0.1rad/s due to mechanical wear. According to the formula, in order to keep the measurement accuracy of the absolute gravimeter at the original level, the falling body needs to be adjusted, so that the offset of the optical center of the pyramid prism in the falling body and the mass center of the falling body is reduced by one hundred times. In order to make the rotation error less than 0.1 μ Gal, the offset between the optical center and the mass center of the abraded falling body is better than 0.1 μm, and the current measurement method is difficult to achieve the level.

Disclosure of Invention

As shown in fig. 2, the falling body coordinate system illustrates: the Z axis is the gravity direction; upward is positive; the X axis is the front-back direction, and the facing direction is positive; the Y axis is the left-right direction, and the right direction is positive.

In view of the above, the present invention aims to provide a measurement scheme that achieves a drop optical center to centroid offset of better than 0.1 μm. In order to achieve the above object, the present invention provides a high-precision method for measuring the optical center and the mass center of a falling body, which adopts a double pendulum structure, can effectively inhibit the non-torsional movement of the falling body, can measure the distance between the optical center and the mass center of the falling body by extracting the double frequency amplitude of the displacement signal of the torsional movement of the falling body along the incident direction of a laser beam through an orthogonal interferometer and the torsional angle of the falling body measured through an angle measuring system, and finally adjusts the offset of the optical center and the mass center of the falling body to a level better than 0.1 μm. The specific technical scheme is as follows:

a falling body optical center mass center measuring device comprises a machine shell and a torsion testing unit, wherein a falling body torsion swing mechanism is arranged in the machine shell, the machine shell is a vacuum cavity, and the torsion testing unit comprises a quadrature interferometer and a photoelectric autocollimator; the side wall of the vacuum cavity is provided with a window for the light of the torsion testing unit to enter and exit; the window of the photoelectric autocollimator is arranged at a position corresponding to the reflector on the surface of the falling body; the window of the orthogonal interferometer is arranged on the inner wall of the vacuum cavity and corresponds to the pyramid prism in the falling body, laser emitted by the orthogonal interferometer is divided into at least two paths of reflection signals with 180-degree difference through a light path, and the reflection signals are output to the orthogonal interferometer; and the photoelectric autocollimator outputs the torsional pendulum data of the falling body.

Further, the falling body torsional pendulum mechanism comprises a vacuum guide, a damping system suspension wire, a damping system, a falling body suspension wire and a falling body; the vacuum guide is arranged at the top of the vacuum cavity; the upper end of the damping system suspension wire is connected with the bottom end of the vacuum guide, and the lower end of the damping system suspension wire is connected with the upper end of the movable part of the damping system; the upper end of the falling body suspension wire is connected with the lower end of the movable part of the damping system; the lower end is connected with the top end of the falling body; the fixed part of the damping system is fixedly connected with the inner wall of the vacuum cavity through a bracket; the damping system suspension wire is a tungsten wire; the falling body suspension wire is quartz wire.

Further, a torsion amplitude control unit for controlling the torsion amplitude of the falling body is arranged in the shell; and the electromagnetic coils of the amplitude-twisting control unit are symmetrically arranged on the vertical plane where the falling body central shaft is located when the electromagnetic coils are in the static position.

Furthermore, the optical path of the orthogonal interferometer is a four-path signal single-frequency laser interference optical path.

Furthermore, the fixed part of the damping system adopts an electromagnet as a damping magnet.

Further, the reflector of the photoelectric autocollimator is arranged on the top end face of the nut adjusting knob.

The invention also discloses a using method of the falling body optical center and mass center measuring device, which comprises the following steps:

1) the falling body torsion swing mechanism is used for exciting the falling body to generate torsion swings with angles larger than a specified angle;

2) obtaining torsional pendulum data of the falling body through the torsion testing unit;

3) the torsion swing angle of the falling body is reduced to the designated angle, and the torsion angle of the falling body is recorded through the photoelectric autocollimator; simultaneously, recording the displacement of the falling body through the orthogonal interferometer;

