OPTIMAL VIBRATION ISOLATION FOR SHOCK-SENSITIVE MACHINES

Jan Hansen-Schmidt1, William Granchi2, Tina M. Barth1 1Bilz Vibration Technology AG, Leonberg, Germany

2Bilz Vibration Technology, Inc., Cleveland, Ohio, USA Introduction

The world is in motion. This fact is being taken for granted by most of us in daily life. But it can be a challenge for those working in precision engineering or related industries when it comes to problems caused by vibrations. Vibrations can be caused by the machine itself or the surroundings, such as a nearby highway, railroad or other machinery. Today, we have to face drastically increasing demands regarding quality and precision in nearly all fields of the engineering process. Therefore, measuring de-vices cannot only be found in laboratories, but also closer to the production lines. This allows continuous monitoring of quality parameters as well as evaluation of selected product samples throughout the entire manufacturing process. Depending on the requirements regarding preci-sion and accuracy, vibrations can be of great negative impact for the results of a production or measurement process. In general, the more precision is required, the more efficient the vi-bration control has to be. Over the last few years, air spring systems in combination with high-quality mechanical level control have been the first choice of an optimal compromise between achievable accuracy and cost effort. By using electronic valves, that enable precise and accurate adaption of the system control parameters, isolation efficiency and transfer function can be drastically improved. As me-chanical and electronic level control systems can be combined with the same type of air spring elements, no constructive modifications at the machine bed or at the support of the isola-tors are required. Moreover costs of the entire isolation system do not increase disproportion-ately. There are numerous advantages of air spring technology compared to steel springs, electro-magnetic actuators or linear motors. The linear correlation between applied air pressure and resulting load capacity of air spring isolators leads to a high flexibility and easy adaption for different load distributions. Basic properties like natural frequency and dampening are nearly constant within a reason-

able range of operation. Moreover air spring elements are characterized by a very high me-chanical stability with low maintenance effort. They also do not require any additional damping elements. Their very low energy consumption is essential to avoid any heat generation and mag-netic variation, which is essential for many appli-cations, like e.g. electronic beam microscopes. Fundamentals The efficiency of any vibra-tion isolation system signifi-cantly depends on the matching ratio η between excitation frequency and natural frequency of the isolator, see [1]. In general, the efficiency of vibration isolation increases as the natural frequency of the insulator drops. The graph in Figure 1 shows that the system only

isolates when this ratio exceeds √2. If the ratio

is less than √2, the vibration will be amplified due to resonance effects. The damping factor D determines the amplification resonance and the transmissibility with high frequencies.

FIGURE 1. Transmissibility of a vibration isolator depending on damping and matching ratio

Typically, the objective is to achieve a ratio of

between 3 and 4 [2], whereas a ratio of 3 is con-

sidered to be the minimum effective target value

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V=� �������������������

where an isolation efficiency of nearly 80% can

be achieved, and a ratio of 4 to be an economic

limit.

Electronic Pneumatic Position Control Effective and cost-efficient solutions for nearly every problem caused by vibration – that is the claim Bilz Vibration Technology meets every day and very successfully worldwide. More than 45 years of experience in the field of vibration isola-tion have made the company based in southern Germany the European market leader today. Bilz offers a wide range of products for vibration isolation including: simple bonded rubber isola-tion pads, rubber and membrane air springs, mechanical and electronically controlled level control systems and both semi-active and active vibration cancellation systems with 3 or 6 De-grees of Freedom. This is why Bilz is able to provide an optimal isolation concept for nearly every application where vibration issues occur, starting from reducing shock emissions of forg-ing hammers to the point of elastic machine support in the semiconductor industry. The latest product by Bilz, the EPPC™ (Elec-tronic Pneumatic Position Control), was launched in fall 2013. It is a real-time level con-trol system which complements the Bilz product portfolio perfectly. The EPPC™ provides optimal vibration isolation for highly dynamic and shock-sensitive machines. Therefore, the EPPC™ can be installed with high-precision machines, where the focus is on optimal level accuracy of +/- 8 µm over a 12 mm range and little deflection as well as short settling times at load changes. Therefore it can be used to isolate not only measuring devices and microscopes but also test equipment and production machines.

FIGURE 2. Air spring isolator BiAir

® equipped

with position sensor

The system is used in combination with standard BiAir

® membrane air spring isolators, which can

be mounted directly below machine supports, support platform or a massive foundation block. Figure 2 provides a schematic cross-sectional diagram of a BiAir

® element.

The two chamber system design consists of a load and a damping volume. Both volumes are connected by an adjustable mechanical bypass throttle valve. Any deflection of the isolated ma-chine on top of the element leads to a change of the size of the load volume and a resulting air flow from one air volume to the other through the bypass. Due to air friction inside the bypass, energy will be converted into heat and damping up to a maximum value of D = 15% can be cre-ated. The natural frequency of the given air springs ranges between approx. 1.1 and 2.5 Hz in vertical direction, whereas the horizontal di-rection is characterized by a natural frequency of 2.5 Hz. Multiple air spring types and sizes are available to ensure that design and layout of the isolation system perfectly match the individual demands of the specific application. In practice, air spring elements are typically designed to work with an air pressure between 2 and 5 bar and vary be-tween 70 mm and 900 mm in diameter. This results in a total load capacity of less than 20 kg up to 15.5 tons per element.

