Y. Ihara (Japan)
machine tools, numerical controller, linear encoder, rotaryencoder, ball screw, position feedback
There are two ways for position feedback of the slides of machine tools such as machining centers. One is to use the rotary encoder of the AC servo motor, the other is to use an additional linear encoder for position detection. The latter, linear encoder feedback, is called "closed loop" and it has an advantage in high precision machining because the tool position error is smaller. On the other hand, the machine might be less stable from the view of control system. In this paper, the characteristic values of mechanical and control parameters of conventional machine tools are surveyed first. Second, feedback systems with above-mentioned values are simulated so as to find the source of the stability. Third, position deviations of real machine tools in the case of straight line motion, with different feedback methods, are measured. The results show that the machines with the linear encoder feedback system are stable enough when the mechanical structure is well-designed and has enough rigidity. Finally, a hybrid feedback method is introduced. The simulation results show that hybrid feedback method is effective for all conditions.
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