4) the orthogonal interferometer records the amplitude frequency spectrum obtained by Fourier transform of the displacement signal of the falling body, and extracts the fundamental frequency peak value A of the amplitude frequency spectrum₁(ii) a Amplitude frequency spectrum obtained by Fourier transformation of falling body torsion angle signal is extracted to obtain fundamental frequency peak value B thereof₁(ii) a Calculating to obtain the offset delta of the optical center of the pyramid prism and the center of mass of the falling body along the X-axis direction_xThe formula is as follows:

5) fitting the torsion angle waveform of the falling body recorded by the photoelectric autocollimator to obtain theta₀Omega and beta, and correcting the relation between the falling body torsion angle and other parameters through a correction function formula (7);

θ＝θ₀ ²e^-2βtcos2ωt (7)

θ₀the initial angle of torsion, beta is a damping coefficient, omega is the frequency of the torsion angle, and t is time;

6) fourier transform is carried out on the formula (7) to obtain a spectrogram, and a frequency doubling peak value B of the spectrogram is extracted₂Substituting into the calculation of formula (8); obtaining the optical centre of the cube-corner prism and the centroid of the falling body along the Y-axis directionOffset d_y：

Wherein A is₂Measuring the double frequency amplitude, B, of the displacement signal for the quadrature interferometer₂The correction function curve is a frequency doubling peak value obtained by Fourier transform;

7) calculating formula (4):

d_yis the displacement of the optical center of the pyramid prism along the direction of the Y axis.

Further, in the step 2), the torsional pendulum angle of the falling body is reduced to the specified angle through the torsional amplitude control unit; the specified angle is 1.6-2.4 degrees.

Further, in the step 2), an electromagnet is adopted as a damping magnet in a fixing part of the damping system; and when the simple pendulum movement and the shaking of the falling body are less than 0.1 degree, the electromagnet is powered off.

According to the falling body optical center mass center measuring device, the shell is set to be the vacuum cavity, so that torsional oscillation damping of the system is reduced; two paths of reflected signals with a phase difference of 180 degrees are obtained through the combination of the beam splitter prism, the 1/4 wave plate and the 1/2 wave plate, the generation of common mode noise is effectively inhibited, and the optical path layout adopts an optical path difference amplification technology, so that the resolution of an interference system is improved.

Drawings

FIG. 1 is a front view of an absolute gravimeter falling body rotation error measuring device according to the present invention;

FIG. 2 is a schematic diagram of an absolute gravimeter falling body coordinate system;

FIG. 3 is a top view of the absolute gravimeter falling body rotation error measuring device of the present invention;

FIG. 4 is a schematic top view of a specific optical path implementation of the quadrature interferometer;

FIG. 5 is a schematic front view of one embodiment of a damping system;

FIG. 6 is a graph of displacement of a piezoelectric translation stage obtained by inversion of interference signals from a quadrature interferometer;

FIG. 7 is a magnitude frequency spectrum of the displacement signal of the piezoelectric translation stage after Fourier transform;

FIG. 8 is a graph of the variation of the falling body torsion angle with time in a high vacuum environment without using an amplitude control system;

FIG. 9 is a graph showing the variation of the falling body torsion angle with time in a high vacuum environment by controlling the torsion angle using an amplitude control system;

FIG. 10 is a graph of displacement of a corner cube prism obtained by inversion of interference signals from a quadrature interferometer;

fig. 11 is a magnitude spectrum obtained by fourier transform of a displacement signal obtained by measurement with the quadrature interferometer;

FIG. 12 is a graph of the measured fall twist angle for an autocollimator;

FIG. 13 is a magnitude spectrum obtained by Fourier transform of a falling body torsion angle measured by an autocollimator;

FIG. 14 is a graph of a constructed correction function;

FIG. 15 is a magnitude spectrum of the constructed correction function after Fourier transform;

FIG. 16 is an error bar graph of an optical center centroid displacement of a falling body calculated by using an initial torsion angle, wherein a torsion wire is a tungsten wire in an atmospheric environment;

FIG. 17 is an error bar graph of an optical center centroid offset of a falling body calculated by substituting a peak value obtained by Fourier transform of a constructed correction function into a formula, wherein a twisted wire is a tungsten wire in an atmospheric environment;

FIG. 18 is an error bar graph of the optical center centroid offset of falling body calculated by substituting a peak value obtained by Fourier transform of a constructed correction function into a formula, wherein the twisted wire is a quartz wire in a high vacuum environment;

FIG. 19 is an error bar graph of the optical center centroid offset of a falling body calculated by substituting a peak value obtained by Fourier transform of a constructed correction function into a formula after a falling body is taught along the gravity direction in a high vacuum environment, wherein a twisted wire is a quartz wire;

fig. 20 is an error bar graph of the optical center centroid offset of a falling body calculated after the falling body is subjected to process and return stroke teaching along the gravity direction after a magnetic damping system is added in an atmospheric environment and a twisted wire is a tungsten wire.

