Application of 3D Spatial Error Compensation Technology for CNC Machine Tools

The application of three-dimensional space error compensation technology for CNC machine tools has been used for a long time since the space precision compensation technology has been applied to the CMM to ensure the high precision requirements of the CMM as a measuring instrument, and its mechanical manufacturing and electrical debugging The accuracy is difficult to meet the relevant requirements.

With the continuous development of CNC machine tools, the requirements for machine tool accuracy are getting higher and higher. The existing machine tool precision is considered from the mechanical design and hardware manufacturing, and has become a common leader in the industry. It is the pioneer of measurement technology in the CMM industry. Renishaw is in the UCC controller of its coordinate measuring machine. After more than ten years of successful application of "space error compensation technology", the "space error compensation technology" has recently been introduced for numerical control systems such as Fanuc and Siemens. Based on Renishaw's mature XL-80 laser interferometer and QC-20 ballbar, the RVC-Fanuc and RVC-Siemens two spatial error correction software were introduced to the market to match the three-dimensional compensation option. Or machining centers of Siemens CNC systems, CNC boring and milling machines and gantry machines to improve their spatial accuracy. The feedback from the actual user use shows that the RVC software is flexible, simple and effective on the relevant CNC machine tools. The bottleneck encountered. The introduction of the space precision compensation technology of the CMM into the CNC machine tool can successfully solve the key problems of improving the accuracy of the CNC machine tool.

Compensation principle

1 CNC machine tool geometric accuracy common 21 errors

In the three-axis moving space of the machine tool, there are 9 translation error parameters, 9 angular error parameters and 3 verticality error parameters, totaling 21 errors. To completely eliminate the influence of 21 errors on the position of the machine space, it is necessary to accurately detect the errors and research and develop the relevant software to convert the detected error data into parameters acceptable to the numerical control system with corresponding functions. Provides system compensation results to improve machine space accuracy. In the actual situation, the error of one machine tool will be the result of the superposition of multiple errors. The single error measurement obviously cannot completely improve the geometric accuracy of the machine tool, especially the accuracy of each direction in the working area of ​​the whole machine.

2 new features of the CNC system

The spatial precision compensation method is used to correct the error generated when the CNC machine is working. As mentioned above, the previous period has been proved to be one of the effective methods to reduce the positioning error of the machine tool in the 3D measuring machine industry. At present, many well-known CNC system manufacturers in the world, such as Siemens and Fanuc, support this method of spatial precision compensation (3D error compensation or VCS) in their high-end CNC systems. This method can be used to generate the entire workspace of the machine tool. The error parameters are used to fully compensate for the deviation of the geometrical accuracy of the machine tool during operation, so as to correct the existing spatial positioning error of the machine tool in real time.

3 Development trends at home and abroad A few years ago, when the high-end CNC system with the space precision compensation function, Siemens 840Dsl (called VCS) and Fanuc31i (called 3D error compensation), was introduced to the market, the manufacturers of high-end CNC machine tools from abroad began to study the relevant space precision. The measurement and error compensation parameters are calculated and a small number of research results are published. From the existing published data, there are laser tracking measurement methods, which measure the spatial positioning point error of the machine tool in different parts of the machine tool, and separate the error source with a certain mathematical model; also use other measuring tools such as laser interferometer and ball bar instrument. According to the 21-item error-by-item detection method.

The use of laser interferometer to test various error sources is a common method at home and abroad. The test results are accurate and traceable. It can measure and check whether the accuracy of the machine tool is accurate, stable and reliable. Check the space compensation effect at any time. The UK's most popular British XL-80 laser interferometer also has an open software interface that allows users to research and develop their own software.

Software and hardware equipment required for space error compensation for Fanuc31i and Siemens840D 1 Detecting equipment XL-80 laser interferometer: measuring linear displacement, straightness, pitch angle, torsion angle, etc., respectively, to provide RVC software with the required calculation compensation parameters Error data file. QC20-W ball bar meter: measure the verticality between each axis; and provide directional diagnosis of machine tool electrical error and mechanical error. RX10 turntable (optional): Measure and provide measurement and compensation of the corner accuracy of the rotary table. Electronic level, etc.: Measure parameters such as machine roll. 2 Spatial error correction software Fanuc three-dimensional space compensation corresponding correction software is RVC-Fanuc, Siemens corresponding correction software is RVC-Siemens. RVC software has the following three functions, each function can compensate for different items of the machine tool to be tested: Ordinary linear error compensation, three-dimensional spatial error compensation (linear displacement, straightness, angle) and triaxial vertical error compensation.

3 CNC system and corresponding space compensation function selection accessories

Fanuc3DCompensation function and SiemensVCS function. Among them 840Dsl1.3 or newer, you need to load the correct ELF file; RVC-Siemens developed by Renishaw is suitable for "VCSplus", "VCSA3" and "VCSA5". To perform the compensation function, the following steps are taken: collecting measurement data in the machine workspace range, evaluating the deviation parameters and saving them as data files; Copy the file into the CNC system sub-directory "Manufact.Cycles" (\CMA); use GUD-variable to activate compensation; the system calculates the compensation result in real time and offsets it to the position according to the actual MCS position of the three geometric axes.

4 Requirements for the basic condition of the machine before the space compensation

Before the spatial error compensation, it is best to use the ballbar instrument to evaluate the comprehensive precision of the machine. If the machine has large electrical errors such as reverse jump and servo mismatch, even if the space error compensation is performed, the machine tool is processed. The accuracy is not improved much. It is especially necessary to adjust the machine tool electrical error to the secondary accuracy before the spatial error compensation (see the QC20-W ballbar instrument for the evaluation of the machine's comprehensive accuracy status). Even if the machine with poor repeatability is compensated for spatial error, the compensation effect is not obvious.

