Loading Control Strategies Of 6-dof Hydraulically Actuated Redundant Parallel Mechanism

The earthquake is one of natural disasters which can cause serious damage to architectural structures.Loading mechanism,the equipment simulating seismic load in lab environments,is mainly used in investigation of the response and failure mechanisms in damaged structures under static and dynamic seismic loads.Sixdegree-of-freedom(6-dof)hydraulically actuated redundant parallel mechanism(HA-RPM)can lower the impact of small size scale model test on the accuracy of evaluating the seismic performance of structures,and thus it can provide theoretical support for the design and construction of structures with characteristics of high,huge,special and novel.Upon the background of the project “the facility for advanced structural test(FAST)” developed jointly by China State Construction Engineering Corporation(CSCEC)and Harbin Institute of Technology(HIT),the corresponding loading control strageties for 6-dof HA-RPLM are researched.For the HA-RPLM,adopting dof control based on the linearization near the zero position can result in motion coupling,adopting joint space control can result in large internal force,and dof control based on kinematic forward kinematics is rarely used due to poor solving accuracy and real-time performance of existing forward kinematics algorithms.In order to reduce the motion coupling and improve the position loading precision,a real-time forward kinematics algorithm based on closed-loop feedback,which converts forward kinematics sloving to the control for closed-loop feedback system,is proposed,and its convergence is analyzed.Meanwhile,dof control based on closed-loop feedback real-time forward kinematcis is also developed.Simulation results show that proposed forward kinematcis algorithm has high accuracy and real-time performance,and the designed loading controller can effectively improve the bandwidth of position control system.Some factors such as inconsistent dynamic of the actuators,zero drift of servo-valves,geometric parameter errors and measurement errors of sensors,usually result in internal forces of HA-RPLM,which can vary along with the poses of the mechanism.In order to reduce the internal force coupling,solving method of internal force space basis is given based on singular value decomposition and the influence of different factors on internal force of the system is discussed.On the basis of the analysis of internal force coupling characteristics and a ccording to the characteristics of the internal force form changing with pose,an internal force control stragtegy based on real-time solving of the internal force space basis is developed.Simulation results show that the proposed control stragtegy can effectively reduce the internal forces of the system.Affected by loading specimen features it can easily cause large fluctuation of vertical force and the P-? effect of the specimen would reducing position control accuracy in the rotation dof under compression and shear condition.Then force control performance of traditional position-based impedance control and mechanical properties of the two kinds of isolation bearing are analyzed under compression and shear condition are investigated.On the basis,a hybrid position/force loading control strategy is proposed.At first,in order to improve force response perfomeance,an impedance control with force error compensation is proposed and control parameter tuning method is given.Then in order to reducing vertical force fluctuation under compression and shear condition,a control method with feedforward compensation of specimen feature is developed.At last,in order to reducing the impact of the P-? effect on position control accuracy in the rotation dof,a disturbance rejection method of the overturning moment is proposed.Simulation results show that the proposed control strategy can effectively improve position/force loading accuracy.Due to nonlinear factors existed in hydraulic system and coupled interaction from specimen,the bandwidth of position and force dof of HA-RPM can be improved in the limited situation,which results in the amplitude attenuation and phase lag of response sinusoidal position/force signals.To improve the tracking accuracy of sinusoidal position/force signals and overcome the shortcomings of the traditional amplitude and phase control(APC),such as complex control structure,huge computation and so on,an APC based on three parameters feedback is proposed through estimating three characteristic parameters of sinusoidal signal and adding control loops of characteristic parameters outside original close-loop control system.In addition,the methods of controller design and control parameter tuning are given.Simulation results show that the proposed control strategy can effectively improve the tracking accuracy of sinusoidal position/force signals.Detailed experimental analysis and research are carried out on 6-dof HA-RPMs for the results of above-mentioned theoretical analysis and the proposed control strategies.Experimental resulats demonstrate that the proposed loading co ntrol strategies of 6-dof HA-RPM are effective.