TAILIEUCHUNG - Theory and Design of CNC Systems Part 2

Tham khảo tài liệu 'theory and design of cnc systems part 2', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 18 1 Introduction to NC Systems The detector can be attached to the shaft of a servo motor or the moving part and the control system can be categorized into four types according to the location at which the detector is attached. Semi-closed Loop The semi-closed loop is the most popular control mechanism and has the structure shown in Fig. . In this type a position detector is attached to the shaft of a servo motor and detects the rotation angle. The position accuracy of the axis has a great influence on the accuracy of the ball screw. For this reason ball screws with high accuracy were developed and are widely used. Due to the precision ball screw the problem with accuracy has practically been overcome. If necessary pitch-error compensation and backlash compensation can be used in NC in order to increase the positional accuracy. The pitch-error compensation method is that at the specific pitch the instructions to the servo driver system are modified in order to remove the accumulation of positional error. The backlash compensation method is that whenever the moving direction is changed additional pulses corresponding to the amount of backlash are sent to the servo driver system. Recently the usage of the Hi-Lead-type ball screw with large pitch for high-speed machining has increased. Closed Loop The performance of the semi-closed loop depends on the accuracy of the ball screw and it is possible to increase the positional accuracy via pitch compensation and backlash compensation. However generally speaking the amount of backlash can be varied according to the weight of the workpiece and location and accumulation pitch error of the ball screw is varied according to the temperature. In addition because the length of the ball screw is limited for practical reasons a rack and pinion driving system is used in large-scale machine tools. However the accuracy of the rack is limited. In this case the closed loop shown in Fig. is applied. In the closed

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