Test On Force-displacement Mixed Control Method Of Loading System With Three Degrees Of Freedom

The loading system with three degrees of freedom achieves the loading control of the horizontal displacement,vertical force and rotation of the specimen through two vertical actuators and a horizontal actuator during the test.The loading system with three degrees of freedom is based on the MTS control system,therefore,the key to guarantee that the test goes on smoothly and obtains accurate test results is the precise loading control of three degrees of freedom of the test system.Because of the complexity of control of the electro-hydraulic servo actuator and the simulation of boundary conditions,higher and higher requirements for the control method of the loading system with three degrees of freedom are put forward.Because of the gap and the slip between specimen and actuators,the control effect of the present force-displacement mixed control methods to the rotation angle is not ideal enough,and when the desired displacement and rotation angle of the specimen are larger,it will produce a relatively large error because of the change of the stiffness and geometry of the specimen in the process of the test,then lead to an inaccurate evaluation of structural seismic performance.Therefore,this paper will regard improving the control precision of the test system as the goal,further research the control method based on the current force-displacement mixed control method to solve these problems given above.The main research contents and conclusions of this paper are as follows:(1)In order to improve the control precision of the loading system with three degrees of freedom for the rotation of the specimen,research the force-displacement mixed control method based on the feedback of the external displacement sensors,namely,control based on the measured values of the external LVDT high precision displacement sensors.Theoretical analysis and numerical simulation results show that the control method is feasible,the control precision is improved compared with previous control method.(2)In order to consider the geometric nonlinearity of the loading system with three degrees of freedom,further put forward the force-displacement mixed control method considering the geometric nonlinearity.Analyze the stability and steady-state error of this method,and verify the feasibility and effectiveness of the control method through simulation analysis.The results show that the system is stable and without steady-state error when selecting the appropriate controller parameters;the control method obviously improves the control precision of the system compared with the control method without considering the geometric nonlinearity.(3)Research the implementation scheme of the force-displacement mixed control method based on the built-in calculation editor of MTS and Visual Basic,and complete the corresponding control program and software.The results show that both approaches are feasible;the computing power of the method based on calculation editor is limited,so the method based on VB is more suitable for complex structure.