Advanced Materials Research Vols. 211-212 (2011) pp 666-670 Online available since 2011/Feb/21 at www.scientific.net (2011) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/amr.211-212.666 Design and Analysis of Hydrostatic Bearing Slide Used Linear Motor Direct-drive Guoan Hou 1, a, Tao Sun 1,b 1 Center for Precision Engineering, Harbin Institute of Technology, Harbin, 150001, China a houguoan@gmail.com, b spm@hit.edu.cn Keywords: Hydrostatic bearing; Slide; Finite element analysis Abstract. This paper discusses the design and analysis of a direct-drive linear slide used linear mortor. The alloy steel carriage is fully floated by twelve hydrostatic bearings, and it is force-closed. It has very high stiffnesses, 1700N/µm in vertical direction and 690N/µm in horizontal direction. The working stroke of slide is 200 mm. The slide is driven by a ironless linear motor. The optic linear encoder was used for the measurements and feedback. To analyze the static and dynamic performance of the slide, the modelling and simulation process, using the finite element analysis method, is presented. Introduction Linear slide for the Ultra-precision machine tools have to meet the following requirement: high linear motion precision; no crawling; stiffness to meet the conditions of use; less heat; repair and maintenance easy. Aerostatic bearings and hydrostatic bearings have been applied frequently in the slides of the ultra-precision machine tools[1,2]. They can provide extremely smooth and accurate motion and positioning. Compared to the aerostatic bearings, hydrostatic bearings have the better static stiffness, dynamic stiffness and damping capacity. This is very important for the ultra-precision machine tools, which directly contribute to the surface finish and accuracie of the workpiece[3]. So, in the state-of-the-art technology, hydrostatic slides are applied in almost all the ultra-precision machine tool. The goal of this research is to build a hydrostatic slide, used in an ultra-precision machine tool. In this paper, structure and design parameters of the slide are presented. Finite element modeling and analysis is used to predict the performance. Design of the slide The whole slide system is divided into three main sub-systems: the linear slide, the actuator and the metrology system. Fig. 1 shows a schematic diagram of the hydrostatic bearings slide. The carriage is made of alloy steel and contains several holes to supply oil to the bearings. The carriage mounting surface size is 380mm x 370mm. It is sustained by twelve hydrostatic bearings symmetric distributed on the horizontal and vertical plane, and it is force-closed. The hydrostatic bearings adopt hole throttling. The gaps in hydrostatic bearing were set to be 15µm for high bearing stiffness and small oil flow rate. The oil supply pressure is 0.5 MPa, if pressure is too low the stiffness is little, and too high easy to produce vibration. The slide is also made of alloy steel, is supported by a natural granite base which have high temporal stability characteristics, high damping properties and low coefficient thermal expansion[4]. The stiffnesses of the slide is 1700N/µm in vertical direction and 690N/µm in horizontal direction. The working strokes of slide is 200 mm. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, www.ttp.net. (ID: 130.203.136.75, Pennsylvania State University, University Park, USA-10/03/15,06:31:15)
Advanced Materials Research Vols. 211-212 667 Vertical bearing Linear motor Horizontal bearing Linear encoder Slide Carriage Machine base Fig.1 Schematic diagram of hydrostatic bearings slide On the drive of the side, there are several feed drive mode such as: ballscrew, rack and pinions, friction drives and linear motor. The ballscrew and rack and pinions need conversion mechanisms and easy to produce backlash, effecting position precision. Friction drive is widely used for precision transmission because it has the characteristics of movement smooth. However, there are some restrict in the application, such as thermal capacity. so, it is difficult for a friction drive to achieve high-speed operation at a heavy load. The direct-drive system, using linear motors for linear slide is very popular in fulfilling the needs in such requirement, in the state-of-the-art technology. Permanent magnet ironless linear motors have the advantage to significantly improve position precision. Due to their inherent smooth motion, they are a natural choice for applications requiring tight velocity and position control.the main vantages of direct drives can be summarised as follows[5,6]: no backlash, no lead-screw error and less friction, resulting in high motion accuracy; mechanical structure is simple, easy to install and maintain, high stiffness. In this design, the slide is fitted with an Aerotech U-channel BLM brushless linear motor, that eliminates cogging and magnetic attraction to allow for extremely smooth motion and very tight velocity and position control. On the choice of linear position detection device, in order to prevent the interference of the linear motor electromagnetic fields, we select linear encoder as the displacement and velocity detection device. It can be further improved resolution and accuracy of position detection by frequency-doubled signal. The slide is