Vibration and acoustic analysis of brake pads for quality control

Paolo Castellini, Paolo Chiariotti, Milena Martarelli and Enrico Primo Tomasini Department of Mechanical Engineering, Università Politecnica delle Marche, Via Brecce

Bianche, 60131 Ancona, Italy

ABSTRACT Two procedures have been used for the damage detection of brake pads presenting two kind of defects, i.e. cracks and braking material detachments. Both the procedures are based on the monitoring of the pad structural resonances that changes in presence of defects. The first procedures employs a Laser Doppler Vibrometer (LDV) for the measurement of the vibration velocity of the pad forced into vibration by means of an impact hammer. The second one, instead, uses a system based on four microphone probe developed by SenSound and called SenQC system since is made for quality control. Both the techniques allowed to recognized damaged pads from the good ones from a sample of 37 elements. The sensitivity of the procedure to environment temperature has also been studied, since the structural resonance can be influenced by this parameter. The sensitivity analysis to temperature has been performed by using the LDV technique and by repeating the measurement on four different pads (two good and two damaged). The temperature has been varied from 15 to 30°C with a resolution of 2°C in order to monitor the complete range of the environmental temperature that can be reached in working conditions in the production line.

1. Introduction Quality control of brake pads is an important issue, being the pad a key component of the braking system. Up to now, the research has been addressed to the measurement and modelling of the whole system, consisting of disk, pad and calliper, all the parts, together, concurring to the formation of the annoying acoustic phenomenon known as squeal [1]. Most of the work has been focused on the improvement of the design of the brake system for the suppression or, at least, attenuation of the squeal [2]. The pad, therefore, has been studied only as part of the whole system, since its dynamic behaviour, in terms of natural frequencies that could be coupled with the disk ones and damping, influences the formation of the squeal. Larger is the brake damping higher is the capacity of the pad to damp out the vibration and to reduce the squeal. Several studies have been thus addressed to the definition of methods to accurately measure the pad damping in terms of Q factor or damping ratio. In particular in [3] a contactless excitation and measurement system has been designed, based on electromagnetic non contact shaker, to force into vibration the pad, and a microphone, to measure the pad response. Concerning the testing of the brake pad to perform quality assurance, i.e. to test structural integrity loss (like presence of cracks or detachments of the braking material from the backing steel plate), not many works are present in literature. Some of them [4] employ ultrasonic measurement systems to determine the mechanical properties of the pad, i.e. the in-plane, out-of-plane and shear elastic modulus. This paper shows the applicability of non contact measurement systems as Laser Doppler Vibrometer (LDV) and array of microphones for detecting damages in pads, in particular detachments of the adhesive layer connecting the pad with the backing plate. A statistical significant sample of undamaged and damaged pads has been tested with both methods, based on the pad’s dynamic characteristics variation due to the detachment.

2. Dynamic characterization of brake pads The dynamic behaviour of the pad has been first investigated by measuring on a grid of 63 points over the friction material. The measurement has been performed via a Scanning LDV (SLDV), a contactless sensor based on interferometry technique sensing the surface vibration velocity as a Doppler shift of the laser light incident on

Proceedings of the IMAC-XXVIIIFebruary 1–4, 2010, Jacksonville, Florida USA

©2010 Society for Experimental Mechanics Inc.

the surface itself. The pad has been forced into vibration by means of an impact hammer whose input force has been measured. The Frequency Response Functions (FRF) between the response velocity measured by the SLDV and input force, given by the hammer, have been acquired. An average FRF obtained by summing the FRF acquired all over the measurement points is given in Figure 1, where both the FRF attained for undamaged and damaged pad are reported.

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Figure 1 Averaged FRFs for undamaged and damaged pad

It is clear that the damage induces an evident shift on the first resonant frequency, indicating a stiffness reduction produced by the detachment. In Figure 2 some of the operational deflection shapes measured for the undamaged and damaged pads are given. With the exception of the first mode, the amplitude of the mode shapes decreases when the pad is damaged since the damping increases. Moreover, the damage provokes a phase scatter of the mode shape, thus it increases its complexity.

