| Accurate measurement of gravity acceleration g is of great practical value and scientific significance.The high precision atom gravimeter can be used in the fields of mineral resources exploration,geological structure research,oil and gas survey,determination of general knowledge constants in scientific field,gravity between substances and other precision engineering surveying fields.High precision cold atom gravimeter is characterized by continuous and high precision measurement for a long time,but a series of complex technical problems need to be solved in order to improve the experimental accuracy of gravity acceleration measurement.In practical measurement,the accuracy of atomic interferometry is affected by such factors as ground vibration noise,Raman phase noise and detection noise.Among them,the ground vibration and noise are the most important factors affecting the atom gravimeter.The cold atom gravimeter uses the free falling cold atoms in the gravity field as the detection mass,and obtains the local gravity acceleration by measuring the acceleration of the cold atoms relative to the Raman reflector.However,due to the influence of the outside world,the mirror itself has vibration acceleration in the vertical direction,and the mirror is a non-inertial system with inertial acceleration.In practice,it is impossible to distinguish the vibration acceleration of the reflector from the measured gravity acceleration signal.In the working process of the cold atom gravimeter,the vibration of the Raman reflector caused by ground vibration will affect the phase size of the laser-atom interaction,so the ground vibration noise greatly limits the further improvement of the accuracy of the atomic gravimeter.A key technology and difficulty of atom gravimeter is the effective isolation of ground vibration and noise.In order to reduce the influence of ground vibration on the measurement of atom gravimeter,this paper designs an active vibration isolation mechanism,and designs related circuits.The mathematical model of the system is verified by experiments.The lead-lag algorithm,sliding mode control algorithm and linear ADRC algorithm are designed to realize the performance of the ultra-low frequency active vibration isolation experimental system.Vibration isolation effect is verified by measuring vibration noise power spectrum.The main research contents and innovations of this paper are as follows:(1)The active vibration isolation system for atom gravimeter is designed and implemented.The principle and design method of active vibration isolation are described in detail,including the mathematical model,mechanical structure design and circuit design of active vibration isolation system.The system includes passive vibration isolation platform,stress vibration isolation platform and seismometer.The experimental platform is simulated and tested.The basic working principle of each part of the system is described in the mechanical structure design.The circuit design includes low noise amplifier circuit and voltage-to-current circuit design.(2)The lead lag control algorithm is applied to the atom gravimeter ultra-low frequency vibration isolation system to reduce the influence of ground vibration on the Raman light mirror.The difficulty in designing the lead lag control algorithm is that the designed controller can make the vibration isolation system maximize the gain or bandwidth of the system under a certain phase margin.In this algorithm,the parameters of the feedback filter are first designed,and then input into the feedback execution program of the FPGA.After the closed loop of the active vibration isolation control system,the power spectral density of the residual vibration noise is analyzed.so that the effect of active feedback can be viewed directly.Compared with the pure passive vibration isolation platform,the vibration noise in the 0.8Hz frequency band is suppressed by more than 100 times,the vibration noise from 1 to 2 Hz is suppressed by more than 3 times,and the vibration noise from 0.3 to 0.6 Hz is suppressed by about 6 times.Compared with the passive vibration isolation effect,the lead lag compensation active vibration isolation performance is superior.(3)An ultra-low frequency active vibration isolation system for atom gravimeter based on sliding mode robust control is designed and implemented.The control structure consists of an exponential reaching law that guarantees the system to reach the sliding surface in finite time and an equivalent control term that guarantees the system state on the sliding surface.In order to reduce the chattering of the system,the saturation function is used instead of the sign function.The simulation and experimental results show that the algorithm has good control effect on vibration.Compared with the passive vibration isolation platform,the system can achieve 1000 times vibration suppression level at the resonance frequency point of 0.8 Hz,50 times vibration suppression level in the range of 0.4-0.6 Hz and 10 times vibration suppression level in the range of 2-3 Hz.and by 6 times as much as that of the lead lag compensation control method implemented on our vibration isolation platform.The sliding mode robust control algorithm also has the characteristics of less tuning parameters and strong anti-interference ability.(4)The ultra-low frequency active vibration isolation system of atom gravimeter based on linear active disturbance rejection control is designed and implemented.Based on the research of linear ADRC technology,the ground random disturbance is estimated by extended state observer,and the complexity of the controller is effectively reduced by linearization method in the controller design,which is convenient for engineering implementation.When the feedback is closed,the equivalent resonance period in the vertical direction is 66 seconds.Compared with the lead-lag compensation control implemented on our platform,the power spectrum of vibration and noise in 0.1-0.3 Hz band decreases by 7 times.Compared with the sliding mode robust control implemented on our platform,,the power spectrum of vibration and noise in 4-6 Hz band decreases by up to 2 times.Finally,the active vibration isolation technology of atom gravimeter is summarized,the imperfections of the current vibration isolation system are pointed out,and some future improvements are prospected. |
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