Vibration Reduction of Audio Visual Device Mounted on Automobile due to Gap Vibration

Transcription

1 Vibration Reduction of Audio Visua Device Mounted on Automobie due to Gap Vibration Nobuyuki OKUBO, Shinji KANADA, Takeshi TOI CAMAL, Department of Precision Mechanics, Chuo University Kasuga, Bunkyo-ku, Tokyo, , Japan Abstract A MD device, as AV device mounted on automobie, causes reading error because the optica pickup misses the data on MD due to externa body vibration. This paper describes that the cause of reading error depends on the gap vibration at high frequency range in spite of sinusoida input at ow frequency using actua MD device. Next a new method is proposed to identify the ocation of objective gap among possibe gaps by good use of rea time animation in time domain based on responses with and without gap vibration. Aso a simpe pate is used to verify the effectiveness of proposed method. Then new design method is proposed to prevent the vibration at optica pickup of MD device from the gap vibration at identified ocation. First the gap itsef is changed smaer and thus an great improvement of reading error can be achieved. On the one hand, mode shapes at high frequency range to contribute the gap vibration are identified and sensitivity anaysis in terms of vibration ampitude is appied to point out the sensitive part for mass structura modification, which can improve the gap vibration and furthermore the reading error. A simpe pate mode is aso used to verify the proposed design improvement against gap vibration and finay the reading error of the MD device is actuay improved. 1. Introduction Nowadays improvements are made to stand against vibration in audio visua device such as MD mounted on automobie, which is required very precise motion under severe externa body vibration on the road. However due to gaps between components the noninear vibration is generated not ony at input frequency but aso at different and higher frequencies, so caed gap vibration. This causes reading error since the optica pickup misses the data on MD[1-3]. In this paper, an actua MD device and a simpified mode are used to investigate the reation between reading

2 error and gap vibration and furthermore the generation mechanism of gap vibration. Then a new method is proposed to identify the ocation of objective gap among possibe gaps. Finay based on sensitivity anaysis with respect to vibration ampitude an effective method to improve the gap vibration and reading error is described[4]. 2. Cause of Reading Error The actua MD device shown in Fig.1, which remains the chassis part as main functiona one and removes casing, etc., is under test. The chassis part consists of the side base component with optica pickup and the ever component with driving motor, which is supported by chassis instaation jig through four dampers at the corners, that is, under simiar supporting condition for vibration isoation. The jig is excited by an exciter at the center with impedance head and the acceeration responses at 78 points are measured with non contact Laser Dopper Vibrometer. First the dynamic characteristics of reading error and gap vibration in function of frequency are evauated. Fig.2 shows the reading error chart, in which pure sinusoida input is appied at each frequency and threshod input acceeration at impedance head is recorded when the reading error occurs by graduay increasing input eve. The reading error is defined as sudden oss of memorized data in data storage for pre-recording system. Two vaeys at specific frequencies are observed where the reading error is apt to occur. Aso the position of track of optica pickup, inside or outside is not infuenced to the reading error. At the same time, the threshod input acceeration eve at evauation point No.78 in Fig.1 cose to the optica pickup is recorded when it exceeds a certain eve at higher frequencies during the pure sinusoida input eve is graduay increased. As seen in Fig.3, so caed gap vibration error chart, the same tendency is observed and it is concuded that the cause of reading error is due to gap vibration at higher frequencies. In fact, the acceeration response at point No.78 in time domain is shown in Fig.4 when the reading error is occurred under pure sinusoida 90 Hz input. If the responses stay within the servo contro standard vaue range, which is the coincident frequency component of the response, namey 90 Hz, the reading error is not occurred. But as seen in the figure, sudden peaks are observed and repeated periodicay, which contain higher frequency components. This is the instant when a coision is taken pace at somewhere in the gap and consequenty the optica pickup misses the track. Therefore this peak shoud be improved. 3. Gap Vibration in Simpified Mode Before discussing about actua appication to compicated MD, a simpe measurement mode is used as shown in Fig.5, which consists of an auminum pate with gap mode. As shown in the figure, a gear ocated at point No.27 has a gap, X which behaves the coision. Point No.9 is excited by an eectromagnetic exciter and the acceeration at point No.29 is taken into consideration. By suppying pure sinusoida input and increasing its eve graduay, the gap vibration error chart is obtained as shown in Fig.6, in which at 40, 160, 180 and 220 Hz the gap vibration is apt to occur. In Fig.7 the frequency

