the difference between the full closed loop and semi-closed loop of CNC machine tools

In the field of precision manufacturing, the positioning accuracy of CNC machine tools directly determines the upper limit of product quality. The servo system's feedback method-full closed loop or semi-closed loop-is crucial for influencing this core metric. The former acts as a "global eye" for the machine tool, capturing the actual position of the actuator in real time; the latter relies on indirect calculations from the drive components, striking a balance between response speed and cost. The choice between the two is not only a trade-off between technical approaches, but also a key decision point for adapting to different processing needs.

Full-closed-loop control, on the other hand, uses a sensing element (such as a linear scale or magnetic scale) mounted directly on the worktable or moving component to directly detect the actual position of the final actuator. The control logic is as follows: After the system command is transmitted to the worktable via the drive component, the sensing element collects the actual displacement of the worktable in real time, performs a closed-loop comparison with the command value, and directly corrects for errors in all aspects of the process, including the motor, screw, and guideway. Because it can compensate for the inherent errors of the mechanical transmission chain, the full closed-loop accuracy is higher and the positioning accuracy can reach 0.0001-0.001mm. It is mostly used in high-precision processing scenarios (such as molds and precision parts processing), but it has higher requirements for the accuracy of detection components and system debugging.

For Semi-closed loop

In semi-closed-loop control, a position sensing element (such as an encoder) is mounted on the end of a servo motor or ball screw, detecting only the motion state of the drive component. The control logic is as follows: After the CNC system issues a command, the sensing element provides real-time feedback on the motor/screw rotation angle. The system then calculates the theoretical displacement of the worktable, compares it with the command value, and corrects any deviations. However, this method cannot detect mechanical errors such as screw transmission error and guideway clearance. Its accuracy depends on the level of mechanical manufacturing. It is typically used in mid-range machine tools, with positioning accuracies typically ranging from 0.001 to 0.01mm.

The rigidity and damping characteristics of the mechanical transmission chain will directly affect the stability of the system. It is easy to cause closed-loop oscillation due to mechanical vibration, thermal deformation, etc., which is difficult to debug and requires higher mechanical design and installation accuracy.

Semi-closed loop

The feedback does not include the mechanical transmission chain. The control system only needs to adjust the motor and the screw. It has higher stability and lower requirements for mechanical components. It is suitable for high-speed and high-dynamic response scenarios.

High cost: It needs to be equipped with high-precision linear detection elements (such as grating rulers) and complex mechanical structure (transmission clearance needs to be eliminated).

Application: High-precision machine tools (such as coordinate boring machines, precision grinders), and high-precision shaft systems of machining centers.

Semi-closed loop

Low cost: It uses rotary encoders and has a simple mechanical structure (no linear detection elements are required). Application: Popular CNC machine tools, lathes, milling machines, small and medium-sized machining centers.

It is necessary to repeatedly debug the rigidity of the mechanical transmission chain and the control system parameters (such as gain, filtering), and the maintenance cost is high (the detection element needs to be calibrated regularly).

Semi-closed loop

The debugging is mainly for motors and servo systems, and the maintenance is simple (no need to calibrate mechanical parts frequently).