Computer numerical controlled (CNC) machine tools or simply machine tools play one of the most fundamental roles in modern society to produce various types of industrial and commercial products. In order to obtain high performance products, there has been a continuous requirement for high-performance parts that require tight tolerance. Thus the accuracy requirement for the machine tools has become higher and higher. As of 2020, CNC machine tools with 1μm of positioning accuracy are commercially available, and further requirement for higher accuracy is expected.
In order to achieve high accuracy of CNC machine tools, machine tool engineers and researchers have developed various technologies and techniques to eliminate the error sources at the design and assembly stage. Even though, achieving the accuracy in the order of mi- crometer is still challenging. Therefore, modern CNC machine tools are equipped with the functionality so called error compensation to cancel the tiny errors by slightly adjusting the motion of the machine. However, these error compensation functions are not designed to keep track of the dynamic change of accuracies, such as thermal distortion or aging dete- rioration, during a machining process. As machine tools are expected to produce a large number of parts with a constant quality of accuracy continuously, the accuracy deterioration must be compensated as soon as possible. However, most of the conventional error compen- sation procedures require manual labor to set up measurement equipment, time to execute a measurement program, data analysis, and manual updating the compensation parameters. Therefore, this study aimed to develop a novel error compensation system that enables ma- chine tools to measure the accuracy and update the compensation parameters automatically. As a key to the automation system, a built-in automatic measurement and compensation system based on dual linear scales is proposed, and the following four points are investigated to realize the system.
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1. The principle of the integrated measurement system
2. The design methodology of the built-in sensor system
3. The feasibility of the proposed system on an actual machine tool
4. The remaining technological problem to be solved and an idea of the solution for future study
Based on the analysis, a physical prototype was developed, and its feasibility was inves- tigated by a series of experiments. The results showed that the measurement with the linear scale system agreed with a conventional measurement method within the uncertainty esti- mated at the design stage. Based on the accuracy deterioration detected by the scale system, the compensation of the accuracy deterioration was attempted. With the compensation, the maximum accuracy deterioration was successfully suppressed from 22 to 3 μm, which is ver- ified by the laser measurement. This indicates that the accuracy of a machine tool can be maintained by the dynamic measurement and compensation system. Future perspective of this study is also discussed. Based on the theory of magnetic field intensity, it was explained that six DOF measurement would be possible in principle by arranging multiple read heads on the moving component. Also, in order to realize a dynamic error measurement, a data analysis scheme with k nearest neighbor algorithm was introduced, and its feasibility was assessed with a sample data. From these results, the feasibility of the proposed dual linear scale system has been verified for yaw measurement. The same kinematic model and data analysis scheme is also applicable to the remaining five DOF elements. Since the scale system has the scalability of degrees of freedom of measurement, further development of the linear scale technology still remains to improve the maturity of this system.
