| Aiming at the operational needs of pick-and-place operations in food medicine and modern logistics automatic production line,this dissertation investigates into the kinematics analysis,rigid dynamics modeling,dimensional synthesis,error modeling and accuracy design based on a 2-R(SS)2-R(RR)2(RR)2 parallel mechanism,which has two operation modes of two degrees of freedom and three degrees of freedom.The following contributions have been acquired:Based on the structural characteristics of the 2-R(SS)2-R(RR)2(RR)2 parallel mechanism,the switching method for the two operation modes of the mechanism is proposed,and the degrees of freedom of the two modes are calculated.By means of the vector method,the inverse position model and velocity mapping model are established.Based on the mathematical characteristics of the velocity Jacobian matrix,two types of spatial pressure angles with clear physical meaning are defined,and the influence of dimensional parameters on spatial pressure angles is revealed by using the monotonicity analysis method,which provides the basis of dynamic dimensional synthesis.A complete rigid dynamic model and a simplified model of the two operation modes of the 2-R(SS)2-R(RR)2(RR)2 parallel mechanism are established by using the principle of virtual work,and the simulation demonstrates the validness of the simplified dynamic model.Three kinds of dynamic performance indexes of inertia,velocity and gravity components of the driving torque for each limb in different operation modes are proposed on the base of simplified model.Combined with the 2-R(SS)2-R(RR)2(RR)2 parallel mechanism dimensional constraint and kinematic spatial pressure angles constraint,a dynamic dimensional synthesis model is established with unified two operations modes.and this method can effectively guarantee the kinematics and dynamics performance of the parallel mechanism.The error models of the R(SS)2 and R(RR)2(RR)2 limbs are established respectively by the vector method,then the unified two-operation mode 2-R(SS)2-R(RR)2(RR)2 parallel mechanism moving platform end error model is carried out.Then,the uncompensated errors of the parallel mechanism in different operation mode are extracted,and the mapping relationship between the uncompensated errors and each error source of the whole mechanism is established,and this result will provide a theoretical reference for sensitivity analysis and accuracy design.Using the sensitivity index method,the probability model of the error source is established and the uncompensated errors of the moving platform end in the two operation modes are defined.Based on the sensitivity analysis,the influences of each error source with respect to the uncompensated errors of the moving platform end in the two operation modes and the key error source are obtained.Considering the two operation modes end accuracy of the mechanism,the accuracy optimization model is proposed,whose objective is the maximum sum of the tolerances of each part of the mechanism and weight coefficient is the sensitivity coefficient value.Through the example,the tolerance values are obtained under the given accuracy condition,and this result will provide a theoretical reference for manufacturing and assembly. |
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