A DESIGN ASSESSMENT TOOL FOR SQUEAK AND RATTLE PERFORMANCE

Transcription

1 A DESIGN ASSESSMENT TOOL FOR SQUEAK AND RATTLE PERFORMANCE David E. Soine, Harold A. Evensen, and Charles D. Van Karsen Center for Applied Researh in Dynai Systes Mehanial Engineering-Engineering Mehanis Departent Mihigan Tehnologial University 14 Townsend Drive Houghton, MI ABSTRACT An experiental tehnique for evaluation and validation of global syste design for Squeak and Rattle perforane is introdued. Based upon siple slip and rattle odels, the tehnique utilizes analysis tools adapted fro Chaos Theory to establish threshold levels of otion whih ontribute to Squeak and Rattle. The tehnique is deonstrated with a siple analytial odel, then with a siple struture under dynai load. The goal of this work is to develop a ethodology for benharking and experiental produt developent as well as oputer aided design and analysis. NOMENCLATURE ass k spring stiffness daping p spring preload X, Z displaeents (I) frequeny INTRODUCTION Investigation of Squeak and Rattle is the study of unexpeted events, usually noises of a short tie duration, that are pereived by a listener to stand out fro bakground (expeted) noise. In the autootive industry, Squeak and Rattle perforane is known to affet ustoer pereived quality [!] and, based on the authors' experiene, has a signifiant ipat on warranty osts. A squeak ours when the relative otion of vehile oponents (attahed or otherwise ontating eah other) overoes soe ontat frition fores between the oponents and generates a pereptible noise. Fators affeting squeak noise inlude aterial obination, frition oeffiient, preloads and stiffness of the oponents, as well as operating environent (huidity, teperature.) Rattle ours when vehile oponents (not neessarily attahed or in ontat) ipat eah other and generate a pereptible noise. Fators affeting rattle inlude ipat speed, relative otion between oponents, preloads, properties of the ontat surfaes, et. [I,2] At the vehile level, Squeak and Rattle is onsidered a sound quality issue. At least one sound quality etri has been proposed that relates sound easureents to subjetive vehile assessents for Squeak and Rattle. [2] Other work has onentrated on loating Squeak and Rattle noise soures using sound easureents, irophone arrays, et. [3] At the oponent level, squeak and rattle analysis fouses on reduing the relative otions of oponents and finding aterials that exhibit better frition and surfae harateristis. One approah that has been applied with soe suess is to assue a speifi rattle odel (say, the part will rattle at a ertain level of dynai otion) for soe oponent like a glove opartent door. One the level has been established, a linear syste odel an be solved and soe input applied, with rattle indiated when the relative otion of the oponents of interest reahes the established level. This approah is interesting beause it is one of the few that addresses the proble of dynai otion leading to Squeak and Rattle at the design level. [4,5] EVALUATION TOOLS The goal of this work is to develop testing tehniques that evaluate Squeak and Rattle perforane with an ephasis on inforation that an lead to iproved syste design. In this paper, only "rattle" perforane will be addressed, however the results and diffiulties enountered will extend well to "squeak" perforane. In general, squeak or rattle noise in an autoobile does not our until the vehile is oving. Therefore, if soe squeak or rattle noise ours, it akes sense to onlude that there is soe threshold level of otion that has been exeeded to ause the noise. The otion ould be due to engine vibration, road input through the suspension, operator input (turning the steering wheel or pressing the brake pedal), or soe other internal or external exitation. The otion ay or ay not be affeted by dynai odes of the vehile. Establishing a threshold level of otion is the key to aking a Squeak and Rattle evaluation tool useful for design, sine it really is a easure of how sensitive squeak and rattle ehaniss are to syste otion. Threshold level data an therefore be used as a easure of perforane for oparing or benharking different designs and is useful in validation of oputer-generated odels. Figure I depits a siple rattle odel, in whih a spring holds a ass() against a oving frae. The ass is held in plae against the frae by preloading the spring. A rattle event is assued to our when the ass separates fro the oving frae. Sine the 1428