In the figure: 1. guiding in vacuum; 2. a tungsten filament; 3. a damping system; 4. quartz wire; 5. falling body; 51. a center of mass; 52. a light center; 6. a magnetic excitation system; 7. a vacuum chamber; 8. an angle measurement system; 9. a mirror plate; 10. a nut adjusting knob; 11. a falling body shell; 12. a pyramid prism; 13. a quadrature interferometer; 14. a laser; 15. a first beam splitting prism; 16. a first 1/2 wave plate; 17. a second 1/4 wave plate; 18. a third 1/4 wave plate; 19. a reference mirror; 20. a second polarization beam splitter prism; 21. a non-polarizing beam splitter prism; 22. a first 1/2 wave plate; 23. a third polarization beam splitter prism; 24. a fourth 1/4 wave plate; 25. a fourth polarization beam splitter prism; 26. a first electromagnet; 27. a second electromagnet; 28. a third electromagnet; 29. a fourth electromagnet; 30. a damping disc.

Detailed Description

The present invention will now be more fully described with reference to the following examples. This invention may, however, be embodied in many different forms and should not be construed as limited to the exemplary embodiments set forth herein.

For ease of description, spatially relative terms, such as "upper," "lower," "left," "right," and the like, may be used herein to describe one element or feature's relationship to another element or feature as illustrated in the figures. It will be understood that the spatial terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as "below" other elements or features would then be oriented "above" the other elements or features. Thus, the exemplary term "lower" can encompass both an upper and a lower orientation. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly.

The invention provides a falling body rotation error measuring device of an absolute gravimeter, which can enable the optical center and the mass center of a falling body to reach the offset smaller than 0.1 mu m; as shown in fig. 1, the structure comprises a vacuum chamber 7, and a vacuum guide 1 mounted on the top of the vacuum chamber 7, wherein the bottom of the vacuum guide 1 is inserted into the vacuum chamber 7; the upper end of the movable part of the damping system 3 is suspended at the bottom of the vacuum guide 1 through a tungsten wire 2, and the falling body 5 is suspended at the lower end of the movable part of the damping system 3 through a quartz wire 4; the fixed part of the damping system 3 is fixedly arranged in the vacuum cavity 7 through a bracket.

Two amplitude control systems 6 are arranged on two sides of a plane where the swing track of the falling body 5 is located. Vacuumizing the inner cavity of the vacuum cavity 7 by using vacuum equipment to ensure that the pressure in the cavity does not exceed 1x10^-5Pa。

As shown in fig. 4, the falling body is composed of a reflector 9, a nut adjusting knob 10, a falling body shell 11 and a pyramid prism 12, the reflector 9 is adhered on the top end surface of the nut adjusting knob 10 so as to do torsional movement along with the pyramid prism 12 in the falling body, and the rotating angles of the reflector 9 and the falling body shell 11 are consistent; the deflection angle of the drop 5 is measured by means of an angle measuring system 8 emitting laser light to a mirror 9 and receiving the reflected light. The angle measuring system 8 is a photoelectric autocollimator, has high measuring precision and can accurately measure the torsion angle of the falling body.

As shown in fig. 5, the optical path portion of the quadrature interferometer 13 is a four-signal single-frequency laser interference optical path: the laser 14 emits laser light and the laser light passes through the first beam splitter prism 15 to obtain linearly polarized light beams; the intensity of the transmitted light and the reflected light is adjusted by the first 1/2 wave plate 16; after adjustment, the intensity of the light entering and returning to the two arms is consistent, so that the alternating current reproduction of the interference signal is maximized. The transmitted light passes through the second polarization beam splitter prism 20, passes through the third 1/4 wave plate 18, is reflected by the reference mirror 19, passes through the third 1/4 wave plate 18, and is reflected back to and transmitted through the second polarization beam splitter prism 20 to form first transmitted light; the reflected light passes through the second 1/4 wave plate 17, is reflected by the corner cube 12 in the falling body, passes through the second 1/4 wave plate 17, and is reflected back to the second polarization splitting prism 20, and the reflected light is the first reflected light.

