Research On Actuating Input Selection And Kinematic Calibration Of A Novel 5-DOF Parallel Machine Tool

The actuating input selection is an important basic problem for the parallel mechanism. The reasonable of actuating input selection will make strong impact on the kinematic performance of the parallel mechanism. The improper selection will probably produce serious effects. It is the machining precision that limits the commercialization and industrialization of the Parallel Machine Tools (PMTs) greatly. The kinematic calibration is an economic and efficacious strategy to improve machining precision. The research on the actuating input selection and the kinematic calibration of the 5-UPS/PRPU PMT is presented and the main contents of the paper are as follows:Based on the relativity of the constraint screws, the actuating input selection of 5-UPS/PRPU PMT is studied. There are 6 actuating input combinations in the 5-DOF PMT because it has 6 P-pairs can be acted as actuating-pair conveniently. The constraint screw can be got after locking a kinematic pair in any limb, which can be taken as actuating wrench acted on the moving platform of the parallel mechanism. The constraint screw matrix is composed of the structure constraint screws and the constraint screws of the actuating pairs. The reasonableness of input selection can be judged by the rank of the constraint matrix. The result of selection shows that there are 5 reasonable input combinations for the 5-UPS/PRPU PMT.The constraint force/torque dexterousness is defined as the ratio of minimum singularity value to maximum one of the force/torque constraint matrix, by which the performance of the combinations of actuating inputs is evaluated. Then, three kind of typical movement of the moving platform are designed and the performances of these combinations under any of these three circumstances are analyzed by mathematic software. Finally, the demands of actuating force are researched under any reasonable actuating inputs combinations with ADMS. The result shows that the best combination is the one using prismatic pairs in the five UPS limbs as the actuating inputs.The error source and the strategies to increase the precision of PMTs are analyzed. Two kinds of calibration algorithms are presented combining the characteristic of the5-UPS/PRPU PMT that the PRPU limb can detect the pose of the moving platform. The first kind of arithmetic—two-step calibration method: The kinematic calibration of PRPU limb is carried out; then the pose of the moving platform can be measured by this limb; at last, the kinematic calibration model is established by using the reverse kinematic solution of the UPS limbs. The second one—global calibration method: Eight different kinds of pose of the moving platform are measured directly, then using the forward solution of the PRPU limb and the reverse solution of the UPS limbs, the calibration model is set up including all parameters of the PRPU limb and the 5 UPS limbs. Genetic Algorithm (GA) is applied to identify the parameters of the PMT calibration model, which can avoid the complicated partial differential calculation in common methods. Thus, the calibration algorithm is greatly simplified.The untouched homing switch is designed, which is very suitable for the 5-UPS/PRPU PMT. Two- step homing method is put forward to realize the PMT homing function. First, make all the driving limbs move to the standard potion and refresh the values of the pole length as the standard value, respectively. Then drive the limbs to the initial pole length, thereby the PMT come to the home position. It is reliable and convenient in application. The calibrating control system of the 5-UPS/PRPU PMT is developed, with which the kinematic calibration experiment has been executed and the positional accuracy before and after the calibration of the PMT is detected, respectively. The result indicates that the positional accuracy of the PMT after calibration has been greatly improved, and the validation of the calibration algorithm presented in the paper has been proved.