Rotary encoder featuring ultra high accuracy and resolution

Transcription

1 Rotary encoder featuring ultra high accuracy and resolution - Advance angle control ability for robot and machine tools - Apr. 23, 2014 Magnescale Co., Ltd. Outline The application of AIST original technology into the high accuracy rotary encoder has enabled angular position measurement at ultra high accuracy Angular error can be detected even after installing encoders to the equipments This technology contributes to improve accuracy and productivity of the equipments such as machine tools, which requires highly accurate angular position control Summary Tsukasa Watanabe (Chief senior researcher, Dimentional Standards Section, Lengths and Dimensions Division) of National Metrology Institute of Japan in the National Institute of Advanced Industrial Science and Technology AIST; President: Ryoji Chubachi,in collaboration with Magnescale Co., Ltd. Magnescale; President: Toru Fujimori, has developed a rotary encoder achieving ultra high accuracy and resolution. The encoder integrates SelfA (angle detector with self-calibration function) technology developed by AIST into high resolution rotary encoder from Magnescale. The new encoder can realize division of 2^33 (approximately 8.6 billion division) per cycle and accuracy of ±0.03 per square arc second, which had not been achieved by existing encoders in the market. Machine tools built with this encoder can improve form accuracy as well as surface roughness at the process of complex objects such as engine blade. The object might be able to skip grinding process before mirror finishing. Moreover, the new encoder could enhance process accuracy and productivity of objects which require both larger size and precise machining, for example turbine parts and gear wheel of wind generation. The details of this research will be published online as an article in Measurement Science and Technology on Apr. 16, 2014(GMT) Refer to Glossary Open a future contributed to Japanese Monozukuri by angle measurement featuring ultra high accuracy and resolution

2 Social Background of Research & Development Rotary encoder must achieve high accuracy, resolution and response speed for the use in high resolution angle measurement of machine tool, direct imaging system for semi conductor, precise measuring instruments, and optical component processing instruments. However, it is considered that measurement and control with rotary encoder at 0.1 per square arc second or less is difficult due to angular error factors such as eccentric error at the installation of encoder to rotation axis of instrument as well as grating error. History of Research and Development Magnescale Co., Ltd. develops rotary encoders with high resolution and response speed. The resolution of angular signal, that is the number of angular signal pulse, out of the disk-shape grating (diameter of 167mm) typically remains from a few ten thousand to a hundred thousand pulse per revolution. In contrast, rotary encoder from Magnescale can output analog signal at 2 million pulses or more. Digital angular signal can reach 10 billion pulse or more through the interpolation of analog signal. However, grating scale and detection units are separate module so that the installation of the encoder to the tool ( e. g. motor or machine tool) leads eccentricity between the center of the rotating shaft of the tool and the center of rotating axis of encoder. The existing encoder by Magnescale had achieved ultra high resolution but not high accuracy, because estimation of accuracy at use is difficult due to the existence of eccentricity as a factor of angular error. AIST had been developing rotary encoder with accuracy of 0.1 per square arc second through the technology of SelfA, but it focused the resolution of less than a few hundred thousand pulses and had not performed an experimental study of SelfA in combination with ultra high resolution rotary encoder. Then, AIST and Magnescale undertake joint research of the application of SelfA into ultra high resolution encoder. Details of Research and development The joint team has developed rotary encoder with the function of SelfA by using grating scale (number of analog sine signal per rotation : 2^21=2,097,152) with diameter of 167mm which Magnescale has newly developed shown as figure1(left figure) and multiple, separate scanning units shown in figure 1(right figure). Self- calibration function by SelfA enables detection and correction by rotary encoder itself of angular errors included in angular signal from scanning head. Rotary encoder can detect not only grating error but also eccentric error at the time of installation which was difficult to measure by separated type rotary encoder. In other words, error after installation can be measured by Rotary encoder itself. The grating interval on the scale is 1μm. Scanning unit detects the grating and outputs 250 nm cycle analog sine wave as the 1/4 of the grating pitch. Scanning unit uses the principle of grating interferometer method, therefore signal distortion of analog signal is very small and analog signal could be sub-divided and converted to digital signal with high accuracy through interpolation circuit. In this study, each of 8 scanning units has own dedicated interpolation circuit and each circuit divides analog signal by 4096 times then convert it to digital signal. Furthermore, Self-calibration through SelfA has been applied to each of 8 angular signals in order to realize accurate detection of angular errors in digital signal. This newly developed rotary encoder has been installed on the National Standard equipment owned by AIST shown as figure 2 for the verification of the performance of angular error detection. The result proved that angular error detected by self-calibration at rotary encoder itself is valid at the accuracy of ±0.03 per square arc second over all angle domain of 360. The measurements after replacing rotary encoder to other units also proved same accuracy. Therefore, this rotary encoder can achieve±0.03 per square arc second over all angle domain through self-calibration by SelfA after installing it to machine tool, direct imaging system for semi conductor, precise measuring instruments, and optical component parts processing instruments. In addition to overall accuracy, the researcher found that the angle error of interpolation signal within one grating is less than ± per square arc second even after change of measuring position

