This article briefly describes the design principles and interface design features of a PC-based debugging software for AC servo systems . It details the various functions of the software in the debugging process of AC servo systems, and solves the key problem of the universality of CNC system software for servo systems.
Servo drives are a crucial component of CNC systems, used to achieve feed servo control and spindle servo control in CNC machine tools. Their function is to convert the command information from the CNC device, after power amplification and shaping, into linear or angular displacement motion of the machine tool's actuators. Since the servo system is the final stage in a CNC machine tool's operation, its performance directly affects the machine tool's accuracy, speed, and other technical specifications. Therefore, servo drives are required to have excellent rapid response performance, accurately and sensitively track the digital command signals issued by the CNC device, and accurately execute the commands from the CNC device, thereby improving the dynamic following characteristics and static tracking accuracy of each motion axis of the machine tool.
The AC servo system PC debugging software (ServoSelfTestTools) developed in this paper is mainly used for online operation debugging and diagnostic operation of machine tools equipped with Huazhong 8 CNC system, and can also be used as an offline data analysis tool.
1. AC servo debugging software design
The machine tool generates a large amount of data during operation. Among them, the current signal is the most important data that objectively reflects the machining status and performance of the machine tool. The servo debugging software, as a comprehensive debugging software for servo-controlled motion axes, mainly collects current signals, comprehensive instruction information and G-code running trajectory, and can monitor the actual displacement values of the coordinates of each motion axis of the machine tool (see Figure 1).
The servo debugging software can intelligently analyze the collected data and provide CNC system engineers with reasonable parameter values through the software's interface dialog window. Simultaneously, the data collected by the servo debugging software can also be automatically uploaded to a "cloud server." All data is centrally hosted in a "cloud data center," allowing R&D engineers to perform real-time analysis and quickly resolve on-site user issues.
The working principle of the servo debugging software is shown in Figure 2. First, the relevant parameters are set in the parameter window, and the program window sends out the debugging program for execution. Then, the feedback information is obtained through the CNC data buffer and displayed in the graphics window. Finally, the above method is used to repeatedly debug until the servo and spindle parameters are adjusted to the optimal level.
2. Features of Servo Debugging Software Interface Design
(1) Data sampling: Provides users with quick sampling of basic data (position, speed and current) and user-defined data (arbitrary data). The debugging software will present these data to the user in the form of time domain waveform or command domain waveform.
(2) Measurement function display: including roundness test, rigid tapping test and contour test. In roundness test mode, it can output the roundness error waveform of any two axes and the corresponding quantitative index; in rigid tapping test mode, it can output the time domain waveform of rigid tapping synchronization error and the corresponding quantitative index; in contour test mode, it can output the contour graphic of any two axes in a two-dimensional plane.
(3) Graphical operation: Users can zoom, select and zoom in on local areas and replay the waveform curves to perform global and local analysis of the sampled feature points.
(4) Data analysis: The debugging software will draw the corresponding waveform curves and intelligently analyze a series of quantitative indicators based on the waveform data. For example, under basic sampling, it will output the maximum and minimum values of tracking error, speed fluctuation, acceleration and rapid traverse acceleration and deceleration; under roundness test, it will output the servo mismatch degree, axis acceleration and deceleration time, etc.; under rigid tapping, it will output the maximum and minimum values of the synchronization error between the Z axis and the C axis.
Users can modify the parameters of the CNC system and servo drive through waveform curves and index data. Multiple sampling and adjustments are made to continuously optimize the parameters of each motion axis of the machine tool, enabling the machine tool to achieve better operating conditions and produce higher-quality part models.
(5) Parameter adjustment: Supports online reading of CNC system parameters and can adjust parameter data.
(6) File import and export: Users can save the sampled data and import the sampled data file in offline mode to observe the waveform and perform arbitrary zooming in and out of the waveform for data analysis.
(7) Graphical comparison: It supports the comparison of graphical data between two oscilloscope files, and also supports the comparison of online acquired waveform files with offline saved waveform files.
The initialization interface and auxiliary interface of the servo debugging software are shown in Figures 3 and 4.
3. Servo debugging software functions
Servo debugging software can collect data from various aspects of servo drive devices, including speed loop, position loop, roundness adjustment, rigid tapping, notch filter, gantry synchronization, spindle speed increase/decrease, frequency converter rigid tapping, tool change time, custom graphic sampling, full closed-loop diagnosis, diagnostic records, and machine debugging reports.
The "three-ring" control of the servo control system, from the inside out, consists of current control, speed control, and position control (see Figure 5).
Therefore, the servo debugging software first collects data information from the current loop, speed loop, and position loop. Based on this data, engineers adjust the parameters of the CNC system and driver, namely: reasonably increasing the servo gain to ensure that the servo system does not oscillate, and enabling the servo system and CNC device to work harmoniously with each other under high response and high rigidity; secondly, the servo debugging software collects the acceleration and deceleration time constants of the servo-controlled axes, and engineers adjust the parameters of the CNC system and driver based on this data to achieve high speed and high precision when machining parts on the machine tool.
