Abstract:
A method and apparatus for determining the vibration damping characteristics of an automotive brake structure measures the vibration response of a brake structure during application of random-frequency wide spectrum excitation, followed by measuring vibration of the structure at particularly noted modal frequencies, using a confined bandwidth random-frequency vibratory excitation. The method and apparatus may be used also to assess the bonding or attachment characteristics of damping materials.

Description:
This Application claims the benefit of U.S. Provisional Application No. 60/430,583 filed on Dec. 4, 2002. 

   BACKGROUND OF INVENTION 
   1. Field of the Invention 
   The present invention relates to a method and apparatus for determining the vibration damping characteristics of an automotive component such as a brake structure, e.g. a brake pad or brake shoe. 
   2. Disclosure Information 
   Automotive brake systems present unique challenges to designers inasmuch as such systems must be capable of stopping a vehicle reliably during a long service life. Vibration-induced noise in disc brake pads is an issue which asserted itself since the dawn of the disc-brake age in the annals of mass produced automobiles. 
   The production of audible noise by automotive disc-brake systems is greatly affected by the presence (or absence) of self-damping capability offered by the disc-brake pads themselves. Not surprisingly, it is therefore desirable to produce pads having self-damping characteristics in the vibration regimes which would otherwise produce noise. The assessment of a brake pad&#39;s self-damping characteristics have been the subject of previous inventive activity. As a result, several test methods and types of apparatus have been proposed, such as that shown in U.S. Pat. No. 6,382,027. The method and apparatus of the &#39;027 patent uses an exciter which is pulsed on and off, with the actual vibration damping being measured while the exciter is in the deactivated mode. This type of operation is not suitable for high speed assessment of parts because the vibration assessment must be accomplished serially. In other words, the vibration response is not measured while the exciter is providing vibratory energy to the disc-brake pad. Thus, more time is required to obtain the assessment. Moreover, the capabilities of the method disclosed in the &#39;027 patent are wanting because of limitations inherent in the time decay process. Moreover, other test methods such as the Oberst Bar test are capable of measuring damping only at lower frequencies. 
   It is advantageous that a method and apparatus according to the present invention overcomes problems associated with prior art methods and apparatus for determining the inherent damping characteristics of automotive brake structures, including brake pads, as well as other components. 
   SUMMARY OF INVENTION 
   As used herein, the term “insulator” means the friction material applied to the backing plate of either a disc brake pad or a drum brake shoe. According to the present invention, a method for determining the vibration damping characteristics of an automotive brake structure includes the steps of resiliently setting or mounting the brake structure to a stationary base, applying broadband random-frequency vibratory excitation to the brake structure, measuring the vibration response of the brake structure during application of the random-frequency excitation, including responsive vibration occurring at not less than one modal frequency, and applying a confined bandwidth random-frequency vibrator excitation to the brake structure, with the confined bandwidth being selected to correspond to at least one modal frequency. The present method further includes the steps of measuring the vibration response of the brake structure during application of the confined bandwidth signal, and using the measured vibration response of the brake structure to the confined bandwidth signal to calculate the damping value of the brake structure. 
   A method according to the present invention preferably utilizes broadband random-frequency excitation in the frequency range from 10 Hz to about 15 kHz. The brake structure is preferably excited by a variable reluctance actuator, which is a non-contacting actuator. The vibration response of the brake structure is preferably measured by a non-contacting sensor system such as a laser velocimeter. The confined bandwidth excitation preferably has a bandwidth of about 200 Hz to about 400 Hz, and a center frequency equal to one of the observed modal frequencies develop during broadband sweep. 
   According to another aspect of the present invention, a system for determining the vibration damping characteristics of an automotive component such as a brake structure includes a broadband, random-frequency vibratory exciter for inducing vibration in the component, and a non-contacting vibration measuring device for determining the frequency response of the component during the vibratory excitation, with the frequency response including the identification of a plurality of modal frequencies. A confined bandwidth random-frequency vibratory exciter then induces vibration in the test component at frequencies corresponding to the plurality of modal frequencies. Then, a processor determines the vibration damping characteristic of the component as the ratio of the difference in the frequency of the vibrations at a 3 dB vibration magnitude bandwidth, as a fraction of the frequency of vibration at the center of the confined bandwidth. 
   The present test apparatus may further include an environmental chamber for housing the component being tested, with the chamber having a ports for accommodating the vibratory exciter and the vibration measuring device. 
   The present vibration testing method and apparatus is advantageous because the time required to perform an assessment of the vibration is sufficiently brief that the present system may be used to assess high-volume production parts. 
   It is a further advantage of the present invention that the present method and system may be employed to nondestructively determine whether a disc brake pad insulator or shim friction material is properly bonded, or otherwise attached to a brake backing plate. This is important because the bonding characteristics of the pad insulator are an important determinator of brake pad vibration damping. 
   Other advantages, as well as objects and features of the present invention, will become apparent to the reader of this specification. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  is a systematic representation of a vibration damping test system according to the present invention. 
       FIG. 2  is a frequency vibration plot developed during a broadband sweep of an automotive component according to the present invention. 
       FIG. 3  is a frequency vibration plot produce by a confined bandwidth random-frequency vibratory excitation according to an aspect of the present invention. 
       FIG. 4  illustrates a brake pad having modal vibration pattern indicating a consistent bonding pattern between the pad insulator and the brake backing plate. 
       FIG. 5  illustrates a brake pad having an irregular distribution of bonding between the pad insulator and the backing plate. 
   