FIGURE 3. EPPC™ level control incl. position sensors and servo-valves (3 units each) as well as the electronic control unit

The EPPC™ can be combined with three (refer to Figure 3) or six groups of air springs to control up to 6 degrees of freedom. It monitors the ma-chine position for every degree of freedom as well as internal air pressure of the air springs to control the dynamic behavior of the system. The performance of the passive air spring itself is significantly improved by using a high-performance 14bit AD converter and a 16bit PID controller which allow nearly noise free regula-tion. Servo-valves are mounted very close to the

air springs to eliminate control degradation through pressure losses in the tubes.

All these features enable the EPPC™ level con-trol system to act as a semi-active system and create a very high damping factor up to 30% in the resonance frequency range. This leads to a significant reduction of the corresponding ampli-fication or transmissibility factor.

In addition to the general advantages of air spring technology compared to systems driven by electro-magnetism (see section “Introduc-tion”), the CAN-Bus topology allows a distance of up to 20 m (66 ft) from the isolation system to the electronic control unit itself. This way the isolation system can even be applied at highly sensitive locations and surroundings e.g. clean room environment and laboratories etc.

Measurements and performance Figure 4 shows a measured disturbing frequen-cy spectrum (Boden) and the resulting vibration frequency spectrum remaining on a Bilz EPPC™ isolated system (System).

FIGURE 4. Frequency spectrum of vibration measurement with (System) and without (Bo-den) Bilz EPPC™ isolation system The isolating air spring element, which is part of the given system, is characterized by a natural frequency of approx. 2.5 Hz in vertical direction. This leads to resonance amplification of approx.

+100% within the √2 range around the natural frequency, see bottom diagram of Figure 4.

This value corresponds to the amplification fac-tor given by the formal expression and its plot shown in Figure 1 in the “Fundamentals” sec-tion: The curve named “30%” represents the theoretical transmissibility function of the EPPC

TM system. This system performance can

be achieved when the maximum damping value of 30% is applied within the electronic system setup. In this case a maximum transmissibility value of 2 results in the resonance case (η = 1), which equals an amplification of 100%.

For frequencies higher than 3.5 Hz, the reduc-tion of the vibration passing from the ground through the isolating system can be clearly ob-served. At 7.5 Hz (η = 3) a vibration amplitude of approx. 90 µm is reduced to nearly 15 µm which equals a vibration efficiency of around 85%.

In comparison to currently used mechanical pneumatic levelling systems (e.g. Bilz MPN) there is a significant increase in performance. The maximum damping factor of the mechanical systems of approx. 15% leads to a much higher resonance amplification, see again Figure 1. The settling time, which is required to reach and stay within a certain range, is drastically re-duced, see Figure 5. For excitation amplitude of 80 µm, the response curve of the respective isolation system is shown. To reach a stable position within a range of e.g. 15 µm, the settling time reduces by 40% from approx. 1.25 s to 0.75 s.

FIGURE 5. Settling time of electronic (EPPC™) and mechanical (MPN) pneumatic position con-trol For applications with very high dynamic loads due to rapid movement of work pieces or ma-chine components like scanning units, tool changing units etc. with very high requirements for settling time and constant leveling, the per-formance of an EPPC

™ and air spring based

isolation system can be further improved by adding additional mass to the system.

FIGURE 6. Vibration analysis of CMM with (System) and without (Floor) vibration isolation Figure 6 shows the measured values obtained by a vibration analysis of an isolated coordinate measurement machine (CMM). In the described application, the footprint of the CMM is approx. 2.0 m x 5.5 m. The maximum height of the ma-chine itself is more than 5.0 m. The weight of the CMM incl. work piece is more than 40 tons. High masses and large dimensions of machine com-ponents and work pieces desire a high quality isolation concept to ensure constant reproduci-bility of measurement results as well as optimal duration of the measurement process. In the same time, the foundation block helps to avoid any torsion of the machine bed due to the elastic installation of the machine. Moreover the static layout of the system is improved by lower-ing the center of gravity of the complete setup. Therefore, the isolation system was realized with the EPPC™ in combination with foundation iso-lation. In this example of an indirect isolation concept, a massive concrete block is put be-tween the isolators and the isolated object. The concrete mass in the given project was around 100 tons. In order to meet the requirements according to the limit values given by the CMM manufacturer and the vibration scenario coming from the envi-ronment from the CMM, the foundation block was put on Bilz BiAir

®-HE air spring elements

which are characterized by a reduced natural

frequency of 1.7 Hz. The reduction is achieved by increasing the volume of the air springs. Similar to the example discussed previously, the resonance amplification can be observed. The achievable high dampening of the EPPC™ re-sults in a resonance amplification of less than factor 2. For excitation frequencies above 2 Hz, isolation efficiency is drastically improving with increasing frequencies. Conclusion The EPPC™ level control system provides high-ly effective vibration isolation in the field of pre-cision engineering. The combination of high-performance electronic and pneumatic devices, optimized pneumatic design and technically mature air spring technology facilitates daily work with optimal results even for critical appli-cations.

REFERENCES [1] P. Alabuzhev, A. Gritchin, L. Kim und G.

Migirenko, Vibration Protecting and Meas-uring Systems With Quasi-Zero Stiffness. Taylor & Francis Inc. 1989

[2] J. Milberg, G. Reinhart, U. Motz, Dynami-sches Verhalten von Werkzeugmaschinen. Institut für Werkzeugmaschinen und Be-triebswissenschaften IWB, München. 1995