Undamaged 2769 Hz Damaged 2150 Hz

Amplitude -15÷21 dB (dB ref 1.95 10-3 m/s/N) Amplitude -7÷22 dB (dB ref 1.95 10-3 m/s/N)

Phase -180÷180° Phase -180÷180°

Undamaged 3538 Hz Damaged 2450 Hz

Amplitude -10÷24 dB (dB ref 1.95 10-3 m/s/N) Amplitude -7÷21 dB (dB ref 1.95 10-3 m/s/N)

Phase -180÷180° Phase -180÷180°

Undamaged 5338 Hz Damaged 3544 Hz

Amplitude -12÷19 dB (dB ref 1.95 10-3 m/s/N) Amplitude -19÷15 dB (dB ref 1.95 10-3 m/s/N)

Phase -180÷180° Phase -180÷180°

Undamaged 5894 Hz Damaged 5344 Hz

Amplitude -18÷12 dB (dB ref 1.95 10-3 m/s/N) Amplitude -14÷11 dB (dB ref 1.95 10-3 m/s/N)

Phase -180÷180° Phase -180÷180°

Undamaged 7931 Hz Damaged 6581 Hz

Amplitude -28÷12 dB (dB ref 1.95 10-3 m/s/N) Amplitude -24÷7 dB (dB ref 1.95 10-3 m/s/N)

Phase -180÷180° Phase -180÷180°

Figure 2 Operational deflection shapes for undamaged and damaged pad

3. Quality assessment based on dynamic measurements Being clear that the detachment influences mostly the first mode of the pad, it has been concluded that the parameters, useful for the discrimination between damaged and undamaged pads, are the first resonance

frequency and the related damping. For the quality assessment, thus, the pad has been supported along the two nodal lines of its first natural mode in order to not influence its dynamics, see Figure 3. The measurement set up remained as the one described in Section 3, with the only exception that a single point measurement has been performed.

Figure 3 Experimental set-up for the pads quality control The time histories of the input force and the output vibration velocity have been acquired. The calculated FRFs for two damaged pads and for two undamaged ones are illustrated in Figure 4. The frequency shift due to the detachment is evident, see plots black and grey.

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Pad n.1 (undamaged)Pad n.2 (undamaged)Pad n.1 (damaged)Pad n.2 (damaged)

Figure 4 FRFs measured on two undamaged pads (black) and two damaged ones (gray)

Having measured 18 good pads and 19 damaged, a statistical analysis has been performed in terms of both first natural frequency and related damping. The damping has been measured from the response time histories as the decay ratio (σ) that, for light damped structures, is related to the damping ratio ξ by the natural frequency ωn ( .

From the bar plots shown in Figure 5 an 6, it can be noticed that the first natural frequency is always below 2500 Hz when the pad is damaged and there is a large spread between different pads. In fact, the standard deviation on the damaged pads is 177 Hz while in the good ones is 31 Hz. It is not the same for the damping of the first natural mode which cannot be used to identify the damage. The analysis has been performed also at different temperatures of the pad between 15 and 30°C. Four pads have been measured: N.7 e 17 good and N.11 e 17 damaged. The variation of the first natural frequency with the temperature has been studied. In the bar plots in Figure 7 and 8 the natural frequency is plotted against the pad temperature.

LDV spot hammer

support lines

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Figure 5 First natural frequency measured for 18 good pads and 19 damaged

Figure 6 Damping ratio related to the first natural mode measured for 18 good pads and 19 damaged

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Figure 7 Undamaged pad (N. 7)

Figure 8 Damaged pad (N. 17)

The variation of the temperature within 15-30°C cannot be considered an important interfering input, since the natural frequency remains always above 2500 Hz for the good pad and below 2500 Hz for the damaged one. A marked separation is kept between good and damaged pads also when the temperature oscillates between 15 and 30°C. This is demonstrated by the first natural frequency distribution measured with variable temperature, see Figure 10, even if the probability distributions of both the undamaged and damaged pad are enlarged, see Figure 9.