2 spring is attahed to the frae, this rattle odel will be alled the "oupled" rattle odel. It is siple to show that the ass will separate fro the frae when the aeleration of the frae exeeds the spring preload (P) divided by the ass: p zthreshold = - This level of aeleration ould be used as the threshold level of otion for this odel and would be the sae regardless of the nature, whether deterinisti or stohasti, of frae exitation. Although this rattle odel is siple to analyze and aesthetially pleasing, it is not the only plausible odel for a rattle syste. ~ I z(t) )> ;,, Figure 1. Coupled rattle odel. ~ 1--l-z(r_)~)> Figure 2. Unoupled rattle odel. Figure 2 depits what we will all an "unoupled" rattle odel. Instead of being attahed to the oving frae, the spring is attahed to ground. If daping is negleted, a siple analysis shows that for separation the threshold level of aeleration varies linearly with position of the frae aording to: P-kx Z threshold = The onept of a threshold level of otion for rattle is now ore opliated than it was for the ase of the oupled rattle odel. The threshold level of otion for the unoupled rattle odel is a funtion of position and aeleration. To illustrate this, onsider the ase of pure sinusoidal otion. We an show that the threshold level of aeleration is now a funtion of frequeny and, interestingly, that the level approahes infinity for exitation near the natural frequeny of the ass-spring syste: p Zrhreshold = ---k -- (.2 We an generate other vanatwns on these rattle odels. For exaple, instead of a spring attahed to ground, the spring ould be attahed to soe other oving oponent, resulting in a threshold aeleration for rattle dependent on displaeent of both the oving frae and the oponent to whih the spring attahed. If daping is inluded in an unoupled odel, the threshold aeleration will also be dependent on veloity. The signifiane of the differene between odels is apparent one we attept to develop a tool to easure threshold levels of otion in real world systes. Consider the rattle ehanis to have at least two "states," a nonrattling state and a rattling state. A phase spae plot is a graphial tool for visualizing the "state" of a syste. Often used in haos analysis, instead of plotting a syste variable versus tie, the value of the syste variable is plotted versus the value of soe other syste variable at the sae instant in tie. As tie progresses, the suessive points for a trajetory whih wiii hange if the syste hanges state (provided the variables have been hosen properly.)[6] For the previously desribed rattle odel, we ould use the displaeents x and z as our state variables. The non-rattle state trajetory would be a line biseting the syste variable axes (x = z). The rattle state trajetory would be any trajetory that ontains points where x :t z. For this study, instead of x and z, we desire a sipler "variable" that represents separation- say, takes a value of when x = z and I when x :t z. Sine the fous of this study is on threshold levels of otion, the "variable" used is siplified further to give a value of 1 at the instant when the syste transitions fro x = z to x :t z, and zero otherwise. We get an "ipulse" at the instant when the "ass" separates fro the "frae" in our siple rattle ehanis. This variable is naed "onset of separation." "Onset of separation" is then plotted versus a variable representing syste otion; i.e. aeleration. If the onset of the separation event an be identified and plaed in tie, a siple phase spae plot of onset of separation versus otion wiii show the threshold level(s) of otion for arbitrary exitation of the syste. In the ase of a oupled rattle syste, a two-diensional phase spae plot is suffiient (onset of separation versus aeleration), sine any aeleration above the threshold level wiii produe separation. For other rattle odels, it ay be neessary to use phase spae plots of higher diension to fully haraterize the threshold levels of otion. In general, the atual onset of the separation event leading to rattle noise annot be diretly deterined in the field; on the other hand, sound easureents an be easily ade. Therefore it is worth investigating whether sound data ould be used to infer the onset of rattle, possibly in onjuntion with soe otion data. This work begins to explore that possibility. EXPERIMENTAL RESULTS An experient was onduted to deterine whether the separation harateristis of two siple rattle ehaniss based on the oupled and unoupled odels would obey the odeled results. In addition, the relationship between rattle noise and separation harateristis was investigated. 1429

