Application of the Transfer Path Analysis to Vehicle Doors Thomas Michaelis 1 Technical University of Munich, Germany; TUM School of Engineering and Design, Department of Engineering Physics and Computation Boltzmannstraße 15, 85748 Garching b. München University of Applied Sciences Würzburg-Schweinfurt Ignaz-Schoen-Str. 11, 97421 Schweinfurt Steffen Marburg 2 Technical University of Munich, Germany; TUM School of Engineering and Design, Department of Engineering Physics and Computation Boltzmannstraße 15, 85748 Garching b. München Stefanie Retka 3 University of Applied Sciences Würzburg-Schweinfurt Ignaz-Schoen-Str. 11, 97421 Schweinfurt

ABSTRACT Transfer path analysis (TPA) is a proven method for identifying critical structure-borne and airborne sound paths. The basic idea here is to divide the overall system into an exciting active component (source) and a passive component, e.g., the mechanical structure to be investigated, with their respective measurement points. By separating them, two independent systems are created, whereby on the one hand the excitation behavior can be characterized and on the other hand the transmission behavior can be assessed. Finally, it is checked whether both systems harmonize with each other or whether changes to the source or structure are necessary. The advantage here is the simple description as a black box without complex modeling. In this contribution, this systematic is applied to window regulator systems found in vehicle doors. The focus is on a hybrid approach between experimental and numerical transfer path analysis. For this purpose, the basic workflow of suitable TPA methods is discussed. This includes evaluation of the test rig, the calculation of the contact forces and the numerical analysis of the transfer paths. 1. INTRODUCTION

Noise, Vibration, and Harshness (NVH) is an essential topic in modern vehicle development. Due to more and more electrification of the drive units and lightweight designs, it is easier to hear and feel acoustic issues for the customer. Classic transfer path analysis methods provide a tool to identify critical paths in a late stage of development [1]. However, changes of the design lead to high effort and costs. Component-based TPA attempts to decouple the source-receiver system, e.g., to perform experimental tests on separate test rigs. This allows changing the single components of

1 thomas.michaelis@fhws.de 2 steffen.marburg@tum.de 3 stefanie.retka@fhws.de

Jai. inter noise 21-24 AUGUST SCOTTISH EVENT CAMPUS ? O ? . GLASGOW

an assembly, e.g., window regulator motors in vehicle doors, to achieve an optimized system. The process of characterizing has already found its way into international standards [2]. System suppliers can use this method in an early development phase as a prediction tool. For example, different motors from tier three suppliers can be characterized and (virtually) coupled with their own receiving structures. In this case, a set of frequency response functions (FRF) is required. They can be achieved either by experimental testing or by calculating numerical models. Generally, few physical prototypes are available in an early phase, limiting the optimization process. In numerical models, design parameters can be changed easily but need high effort to be modeled correctly. This contribution attempts to establish the connection between experimental and numerical models of vehicle door components, e.g., lightweight carriers inside the door and window regular systems. The connection aims to set up virtual acoustic prototypes, which can predict the acoustic characteristics of the assembled system. For this purpose, we will show and discuss the workflow based on the ISO standard. Afterwards, we present an evaluation of the existing test rig to find whether the blocked force or in situ method can be applied. The last topic includes the description of the numerical model. 2. THEORY

In order to characterize the active component of a source-receiver system (Figure 1) a variety of concepts based on the component TPA are shown in [1]. All those methods are trying to follow the equivalent source concept to characterize the internal excitation 𝒇 𝒔 . The aim is to find a set of equivalent forces 𝒇 𝒆𝒒 in the interface 𝒖 𝒊 such that all responses 𝒖 𝒓 of the entire assembly can be determined. Subsequently, detailed information on the blocked force and in situ TPA will be shown.

Figure 1: Source-Receiver System.

