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What is the difference between open loop, closed loop and semi-closed loop of CNC machine tools?

Jun 25, 2019

1.Open loop control CNC machine tool

The CNC machine tool of this type has no position detecting component in its control system, and the servo drive component is usually a reactive stepping motor or a hybrid servo stepping motor. Each time the CNC system issues a feed command, after the power of the drive circuit is amplified, the stepping motor is driven to rotate by an angle, and then the screw is rotated by the gear reduction device, and converted into a linear displacement of the moving component by the screw nut mechanism. The moving speed and displacement of the moving part are determined by the frequency of the input pulse and the number of pulses. The information flow of such CNC machine tools is one-way, that is, after the feed pulse is sent out, the actual movement value is no longer fed back, so it is called open-loop control CNC machine tool.

The CNC machine tool of the open-loop control system has a simple structure and a low cost. However, the system does not monitor the actual displacement of the moving parts, nor can it perform error correction. Therefore, the stepping motor's out-of-step, step angle error, gear and screw drive errors will affect the accuracy of the parts being machined. The open-loop control system is only suitable for small and medium-sized CNC machine tools with low machining accuracy requirements, especially simple and economical CNC machine tools.


2.Closed loop control CNC machine tool

The closed-loop control CNC machine tool is to install a linear displacement detecting device on the moving parts of the machine tool, directly detect the actual displacement of the table, feed back the measured actual displacement value to the numerical control device, compare it with the input command displacement value, and use the difference. The value controls the machine tool to move the moving parts according to the actual required displacement, and finally realizes the precise movement and positioning of the moving parts. Theoretically speaking, the motion accuracy of the closed-loop system mainly depends on the detection accuracy of the detection device, and has nothing to do with the error of the transmission chain, so its control precision is high. Figure 1.9 shows the system block diagram of a closed-loop control CNC machine. In the figure, A is the speed sensor and C is the linear displacement sensor. When the displacement command value is sent to the position comparison circuit, if the table does not move, there is no feedback amount. The command value causes the servo motor to rotate. The speed feedback signal is sent to the speed control circuit through A, and the actual displacement of the table is fed back through C. Going back, in the position comparison circuit, compared with the displacement command value, the position difference is obtained by using the difference obtained after the comparison until the difference is zero. This kind of controlled CNC machine tool is called closed-loop control CNC machine tool because it incorporates the workbench into the control link.

Closed-loop control CNC machine tools have high positioning accuracy, but debugging and maintenance are difficult, the system is complex, and the cost is high.

3. Semi-closed loop control CNC machine tool

The semi-closed-loop control CNC machine tool means that an angular displacement current detecting device (such as a photoelectric encoder) is mounted on the shaft of the servo motor or the driving screw of the numerical control machine tool, and the actual displacement of the moving component is indirectly detected by detecting the rotation angle of the screw, and then Feedback is made to the numerical control device and the error is corrected.

Whether it is open loop, closed loop, semi-closed loop, they have their own characteristics, want to know more? Please click on SHADONG ALLES ENERGY TECHNOLOGY CO.,LTD. ALLES CNC's 24-hour quality online service will answer your questions.

What is the difference between open loop, closed loop and semi-closed loop of CNC machine tools

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