Air element suspension

Dynamic characterization of air spring for AVC applica-tions

L. Bregant, M. Spagnol, R. MorasLVA, Mechanical Engineering Dept. - University of Triestevia Valerio 10, 34127 Trieste, Italye-mail: [email protected]

AbstractMany engineering applications requires for the use of resilient mounts whose main scope is to decouplethe vibration flow from the source to the receiving structure. In most of the cases, the provided designcharacteristics of these components are limited to the static stiffness and no information on the their dynamicbehavior is provided. This represents a drawback while optimizing the resilient mounting of large vibratingmachinery where every knowledge of the dynamics of the vibration paths plays an important role.In the years, to improve isolation performance of the mounts, air springs, instead of the full rubber mounts,have been adopted. These consists of a rubber sleeve, with appropriate endings, filled up with air at theappropriate pressure. Again the manufacturers do not provide much of the information, the engineers wouldneed to use those devices at their best. In this work the authors describe the testing machine that has beenbuilded at the LVA to derive the force vs frequency vs pressure plots of a set of commercial air spring.The machine can measure the forces both above and below the device, allowing to determine, directly, inthree directions, the transmissibility as function of the frequency and of the applied pressure. The preloadacting on the spring, as well as the internal pressure, can also be modified allowing for a more accuratesystem description. The characteristics curves derived for the air-spring in such a way, will be used withina simple SDOF AVC system. The performances of this solution will be compared with the performance ofan analogous system using only the commonly available static deflection value. The work clearly exhibitsthe importance of the accurate measure of the dynamics properties of the mounts, and the relatively simpleset-up with which these data can be acquired.

1 Introduction

The purpose of this report is to investigate the behavior of air spring. Contitech air spring systems offerssolutions for the isolation of vibration generating machines. From the technical design all the way through tothe harmonization of individual system components. Especially for the application field of vibration isolationfor heavy foundations, Contitech air springs are able to realize a maximum load capacity up to 33800 kg,with a very low frequency and excellent horizontal stability. In the described test, a Contitech SK19-4(Fig. 1) was used. The initial characterization of the air spring consisted in measuring the acoustic cut atdifferent pressures, pre-loads and frequencies. Consequently the characteristic data were used to improvethe performance of an active vibration control for a SDOF system trying to minimize acceleration of themoving masses (top end of the spring). In a second time the force transmitted to the foundation will beminimized. It was necessary to design & build an appropriate testing machine to achieve the requestedresults.

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Figure 1: Contitech SK19-4 Figure 2: Test machine

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ContiTech Luftfedersysteme GmbH Edition: 2006-10-05

SK 19-4 CONTI® Sleeve Type Air Spring

SK 19-4 Force-height diagram

Purchase order data Technical data

Type Order No. Min. pressure 0 barWith G 1/8 air inlet 61956 Return force to min. height ≤ 22 N

0.07 kg

Additional types on request Vibration isolation - dynamic characteristic valuesDesign height H: recommended 45mm, minimum 30mmPressure p [bar] 3 4 5 6 7 8 Vol V [l]Force (Load) [KN] 0.45 0.59 0.72 0.88 1.02 1.15Spring rate [N/cm] 210 260 310 360 410 460Natural frequency [Hz] 3.4 3.3 3.3 3.2 3.2 3.2

Pneumatic application - static characteristic valuesForce F [kN]Pressure p [bar] 3 4 5 6 7 8 Vol.( l )Height H [mm] 55 0.33 0.43 0.53 0.64 0.74 0.84 0.08

45 0.45 0.59 0.74 0.88 1.02 1.15 0.0735 0.53 0.70 0.86 1.03 1.19 1.35 0.05

Service instructions

M8 = 3-5 Nm

M16 = 11-14 Nm

Further information available by fax +49 (0)511-938-5162 or under http://www.contitech.de/luftfedersysteme

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1718 PROCEEDINGS OF ISMA2010 INCLUDING USD2010

1.1 Test Structure

To define the overall design of the testing machine, a preliminary investigation was conducted in ANSYS.The resulting geometry consists of a package of circular plates and rings. Three columns guide the pre-loadmasses and support an electrodynamic shaker (Fig. 2). The test specimen is fixed to the bottom plate and themoving pre-load masses (Fig. 3). These are put in motion by the above mentioned shaker. The maximumpreload is around 40 kg. To gain the best performance, the structure is CNC machined with close tollerance.The air supply circuit consists of: an air compressor with pressure regulator, filter and moist separator, aproportional pressure valve REXROTH ED02, a two port solenoid valves SMC VQ20 (high speed response,less 5ms) with air tank and retroaction control using pressure sensor SMC PSE540.

