567. Research of Dynamics of a Vibration Isolation Platform

Transcription

1 567. Research of Dyamics of a Vibratio Isolatio Platform A. Kilikevičius, M. Jurevičius 2, M. Berba 3 Vilius Gedimias Techical Uiversity, Departmet of Machie buildig, J. Basaavičiaus str. 28, LT Vilius, Lithuaia akilikevicius@gmail.com; 2 midaugas.jurevicius@vgtu.lt; 3 mberba@ktl.mii.lt (Received 30 Jue 200; accepted 0 September 2009) Abstract. Performace of egative-stiffess vibroisolatig table Mius K500BM- was aalyzed i a frequecy rage of 2-0 Hz. The paper also reviewed other possible ways of vibroisolatio. Performed experimetal vibratios trasmissibility tests were reported as well. A special vibratio excitatio equipmet was tested i research facilities. Subsequetly, experimetal test methods were described together with presetatio of results ad coclusios of experimetal aalysis. Keywords: egative-stiffess, payload, HVAC systems, actuators, elastomers. Itroductio Efficiet vibratio dampig systems are eeded i order to be able to effectively cotrol the accuracy of the measuremet results, particularly i the case of sesitive laboratory measuremet equipmet. Accordig to Erico L. Colla (2003), a payload (term used i this work to refer to the object to be isolated from vibratios) ca geerally be subject to four sources of perturbatios arisig: from groud vibratios; from ambiece, i.e. acoustic oise or air flow; from mechaically coupled exteral equipmets ad from iteral mechaisms. Measurig equipmet from extraeous vibratios ca be isolated usig active or passive vibroisolatio systems. A active vibratio isolatio systems creates equal, but opposite directio forces to directly cacel uwated vibratio from the system by usig actuators, sesors ad cotrollers. With passive methods of isolatio, by usig a mechaical coectio, the vibratio eergy is dissipated or redirected before reachig the isolated object. Passive systems may employ elastomers, sprigs, fluid or egative stiffess elemets. Oe of the mai passive vibratio isolator is a sprig that is placed betwee the trasmittig shock or vibratio surface ad a isulatig object. The sprig resists to impulse ad absorbs a lot of eergy through deformatio. Fluid or elastic elemets are attached to the sprig to perform the fuctio of ihibitio. A represetative example is a shock-absorber i a car. I this case, the mechaical eergy from shock or vibratio operates i the fluid ad is coverted ito heat eergy, thereby reducig the trasmissio eergy exerted oto vehicle body. Passive vibroisolatio systems usually cost less tha the active oes ad their relative simplicity makes them more reliable ad safe. Passive systems are classified ito the egative stiffess systems that ca isolate the effects of vibratio alog all six degrees of freedom whe the resoat frequecy is 0,5 Hz or eve less. These vibroisolatio systems are better times i compariso to most air tables i 5-0 Hz rage. Cosiderig the ature of vibratios, several vibroisolatig systems may be used which act as oe. 36

2 Fig.. Most relevat vibratio sources for a payload (hexagoal object). a) groud vibratios, b) acoustic oise, c) exteral ad d) iteral vibratio sources The object of research For curret research a egative-stiffess vibroisolatig table "Mius K 500BM- is used, which isolates low frequecy ad amplitude vibratios geerated i buildigs ad floors. This vibroisolator is fully mechaical ad isolatio is achieved by usig sprigs ad a egative stiffess mechaism. Fig. 2. Negative-stiffess vibroisolatig table Mius K500BM- Accordig to Masaki Hosoda (2009), a particularly importat coditio occurs whe the frequecy of the periodic force early or exactly correspods to the atural frequecy f 0 of the device. I the regio aroud f 0, the trasmissibility is over, which meas the amplified vibratio of a groud motio trasmits to the equipmet. If the trasmissibility is lower tha, the equipmet is isolated from the groud motio. The atural frequecy f 0 of the equipmet is give by the equatio: f o k M = () 2π Accordig to this equatio, the atural frequecy may be small, if oe desigs the equipmet of low stiffess ad mass. But it is ot a reasoable way. Istead of chagig the desig of the equipmet, the atural frequecy ca also be reduced by the meas of vibroisolatig system, which has to filter floor (base) vibratios. Isolatio is attaied primarily by maitaiig the proper relatioship betwee the disturbig frequecy ad the system atural frequecy. The damped atural frequecy of the system is described by the equatio: 362