3 response at point No.29 is shown when the gap vibration is occurred. In addition to input frequency component at 160 Hz, higher frequency components at specific frequencies are observed. And these specific frequencies are the same when input eve and frequency are changed. Parae to that, the moda testing is carried out to measure FRFs and the FRF between point No.27 at the gap and No.29 at evauation point is shown in Fig.8. At ow frequency range, natura frequencies at 40, 160, 179 and 222 Hz are found which coincide with those observed in the gap vibration error chart. This means that the gap vibration error is apt to occur at natura frequency. Besides higher frequency components of gap vibration match the natura frequencies at high frequency range, which indicates that the coision occurred at the gap generates an impusive secondary force with broad band frequency content. 4. Identification of Gap Location There are many possibe gaps existed in a MD device so that it is important to identify the ocation of objective gap which exhibits the coision for higher frequency responses. It is difficut to identify just by watching the mode shape animation or rea time animation because of mixture of mode shape and gap vibration. To separate the gap vibration ony, the foowing method is proposed, A g A R F n n A R / F (1) First the responses in inear range, A are measured when the sinusoida input eve, F is ow enough not to occur the gap vibration. Then when the input eve becomes higher, to occur the gap vibration, the responses, A are measured. Lasty the pure gap vibration, A g is obtained based on equation (1) using the ratio of input n eve, R. This procedure is processed to a measurement points and then the rea time animation of A g is watched to identify the ocation of objective gap. In Fig.9, the rea time animation when input frequency is at 160 Hz is shown. The input eves are 0.03 N for inear range and 0.6 N for gap vibration, that is the input ratio is equa 20. It is very obvious that the gap vibration is ony observed at point No.27, which is correct identification of the ocation of gap. Aso at other frequencies, or muti gaps, this method works we. F n 5. Structura Modification Based on Sensitivity Anaysis for Gap Vibration At each specific frequency, the sensitivity anaysis with respect to vibration ampitude is appied to improve the gap vibration error. In case of the above simpified pate mode, three sets of moda parameters at 1171, 2177 and 2302 Hz are used to suppress the evauation response at point No.29. In Fig.10, sensitivity maps are shown where the mass sensitivities are indicated, that is, when unit mass is added

4 at each point, how the evauation response at point No.29 is varied is interpreted. The darker the circe becomes, the more the response is decreased. Taking into account three sensitivity maps, point No.26 marked in square is found as appropriate point to reduce the response at point No.29. Based on this prediction, a umped mass, 0.02 Kg is added at point No.26. Fig.11 shows the comparison of frequency response at evauation point No.29 before in (a) and after the mass addition in (b). Higher frequency components are significanty suppressed by the mass addition according to the prediction. After the mass addition, the response becomes coser to pure sinusoida signa at 160 Hz input frequency, which is very easy to contro without any sudden peak. In deed as shown in Fig.12, the gap vibration error chart after the umped mass addition in dark bar is compared with that before in bank bar. At each frequency effective improvement is achieved. 6. Appication to actua MD The proposed method is now appied to the actua MD shown in Fig.1. First the sudden peaks of response in time domain in Fig.4 shoud be improved to identify which gap infuences. Equation (1) is used to extract ony the gap vibration based on the response at 90 Hz with 0.05 G input eve and that at 90 Hz with 0.5 G input eve. Fig.13 shows the time response of ony gap vibration, which suggests the gap vibration at time instant (a) and (b) shoud be focused. Fig.14 shows the rea time animation at time instant (a), where side base at point No.76 comes into the first coision, and then that at time instant (b), where ever at point No.23 comes into the secondary coision after the gap vibration is transmitted through driving gears of side base. Therefore it is suggested to suppress the first coision at side base for improvement of gap vibration error and further reading error. Then the moda testing is carried out to measure the mode shapes at higher frequency where the resonance at FRF between the evauation point, namey optica pickup and the gap identified is dominant. As a resut, three mode shapes, 395, 439 and 672 Hz are extracted and used to provide sensitivity maps of mass addition with respect to the response at side base point No.78 as shown in Fig.15. Consequenty point No.4 is found appropriate to suppress the response at side base. Finay 0.01 Kg mass is added at point No.4 and again the reading error chart is examined as shown in Fig.16. Compared with that before the mass addition in diamond mark, the reading error after the mass addition in back circe is improved at each frequency as predicted by proposed method. 7. Concusions (1) The cause of reading error is due to the gap vibration at higher frequencies occurred at resonance dominant at FRF between the evauation point and the objective gap ocation. (2) New rea time animation method of gap vibration ony is proposed to identify the objective gap ocation and verified by simpe mode.

5 (3) Based on sensitivity anaysis with respect to vibration ampitude at evauation point, the gap vibration error can be suppressed and furthermore the reading error cab be improved. References [1] Shintani,eta,Trans.of JSME,C,64-619,pp ,1998 [2] Kou,eta,Trans.of JSME,C,65-631,pp ,1999 [3] Yueqiang,Proc.of 12thIMAC,pp ,1994 [4] Kanada,eta,Trans.of JSME,C,68-676,pp ,20 Fig. 1 Actua structure measurement setup Fig. 2 Reading error chart Fig. 3 Gap vibration error chart Fig. 4 Time response(input:sine90hz 0.5G) Measurement point:a 35 point Fig. 5 Simpe measurement mode with gap mode Fig. 6 Gap vibration error chart

6 Fig. 7 Frequency response(input:sine160hz 0.6N) (a) Before umped mass Fig. 8 FRF of auminum pate (Response No29 / Input No27) (b) After umped mass (0.02[kg]) Fig. 11 Comparison of frequency response by umped mass (Input:SINE160Hz 0.6N ) Fig. 9 Rea time animation of ony gap vibration (Gap:No.27, input frequency:160hz) r m j Fig. 12 Comparison of gap vibration error chart by umped mass (0.02 [kg] at No.26) (a) 1171Hz (b) 2177Hz (c) 2302Hz Fig. 10 Sensitivity anaysis for mode shape of No.29 Fig. 13 Time response of ony gap vibration (Input:SINE90Hz 0.5G)

7 (a) [s] (b) [s] Fig. 14 Rea time animation of ony gap vibration (Input:SINE90Hz 0.5G) (a) 395Hz (b) 439Hz (c) 672Hz r m Fig. 15 Sensitivity anaysis for mode shape of No.78 Fig. 16 Comparison of reading error chart by umped mass (0.01[kg] at No.4) j