Research and application status and key technology of NC machining adaptive control
Before adaptive control is applied to the CNC system, process parameters such as spindle speed, feed rate, etc. are pre-specified by the part's programmer, so their experience and knowledge determine the merits of the selected process parameters. In order to protect tools and machine tools, even in the most favorable conditions, part program compilers prefer to choose conservative process parameters, which of course reduces the processing speed of the system. With the rapid development of integrated circuit technology and the increasing requirements for processing efficiency and processing quality, the research on adaptive control of CNC machining process began in the late 1950s and early 1960s.
The adaptive control system can be defined in this way: an adaptive control control system needs to measure a certain performance index of the control system, and the adaptive mechanism modifies the parameters of the adjustable regulator according to the deviation between the expected performance index and the measured performance index. Or control signals so that the performance of the system remains within the desired range. A conventional feedback control system can reduce the disturbance acting on the manipulated variable, but its dynamic performance will change when the parameter is disturbed. In addition to the feedback loop of an adjustable parameter regulator, an adaptive control system has an additional loop that relates to the regulator parameters in order to maintain system performance in the event of process parameter changes. This additional loop is also a feedback structure in which the controlled variable is the performance of the control system itself.
1 Research on Adaptive Control of CNC System The adaptive control system of CNC machine tools can be divided into two categories: optimized adaptive control (AC0) and constrained adaptive control (ACC). ACO means that one or several given performance indicators (generally economic indicators) in the system are governed by process and system constraints (such as the maximum allowable cutting force, etc.). Although a lot of research has been done in the development of ACO system, it is impossible to establish the machining process model and the optimization index model with sufficient precision because it is impossible to measure the wear of the tool on-line, and the research on the adaptive control of the CNC system is combined with Adapt to control! “The method of indirect measurement of cutting force by lis current and power is detailed. Netbookmark3 lacks online optimization of robust algorithms, so the ACO system has so far only been used in grinding and EDM without measuring tool wear. STC is a model adaptive control system developed synchronously with MRAC. The STC system uses the input and output of the process to estimate (identify) a model in real time to replace the process model. STC has two main functions: online identification of cutting process parameters. And real-time control. Since the model is online, it can suppress the influence of uncertain factors on system control performance. Intelligent adaptive control system, variable structure control 15, coupled control (CCC), zpetc and ikf, etc. But the current commercial CNC What has been used in the system is the "old" PID plus feedforward control. The reason is actually very obvious: the basis of these 'excellent' servo control algorithms is based on the "process model". Although the conventional feedback control system itself has the ability to resist interference and resist parameter changes when the controlled object (machine tool) changes autonomously, such as increased cutting force, temperature change, etc., this capability requires changes in system parameters. The range does not exceed a certain limit and this ability comes at the expense of loss of machining accuracy, ie the ability itself is limited.
The precision required for high-speed CNC machining is higher, and the limitation of the parameter variation range of the numerical control system is obviously stricter. If the design that limits the machining range of the machine tool is used to obtain the design that meets the accuracy requirements, it is difficult to improve the application range of high-speed cutting and play real-time. The detection of the parameter changes of the numerical control system and the adaptive control of the modified controller parameters must become the guarantee for the wide application of high-speed CNC machining. The practical research on the adaptive control of high-speed CNC system has become the inevitable research focus.
At present, the research on high-speed cutting adaptive control can be divided into the following aspects: the adaptive control integrated system research machine tool numerical control system gradually moves from decentralized control to integrated control. The most representative development is the modularization of CNC machine tools and the IPC-based open CNC system that is gradually coming to market.
The integrated research and application of numerical control system is one of the hotspots and frontiers. Many large companies at home and abroad have spent a lot of manpower and material in this area and their momentum has increased. Such as: SIMENS, FUNAC, CINCINNATI and other foreign giants and Huazhong CNC, Shenyang Blue Sky and other domestic more powerful CNC companies.
Much progress has been made so far, but there is clearly less integration in adaptive control. At present, the controllers for adaptive control of cutting loads applied to CNC machine tools are mostly plug-in forms, such as OMAT in Israel, that is, adaptive controllers are not an integral part of CNC machine tools. Machine tool CNC systems generally do not contain loads and other machining dynamics. Adaptive controller for parameters. The external adaptive controller is generally versatile and does not take into account the actual conditions and performance characteristics of the machine. Therefore, the optimization of the control results is very limited. In addition, such a controller can only change the feedrate by the feed rate override switch. Due to the limitation of the number of digits of the override register, only the level adjustment can be performed, and the accuracy cannot meet the requirements of high-speed cutting. Therefore, the research goal of high-speed cutting adaptive control is to integrate the adaptive control of various types (such as roughing, finishing, etc.) into the numerical control system based on the idea of ​​integration, which is more suitable for Cutting conditions for high-speed machining: Uniform cutting forces and smoother, higher cutting speeds optimize system control performance.
Coarse and fine processing adaptive control research High-speed cutting high-speed, high-efficiency precision features superior control system control requirements before / / must Muetbookmark4 With the high-speed CNC machine tool electric spindle power increase, suitable for high-speed cutting tool materials Widely used, the machine tool components and overall stiffness increase (such as the use of marble bed) and the improvement of the stability and stability of the CNC system, high-power high-speed cutting machine tools can be used in the processing of common metals without the need for finishing and semi-finishing Machining will be directly finished, cutting the blank to the required surface accuracy and dimensional requirements at a time. This high-speed cutting method is defined here as "coarse and fine" processing. At this time, the uniformity of the cutting force and the uniformity of the feed rate need to be considered at the same time, which is a new subject for high-speed numerical control machining. "Coarse and fine" processing can greatly improve the processing efficiency of high-speed cutting, and is the inevitable goal of high-speed cutting development. Therefore, high-speed numerical control system is required. (3) In addition, the development of cutting force sensor suitable for CNC machine tool application, research on fast, stable and adaptive comprehensive control algorithm will also be the key research problem of adaptive control of numerical control system. one.
4 Conclusion Although the research on adaptive control of numerical control system has been extensive, the combination with numerical control system is far from uniform, and the gap is still very large, which leads to the application of adaptive control of numerical control system is still not perfect. According to the mature adaptive control theory, further research on the application of CNC system, especially the high-speed NC integrated adaptive control system is an important guarantee for the practical control of CNC system.
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