3 f = 2π k M C C C 2 ; (2) Here C is a dampig coefficiet with a uit of [Kgsec/m]; C C is a critical dampig with a uit of [Kg-sec/m]; C/C C is a dampig factor. Dampig is advatageous whe the mouted equipmet is operatig at or ear its atural frequecy because it helps to reduce trasmissibility. Icreasig the dampig factor C/C C reduces atural frequecy of the isolatio system. Trasmissibility is give by the equatio: x T = ; (3) y Trasmissibility ca also be expressed i decibel (db): T y = 20log db ; (4) x Methodology ad experimetal research Vibratio trasmissibility ad other dyamic parameters were ivestigated for the egativestiffess vibroisolatig system Mius K 500BM-. A 64 kg miimum load ad hoeycomb composite plate were employed. The experimets eabled determiatio of: - Vibratio trasmissio characteristics by applyig impulse excitatio; - Vertical vibratio trasmissio characteristics actig i the excitatio frequecy 2-0 Hz; - Dampig efficiecy for low frequecies. Tests were performed with Daish Bruel & Kjaer dyamic measuremet ad aalysis equipmet: movable measurig equipmet Machie Diagostics Toolbox Type 9727 with computer DELL; Vibrometer 25; Seismic accelerometers 8306 ad 836; Amplifier 2706; Geerator 027; Oscillator 480; Shock hammer 8202; Measuremet sigals were processed i PC with Origi 6 ad Pulse software packages. The measuremet results were aalyzed ad sigal spectra as well as statistical parameters (arithmetic mea, the stadard deviatio ad the stadard deviatio of the mea) were calculated usig the followig formulae: 2 x = x i, S X = ( xi x), i= i= 2 ( x x), S X S = = X ( ) (5) i i= here is the umber of measuremet results, x i measuremet result. Vibratio measuremets (fig. 5) were performed o the egative-stiffess vibroisolatig table surface (9) attached orietatio log (7) with seismic accelerometer 8306 (6). Other seismic accelerometer 838 (2) was mouted o the rigid optical table (4). Negative-stiffess vibroisolatig table was mouted o a rigid optical table. Orietatio log allows fixig or reversig a accelerometer i the right directio. 363

4 Fig. 5. Oscillatio caused by vibrator. Oscillator 480; 2 Amplifier 2706; 3 Geerator 027; 4 Optical table; 5 Mass 64 kg; 6 Seismic accelerometer 8306; 7 Orietatio log; 8 Fix board; 9 Negative-stiffess vibroisolatig table Mius K 500BM- ; 0 Vibrometer 25; Movable measurig equipmet Machie Diagostics Toolbox Type 9727 with computer DELL; 2 Seismic accelerometer 838 Durig research with oscillator exerted harmoic vibratios i 2-0 Hz rage i vertical directio. Research Results Obtaied experimetal results were processed statistically ad are listed i Table. The experimetal results are preseted i Figs Table. Statistical parameters of the vibratio acceleratio measuremet i harmoic oscillatio usig the excitatio sigal Stadard The stadard Miimum Maximum Excitatio deviatio S Accelerometer x, deviatio of value x mi, value x max, frequecy, Hz m/s 2 average S, m/s 2 m/s 2 m/s ,98E-4 3,E-6-0,005 0, , ,E-5-0, , ,0049,7E-5-0,0028 0, ,0268 2,0E-4-0,395 0, , ,64E-5-0,02 0, ,372 0,0029 -,84, ,04,09E-4-0,0372 0, ,65 0,026-4,09 3, , ,54E-5-0,0094 0, ,380 0, ,002 0,799 x 364

5 Fig. 6. Temporal sigal ad spectrum. Vertical excitatio vibratio with 2 Hz frequecy o the optical table Fig. 7. Temporal sigal ad spectrum. Vertical ihibitio vibratio with 2 Hz frequecy o the Mius K 500BM- table Fig. 8. Temporal sigal ad spectrum. Vertical excitatio vibratio with 80 Hz frequecy o the optical table Fig. 9. Temporal sigal ad spectrum. 80 Hz ihibitio vertical vibratio o the Mius K 500BM- table 365

6 Fig. 0. Groud temporal vibratio sigal ad spectrum o the optical table. Seismic accelerometer 838 Fig.. Groud temporal vibratio sigal ad spectrum o the Mius K 500BM- table. Seismic accelerometer 8306 Fig. 2. Trasmissibility curve of the Mius K 500BM- table. Excitatio vibratio with 2 Hz (a) ad 3 Hz (b) frequecies Fig. 3. Trasmissibility curve of the Mius K 500BM- table. Excitatio vibratio with 4 Hz (a) ad 5 Hz (b) frequecies Test results idicate that the best dampig is obtaied at a frequecy rage of Hz. The worst dampig is observed till 0 Hz ad i a rage of Hz. 366

7 Coclusios. A methodology ad equipmet were developed for vibratio table testig. A experimetal research was coducted o a egative-stiffess vibroisolatio table "Mius K 500BM-". The paper preseted the obtaied excitatio ad ihibitio sigals ad their spectrums. 2. Vibratio trasmissibility characteristics were determied for the egative-stiffess vibroisolatig table i the frequecy rage of 2-0 Hz of vertical vibratios. Refereces [] Colla L. E. Piezoelectric techology for active vibratio cotrol, [2] Ogado J. Buildigs really do shake ad sway uder our feet, Desig News, [3] Hosoda M. Selectio of vibratio isolators, [4] Kasparaitis A., Vekteris V., Kilikevičius A. Lie scale comparator carriage vibratios durig dyamic calibratio, Joural of Vibroegieerig, 0(3): ISSN , pp [5] Kausiis S., Kasparaitis A., Barakauskas A., Barauskas R., Jakstas A., Kilikevicius A. Lie scale calibratio i o ideal measuremet situatio, Solid State Pheomea, ISSN , 2009, pp [6] Kilikevičius A. Sabaitis D. Stability of legth measuremet comparator s carriage geerative uatural excitatio, Ultragarsas, (63), ISSN , 2008, pp