After the servo "three rings" have been running stably under high rigidity, and the relevant event constants have been set correctly, it is necessary to continue running the servo debugging software to collect the machining error information of typical parts in order to ensure the machining accuracy when the servo and the machine work together. The machining shapes that are the focus of verification are circles, squares and 90° broken lines connected to 1/4 arcs.
(1) Use servo debugging software to adjust the roundness, including the adjustment of roundness, size and quadrant.
Roundness adjustment: When machining arc shapes, the outline of the circle becomes elliptical. The main reason for this is a dynamic mismatch between the two motion axes of the interpolation. The reasons for this problem in terms of parameters include the following aspects.
① The types and magnitudes of the acceleration and deceleration time constants of the two axes involved in the interpolation are inconsistent (including before/after interpolation in general mode and high-speed/high-precision mode).
②Whether the feedforward function is used or not, and whether the feedforward compensation coefficients are consistent.
③ Are the position ratio gain settings consistent? Engineers use servo debugging software to collect data on the above-mentioned factors affecting roundness, and at the same time use servo debugging software to analyze the data and adjust the parameters affecting roundness to ensure that the dimensions of the machined round parts are qualified (see Figure 6).
Roundness adjustment: Roundness size, relative to roundness issues, generally has a small impact on machining accuracy. The main cause is machining shape error caused by servo lag. In servo debugging software, the machining shape error caused by servo lag can be improved by using the feedforward compensation function and appropriately setting a smaller interpolation time constant.
Adjustment of the circular quadrant: Adjusting the circular quadrant is one of the more difficult aspects of servo tuning. In actual machining, there are many reasons why quadrant stripes appear at the machining cross-quadrant position. One cause of cross-quadrant is that during the transmission of the machine tool's feed axis, factors such as backlash and friction cause the motor to lag during reverse motion, resulting in machining delay. In this case, raised stripes are left at the quadrant transition point of the machined arc. Engineers use the backlash acceleration adjustment function in the servo tuning software to first adjust the position loop and speed loop gains of the machine tool's feed axis to reasonable values; secondly, they set backlash compensation amounts in the speed loop to improve the lag caused by the transmission links and reduce the deviation at the reverse position (see Figure 7).
(2) Use servo debugging software to debug the square machining accuracy.
When machining square parts, especially at corners, a balance must be struck between high machining speed and good machining quality. This is achieved by collecting data using servo adjustment software, allowing engineers to select appropriate parameters to ensure high-speed, high-precision part machining (see Figure 8).
(3) Use servo debugging software to collect the arc trajectory of a 90° broken line connected to 1/4.
The 90° zigzag line connecting to a 1/4 arc program is mainly used to confirm the machining accuracy at the transition point when a straight line is tangent to an arc or an arc is tangent to a straight line. The system mainly uses the arc radius deceleration function to confirm the relationship between speed and accuracy.
4. Application of AC servo debugging software
Servo debugging software is currently being used for debugging machine tools on-site, in automated production lines, and in smart manufacturing workshops. It has also been applied in long-term continuous reliability testing environments. When used for on-site debugging, the software primarily meets the user's requirements for high-speed and high-precision cutting of parts. It can also collect information on problems encountered during machining, utilizing intelligent sensing and cloud computing technologies to gather information and directly upload it to a "cloud data platform" as a foundation for big data analytics. When other users encounter similar problems, customer service can use the information from the big data platform to resolve the issues quickly and appropriately.
In reliability testing, servo debugging software can collect the actual operating position of the servo-controlled motor and the current signal of the servo drive. Test engineers can periodically review the machine tool or electrical cabinet data during reliability testing to facilitate monitoring the overall operating status of the CNC system.
The automated testing system will be applied in smart manufacturing demonstration factories in the future. The servo debugging software will monitor the correctness of the G-code module in actual production and processing and the stability of the functional module when it is used continuously through mobile APP or web homepage. It can also monitor the operating status of each CNC machine tool to ensure the safety and reliability of parts processing in smart factories.
5. Conclusion
The servo debugging software is easy to connect to and collects rich data, enabling monitoring of various servo system operating states, such as displaying actual position, speed, and current signals. It also provides convenient and quick debugging methods, offering real-time data updates through the software interface. Engineers can quickly resolve on-site problems using this software, ensuring stable part processing performance and safe and reliable machine operation in actual production environments. The collected data is directly uploaded to a "big data center in the cloud service," allowing for longitudinal comparison of historical data from individual machines and horizontal comparison of machine tool cluster data to monitor machine tool health and ensure machine tool health. This servo debugging software can be widely applied in various industries within intelligent manufacturing .