1.2 Test Setup

The excitation is obtained using a permanent magnet electrodynamic shaker, LDS V406/8, driven by itsamplifier LDS PA 100E. This combination is sufficient to have a displacement of 2 cm in a range of frequencyfrom 2.5 Hz to 12 Hz.To control the internal pressure of the spring, a proportional valve Rexroth ED02 was used. This valvehas fast response and an internal control system to maintain constant the internal pressure of the pneumaticcircuit. The data acquisition is entrusted to a system DSPACE DS1104. It has analog and digital input/outputso it is possible to control the valves, acquire and store the sensors’s measurement.The acquisition test equipment is composed of two load cell (one PCB 260A01 is placed under the springand the other, PCB 208A03, above the moving masses), one accelerometer, PCB 356B18, on the movingmasses and one pressure sensor SMC PSE540. The signals are amplified by PCB 482A20 amplifier. It wasnecessary to design and build specific hardware to condition the signals and pilot the valves. The springreaction force depends from the internal pressure and the internal volume of the air chamber.It can be expressed as:

Fi = priAwi

where Fi is the air spring force, pri is the relative pressure in the air spring. Awi , the effective area on whichthe pressure acts. It is a non-linear function of the spring height. In order to calculate the internal area, astatic load was applied while measuring the internal pressure with the SMC sensor.The internal volume depends from the air mass and pressure variation inside the air spring. From thermody-namics and ideal gas flow:

privi = nRT

The initial volume is v0 = 0.08 lt, and vi is the volume at the pressure pri. In our test, the number of molesinside the spring has been kept constant closing the air circuit. So once the pressure is set, no other exchangeof air mass with external ambient can occur.

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v0 = 0.08 lt R = 0.082 lt atm K−1 mol−1 T = 293.15 K pr0 = 2atm

The static deflection test presents good agreement with Contitech datasheet (Fig. 4(d)). This data permit-ted to calculate internal area Awi Fig. 4(b) and air volume vi Fig. 4(a), used afterwards for modeling, viaSimulink, the behavior of the spring and define the active control parameters.

DYNAMIC TESTING: METHODS AND INSTRUMENTATION 1719

2 Experimental characterization

For dynamic characterization of the spring, a variable frequency load has been applied using the electrody-namic shaker. The frequency was swept between 2, 5 Hz and 12 Hz, with a step of 0, 1 Hz. Also the pressure,inside the spring, was varied from 2, 4 bar to 6 bar, with 0, 2 bar steps.The test involve a cyclic excitation (sine wave) for 3 seconds of acquisition. Between one step frequencyand the other there is a stabilization time of 4,5 seconds. The response surfaces and top view are reported inFig. 5, respectively for a pre-load of 35.96 and 30.96 kg. It can be noticed that the natural frequency variesbetween 4.0 and 6.5 Hz in the first pre-load case and between 4.2 and 7.3 Hz for the second case. The damp-ing factor, calculated with half amplitude method, is ξ = 0.07. It could be seen that the variation of resonancefrequencies is quite linear with the pressure. The maximum acoustic cut, defined as AC = 20 log(Fdw/Fup)(where Fdw is the force measured from the load cell under the spring and Fup is the one from the upper loadcell), is −18 dB for 35, 96 kg and −16.7 dB for 30, 96 kg.

2.1 Temperature effect

In order to understand the effect of temperature on the characteristic of the spring, a long duration test wasperformed. It is expected that due to rubber friction and repeated compression, the air temperature containedin the spring would rise.The test requires the inflation of the air spring at the pressure of 5 bar, the excitation of the moving preloadmasses for about 2 hours and the evaluation of the acoustic cut each 30 minutes ever since.The test were conducted both at constant pressure (reinflating the system when necessary) and without ”re-inflation”. The range of acquisition was set from 6.4 Hz to 6.9 Hz, with step of 0.005 Hz. These tests weredone to have an high resolution near the resonance frequency. It could be noticed that the resonance fre-quency vary more in the second case than the first. This may attributed at a reduction in internal pressurecaused by rubber heating.The results and reported in Fig. 6. Refilling the pressure every time, it is possible to have a repeatability ofthe results, maintaining the resonance frequency almost constant. Instead, leaving the system get warm, theresults change in time, obtaining lower resonance frequency.In the future, further investigations will be focused on the definition of the thermal effects on the dynamicstiffness of the spring.

