Innovative Prediction Technique Tackles Brake Squeal at General Motors

Innovative Prediction Technique Tackles Brake Squeal at General Motors

There is nothing more irritating than crossing a crowded shopping street or pulling onto your driveway with heavily squealing brakes. The loud monotonous sound generated by brake systems is unanimously perceived negatively and technically very difficult to overcome; making brake squeal a persistent quality issue for automotive OEMs and brake system suppliers. General Motors Corporation (GM), together with LMS, developed a new process that effectively combines advanced testing with systematically correlated static and dynamic simulations in order to identify and help fix brake squeal early in the design process. The world’s largest vehicle manufacturer validated this method by characterizing a 13 kHz intermittent squeal and has since used it in several development programs. GM engineer Mark Riefe commented, “The ability to measure and model brake squeal dynamics provides a dramatic step forward in our noise and vibration reduction efforts, generating substantial savings in engineering cost and development lead time.”

Brake squeal, noise generated by brake systems under operation, is an extremely complex phenomenon and therefore difficult to model. The characteristics of the different brake system components, the way these parts are put together, the grease that is used to lubricate parts and connections, the temperature at the brake pads, and the friction between brake pads and rotors can all affect the tendency of the brake to squeal. In addition, the resonances of the individual components interact heavily with each other, making the phenomenon even more difficult to control. Brake squeal has traditionally been addressed with an iterative design-build-test process because regular Finite-Element (FE) analyses are not accurate enough. With this time-consuming approach, problems are extremely expensive to fix since they are usually not identified until relatively late in the design process.

GM engineers Mark Riefe and Tinghui Steven Shi, together with Steven Dom, LMS consulting engineer, successfully developed a systematic and rigorous correlation and updating process to reproduce and predict high-frequency brake squeal dynamics. The process begins with the creation of an initial FE model of the complete brake corner, which is used to determine an optimal set of measurement locations on the brake corner assembly. Experimental modal analyses performed on the main individual components of the brake system are used to update their respective FE component models, by means of manual tuning and automatic optimization routines. Then, a similar sequence of modal testing and model updating is executed on the entire brake assembly. As a final step, an Operating Deflection Shape (ODS) of the brake assembly is measured while the brake is squealing. This ODS is compared to the stability results from a complex eigenvalue FE analysis, and once these results match up, engineers can be virtually certain that the unstable modes gained from the analysis depict the same squeal mechanisms as those indicated on the ODS.

Validating the new method

A brake corner exhibiting a 13 kHz squeal was used to validate this new prediction methodology. In this particular case, a short squeal occurred once to twice per revolution of the brake rotor.

The first step consisted of creating the FE model, selecting measurement locations, and generating the test geometry using LMS Gateway. The modes of the original FE model were used to determine whether the mesh was sufficiently detailed to support accurate modal simulations and helped determine the appropriate number of degrees of freedom for the test geometry and their spatial distribution. The software also generated Modal Assurance Criteria (MAC), a measure of resemblance or co-linearity, for each pair of modes. The goal of selecting the correct measurement points was to minimize the value of the off-diagonal MAC terms in the resulting matrix. Achieving this assured that the data acquired from the selected measurement points would actually provide complete and accurate modal results. For each cheek of the brake rotor, two concentric circles with 36 measurement points were selected. Other measurement points were located on the brake pads, caliper, brackets and axle.

For convenience reasons, the ODS measurement was performed next, instead of at the end of the process. The Running Modes module of LMS CADA-X was used to execute the ODS assessments. A brake noise dynamometer enabled the engineers to manually replicate the operating conditions required to produce the squeal. A Laser Doppler Vibrometer (LDV) was used to capture the out-of-plane rotor deflections, while small lightweight accelerometers were used to track in-plane rotor deflections and deflections on non-rotating components. The axle and associated fixture were attached to a large stationary mass and brake pressure was applied at three different levels, corresponding to the pressure range under which squeal occurred. The conditions of the system, particularly the rotational wheel speed and brake pressure, were kept constant throughout the tests. Measuring all points on the complete brake assembly required different measurement runs with one common reference channel, which corresponded to one specific point on the caliper. The analysis was split into three parts: the outboard and inboard cheeks of the disk (both out-of-plane) and the in-plane measurements. This split was needed because the front and rear cheeks could not be measured simultaneously by the LDV. A 48-channel LMS data acquisition system (including LMS CADA-X TMON and SMON post-processing), a microphone trigger, accelerometers, and the LDV were used to acquire the data for the ODS. The measured data were saved as cross-powers of the responses with respect to the reference signal and auto-power of the reference signal. The amplitude-corrected cross-powers were animated to create the ODS. While acquiring in-plane data, it was also required to track the position of the disk with respect to the timing of squeal events to be able to accurately position the measurements in space.

The same locations as those previously used for the ODS measurements were also used for the individual experimental component assessments. LMS CADA-X Modal Analysis was used for this series of assessments. Through the modification of the geometry and material properties of an individual component model, the system model was tuned until a good correlation with the measured modal responses was obtained.

Once all component models were validated and updated, the engineers analyzed the modal characteristics of the entire assembly, using similar operating and boundary conditions that were previously applied to generate the squeal during the ODS measurements. The correlation of the complete assembly was performed starting with the lower frequencies where the interactions of the components with each other tended to dominate. The stiffness of all the connections was tuned using the LinkSolver routine in LMS Gateway. Again, complex eigenvalue analysis was performed to identify the unstable modes, which were subsequently compared to the previously measured ODS. The FE model of the considered brake corner showed three unstable modes with a nearly identical shape, and all these modes stayed within 150 Hz of the frequency range of the experimental ODS. Both the test and the analysis shapes were very similar and exhibited a relative walking motion in between the two rotor cheeks.

These results demonstrate that, with proper validation, analysis can serve as a predictive and diagnostic tool to help address brake squeal problems as high as 15 kHz relatively early in the design process. GM engineer Tinghui Steven Shi stated, “The key to the success of the process is the systematic and rigorous correlation between physical tests and virtual FE modeling, at both component and system level, and under static and dynamic conditions. Once we have thoroughly verified the analytical model, we can use it to identify and fix brake squeal problems much earlier in the design cycle. After validating this method on the application described in this article, we have moved it into GM’s mainstream development process and used it on several actual programs.”