2.1. Blocked Force TPA To achieve the blocked forces, the source is coupled to a rigidly fixed support (Figure 2). The internal excitation 𝒇 𝒔 is portrayed in the reaction forces such that the displacement 𝒖 𝒊 on the active component is zero [1]. The equivalent forces 𝒇 𝒆𝒒 can be found by equation (1) with 𝒀 𝒔,𝒊 (FRF internal source to the interface), 𝒀 𝒊,𝒊 (FRF of the interface of the active component), and the internal excitation 𝒇 𝒔

ି𝟏 𝒀 𝒔,𝒊 𝒇 𝒔 . (1)

𝒇 𝒆𝒒 = 𝒀 𝒊,𝒊

Theoretically, this is an easy way to find the equivalent forces, although it is hard in practice. The methods impose the boundary to be infinitely stiff. Furthermore, the direct measurement of the blocked force needs a force sensor, which is expensive and unable to measure rotational degrees of freedom.

Figure 2: Schematic overview of the blocked force TPA.

2.2. In situ TPA The in situ TPA (proposed in [3-5]) is a method to obtain the equivalent forces regardless of the receiver system. The forces are achieved by measuring kinematic quantities while a connected source operates. Theoretically, the forces can be transferred to the original receiver system.

Figure 3: Schematic overview of the in situ TPA.

To calculate the forces in the interface 𝑓 ௘௤ , indicator points 𝑢 ௜௡ௗ are placed near the interface. Those points will measure the kinematic quantities 𝒖 𝒊𝒏𝒅 while the source is operating. Trough inverse calculation of the transfer matrix 𝒀 𝒊𝒏𝒅,𝒊 (indicator to interface points) the equivalent forces can be determined (Equation 2). A detailed workflow on how to perform the measurements etc. is shown in [5].

Source

ି𝟏 𝒖 𝒊𝒏𝒅 (2)

𝒇 𝒆𝒒 = 𝒀 𝒊𝒏𝒅,𝒊

3. BLOCKED FORCE TEST RIG

The fundamental step of characterizing the active component is to design a valid test rig for either in situ or direct blocked force measurements. Besides requirements regarding the equivalent source concept, the test rig must also simulate different loads of the window regulator system. Therefore, an existing test rig is being used (Figure 4). This consists of a block (1) with an adapter plate for varying motors (2). The adapter plate is designed to have enough space for accelerometers or direct force sensors (depending on the measurement method). Part (3) shows the coupling to the load machine, which is another servo motor. Attention has to be paid to the alignment of the block to the load machine. In the case of a shaft offset between motor and load, constraint forces can affect the accuracy of the force measurement. Part (4) is designed to measure the airborne sound of the motor. Microphones can be placed at certain distances to measure airborne transfer paths. To ensure which component-based TPA Method works best with our test rig, evaluations regarding the block dynamics must be carried out. The evaluation includes an experimental modal analysis and the measurement of the accelerance at the connection points on both block and motor. Both methods will be shown subsequently.

Source Receiver

Figure 4: Test rig for source characterization.

3.1. Modal Analysis In order to find the dynamic behavior of the block, an experimental modal analysis is performed. The test setup consists of two labCOMPACT12 front ends, six triaxial accelerometers, and a manual impact hammer. For data acquisition, the block is discretized into 47 points (Fig. 3) and measured by the roving hammer method. The first five modes with the corresponding shape are shown in Table 1. These dynamic properties of the block have to be taken into account when evaluating the stiffness of the block, according to chapter 2.1.

Table 1: Results of the experimental modal analysis

Mode 1 2 3 4 5 Frequency 133 Hz 373 H z 588 Hz 839 Hz 1069 Hz

Shape

3.2. Accelerance measurements As proposed in [6], if a deviation of 1dB is acceptable, the effective mass ratio at the connection points must be higher than 10. For this reason, a measurement setup for evaluating the connection points is established (Figure 5). Here we place a screw in each connection point, which allows us to induce impacts in all translational directions. Close to that, two accelerometers are placed to determine that point's frequency response function (FRF) (. For the comparison, we will average the signals over frequency. As proposed in [2], the FRF of the active component should be measured in free boundary condition. Therefore, the motor was fixed with two ropes, allowing moving in all directions. From both setups, the effective mass for the block 𝑚 ஻௟௢௖௞ and 𝑚 ெ௢௧௢௥ is calculated from the measured acceleration 𝑎 ଵ , 𝑎 ଶ and the hammer signal ℎ

