Design of a Highly Dynamic Hydraulic Actuator?for Active Damping Systems in Machine Tools

The positioning accuracy of conventional servo hydraulic cylinders is limited by oil compressibility, leak oil, nonlinearities, hysteresis effects, etc. This affects the control quality of the actuator, which is essential for a use in high dynamic positioning tasks, such as applications in active damping systems for machine tools. The presented hydraulic actuator design is a new approach to extend those limitations by using membrane based piston guidance and casing of pressure chambers. The actuator design is based on a linear mathematical model and FE-Simulations. The developed linear actuator model is integrated into a coupled multi body simulation of an existing active damping system. As the results of the coupled multi body simulation were promising, the actuator was manufactured and put into operation. A first insight into the behavior of the actuator and the verification of the linear actuator model is provided. DOI: 10.4018/ijimr.2012100102 16 International Journal of Intelligent Mechatronics and Robotics, 2(4), 15-26, October-December 2012 Copyright © 2012, IGI Global. Copying or distributing in print or electronic forms without written permission of IGI Global is prohibited. damping systems can be implemented within a machine tool already in production. Because of the fact that passive damping systems act only in a certain frequency bandwidth, active damping systems are developed. Active damping systems enable the user to act in a wider bandwidth, as those systems merge sensor, control and actuator into one system, and therefore adapt to the actual state of the machine tool. In order to work against the occurring vibrations, active damping systems load the structure with additional damping forces. The efficiency of an active damping system is therefore determined by the used actuator principle. Hence, some basic requirements have to be defined for an optimal dimensioning of the used actuator. Due to economic requirements and the increasing demand for resource efficient machine tools, an additional active damping system should be used within the machine tool’s available energy sources (e.g., electric or hydraulic), and without the need of expensive peripheries. From the technical point of view, the used actuator requires a large frequency bandwidth and positioning velocity. At the same time, the actuator should be able to provide the necessary travel range and positioning accuracy. A high stiffness of the actuator is necessary if it is placed in the direct force flux. Moreover, nonlinearities are not desirable in order to get a high control quality. Regarding the integration into an active damping system, a compact actuator design is necessary. Many active damping systems for machine tools use piezoelectric actuators to generate damping forces (Ast et al., 2008; Brecher et al., 2009; Munzinger et al., 2010; Simnofske et al., 2009). Isermann (2008) observed that piezoelectric actuators have a high power density (watt/kilogram), a compact design and an excellent dynamic behavior. But they are limited regarding their travel range. According to PI (2012), piezoelectric stack actuators are limited to approximately 2 ‰ of the stack length. In addition, piezoelectric actuators need expensive amplifiers, to generate the necessary power supply as a function of the calculated control signal. These drawbacks are behind the motivation to consider other actuator principles, for a use in active damping systems in machine tools. In large scale machine tools, large levers and overhanging structural components lead to a need for long travel ranges of the actuator. For example portal milling machines may have a clearance height up to 10,000 mm and a travel of the RAM of up to 4,000 mm. Those structural components create a significant weak spot in the machine tool. Brecher, Manoharan, and Witt (2008) detected this weak spot to be a bending oscillation of the overhanging structure, which leads to a deviation between the work piece and the tool center point (TCP) of the machine. In addition to that, an unstable production process may occur due to pronounced negative real parts of the machine’s dynamic compliance. A hydraulic cylinder is characterized by a long positioning range and a high power density (Isermann, 2008). In addition, hydraulic cylinders benefit from a cost-effective periphery, with hydraulic energy being available in most of machine tools today (Drechsler, 2007). This actuation principle is therefore a possible alternative to the piezoelectric actuator mentioned. However, hydraulic cylinders have some disadvantages. The buildup of pressure is nonlinear due to the change of the piston position, the compressibility of the oil and the elasticity of components like connection pipes or assembly parts. Moreover, the flow of leak oil over the gliding gasket between piston and cylinder is dependent on the pistons velocity and oil pressure. These disadvantages limit the performance of a hydraulic cylinder. Furthermore, friction forces at the gliding gasket lead to a lower positioning accuracy by the hysteresis phenomenon. In particular, a high oscillation at short piston strokes may cause thermal damages to the gasket (Murrenhof, 2002). Abicht (2002) developed a promising hydraulic actuator for an active isolating of telescopes against vibrations. In an active damping system for machine tools, it is essential to reduce these negative effects described. 10 more pages are available in the full version of this document, which may be purchased using the "Add to Cart" button on the product's webpage: www.igi-global.com/article/design-highly-dynamic-hydraulicactuator/74807?camid=4v1 This title is available in InfoSci-Journals, InfoSci-Journal Disciplines Engineering, Natural, and Physical Science. Recommend this product to your librarian: www.igi-global.com/e-resources/libraryrecommendation/?id=2