The first transmitted light and the first reflected light are directed to the non-polarizing beam splitter prism 21, and the reflected light passes through the fourth 1/4 wave plate 24, and is finally reflected and transmitted by the fourth polarizing beam splitter prism 25Obtaining two paths of interference signals I with 90-degree phase difference₃And I₄Similarly, the transmitted light passes through the first 1/2 wave plate 22 and is finally reflected and transmitted by the third beam splitter prism 23 to obtain two paths of interference signals I with a 90 ° difference₁And I₂Two groups of signals (I) with 180 DEG phase difference and opposite phases are combined₄And I₁，I₃And I₂) Two signals with orthogonal phases can be obtained by respectively inputting the two differential amplifiers, so that the generation of common mode noise can be effectively inhibited, and the resolution of an interference system is improved by adopting an optical path difference amplification technology in the layout of an optical path.

As shown in fig. 6 and 7, the measurement accuracy of the quadrature interferometer is calibrated by using the piezoelectric translation stage with the resolution of 0.1nm, a sinusoidal driving signal with the frequency of 0.2Hz and the voltage amplitude of 1V is generated by the signal generator to drive the piezoelectric translation stage to move, the resolution is limited by the signal generator and the piezoelectric displacement stage, the noise at the bottom of the signal in the final spectrogram is about 0.1nm, and the resolution of the quadrature interferometer is actually better than 0.1 nm.

The simple pendulum period of the falling body is related to the length of the quartz filament 2, and the formula is as follows:

the torsion period of the falling body is related to the moment of inertia of the falling body and the torsion elastic coefficient of the quartz wire, and the torsion elastic coefficient of the quartz wire is related to the shear elastic modulus, the diameter and the length of the torsion wire, and the formula is as follows:

selecting a quartz wire with proper length and diameter as a torsion wire, so that the frequency of the falling body simple pendulum motion is 10 times higher than the frequency of the torsion pendulum motion and the shaking motion; in this way, the frequency of the falling body simple pendulum movement is conveniently separated from the frequency of the torsional pendulum movement and the shaking movement.

The working principle of the absolute gravimeter falling body rotation error measuring device is as follows: when the falling body is subjected to external excitation guided by vacuum, the motion mode of the falling body can be divided into torsional pendulum motion, simple pendulum motion and shaking, the simple pendulum motion and the shaking are reduced or even eliminated through a damping system, when the falling body only does torsional pendulum motion, due to the influence of gravity, the extension line of the torsional wire necessarily passes through the mass center of the falling body, namely the falling body does torsional pendulum motion around the quartz wire. Measuring the rotating angle of the falling body in torsional pendulum motion through an angle measuring system 8; the displacement of the optical center of the pyramid prism 12 along the Y-axis direction (i.e. the falling body falls along the gravity direction in the absolute gravimeter) is measured by the quadrature interferometer 13, and the frequency doubling amplitude of this displacement signal is related to the offset of the optical center of the pyramid prism 12 and the center of mass of the falling body along the Y-axis direction, and its formula is:

d_yis the displacement of the optical center of the corner cube 12 in the Y direction, delta_yIs the offset of the optical center of the corner cube prism 12 and the center of mass of the falling body along the Y-axis direction, delta_xIs the offset of the optical center of the corner cube prism 12 from the center of mass of the falling body along the X direction₀The initial angle of the rotation of the body doing the torsional pendulum motion is beta, the damping coefficient of the torsional pendulum motion is beta, and the angular frequency of the body doing the torsional pendulum motion is omega.

