Research On Structure Optimization And Natural Frequency Of A 3-DOF Parallel Manipulator With Folders

The main method of evaluating the performance of the military hardware,the state ready for war,reliability and completing the acceptance of weapons is to test the equipment in a mechanical environment.Experiment platform of mechanical environment simulation is an equipment providing necessary stress environment for the military hardware,including noise,vibration simulation,rolling simulation,space microgravity simulation,impact simulation etc.Parallel manipulator has a great stiffness,stable structure,better accuracy,heavier loading capacity and no accumulation error,therefore it is widely used in the field of motion simulation.A 3-dof parallel manipulator with folder is proposed.Compared with Stewart parallel mechanism,the mechanism has the advantages of large bearing capacity,high rigidity,large reachable space,and wide range of motion.In view of its advantages,it is necessary to study its basic theory of structure,kinematics,dynamics,frequency response etc,so as to realize industrial application more quickly.The three DOF of adjoint motion and folding structure make it difficult in mechanism analysis.The constraint equations are divided into two forms: length constraint of folders and mechanism structure constraint.Focusing on these issues,firstly,the paper proposes a method of projected branched coordinate system.This novel method brings great convenience to kinematics modeling.With this approach,the velocity and acceleration of all components have the same form which can be written as combination jacobian matrix with generalized velocity and the derivative of jacobian matrix with generalized acceleration.Secondly,a novel method of solving the derivative of jacobian matrix is proposed,which greatly reduces the computation and is easy to program.Finally,Multi rigid body dynamics model is established based on Kane method.Here,Multi rigid body dynamics model of all mechanisms can be boiled down to three stylized processes: searching for restraint equation,solving jacobian matrix with its derivative and establishing dynamics equation.Establishing parallel mechanism model is not the ultimate goal of mechanism analysis.We expect to complete the mechanism characteristic analysis,structural optimization and control system design through dynamic model.Thus,to optimize the 3 DOF parallel mechanism,firstly,the performance evaluation indexes of the parallel mechanism,such as dexterity,force transmission chara cteristics,reachable space etc,are proposed.As for the performance index of the reachable space of the mechanism,the effect of structure parameters on the reachable space boundary of the mechanism in the neutral position is analyzed,and the suitable s tructure parameter range of the large range motion of the mechanism is given.Then,the relation between the condition number of jacobian matrix and the static force of actuator in the ultimate limit pose is studied,and the singular judgment condition of the mechanism is given.Finally,based on the aforementioned structural parameters,the optimization process of the structural parameters of the actuator is carried out with the maximum stroke,the maximum output and the compact structure of the actuator a s the target.And the optimized structural parameters are analyzed.As for the limited-DOF parallel manipulators,the natural frequency will have different expressions and matrix dimension when different generalized velocities are selected.In order to know the problem clearly,firstly,the hydraulic power mechanism of the parallel mechanism is designed on the basis of the aforementioned structural parameters.Then,the model of natural frequency aiming at different generalized velocities is established.Finally,the simulation of natural frequency is accomplished.Simulation results of multi rigid body dynamics model and natural frequency are validated in the SimMechanics and Adams environment.