3 Disk-shape grating Figure 1 167mm diameter grating scale, newly developed by Magnescale(left figure) and Rotary encoder with self-calibration function and 8 scanning units developed by Mangescale.(right figure) SelfA rotary encoder developed for this research National Standard equipment for angle Figure 2 Evaluation of newly developed rotary encoder(figure 1, 2) on the National Standard equipment for angle Future plans As principle development had been already completed in this study, Magnescale investigates the possibility of release to the market as the unified system of scanning head and interpolation circuit with the consideration of user convenience. In addition, Magnescale addresses further improvement of accuracy toward ±0.01 per square arc second of angular error measurement ability which is currently ±0.03 per square arc second

4 Contact Tsukasa Watanabe, Chief senior researcher National Metrology Institute of Japan Lengths and Dimensions Division Dimensional Standards Section Tsukuba Central 3, Umezaono 1-1-1, Tsukuba, Ibaraki, , Japan TEL: FAX: Press release/inquiry Tomoko Kitazawa, Public relations Tsukuba Central 3, Umezaono 1-1-1, Tsukuba, Ibaraki, , Japan TEL: FAX: Takuji Tanaka, President office Magnescale Co., Ltd. Suzukawa 45, Isehara city, Kanagawa, , Japan TEL: FAX: Glossary Rotary encoder Similar to protractor. Installed on the rotation axis of industrial robot, machine tool, motor and so on. Important devices to measure and control angular positions. SelfA(angle detector with self-calibration function) National Standard equipment for angle calibrates external rotary encoder installed on itself through EDA-method (equally divided average method). SelfA is an encoder that accomplishes EDA-method as stand-alone device with built-in national standard reference. Nevertheless, structure of SelfA is simple. Several detection units are installed at equally divided intervals. Engine blade Turbine blade inside of engines for jet airliner Mirror finishing Process to smooth the surface of the work piece like mirror. Metalworking process usually leaves machining marks. Additional process such as buffing will remove the marks and changes the surface like mirror. However, highly accurate process does not leave machine marks and makes the surface like mirror without further processes. Angular error factors Rotary encoders detects grating on the scale, same as proctors, count the number of gratings then output angle position. The cause of angular error factors from rotary encoder consists of inherent factors, determined during manufacturing process, and acquired factors, determined during the actual operation by end user. Eccentricity error between the center of rotation of encoder and - 4 -

5 center of grating is inherent, whereas eccentricity error at the installation of rotation axis of encoder into the rotation shaft of the tool is acquired. Eccentricity error between encoder and the tool is difficult to detect because the error has appeared during installation. Eccentricity of encoder shaft and rotating shaft of device Rotating shaft of device Bearing Encoder shaft Graduated disk Graduated scale Detection unit Eccentricity of encoder shaft and rotating shaft of device Interpolation Detection unit of rotary encoder outputs two different sine wave analog signals with 90 degree phase difference. Interpolation circuit converts these signals to digital through AD converter then increases resolution per one signal output by calculating the angle inside of Lissajous made by 2 digital signals. The encoder developed in this research generates the number of sine wave signal of 2^21 and one rotation of Lissajous signal is divided by 4096(2^12). As a result, resolution of this encoder is 2^33 which is 2^21 times 2^12 Grating interferometer method Detection method which measures phase of diffracting gratings included in the light diffracted by grating on the scale, through interference. It measures the position of grating by the characteristics of light wave. Comparing to Moire method applied to general optical encoders, grating interferometer method has advantage that resolution is higher since it can measure wavelength from scale grating shorter than wavelength of the light