The measurement precision of the optical center mass center offset of the falling body is related to the initial torsion angle of the falling body in torsional pendulum motion, when the torsion angle is small, after the displacement signal measured by the orthogonal interferometer is subjected to Fourier transform, a double frequency peak value of an amplitude spectrum of the displacement signal can be submerged by bottom noise, and accurate measurement cannot be performed, so that the torsion angle of the falling body needs to be maintained in a proper angle range through the torsion amplitude control system 6. The initial position of the falling body is the same horizontal position as that of fig. 3, and the falling body is simple swung, rocked and twisted in the vacuum cavity due to the initial excitation applied by the vacuum guide; wherein, the frequency of simple pendulum and shaking is higher, and the attenuation is fast. The amplitude control system consists of two energized magnetic coils which, when energized, produce a constant horizontal magnetic field, primarily for limiting the angle of the falling body twist. The relative sectional area of the metal piece in the falling body in the torsional pendulum is also continuously changed in a magnetic field, namely, the magnetic flux of the falling body is changed to generate induced electromotive force, so that the torsional motion of the falling body is quickly attenuated, and the torsional angle of the falling body is quickly reduced; when the autocollimator detects that the pendulum amplitude of the falling body reaches a specified angle, the current is cut off, and the falling body makes free torsional motion by taking the specified angle as an initial torsional angle.

As shown in fig. 8 and 9, if the vacuum guide applies a large external excitation, the swing angle of the falling body is large, and in a high vacuum environment, since the Q value of the system is large, the attenuation is very small, and if it is desired to control the falling body torsion angle at a specified angle, it is necessary to wait for a long time only by natural attenuation. When the amplitude control system 6 is electrified with the current of 12A, the torsion angle is rapidly controlled to be 2 degrees, the current is cut off, and the falling body makes free torsion motion at the initial torsion angle of about 2 degrees, so that the preparation time of each experiment is shortened.

As shown in fig. 10, when the falling body performs torsional pendulum motion in the atmospheric environment, the torsional pendulum motion is greatly attenuated due to the influence of the gas damping and the torsional wire structure damping, which is not beneficial to measurement, so that the Q value of the whole system needs to be increased, the damping coefficient needs to be reduced, and the time of the torsional pendulum motion needs to be delayed. If a high vacuum environment is maintained in the vacuum cavity 7, the influence of gas damping can be ignored, and the high-Q-value quartz wire can greatly reduce the damping of the twisted wire structure, so that the damping speed of torsional pendulum motion is greatly reduced.

As shown in fig. 11, it is apparent from the amplitude spectrum that the various motion modes of the falling body are separated by the frequencies of the torsional pendulum motion, the simple pendulum motion and the shaking motion, but since the non-torsional pendulum motion is not suppressed, the bottom noise at the double frequency is obviously lifted by the simple pendulum motion, and therefore, a damping system is required to suppress the simple pendulum motion and the shaking motion.

The damping system 3 in the embodiment is a magnetic damping system, when two coils of the damping system are electrified to generate a strong magnetic field, when the falling body 5 generates simple pendulum movement or shakes, the damping disc 30 made of metal of the movable part can be driven to do simple pendulum movement or shake, magnetic induction lines along the gravity direction generated by the cutting fixing part convert kinetic energy into heat energy to be dissipated, so that non-torsional movement is restrained, when the simple pendulum movement and the shake are restrained to be almost eliminated, the coils are powered off, the strong magnetic field disappears, and the influence on the amplitude control system is eliminated.

As shown in fig. 5, the damping system 2 is a magnetic damping structure, comprising: a first electromagnet 26, a second electromagnet 27, a third electromagnet 28, a fourth electromagnet 29, and a damping disc 30; the first electromagnet 26, the second electromagnet 27, the third electromagnet 28 and the fourth electromagnet 29 are fixedly connected with the vacuum device through the shell of the damping system 3. The damping disc 30 is preferably a circular disc of equal thickness with a center of mass at the center of the circle and is easy to machine. The central axis of the first electromagnet 26 is coaxial with the central axis of the third electromagnet 28, and the magnetic field directions are the same; the central axis of the second electromagnet 27 is coaxial with the central axis of the fourth electromagnet 29, and the magnetic field directions are the same; so that the magnetic field intensity is higher under the same excitation current. The central axis of the first electromagnet 26 and the central axis of the second electromagnet 27 are centrosymmetric with respect to the central axis of the damping disc 30, so that the damping obtained by the damping disc 30 is more uniform. Vacuum guide 1 is connected with the convex axostylus axostyle of damping dish upper surface through tungsten filament 1, and the convex axostylus axostyle of lower surface is connected with falling body 5 through quartz filament 4, and the vertical axle that damping dish 30 barycenter belongs to makes as the rotation axis, and the torsion of twisting silk drives the damping and coils the rotation axis and do torsional motion. The damper disk 30 serves as a movable part of the damper system 3, and the first electromagnet 26, the second electromagnet 27, the third electromagnet 28, the fourth electromagnet 29, and the housing of the damper system 3 serve as a fixed part of the damper system 3.

