Research On Synchronization Control Of Servo System In Large CNC Hobbing Machines

In the process of CNC gear hobbing machining, both the rotation speeds of the hob and rotating table have to be maintained in a linear relationship to guarantee the precision of gear machining. So the degree of speed match between the hob and rotating table is closely related to the gear machining precision. In the traditional synchronization method of CNC machines, the electronic gearbox is usually adopted to maintain this linear relationship. There are four ways to realize the function of electronic gearbox, which are master-slave electronic gearbox, parallel electronic gearbox, hardware-based electronic gearbox and software-based electronic gearbox. Whichever way the electronic gearbox is implemented, they are all only focused on the operation of single axis, without consideration of synchronization information among multiple axes. Thus, the tracing precision of single axis can be maintained, but the precision of synchronization motion can not be guaranteed under situation of the abrupt or constant change of load, which will deteriorate the performance of CNC hobbing machines. Cross-coupled synchronization control strategy has great advantage on the aspect of contour forming precision. It is consisted of a contour error model and a contour error compensation distributor. Through integration and synthesis of synchronization information among multiple axes, the motion of each axis in the machine will be compensated using a kind of compensation distribution law, which will then improve the whole precision of machining.On the basis of conventional set-point synchronization control strategy, the connection between the hob and rotation table is established in this thesis, with the introduction of cross-coupled control consisted of PI type compensation distributor. With the analysis of control principle of permanent magnet synchronous motors (PMSM) and control structure of servo system of machine, a simplified PMSM mathematical model is created, the speed and current controller are described mathematically, thus the formulas of output speed and synchronization error are deduced.With the mathematical models of control system in CNC machines, the simulation model for the whole motion system is created. The working condition of both no load and load abrupt change are simulated and researched. Conclusion can be obtained from simulation results that the proposed PI type cross-coupled control strategy have better effect, which can significantly reduce the synchronization error and improve the precision of hobbing machining. In the end, the achievement of this paper is summarized, and the suggestions and plans for the further work are proposed.