Investigation Into Some Key Issues For The Development Of Open Architecture CNC System Of Parallel Kinematic Machines

This dissertation deals with some key issues in the development of open architecture CNC system of parallel kinematic machines (PKMs), including interpolation, velocity transition, kinematic calibration and the design methodology of open architecture CNC. The following contributions have been made.Considering the kinematic characteristics of PKMs, a two-step interpolation strategy is proposed. The theoretical trajectory error due to the two-step interpolation strategy and the nonlinear mapping is estimated in the entire workspace. The simulation results show that the upper error bound of the PVT algorithm is far less than the resolution of CNC system for the moderate feed rate.On the basis of the two-step interpolation strategy and linear acceleration rule, a real-time interpolation algorithm is presented. Moreover, the velocity transition technique for the different segments is adopted to enable the machine to achieve sufficient tracking accuracy.The"PC+PMAC"architecture is employed in the development of the hardware of the open CNC system. Meanwhile, the CNC software is designed on the basis of a coordinating mechanism to enhance the communication speed between the master-slave CPUs using the techniques such as interrupt, dual-loop buffer, and real-time dataflow for multiple task handling, etc. A number of embedded modules are also developed such as human-machine interface (HMI), NC code interpreter compatible to ISO standard, Handwheel, Home, Jog, Auto, etc.Taking the 3-HSS parallel kinematic machine as an example, the error mapping function is formulated in such a way that the geometric errors affecting the compensatable and uncompensatable pose errors can be separated. Based upon the previous investigation, a hierarchical approach is proposed to implement the geometric error identification using leaser interferometer. This allows the error compensation to be carried out by modifying the trajectory commands resided in the CNC system. The simulation and experiment results verify the validity and effectiveness of the proposed approach.A real cutting experiment has been carried out on the 3-HSS parallel machine tool. The metrological results show that the accuracy in terms of both dimensions and geometry is fairly good in comparison with the conventional milling machines.