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EuroBrake 2022: Noise, Vibration, Harshness session overview

The Noise, Vibration, Harshness session will take place on Tuesday May 17th and will be chaired by Claus Thomas of Porsche AG and co-chaired by Cristian Malmassari of Brembo S.p.A.

Topics and speakers for the session include:

Proposal for a common measurement standard for the acoustic behaviour of electric parking brakes on component level

Steven Rees, ZF Group

Electric Parking Brakes (EPB) on caliper brakes are an established product. As of today, no common standard has evolved for the measurement of their acoustical behaviour during the clamp and release process.

Each supplier has its own internal measurement standard. This causes friction during requirements discussion between supplier and customer as limits value cannot be referenced to a common measurement standard and limit values cannot be easily transferred.

An SAE committee, including employees of EPB suppliers and vehicle manufacturers, has developed a proposal for a measurement standard. The proposal is closing towards its final draft and is expected to be released before EuroBrake 2022 takes place.

The proposal focuses on the acoustic behaviour on component level (caliper and Motor Gear Unit) thereby excluding vehicle influence.

Two options are proposed for test bench setups that aim to be compatible to known caliper designs.

  • Characteristics of sensors are proposed.

  • Settings for data acquisition are proposed.

Parameters are proposed for measurement and analysis of air borne and structure borne noise.

Psychoacoustics and machine learning methods for the objective rating of creep groan noise

Severin Huemer-Kals, TU Graz

Rating a brake and axle setup’s creep groan performance is a necessity during the development process of a vehicle, be it for the comparison of different components or for the final assessment and release. Objective rating methods present a valuable addition to the often laborious and biased subjective ratings by test drivers.

One solid approach for objective rating can be based on psychoacoustic metrics: These metrics quantify specific auditory sensations of the human being. Based on preliminary evaluations presented at EuroBrake 2021, the psychoacoustic metrics loudness, sharpness, roughness, fluctuation strength and tonality were analysed in detail, both with classical statistical methods as well as with machine learning approaches.

Despite a certain variance in the subjective rating data, it was possible to create accurate rating models. Depending on the used signal source, cabin sound pressure or brake caliper acceleration, the metrics loudness, roughness, and tonality were found to dominate the human sensation of creep groan. Furthermore, the behaviour of different creep groan manifestations was compared. Based on the presented results, accurate objective rating mechanisms can be developed and implemented in future brake NVH development processes.

High damping and high stiffness under layer for squeal noise improvement

Leann Johnson, ADVICS North America

With the emergence of hybrid, electric, and autonomous technology, vehicles have become quieter overall, which has caused more NVH complaints from consumers. Brake squeal noise is among those complaints. Since hybrid and electric vehicles employ regenerative braking, the conditions in which foundation brakes are used have changed. Brakes will now be used in low pressure and low temperature conditions more often. Therefore, NVH requirements for foundation brakes will be more focused on these conditions.

Now, friction suppliers have the challenge of addressing brake squeal issues without compromising other aspects of the brake pad (hereafter “PAD”) performance. Although a relationship between some characteristics of the PAD and squeal noise performance has been confirmed with previous studies, the relationship of damping property of Under Layer (hereafter “UL”) material has not been confirmed. The UL is a material between the friction material and the back plate that is used to improve PAD durability and is widely used in today’s market. In this study, it is confirmed that the damping characteristics of UL in the PAD impacts brake squeal noise, specifically during low pressure and low temperature braking conditions.

Two different types of UL materials were examined based on material structure and dynamic viscoelasticity. PAD damping ratio and squeal noise performance was also studied. The results show the raw material orientation in the UL material, which is created by manufacturing method, influences the anisotropic properties of the material. Therefore, dynamic viscoelasticity is dependent on the direction it is measured. Specifically, the damping element of the dynamic viscoelasticity, the loss modulus, is increased in the direction of the raw material orientation for high damping and high stiffness UL material.

It also was determined that the loss modulus in the direction of the raw material orientation is effective for the bending vibration mode. Therefore, the study focuses on the PAD damping ratio and squeal noise performance in the 1st bending vibration mode of the PAD. It was determined that the PAD squeal noise performance improved due to the damping ratio increasing as loss modulus increases. In conclusion, this study proves that the high damping and high stiffness UL material is effective for squeal noise improvement in conditions that will be increasingly used during regenerative braking.

A structural dynamics modification strategy based on expanded squeal operational deflection shapes

Guillaume Martin, SDTools

To analyse brake squeal, measurements are performed to extract the Operational Deflection Shape (ODS) characteristic of the limit cycle. The advantage of this strategy is that the real system behaviour is captured, but measurements suffer from a low spatial distribution and hidden surfaces, so that interpretation is sometimes difficult. It is even more difficult to propose system modifications from test alone. Historical Structural Dynamics Modification (SDM) techniques need mass normalized shapes which is not available from an ODS measurement. Furthermore, it is very difficult to translate mass, damping or stiffness modification between sensors into physical modifications of the real system.

On the model side, FEM methodology gives access to fine geometric details, continuous field over the whole system. Simple simulation of the impact of modifications is possible, one typical strategy for squeal being to avoid unstable poles. Nevertheless, to ensure accurate predictions, test/FEM correlation must be checked and model updating may be necessary despite high cost and absence of guarantee on results.

To combine both strategies, expansion techniques seek to estimate the ODS on all FEM DOF using a multi-objective optimization combining test and model errors. The high number of sensors compensates for modelling errors and while allowing imperfect test. The Minimum Dynamics Residual Expansion method used here, ensures that the complex ODS expanded shapes are close enough to the measured motion but have smooth, physically representative, stress field, which is mandatory for further analysis.

From the ODS expansion, the two underlying real shapes are mass-orthonormalized and stiffness-orthogonalized using the model resulting in a reduced model with two modes known on the full model. Sensitivity analysis is thus possible and the impact of thickness modifications on frequencies is built. This provides a novel structural modification strategy where the parameters are thickness distributions, and the objective is to separate the frequencies associated with the two shapes found by expansion of the experimental ODS.

The methodology will be illustrated for a recent disk brake test and model.

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