3 "oupled" rattle ehanis - ~';~~;;~~ and foa "spring". ~ /aeleroeter ~ :.:--,_ 1 l t antilevered bea shaker.} +z L ~rophone ~~~{~=====... ~'-1 L rattle ehanis- sall ass and spring attahed to ground Figure 3. Experiental setup sheati. The experiental setup, see Figures 3 and 12, onsisted of a antilevered bea (the "frae") driven at the free end by a sall shaker. A sall ass and grounded spring were used as the rattle ehanis to siulate the "unoupled" ase; a sall ass and an attahed foa isolator were used to siulate the "oupled" ase. The rattle ehanis in eah ase was loated near the idpoint of the bea, and aeleration was easured with an aeleroeter as near to the rattle loation as possible. To detet separation, wire leads were attahed to the bea and rattle ehanis, and the rattle ehanis ated as an on-off swith. 1.5 volts was supplied to the iruit with a dry-ell battery, an(,! the voltage (on-oft) onitored by the data aquisition syste. Upon separation, the voltage aross the "swith" would suddenly inrease fro near zero to approxiately 1.5 volts. Sound pressure near the rattle ehanis was also reorded through a irophone. All data (aeleration, "separation" voltage, and sound) were reorded siultaneously. ;~(a) 1 3 5L..,,.L... J ::~:(b).1 g(o).2l. ;.,...,, I 'fjj~ Figure 4. Typial raw data. (a) aeleration, (b) sound, () separation generated fro the voltage signal, with short pulses plaed where onset of separation ourred in tie. Sine this was a laboratory setup with low extraneous noise, the onset of rattle (ipat) noise ould be identified in a anner siilar to the onset of separation. As an aid to deterining the onset of rattle noise, the sound data was high-pass filtered with a utoff of 2 Hz and the envelope of the filtered signal was plotted on a logarithi sale. Figure 5 shows typial tie traes generated as desribed above. Rattle events are learly identified when the sound envelope trae is plotted on a log sale; a trae with pulses representing the onset of rattle noise an be generated. This "onset of rattle" trae was onfired by oparing it to the audible rattles in the original sound. Eah ase was exited with rando exitation liited to less than I Hz, with exitation level set anually to insure that rattle soeties ourred. Twenty to thirty seonds of tie doain data was aquired, Figure 4 shows a typial portion of data. Aeleration data ontains large low-frequeny oponents, although ipat events are learly evident. The sound data is siilar in nature to the aeleration data, and the separation voltage data shows lear areas of high and low voltage, indiating separation. After aquisition, aeleration data was digitally low-pass filtered at 18 Hz. The onset of separation of the rattle ehanis fro the frae was taken to our at the leading edge of the voltage signal as it rose fro low to high voltage, voltage "spikes" that did not reah full voltage were disregarded. A separation signal was Figure 5. (a) filtered aeleration, (b) onset of separation, () envelope of filtered sound, (d) onset of rattle noise 143

4 ~ Ill.. (f) z (; IJL...I.I.---IU. - ~ Ill.. (f) z Figure 6. Phase spae plot, onset of separation vs. aeleration, oupled ase -.1-4~~-3~---~2~---~1~~~--1~~~2~~3~--~4 Figure 7. Phase spae plot, onset of separation vs. aeleration, unoupled ase This data analysis syste allowed the operator to listen to the sound signal and wath the reorded and generated signals siultaneously, and it was noted that separation did not always lead to audible rattle noise. Rattle noise was not pereived until separation was sustained for a ertain aount of tie (approxiately 5 se for the oupled ase.) A phase spae plot of onset of separation versus aeleration was generated to help visualize the aeleration level at whih the syste hanges fro a non-rattle state to a rattle state. For the oupled rattle ehanis, a lear indiation of a threshold level of aeleration is expeted. This is indeed indiated by the phase spae