The function formula of the falling body torsion angle is as follows:

wherein theta is₀Is the initial angle of torsion, beta is the damping coefficient, omega is the angular frequency of torsion,is the initial phase.

As shown in fig. 12, the torsion angle of the falling body is measured by the photoelectric autocollimator,the autocollimator measured the data in angular seconds and measured an initial twist angle of 2609 ", i.e. 0.725 °. FIG. 13 is an amplitude spectrum obtained by Fourier transform of a falling body torsion angle signal, and its peak B is extracted₁The offset delta of the optical center of the corner cube 12 and the center of mass of the falling body along the X-axis direction can be calculated_x. The formula is as follows:

wherein A is₁Measuring a frequency-doubled amplitude of the displacement signal for a quadrature interferometer, B₁The peak value is obtained by Fourier transformation of the falling body torsion angle signal.

For the torsion angle data, fitting theta by using the falling body torsion angle calculation formula₀And β, ω can be calculated by the period in fig. 12, from which a correction function can be constructed, calculating δ_zMore accurate, its formula is:

FIG. 14 is a constructed correction function curve, and FIG. 15 is a frequency spectrum diagram obtained by Fourier transform of the correction function curve, and the peak B of the frequency doubling is extracted₂Substituted into the calculation of the offset.

Deviation d between the optical center of the corner cube prism 12 and the center of mass of the falling body along the Y-axis direction_yThe calculation formula is as follows:

A₂measuring the double frequency amplitude, B, of the displacement signal for the quadrature interferometer₂And carrying out Fourier transform on the constructed correction function curve to obtain a peak value.

As shown in fig. 16 and 17, the twisted wire is a tungsten wire, the Q value of the system is 600 in the atmospheric environment, and the initial angle of the falling body torsion is selected as the initial angleSubstitution of theta into delta_yWhen in the calculation formula, the deviation between the calculated offset and the true value is too large, and the peak value A obtained by Fourier transform of the constructed correction function₂When the offset is substituted into the calculation formula of the delta y, the calculated offset is closer to the true value. The larger the initial torsion angle of the falling body is, the closer the offset calculated by frequency doubling is to the offset of actual simulation, and the initial torsion angle is generally set to be 1.6-2.4 degrees.

As shown in fig. 16 and 18, when the twisted wire is changed to a high-Q quartz wire, the Q value of the system is increased to 20000 in a vacuum environment, and a frequency doubling amplitude is selected as θ and is introduced into the calculation formula, the calculated offset is very close to the true value, and the measurement accuracy is increased by two orders of magnitude.

As shown in fig. 18 and 19, the Q value of the system is 20000, after the falling body is taught along the gravity direction, the deviation between the optical center of the corner cube 12 and the falling body centroid along the gravity direction is reduced to one tenth of the original deviation, the initial torsion angle is set to 1.6-2.4 °, the deviation between the optical center of the falling body and the centroid along the gravity direction is measured again, and the deviation between the measured value and the true value at this time is less than 0.3 μm.

As shown in fig. 20, after the magnetic damping system is added, the falling body is calibrated in the process and return stroke along the gravity direction, a circle is screwed in the graph, the offset of the optical center and the center of mass of the falling body along the gravity direction is set to change by 54 μm, the offset of the optical center and the center of mass of the falling body along the gravity direction is measured respectively, the measured value has a good linear relationship, the change value of the offset of the optical center and the center of mass of the falling body along the gravity direction obtained by measurement is consistent with the change value of the actual offset, and the maximum deviation is 1 μm.

The above examples are only for illustrating the present invention, and besides, there are many different embodiments, which can be conceived by those skilled in the art after understanding the idea of the present invention, and therefore, they are not listed here.