| Implementing scientific and sufficient simulation experiment on the ground is necessary before space equipment performing in-orbit tasks in outer space.Therefore,the research on the microgravity simulation system has become an important part of the space technology.In this paper,a microgravity simulation experiment system based on hardware in the loop technology is discussed.The control strategy of this system with the hybrid pneumatic-electric actuator is proposed.In order to implement the strategy,research content consists of system modeling,forward kinematics of Stewart platform in real time,control method of hybrid pneumatic-electric actuator,microgravity simulation and experiment.First,in order to build mathematical model which is necessary for hardware in the loop simulation,the kinematic and dynamic analysis of the Stewart platform used in the microgravity simulation platform is carried out.The analysis of kinematics focuses on the research of real-time forward solution.Three methods of the real-time forward solutions of Stewart platform are analyzed.The process of theoretical derivation is analyzed and implement the numerical simulation based on MATLAB.The precision and time complexity of the three kinds of methods were compared.A C++ program of this algorithm using Eigen open source library is developed and tested.The model fitting method with the shortest running time of 260.3 ?s is chosen to be the real-time forward kinematics of the system in this paper,in order to improve the performance of microgravity simulation The Newton-Euler method is used to build a dynamic model for Stewart platform.The correctness of the model is verified by the SimMechanics toolbox,which provides a theoretical basis for implementing hardware in the loop simulation.A kind of hybrid pneumatic-electric actuator is used for the platform in this paper.In order to control the hybrid actuator well,the controllers of the pneumatic and electric system are designed respectively.A Shearer equation is used to model the pneumatic system of the hybrid actuator,which is based on high-speed on-off valve.A model of the electric system consist of permanent magnet synchronous motor and ball screw is built.Considering the disturbance caused by cylinder volume changing,LADRC is used for pressure signal tracking.Compared with PID algorithm,it reduces the adjustment time by 48.8%,reduces the overshoot by 23%,and improves the convergence speed under the disturbance.After that,a velocity feedforward method is used for solving time delay problem in the electric system.The accuracy is increased by 37.12% with this control method.Finally,based on the hardware in the loop technology,the control strategy of the microgravity simulation system is designed,and the experiment of the single axis microgravity simulation and the simulation of the platform are carried out.The results show that if ignoring the friction effect of the cylinder output,pneumatic output force could compensate the gravity of the load completely,which achieves the goal of the ground microgravity simulation well.Secondly,on the premise of ensuring motion precision,the pneumatic system can bear about 85% static load and about 60% dynamic load for a 10 kg payload,while it can bear about 60% static load and 50% dynamic load for a 20 kg payload.Therefore,under the condition that the driving capacity is constant,a smaller size of the motor can be selected by using hybrid actuator,which makes the machine equipment based on parallel mechanism smaller and lower cost. |
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