fitted with A Renishaw RGH25F linear encoder having a resolution of 10 nm. Performance analysis of the slide In design of the slide, analyse of static and dynamic characteristic of the slide structural is very important. So, finite element analysis (FEA) was performed by using ANSYS. We established the integrated 3D FEA model of the slide to accurately evaluate the static and dynamic performance of the entire slide system. The main purpose of statics analysis is to achieve structure deformation of the slide under the joint action of weight and oil pressure. Through dynamic analysis, the fundamental shapes of the vibration modes and the corresponding frequencies is achieved. Static analysis. Due to the oil film separate the carriage and the slide, so they can be analyzed separately, not only make analysis is easy and can ensure the calculation precision. During normal working, oil film thickness of the hydrostatic slide is only 15µm, in order to avoid the oil film
668 Mechatronics and Intelligent Materials thickness is too small or excessive to cause structural deformation, even carriage and slide are touching each other, the gap change of the film should be less than 3µm. Fig.2 Deformation of carriage and slide Fig. 2 shows the deformation of the carriage and slide under the action of its own gravity and oil pressure, when the carriage locate in the middle of the slide. The maximum deformation of the slide is small, for 0.36µm. The maximum deformation of the carriage is3.3µm, but it happen in the lower edge, to have little impact to the hydrostatic slide. The deformation in other parts of the carriage were less than 3um. Dynamic analysis. Through dynamic analysis of the slide, we can understand the fundamental shapes of the vibration modes and the corresponding frequencies.in this slide, there are several hydrostatic bearings; it is difficult to directly simulate the effects of the compressed oil. We must use the equivalent method to simulate them. In the analysis, spring elements (combin 14) were introduced to take into account stiffness of the hydrostatic bearings in the slide. Different parameter spring element is set to simulate the stiffness of slide in vertical and horizontal stiffness. In this analysis, the first six natural frequencies and their vibration mode shapes were extracted, using the block Lanczos method. Table 1 lists the first six natural frequencies and the description of the mode for the slide. Detailed pictorial descriptions of vibration modes are shown in Fig. 3. Mode no. Table 1 First six natural frequencies and mode description of the hydrostatic slide Frequency [Hz] Mode description Mode no. Frequency [Hz] Mode description 1 354.90 Carriage rolling 4 510.20 Carriage bending 2 401.33 Carriage rolling 5 763.40 Carriage bending 3 402.76 Carriage rolling around Y-axis 6 881.56 Carriage rolling around X-axis From Table1 and Fig. 3, we can see, the stiffness of the hydrostatic bearings dominates the natural frequencies and vibration modes of the slide, in the lower frequency range. We can increase the stiffness of hydrostatic bearings, such as an increase in oil pressure within a certain range, to improve the dynamic performance of slide.
Advanced Materials Research Vols. 211-212 669 a)1 st mode b)2 nd mode c)3 rd mode d)4 th mode e)5 th mode f)6 th mode Fig.3 Results of modal analysis Summary In this research, an hydrostatic bearing slide used linear motor direct-drive has been designed. Structure design, design parameter and static and dynamic analysis results are presented. The slide is fitted with an U-channel brushless linear motor and a linear encoder having a resolution of 10 nm. Static analyses predicte the deformation of the carriage and the slide. Dynamic analysis showed that the hydrostatic bearing is the important sensitive component of the slide, its stiffness
670 Mechatronics and Intelligent Materials determines the slide system's low frequencies and modes. We can increase the stiffness of hydrostatic bearings to improve the dynamic performance of the slide system. Acknowledgments The authors are grateful for the support of the 111 project (B07018). References [1] Information on http://www.nanotechsys.com/machines/ [2] Information on http://www.precitech.com/2010_precitech_product_overview.html [3] D. Huo, K. Cheng: A dynamics-driven approach to the design of precision machine tools for micro-manufacturing and its implementation perspectives, Proceedings of the Institution of Mechanical Engineers, Part B: Journal of Engineering Manufacture Vol. 222(1) (2008), p. 1-13. [4] Schellekens, P. and Rosielle, N. :Design for precision: current status and trends. Ann. CIRP, Vol.47, No.2(1998), p. 557-584. [5] Luo, X., Cheng, K., Webb, D., and Wardle, F. :Design of ultraprecision machine tools with applications to manufacture of miniature and micro components. J. Mater.Process. Technol. Vol.167, No.2(2005) p.515-528. [6] Brecher C, Klar R, Wenzel C.: Development of a high precision miniature milling machine. Proceedings of the 3rd International Conference on Multi-Material Micro Manufacture, 4M 2007, p.327 330.
Mechatronics and Intelligent Materials 10.4028/www.scientific.net/AMR.211-212 Design and Analysis of Hydrostatic Bearing Slide Used Linear Motor Direct-Drive 10.4028/www.scientific.net/AMR.211-212.666