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undamaged histogramdamaged histogramundamaged distributiondamaged distribution

Figure 9 First natural frequencies histogram and probability distribution measured at constant temperature (white:

damaged pads, red: good pads).

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undamaged histogramdamaged hisogramundamaged distributiondamaged distribution

Figure 10 First natural frequencies histogram and probability distribution measured at temperature varying between 15 and 30°C (white: damaged pads, red: good pads). The distribution shapes are larger but the

separation is still evident.

4. Quality assessment based on acoustic measurements The analysis has been also performed with the SenQC system, made of a four microphone probe to be placed at a short distance from the pad. This system enables the user to clean acquired signals from background noise: this is obviously very usefull when tests are performed in the production line, where high level of background noise can lead to misleading results, causing bad products to pass and good ones to be rejected. As a consequence of the denoising procedeure no acoustic enclosure is needed. SenQC is also able to show SPL of direct sound from the target and represent an all-rounded non-destructive/non-contact defect detection technique. The denoising algorithm needs a train phase with respect to the target and the environment, while the defect identification requires the definition of pass/fail metrics on denoised signals.

Figure 11 Set-up for SenQC test (left) and close-up on probe measuring distance (right)

The product has been tested in real operating environment (Figure 11), i.e. with background noise reproducing the floor noise present in production line. The effect of the decreasing first natural frequency is appreciated also by using the acoustic measurement. Therefore the same kind of analysis can be used with the SenQC system.

Figure 12 Frequency response between the output, i.e. the acoustic response of the pad (combination of the acoustic pressures measured by the microphones) and the input force.

The learning phase of the system has been performed by measuring 3 good pads and 3 damaged ones. By fixing the discrimination region (the so-called metric) in the range between 2500 and 3000 Hz the good pads can be recognized from the damaged. In this range, in fact, the level of the acoustic emission of the good pad is higher than the damaged one. On this basis the green and red regions have been set. If the measured pad will lie in the green region it will pass, if it will lie in the red it will fail.

Good Damaged

Figure 13 Pass/Fail metric definition

The results are given below (Table 1). All the pads have been correctly recognized. An example of SenQC way to show test results is reported in Figure 14.

Brake Pads % Recognized Good 100 Damaged 100

Table 1 Pass/Fail test results with SenQC system

Figure 14 Results table example

5. Conclusions Two measurement techniques for quality control of brake pads have been tested: the first one based on vibration measurement by a non contact LDV and the second one using a 4 microphones probe and a dedicated signal processing developed by SenSound Acoustic Imaging. Both the systems are non contact and require excitation of the pad by means of an impact hammer. The damaged pads are completely recognized from the good ones by both the techniques. The influence on the quality control system, in particular the vibrational one, of the pad and environment temperature has been investigated, it evidencing that the feature characterizing the damage (i.e. the pad first natural frequency) has got a probability distribution distinctly separated from the same feature distribution relative to good pads.

References [1] R. A. Ibrahim, “Friction-induced vibration, chatter, squeal and chaos. Part II: Dynamics and modelling“,

ASME Applied Mechanics Reviews, 47:227-259, 1994. [2] U. von Wagner, D. Hochlenert, T. Jearsiripongkul, P. Hagedorn, “Active control of brake squeal via ‘smart

pads‘“, SAE Technical Paper Series, 2004-01-2773. [3] R.P. Uhlig, ”Brake pad assembly damping and frequency measurement methodology” United States

Patent No. US 6,382,027 B1, 7 May 2002 [4] IMS inc. Internal Report, ”Quality Assurance Measurements on 4 As-manufatured Brake Pads”