The oscillation of CNC machine tools, and how to eliminate it

Methods for Eliminating Oscillation of CNC Machine Tools

CNC machine tools play an important role in modern industrial production. However, the oscillation problem often plagues operators and manufacturers. The reasons for the oscillation of CNC machine tools are relatively complex. In addition to many factors such as unremovable transmission gaps, elastic deformation, and frictional resistance in the mechanical aspect, the influence of relevant parameters of the servo system is also an important aspect. Now, the CNC machine tool manufacturer will introduce in detail the methods to eliminate the oscillation of CNC machine tools.

I. Reducing the position loop gainThe proportional-integral-derivative controller is a multifunctional controller that plays a crucial role in CNC machine tools. It can not only effectively perform proportional gain on current and voltage signals but also adjust the lagging or leading problem of the output signal. Oscillation faults sometimes occur due to the lagging or leading of the output current and voltage. At this time, the PID can be used to adjust the phase of the output current and voltage.The position loop gain is a key parameter in the control system of CNC machine tools. When the position loop gain is too high, the system is overly sensitive to position errors and is prone to causing oscillation. Reducing the position loop gain can reduce the response speed of the system and thus reduce the possibility of oscillation.When adjusting the position loop gain, it needs to be reasonably set according to the specific machine tool model and processing requirements. Generally speaking, the position loop gain can be reduced to a relatively low level first, and then gradually increased while observing the operation of the machine tool until an optimal value that can meet the processing accuracy requirements and avoid oscillation is found.

II. Parameter adjustment of closed-loop servo systemSemi-closed-loop servo systemSome CNC servo systems use semi-closed-loop devices. When adjusting the semi-closed-loop servo system, it is necessary to ensure that the local semi-closed-loop system does not oscillate. Since the full-closed-loop servo system performs parameter adjustment on the premise that its local semi-closed-loop system is stable, the two are similar in adjustment methods.The semi-closed-loop servo system indirectly feeds back the position information of the machine tool by detecting the rotation angle or speed of the motor. When adjusting parameters, the following aspects need to be paid attention to:(1) Speed loop parameters: The settings of speed loop gain and integral time constant have a great influence on the stability and response speed of the system. Too high speed loop gain will lead to too fast system response and is prone to generating oscillation; while too long integral time constant will slow down the system response and affect processing efficiency.(2) Position loop parameters: The adjustment of position loop gain and filter parameters can improve the position accuracy and stability of the system. Too high position loop gain will cause oscillation, and the filter can filter out high-frequency noise in the feedback signal and improve the stability of the system.Full-closed-loop servo systemThe full-closed-loop servo system realizes accurate position control by directly detecting the actual position of the machine tool. When adjusting the full-closed-loop servo system, parameters need to be selected more carefully to ensure the stability and accuracy of the system.The parameter adjustment of the full-closed-loop servo system mainly includes the following aspects:(1) Position loop gain: Similar to the semi-closed-loop system, too high position loop gain will lead to oscillation. However, since the full-closed-loop system detects position errors more accurately, the position loop gain can be set relatively high to improve the position accuracy of the system.(2) Speed loop parameters: The settings of speed loop gain and integral time constant need to be adjusted according to the dynamic characteristics and processing requirements of the machine tool. Generally speaking, the speed loop gain can be set slightly higher than that of the semi-closed-loop system to improve the response speed of the system.(3) Filter parameters: The full-closed-loop system is more sensitive to noise in the feedback signal, so appropriate filter parameters need to be set to filter out noise. The type and parameter selection of the filter should be adjusted according to the specific application scenario.

III. Adopting high-frequency suppression functionThe above discussion is about the parameter optimization method for low-frequency oscillation. Sometimes, the CNC system of CNC machine tools will generate feedback signals containing high-frequency harmonics due to certain oscillation reasons in the mechanical part, which makes the output torque not constant and thus generates vibration. For this high-frequency oscillation situation, a first-order low-pass filtering link can be added to the speed loop, which is the torque filter.The torque filter can effectively filter out high-frequency harmonics in the feedback signal, making the output torque more stable and thus reducing vibration. When selecting the parameters of the torque filter, the following factors need to be considered:(1) Cutoff frequency: The cutoff frequency determines the attenuation degree of the filter to high-frequency signals. Too low cutoff frequency will affect the response speed of the system, while too high cutoff frequency will not be able to effectively filter out high-frequency harmonics.(2) Filter type: Common filter types include Butterworth filter, Chebyshev filter, etc. Different types of filters have different frequency response characteristics and need to be selected according to the specific application scenario.(3) Filter order: The higher the filter order, the better the attenuation effect on high-frequency signals, but at the same time, it will also increase the computational burden of the system. When selecting the filter order, the performance and computational resources of the system need to be considered comprehensively.

In addition, in order to further eliminate the oscillation of CNC machine tools, the following measures can also be taken:Optimize mechanical structureCheck the mechanical parts of the machine tool, such as guide rails, lead screws, bearings, etc., to ensure that their installation accuracy and fit clearance meet the requirements. For severely worn parts, replace or repair them in time. At the same time, reasonably adjust the counterweight and balance of the machine tool to reduce the generation of mechanical vibration.Improve the anti-interference ability of the control systemThe control system of CNC machine tools is easily affected by external interference, such as electromagnetic interference, power fluctuations, etc. In order to improve the anti-interference ability of the control system, the following measures can be taken:(1) Adopt shielded cables and grounding measures to reduce the influence of electromagnetic interference.(2) Install power filters to stabilize the power supply voltage.(3) Optimize the software algorithm of the control system to improve the anti-interference performance of the system.Regular maintenance and upkeepRegularly perform maintenance and upkeep on CNC machine tools, clean various parts of the machine tool, check the working conditions of the lubrication system and cooling system, and replace worn parts and lubricating oil in time. This can ensure the stable performance of the machine tool and reduce the occurrence of oscillation.

In conclusion, eliminating the oscillation of CNC machine tools requires comprehensive consideration of mechanical and electrical factors. By reasonably adjusting the parameters of the servo system, adopting high-frequency suppression function, optimizing the mechanical structure, improving the anti-interference ability of the control system, and performing regular maintenance and upkeep, the occurrence of oscillation can be effectively reduced and the machining accuracy and stability of the machine tool can be improved.

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Company Name: Qingdao Taizheng Precision Machinery Co., Ltd.
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Phone: 13605328581
Address:C-101, Building 31, Zhongnan High tech Jimo Technology Innovation Industrial Park, No. 76 Dazhong 1st Road, Jimo District
City: Qingdao
Country: China
Website: https://www.ncmillingmachine.com/