3 FEM analysis

Due to the nature of the part, before evaluating the modal parameters and the harmonic response, the staticanalysis with large deformation option was performed, an example of the sleeve deformation is reported in(Fig. 7(d)). This analysis allowed to define the static response surface of the air spring as function of theinternal pressure and the pre-load (Fig. 7(a),7(b)).The material characteristics of the rubber sleeve were optimized performing a multi modal updating withinFEMTools. Circumventing the fact that this tools does not allow for large deformation analysis, a goodagreement between the numerical and experimental first natural frequency was obtained (axial mode). Thesecond one (torsional mode) is at around 200 Hz well above the available testing machine capabilities.The harmonic analysis was also performed to derive the transmissibility as function of the frequency and thedifferent pre-loads. Fig. 7(c) shows the overlapped response surfaces highlighting the non-linear effect ofthe pressure variation. Fig. 7(e) shows the good agreement between the curves obtained at LVA (red lines)and at Contitech (blue lines), while the comparison between Fig. 7(d) and Fig. 7(f) shows how close thesimulation of the static deformation is to the real one.


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Figure 6: Long testing - temperature effect: a) pressure refill every cycle; b) pressure set at test start


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4 Active control

The responses of the system excited with a variable frequency excitation were compared. The analysis wasfocused on the first mode that is the most common spring working condition.The model, created according to bibliogharphy[4], was tested first via SIMULINK and afterwards ran withHIL in the dSPACE ambient.The control strategy is based on the minimization of a quadratic function containing the state of the system(acceleration and velocity), acting on the value of the supplied pressure to the air spring. At the moment,only one pre-load mass and one internal start pressure setting (5 bar) was tested, but in the near future, thepossibility of the system will be further investigated. The correction pressure can vary from 2 bar to 6 bar.Due to the setup with a fixed shaker, it was decided to excite the system with a variable sine wave, changingits frequency or its amplitude to obtain a transitory effect. The results are plotted in Fig. 8, where the blueline represents the system response with the constant spring stiffness, while the green line is the response ofthe controlled one.It is evident that the major effect is achievable near resonance frequency: changing the internal pressureallows to reduce the acceleration response of the system. It is under study a control strategy capable toreduce the force transmission to the ground, and thus incrementing the acoustic cut as requested by theindustrial application in which the vibration source as to be isolated from the foundation.Research activities will be focused at implementing a robust control and take advantage of the full springresponse surface.

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Figure 8: Active vibration control: a,b) stepping frequencies c,d) stepping amplitudes


5 Results

According with our opinion, the behavior of air spring can help in industrial vibration control application. Infact it is possible to tune the internal pressure to the best acoustic cut zone for specific frequency range. Oursimple SDOF control put in evidence the fast response of the spring and the possibility offered by generalpurpose air flow control.Off course some investigation will be required to define the effect lateral stiffness of this kind of devices ona MDOF test case, but there is a strong believe that for large applications with low resonant frequencies, airspring would deliver interesting solutions.

References

[1] Cyril M. Harris, Ed., Shock and Vibration Handbook, The McGraw-Hill Companies, (1987).

[2] W. Bolton, Control engineering, Longman, (1998).

[3] Michael R. Hatch, Vibration simulation using MATLAB and ANSYS, Chapman Hall/CRC, (2001).

[4] F. Chang, Z-H Lu, Dynamic model of an air spring and integration into a vehicle dynamics model, Proc.IMechE Vol.222 Part D: J. Automobile Engineering, (2008), pp. 1813-1825.

[5] MathWorks: MATLAB User’s Guide (Manual), (2007).

[6] ANSYS, Inc.: Release 11.0 Documentation for ANSYS (Manual), (2008).

[7] dSpace: dSpace User’s Guide for Release 6.4, (2008).


Air element suspension

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