   DETAILED DESCRIPTION 
   As shown in  FIG. 1 , brake pad  20  is placed on stand  24  within environmental chamber  22 , which may be used to provide heat for conducting a vibration assessment according to the present invention, at a variety of temperatures. Those skilled in the art will appreciate in view of this disclosure that the present method and system could be used for the purpose of determining vibration damping and component material bonding for parts other than automotive brake parts. As used herein, the term “mounting” means placing the part to be tested upon supports located within chamber  22  so that the part merely rests on the supports, or alternatively, attaching the part to the supports. 
   Brake pad  20  is placed upon stand  24 , comprising silicone rubber supports, within environmental chamber  22 . Exciter  36  passes into the interior of environmental chamber  22  through port  32 . Exciter  36  is preferably a non-contact magnetic actuator which generates magnetic force. This type of device, sometimes termed a variable reluctance actuator, is sold by Electro Corporation under the model number 3030HTB. Exciter  36  is driven by high output amplifier  30 , which may comprise a single-channel high output amplifier such as an AVC 790A01 model power amplifier. The amplifier&#39;s controls are operated by controller  28  drawn from a universe of such controllers known to those skilled in the art of vibration testing and suggested by this disclosure. 
   Vibrations of pad  20  are sensed by means of laser doppler velocimeter  26 , which preferably comprises a laser velocimeter such as a Polytech PDV-100 or Polytech PSV-400 having a Class 2 visible helium/neon laser. Controller  28  includes a two-channel signal analyzer drawn from the class of such analyzers known to those skilled in the art and suggested by this disclosure, such as a Hewlett Packard model 35670A. 
   According to the present invention, the experimental method begins as shown in  FIG. 2  with a broadband excitation of pad  20  at frequencies of up to, or even exceeding, 15 kHz. The resulting vibration of pad  20  is measured by laser velocimeter  26 , and a series of modal frequencies is developed, as shown in the FIG.  2 . After the broadband sweep has been performed as shown in  FIG. 2 , controller  28  performs the assessment shown in  FIG. 3 , wherein a confined bandwidth random-frequency vibratory excitation is applied to pad  20 . This excitation is selected to correspond to at least one of the previously developed modal frequencies. The narrow band random excitation signal preferably has a bandwidth of 200 Hz to 400 Hz, with the center frequency being equal to one of the modal frequencies. Then, the modal damping value is calculated using the half powered principle according to the following equation: 
       η   =         f       +   3     ⁢   dB       -     f       -   3     ⁢   dB           f   peak           
 
   Where η is damping loss factor, f+3 dB is the frequency of vibration at 3 dB less than the peak magnitude of vibration at the right side of the power spectra plot, and f−3 dB is 3 dB down from the peak magnitude of vibration on the left side of the power spectra plot. This is termed a “half power” calculation because 3 dB attenuation corresponds to a halving of the vibration power. As seen from the formula, the modal damping value increases as the slopes to and from the peak vibration value become increasingly smaller. In essence, controller  28  determines, for at least one of the modal frequencies, the vibration damping characteristic of a component as the ratio of the difference in the frequencies of the vibrations, at a predetermined off-peak magnitude (in the above example, at a vibration magnitude which is 3 dB less than the peak magnitude), to the value of the modal frequency. 
   As noted above, the present system may be used for assessing the structural integrity of a brake structure by comparing a pattern of modal lines developed at a fixed frequency with a predetermined pattern of such modal lines.  FIG. 4  shows a well-developed pattern of modal lines, M, detected by sweeping velocimeter  26  over the surface of pad  20  while exciter  36  providing constant frequency vibratory excitement to pad  20 . The pad shown in  FIG. 4  has proper bonding between the pad&#39;s insulator and backing plate. In contrast with the situation of  FIG. 4 , the pad illustrated at  FIG. 5  does not have proper bonding between the insulator and the backing plate, as shown by the absence of any coherent modal lines at the center region of the pad. The present invention allows the testing of pad in a non-destructive fashion, which can be used to more easily assess insulator bonding processes and materials. 
   Although the present invention has been described in connection with particular embodiments thereof, it is to be understood that various modifications, alterations, and adaptations may be made by those skilled in the art without departing from the spirit and scope of the invention set forth in the following claims.