Research On The Multi-DOF Loading System Based On Parallel Mechanism

With the rapid development of machinery, construction and other industries, thestress distribution of some large-scale structural components is usually very complex. Inorder to guarantee the reliability of the structure, there need to the corresponding systemto conduct the six dimensional force loading test with the control of the load force.Since parallel platform having structural rigidity, strong bearing capacity, etc., and canachieve six dimensional force/torque loading. The multi-DOF loading system studied inthis thesis used Stewart platform structure, by studying the relationship between theloading force/torque and the driving force of parallel mechanism, the influence ofparallel platform structure parameters on the driving force was analyzed, and theloading force/torque control method was also been studied.Firstly, the mechanics models of the multi-DOF loading test system when the loadforce was static and dynamic respectively were derived, analyzed the influence of theplatform position and attitude change related to the jacobin matrix of the mechanicalmodel on the actuator driving force, as well as the influence of inertia force, coriolisforce, gravity when the load force was change. the relationship between the loadingforce/torque and the driving force of parallel mechanism was established when the loadforce was static and dynamic.Secondly, according to the static loading mechanical model, and with the loadperformance parameters of the large spin dig drill motor arm as an example, analyzedthe influence of upper hinge point radius, lower hinge point radius and platform heighton the driving force of the actuators in different load modes. The method to make thedriving force of the actuators minimum was proposed to be the optimize target, throughthe local optimization of structural parameters, the basic structure of the system and themaximum driving force of the actuators were obtained.Due to the practical problem of measuring the six dimensional force/torque ofparallel loading test system, one load force control mode based on actuator driving forceloop was proposed, according to the maximun actuator driving force, single-cylinderdriving force control system was designed, as well as its model, then the frequencydomain and time domain analysis of single-cylinder driving force control system werecompleted.Thirdly, the mechanical structure model of the force loading system and loadmodel were established by Simmechanics, with the force inverse model obtained fromthe mechanical model and single-cylinder driving force control system modelestablished by Simulink, the complete simulation model of the force loading test system was established. With the system under different load stiffness, the dynamiccharacteristics of each DOF load force/torque were analyzed. To make the vertical-loadforce of the system adapt to a certain range of load stiffness, corresponding forcecontroller was designed on the actuator, which was realized by changing theproportional gain according to the identified load stiffness. In addition, the influence ofthe force controller on the dynamic characteristics of the other DOF load force/torquewas analyzed.As the simulation results showed, the accuracy of the mechanical model betweenthe loading force/torque and the driving forces derived was accurate, when the load wasstatic and dynamic. multi-DOF driving force loading force/torque control based on theclosed-loop of the driving force is feasible and correct. The force controller of drivingsystem could well adapt to the change of the stiffness of the load.