Answer: C, the installation position of the feedback unit.
The main difference between the full closed-loop feed servo system and the semi-closed-loop feed servo system of CNC machine tools lies in the installation position of the feedback unit.
Answer analysis:
1. Full closed-loop feed servo system:
- The feedback unit is directly installed on the workbench to directly detect the actual position of the workbench.
- In theory, the full closed-loop system can eliminate the error, clearance and lost momentum of the entire drive and transmission link, so it has a very high position control accuracy.
- However, since the friction characteristics, rigidity and clearance of many mechanical transmission links in the position loop are nonlinear, it is easy to cause system instability, making the design, installation and debugging of the closed-loop system quite difficult.
- It is mainly used for boring and milling machines, super-precision lathes, super-precision grinders and larger CNC machine tools with high precision requirements.
2. Semi-closed-loop feed servo system:
- The feedback unit is installed on the drive motor (commonly used AC/DC servo motor) or the screw end, and the displacement of the worktable is indirectly detected by detecting the rotation angle of the motor and the screw, rather than directly detecting the actual position of the worktable.
- Since the semi-closed loop does not include or only includes a small number of mechanical transmission links, a relatively stable control performance can be obtained.
- Its accuracy is worse than that of the open-loop system, but not as good as that of the full-closed loop system, because the motion errors caused by the links outside the position loop, such as the pitch error of the screw and the gear clearance, are difficult to eliminate.
- However, such errors can be corrected by compensation technology, so the semi-closed loop system can still obtain satisfactory accuracy.
- The semi-closed loop CNC system has a simple structure and is easy to debug, so it has been widely used in modern CNC machine tools.
In summary, the main difference between the two is that the installation position of the feedback unit is different, which directly affects the detection method and accuracy of the system. Therefore, the correct answer is C. The installation position of the feedback unit.