Research on Suspension Gravity Compensation System of Lunar Rover with Magnetic Levitation Servo

In order to overcome the shortcomings of the traditional sling suspension method,such as complex structure of suspension truss,large running resistance,and low precision of position servo system,a gravity compensation method of lunar rover based on the combination of active suspension and active position following of magnetic levitation is proposed,and the overall design is carried out.The dynamic model of the suspension module of microgravity compensation system was established,and the decoupling control between the constant force component and the position servo component was analyzed and verified.The constant tension control was achieved by using hybrid force/position control.The position following control was realized by using fuzzy adaptive PID(proportional⁃integral⁃differential)control.The stable suspension control was realized based on the principle of force balance.The simulation results show that the compensation accuracy of constant tension could reach more than 95%,the position deviation was less than 5 mm,the position deviation angle was less than 0.025°,and the air gap recovered stability within 0.1 s.The gravity compensation system has excellent dynamic performance and can meet the requirements of microgravity simulation experiment of lunar rover.

Proportional Navigation Guidance Law with Variable Coefficient Gravity Compensation

Based upon a discussion on the merits and limitations of proportional navigation（PN）guidance law in which constant gravity compensation is included as a part,a counterpart having varying compensations,which changes with pitching angle and line-of-sight angle,is substituted.Flight trajectory simulation over a submissile model is conducted,resulting in increased impact angle,shorter miss distance,smaller maximum normal overload and narrower terminal angle of attack.

Magnetic Gravity Compensation for Ferrofluid With Maxwell-Helmholtz Coils

Due tothe important and common problems in optimizing material processing under reduced or micro gravity, bubble dynamic behaviors under thus gravity condition attract more and more attentions.For that purpose,a pair of Maxwell-Helmholtz(M-H)coils is developed to obtain a constant magnetic force acting on ferrofluids to neutralize gravity in a certain volume,which provides a uniform gradient magnetic field as well as a magnetic fieldintensity that is big enough to reach the magnetization saturation of ferrofluids.Afterwards,a finite-element-method based numerical simulation of the M-H coils shows a 20mm×30mm zone for 90%gravity compensation exists.An initial experimentshows that gas bubble is well controlled in a closed Hele-Shaw filled with ferrofluid with the help of magnetic gravity compensation.

Gravity compensation method via magnetic quasi-zero stiffness combined with a quasi-zero deformation control strategy

Gravity compensation refers to the creation of a constant supporting force to fully or partly counteract the gravitational force for ground verification to simulate the spacecraft dynamics in outer space with zero-or micro-gravity. Gravity compensation is usually implemented via a very low stiffness suspension/supporting unit, and a servo system in series is adopted to extend the simulation range to hundreds of millimeters. The error of suspension force can be up to tens of Newton due to the contact/friction in the suspension/supporting unit and the error of the force/pressure sensor. It has become a bottleneck for the ground verification of spacecraft guidance, navigation, and control systems with extreme requirements, such as tons of payload and fine thrust in sub-Newtons. In this article, a novel gravity compensation method characterized by quasi-zero stiffness plus quasi-zero deformation(QZS-QZD) is proposed. A magnetic negative stiffness spring in parallel with positive springs and aerostatic bearing is adopted to form a QZS supporting unit, and disturbance forces, such as contact or friction, can be eliminated. The deformation of the QZS supporting unit is measured via a displacement sensor, and the QZD control strategy is applied to guarantee the force error of gravity compensation to be less than sub-newtons and irrelevant to the payload. The principle of gravity compensation with QZS-QZD is analyzed, and performance tests on a prototype are carried out. The results show that when the spacecraft moves smoothly, the absolute force error is less than 0.5 N, the relative error of gravity compensation is less than 0.1%, and when collisions with other objects occur, the relative errors are 0.32% and 0.65%. The proposed method can significantly improve the gravity compensation accuracy in comparison with conventional approaches.

The gravity and isostatic Moho in North China Craton and their implications to seismicity

The Bouguer gravity is the combination of field sources in different depths. Based on the multi-scale analysis of the Bouguer gravity,we can get the gravity anomaly caused by the Moho undulation. This study presents the various orders of approximation of gravity anomaly in North China Craton（NCC）,the possible source depths with radial logarithmic power spectrum,and the relationship between the deep structure and gravity anomaly. Furthermore,we discuss the isostatic compensation about the Moho depth from gravity and deep seismic sounding profiles（DSS）. The results show that：（1） the fourth approximation could have resulted from the Moho undulation,（2） in contrast to the isostatic Moho,the inverted gravity Moho and the DSS Moho show that most of NCC has been isostatically compensated,and（3） the isostatic compensation rate has some close relation to the seismicity.

Effect of gravity gradient in weak equivalence principle test

A high accuracy test of the weak equivalence principle（WEP） is of great scientific significance no matter whether its result is positive. We analyze the gravity gradient effect which is a main systematic error source in the test of WEP.The result shows that the uncompensated gravity gradient effect from the coupling term of the dominated gravity gradient multipole moment component q_（21） and the relative multipole field component Q_（21） contributes to an uncertainty of 1×10^（-11） on the E otv os parameter. We make a Q_（21） compensation to reduce the effect by about 20 times, and the limit of the test precision due to this coupling is improved to a level of a part in 10^（13）.