For CNC machine tools with an accuracy requirement of up to 5μm, it is recommended that the environment in which they are used should be in accordance with the environment in which the CMM is used. Otherwise, the accuracy of the machine itself due to environmental changes will be to some extent in the long run. Reduce the effect of spatial error compensation. Application case of RVC spatial error correction software 1RVC-Fanuc software application On the FanucRobodrill machine equipped with Fanuc31i used by a British company in daily production, the machine is subjected to three-dimensional error compensation by Renishaw RVC spatial error correction software. According to ISO230-4 "CNC machine circle inspection" standard, the roundness of the XY plane before compensation and the compensation is validated by the ballbar instrument. The roundness error is reduced from 9.1μm to 5.7μm. The machine tool research institute's newly-produced Fanuc31i CNC coordinate boring machine uses the QL20-W ballbar to perform comprehensive precision test on the machine. Before 3D spatial error compensation, the verticality XWY of the XY plane is 24.9μm/m, comprehensive roundness. 11.5μm. After adding 3D spatial error compensation parameters and verticality compensation parameters and making the compensation effective, the verticality error XWY is 2μm/m; the comprehensive roundness error is 5.2μm~6.6μm (including repeated errors of multiple measurements) ).

In order to explain the details of the machine space error compensation, the Y-axis is taken as an example. The accuracy test and compensation effect of the machine tool are described in detail as follows: the Y-axis motion of the machine Y-axis in the X direction is up to 12 〃; the Y-axis pitch error YRZ of the Y-axis is up to 9〃; the Y-axis positioning accuracy YTY is measured at the X-500Z-791.235 position (ie, the end of the spindle), the error is about 14μm. At 450mm from the main axis, The Y-axis positioning accuracy is measured, and the error YTY is about 12μm. However, it is obvious that the Y-axis positioning accuracy is also affected by the angular error of the Y-axis of the machine tool in the X-direction. The accuracy curve varies greatly from the different positions of the spindle end face. After three-dimensional compensation of the straightness YTZ in the Y-axis and Z-direction, the compensation effect is immediately verified, blue is the pre-compensation trend curve (error bandwidth is about 7 μm), and the green curve is the compensated curve (error bandwidth is about ±1 μm). The compensation effect is obvious.

2RVC-Siemens software is applied to the Flymill1000 gantry machine equipped with Siemens840D in Breton, Italy. XL-80 laser interferometer and ball bar are used to measure various geometrical accuracy and complete VCS spatial error compensation. The most obvious improvement in the top three is the X-axis positioning accuracy error XTX reduced from 68μm to 2μm; the Z-axis straightness error ZTY in the Y direction is reduced from 18μm to 3.7μm; the X-axis straightness error in the Z direction XTZ is reduced from 15μm to 1.1μm.

For the above measurement and VCS compensation, the method of measuring linear positioning accuracy in multiple places is used to verify the overall improvement of spatial precision. For example, before the spatial compensation is performed, the position errors of the high, medium, and low positions in a space are 5.8 μm, 3.9 μm, and 8.0 μm, respectively, and after the VCS spatial error is performed with the XL-80 laser interferometer and the ball bar apparatus. The position errors at the high, middle and low positions are 2.7μm, 1.9μm and 2.1μm respectively. The overall positional accuracy of the visible space is improved after the VCS is effective, and tends to be consistent. ISO230-4 The roundness of the ballbar test was also increased by 25%. A Huron machine equipped with the Siemens 840D was tested at the Siemens Technology Center in Erlangen, Germany. Tests have shown that the RVCSiemens software and the VCS function of the Siemens system are fully effective on the machine. The verticality compensation effect is particularly obvious, and the XY perpendicularity XWY is increased from -9.8〃 to -0.1〃; the linear and angle compensation results are also good. The people involved in the test commented that the Renishaw ballbar and the XL-80 laser interferometer are much faster than other similar products, because it is more convenient to see Renishaw's products from the installation and use of the instrument. According to ISO230-2, the Y-axis is compared before and after the linear positioning accuracy compensation, and the blue curve is the compensation error. The roundness accuracy of the ballbar test according to ISO230-4 is improved by nearly 40%.

In view of the difference between the three-axis geometric accuracy compensation technology of CNC machine tools and the rotary axis compensation technology in five-axis machine tools (rotary axis compensation requires additional options and additional test methods for the CNC system), according to the needs of most users, the current RVC software is mainly aimed at Spatial error compensation for three-axis machines. Several views that users are currently generating in test applications:

(1) According to the test feedback of multiple customers, the spatial error compensation test is carried out by using XL-80 laser interferometer and QC20-W ball bar instrument, because it can easily test the various errors of the machine tool and quickly verify the compensation effect. It is also possible to selectively select only some of the key error terms to compensate, so that the method of selective compensation can also be used to save time under the premise of ensuring accuracy.

(2) Linear displacement, straightness, and angular error are tested item by item with ML10/XL80 laser interferometer. It is easy to trace the error and facilitate the judgment of error direction.

(3) Using the QC20-W wireless ballbar instrument, in the case of one installation, the three planes of machine tool XY, YZ and ZX are tested, and the verticality can be measured and compensated quickly.

(4) The test results prove that the three-dimensional error compensation of the Fanuc31i and Siemens 840Dsl machines is performed by Renishaw's RVC software, and the laser interferometer is used according to the ISO230-2 standard or/and the ball bar instrument is verified according to the ISO230-4 standard. The effect before and after compensation is obvious.