plot, Figure 6, with separation ourring above approxiately 14 to 18 rn!s 2. For the unoupled ehanis the phase spae plot, Figure 7, is not so learly defined, with separation distributed over a range of aeleration levels fro approxiately -5 to -25 /s 2 This akes sense, sine the threshold level of otion (aeleration) is dependent on displaeent, whih is not represented on this two-diensional phase spae plot. On the other hand, the range of aeleration ould be desribed statistially. In fat, this data an iediately be plaed in a histogra forat, Figures I and 11, whih will lead to an estiate of the probability of the aeleration being in a a Figure 8. Onset of rattle noise vs. aeleration, oupled ase 1.1 r---r ,----.,.---,r , z -.1 -~~~---:::!:---:lo~---:~---:±--±--~:-----: Figure 9. Onset of rattle noise vs. aeleration, unoupled ase ertain range, given that a separation event is beginning. These histogras learly point out the differene in harater of the two systes: the onset of separation is narrowly distributed about approxiately 2 /s 2 for the oupled ase, and is distributed in a wider, ore Gaussian fashion, about -15 rn!s 2 for the unoupled ase. Figures 8 and 9 depit the phase spae plots of onset of rattle noise versus aeleration for the oupled and unoupled ases, and the orresponding histogras are inluded in Figures I and II. No threshold level of otion is evident, sine the onset of rattle noise alone does not aurately reflet the onset of separation. For both ases, the onset of rattle noise sees to be distributed in a Gaussian fashion about the level of zero aeleration. Another interesting feature evident fro the histogras is that the levels for onset of noise and onset of separation appear to be nearly "exlusive" for the oupled ase, and less "exlusive" for the unoupled ase. Put siply, the onset of a rattle event sees to our at a lower aeleration level than the onset of separation. This ay be useful in inferring the onset of separation leading to rattle fro the sound and aeleration data. If the threshold levels for sound and separation are "exlusive" (with levels for sound being less than the levels for aeleration) then the ipat of the separated parts ust our after soe peak in the aeleration wavefor. Therefore an estiate of threshold level of aeleration 1431

5 6, !'liiOnset of Separationl lEt onset of Noise ~*---~ ~* ~~Onset of Separation t onset of Noise ~~--~ Aeleration level (/sl} I u I Aeleration level {/s 2 ) Figure I. Aeleration at onset of event, oupled ase Figure I I. Aeleration at onset of event, unoupled ase ould be ade by deterining the aeleration peak that ours just prior to the ipat. FUTURE WORK Sound data ontains the inforation neessary to identify squeak and rattle events, even though soe delay an be expeted between the sound and the onset of the event that aused it. If the onset of rattle noise an be aurately plaed in tie then an estiate of the threshold level ay be possible by deterining the previously ourring peak aeleration in the tie doain. Work to relate squeak and rattle sounds to threshold levels of otion is ongoing. REFERENCES [I] Her, J. Y., Hsieh, S., Li, W., Haddow, A., "Quantitative Predition of Rattle in Ipating Syste," SAE paper 97259, Proeedings of the 1997 Noise and Vibration Conferene, pp. I 59-I 5 I 5, I 997. [2] Weish, G., Stiiklshwaiger, W., de Mendona, A., Monteiro, N. T. S., dos Santos, L., "The Creation of a Car Interior Noise Quality Index for the Evaluation of Rattle Phenoena," SAE paper 97218, Proeedings of the 1997 Noise and Vibration Conferene, pp. I , [3] Dubaher, S. M., Brown, D. L., "Evaluation of Noise Soure ID Using Analytial FRFs," Proeedings of the 16th International Modal Analysis Conferene, pp. I 292- I 32, [4] Hsieh, S., Borowski, V. J., Her, J. Y., Shaw, S. W., "A CAE Methodology for Reduing Rattle in Strutural Coponents," SAE paper 97257, Proeedings of the I 997 Noise and Vibration Conferene, pp. I 497- I 52, [5] Yang, B. X., and Rediers, B. E., "Experiental Investigation of a Frition-Indued Squeak Between Head Restraint Fabri and Glass," SAE Paper 95 I 269, Proeedings of the I 995 Noise and Vibration Conferene, pp. 28 I -286, I 995. [6] Willias, Garnett P., Chaos Theory Taed, Joseph Henry Press, Washington D. C., pp , Figure 12. (a) Experiental setup, (b) oupled rattle ehanis, () unoupled rattle ehanis 1432