Research and Realization of a Master-Slave Robotic System for Retinal Vascular Bypass Surgery

Retinal surgery continues to be one of the most technical demanding surgeries for its high manipulation accuracy requirement, small and constrained workspace, and delicate retinal tissue. Robotic systems have the potential to enhance and expand the capabilities of surgeons during retinal surgery. Thus, focusing on retinal vessel bypass surgery, a master-slave robot system is developed in this paper. This robotic system is designed based on characteristics of retinal vascular bypass surgery and analysis of the surgical workspace in eyeball. A novel end-effector of two degrees of freedom is designed and a novel remote center of motion mechanism is adopted in the robot structure.The kinematics and the mapping relationship are then established, the gravity compensation control strategy and the hand tremor elimination algorithm are applied to achieve the high motion accuracy. The experiments on an artificial eyeball and an in vitro porcine eye are conducted, verifying the feasibility of this system.

Ground demonstration system based on in-orbit assembly oriented manipulator flexible force control

To eliminate the load weight limit of carrier rockets and reduce the burden on support structures,in-orbit assembly is a key technology to make design of scattering a large diameter telescope into submirror modules,which requires smooth operation of assembly robots,and flexible force control technology is necessary. A ground demonstration system is presented for in-orbit assembly focusing on flexible force control. A six-dimensional force/torque sensor and its data acquisition system are used to compensate for gravity. For translation and rotation,an algorithm for flexible control is proposed. A ground transportation demonstration verifies accuracy and smoothness of flexible force control,and the transportation and assembly task is completed automatically. The proposed system is suitable for the development of in-orbit assembly robots.

Dynamic Analysis of Three-link Robot

As modem society is developing rapidly, the robots in our lives as well as industrial manufacturing and other aspects have occupied an important position. Requirements for the robot control and accuracy are getting higher and higher, and the dynamic control of the robot has thus been getting the better development. The dynamic problem of the robot is helpful to keep the dynamic characteristics and static characteristics of the robot, which is of great significance to the control problem of the robot. In the process of dynamic analysis of the three-link robot, the system model of the three-link robot is simplified accordingly, and then the classical Lagrangian functional equilibrium method is used to derive the robot dynamics equation. After the kinetic equation, based on the passive characteristics of the three-link robot arm, we use the PD control with gravity compensation for the robot, and use the Lyapunov function to prove that the system has any trajectories in the vicinity of the equilibrium state under any initial condition which is near the equilibrium state, proves the stability of the three-link robot system.

A rapid compensation method for launch data of long-range rockets under influence of the Earth＇s disturbing gravity field

Regarding the rapid compensation of the influence of the Earth＇ s disturbing gravity field upon trajectory calculation,the key point lies in how to derive the analytical solutions to the partial derivatives of the state of burnout point with respect to the launch data.In view of this,this paper mainly expounds on two issues：one is based on the approximate analytical solution to the motion equation for the vacuum flight section of a long-range rocket,deriving the analytical solutions to the partial derivatives of the state of burnout point with respect to the changing rate of the finalstage pitch program;the other is based on the initial positioning and orientation error propagation mechanism,proposing the analytical calculation formula for the partial derivatives of the state of burnout point with respect to the launch azimuth.The calculation results of correction data are simulated and verified under different circumstances.The simulation results are as follows：（1） the accuracy of approximation between the analytical solutions and the results attained via the difference method is higher than 90%,and the ratio of calculation time between them is lower than 0.2%,thus demonstrating the accuracy of calculation of data corrections and advantages in calculation speed;（2） after the analytical solutions are compensated,the longitudinal landing deviation of the rocket is less than 20 m and the lateral landing deviation of the rocket is less than 10 m,demonstrating that the corrected data can meet the requirements for the hit accuracy of a long-range rocket.

Algorithm and experiments of six-dimensional force/torque dynamic measurements based on a Stewart platform

Stewart platform(SP) is a promising choice for large component alignment, and interactive force measurements are a novel and significant approach for high-precision assemblies. The designed position and orientation(P&O) adjusting platform, based on an SP for force/torquedriven(F/T-driven) alignment, can dynamically measure interactive forces. This paper presents an analytical algorithm of measuring six-dimensional F/T based on the screw theory for accurate determination of external forces during alignment. Dynamic gravity deviations were taken into consideration and a compensation model was developed. The P&O number was optimized as well.Given the specific appearance of repeated six-dimensional F/T measurements, an approximate cone shape was used for spatial precision analysis. The magnitudes and directions of measured F/Ts can be evaluated by a set of standards, in terms of accuracy and repeatability. Experiments were also performed using a known applied load, and the proposed analytical algorithm was able to accurately predict the F/T. A comparison between precision analysis experiments with or without assembly fixtures was performed. Experimental results show that the measurement accuracy varies under different P&O sets and higher loads lead to poorer accuracy of dynamic gravity compensation. In addition, the preferable operation range has been discussed for high-precision assemblies with smaller deviations.