Design And Development Of A Small Active Vibration Isolation Platform For MEMS Gravimetry

External environmental vibrations can affect the normal operation of precision instruments such as gravimeters,and MEMS gravimeters are geared towards mobile testing in the field,placing a demand on lightweight,small active vibration isolation platforms.The main problems with miniaturised active vibration isolation platforms are: the need to improve the cross-axis rejection ratio in the vertical direction after the miniaturisation of the structure,the design of control circuits for low background noise,and the optimisation of the control effect for the parameters of the sensors after the miniatur isation.At present,the active vibration isolation platforms designed and customized for gravimeters at home and abroad are large active vibration isolation platforms with large structures and sensors,and there is a lack of research on small active vibration isolation platforms.This paper addresses the problems of small active vibration isolation platforms by optimising the structural design of small active vibration isolation platforms to improve the cross-axis rejection ratio in the vertical direction,then designing the overall control circuit under the requirement of low noise and conducting noise test analysis,and finally optimising the overall control system effect based on the existing sensor parameters by proposing the use of genetic algorithm to optimise the PID parameter adjustment.Firstly,a mathematical and physical model of the active vibration isolation control system is established,and the amplitude-frequency response of the active vibration isolation system with different physical feedback methods is compared to select a suitable active feedback method for the design.The selection of the velocity sensor and the testing of its key parameters,including bandwidth and background noise,were also completed and the test results met the requirements.The voice coil motor was then selected and calibrated for input and output coefficients at different positions to provide a reference for the subsequent development of the overall control model.In the structural design of the small vibration isolation platform,the modal simulation analysis comparing different solutions verifies that the improved solution can effectively improve the cross-axis rejection ratio in the vertical direction,and then the selection of elastic elements and parameter determination are carried out for the designed structure.Secondly,the active vibration isolation control circuit was designed under low background noise requirements,including a high-pass filter circuit,a PID control circuit and a voltage to current conversion circuit.A noise model was also developed to analyse the noise level,and actual noise tests showed that the background noise of the circuit was below 0.5 ng/√Hz between 0.1-10 Hz bands.Finally,the mechanical structure of the vibration isolation platform and the control circuit are debugged,in which a genetic algorithm-based PID parameter optimisation method is proposed to address the instability of the closed-loop control system due to the phase frequency response of the sensors in the vibration isolation system.The PID parameters are solved with the minimum value of the amplitude at 1 Hz in the transfer function of the closed-loop system as the objective function.After simulation and experimental verification,the method is able to obtain the relatively optimal parameters for the vibration isolation effect on the basis of the stability of the active vibration isolation system.The active vibration isolation system has been commissioned and the experimental results show that in the laboratory environment,the vibration isolation frequency range of the active vibration isolation platform is 0.6-100 Hz and the vibration isolation rate reaches-20 d B at 3 Hz.The residual noise of the active vibration isolation table is compared with the background noise of the sensor,and the reason why the vibration isolation effect is not as effective as the background noise of the sensor in the frequency range of 0.1-1 Hz is analysed It is because the intrinsic frequency of the passive vibration isolation structure is large,and to achieve the vibration isolation requirements,the intrinsic frequency of the passive structure should reach below 0.2 Hz.In this paper,the following is achieved by examining some of the problems that exist with small active vibration isolation platforms.Improved cross-axis rejection ratios in the vertical direction through improvements in the design of the passive vibration isolation structure.With low noise design considerations,the overall control circuit design and noise analysis tests have obtained that the background noise of this control circuit is below 0.5ng/ √ Hz in the 0.1-10 Hz frequency band,which meets the system requirements.By proposing a genetic algorithm-based PID tuning optimization method,the optimal PID parameters can be obtained based on the stability of the sensor phase response and the overall control system.In summary,the innovations of this paper are:(1)The cross-axis rejection ratio in the vertical direction is improved by improving the layout design of the spring and voice coil motor of the passive vibration isolation structure.(2)An active vibration isolation control circuit with low background noise is designed.The background noise is below 0.5 ng/√Hz in the frequency band 0.1-10 Hz,and the whole control circuit can meet the vibration isolation demand for vibration isolation systems with a residual noise value of 0.5 ng/√Hz or more in the frequency band 0.1-10 Hz.(3)A genetic algorithm-based PID parameter adjustment and optimisation method is proposed,which can obtain the relatively optimal PID parameters for vibration isolation effect under the condition of stability of the control system for different sensor’s phase frequency response.