ଵ

௔ భ (௙)

௔ మ (௙)

௛(௙) ቁ , (3)

𝑚 ஻௟௢௖௞ (𝑓) =

ଶ ቀ

௛(௙) +

௔ ( ௙ )

௛(௙) . (4)

𝑚 ெ௢௧௢௥ (𝑓) =

Figure 5: Accelerance measurements of the block.

The ratio mentioned before for three connection points is depicted in Figure 6. Here we can directly identify that the requirement of a ratio higher than 10 is for specific frequencies not satisfied. Interestingly, some modes do not affect the ratio as much as expected. In every connection point, the ratio requirement is not satisfied at 250 𝐻𝑧 due to the motor’s high effective mass. The influence of modes can be found in the frequency range 1000 𝐻𝑧 - 1300 𝐻𝑧 , which is near to critical. Summing up both the modal analysis and accelerance measurement, we propose using the in situ method to determine the interface forces for this test rig. In certain frequencies, it cannot be excluded that the block's dynamics influence the accuracy in direct blocked force measurements. For in situ measurement, an on-board validation has to be performed to evaluate the error of the force determination.

100000 10000 3 100 = as 2 10 € 10 250 500 750 1000 1250 Frequency [Hz] 1500 1750 2000 2250 2500 — Point 1-x —Point1-y — Point 1-z

Matock / Mrvotor [-] 100000 10000 1000 100 10 250 500 750 1000 1250 Frequency [Hz] 1500 1750 2000 2250 2500 —Point 2-x —Point2-y — Point 2-z

Figure 6: Ratio of the effective mass for each measurement point

Matock / Motor [-] 100000 10000 1000 100 10 250 500 750 1000 1250 Frequency [Hz] 1500 1750 2000 2250 2500 — Point 3 -x —Point3-y —— Point 3-z

4. Numerical Model and Coupling

The following model was created by Hi-Lex Europe. We adopt it for the virtual acoustic prototype in order to connect with the experimental blocked forces. The input data consists of five parts – the lightweight carrier, motor (not displayed), speaker (point mass here), drum, and housing (Figure 7).

Figure 7: CAD Model of the lightweight carrier and motor.

The entire model is discretized (Figure 8) into 1.5 Million elements with SHELL181 elements for the lightweight carrier, SOLID187 elements for the motor, drum and housing; as well as BEAM188, and MASS21 elements for the speaker connection. For a detailed description of each element, refer to ANSYS Documentation. The connection to the vehicle door is neglected in this stage of method development. Instead of modeling the screws and clamps, we define fixed supports around each connection (Fig below, blue points).

Figure 8: Left: Mesh of the lightweight carrier including the motor and connection to the door.

Right: Coupling points of the equivalent forces.

The coupling of experimental excitation data will occur in the interface between motor and lightweight carriers (Figure 8 right). Here, the measured accelerations must be transferred into forces as stated in Eqn. 2. The complex force is then applied to the interface in the numerical model. Finally, if the modeling is complete, the harmonic analysis can be performed. 5. CONCLUSIONS AND OUTLOOK

In this paper, we want to connect the experimental test data of source characterization with a numerical model to achieve a virtual acoustic prototype for optimization in early phase automotive engineering. For this purpose, the used characterization techniques were briefly explained. Based on an existing test rig for servomotors of window regulator systems, both methods were evaluated regarding their applicability on the test rig. For evaluation, we performed a modal analysis to find mode shapes and eigenfrequencies of the block and an accelerance measurement in the connection points of the motor and block. Here we assess the ratio of the effective mass of the block over the motor. Because of the measurement results, we recommend using the in situ TPA to determine blocked forces. The last chapter describes the coupling of force data and the numerical model. Here, we describe the basic model of a lightweight carrier, which will be used in the future for coupling in virtual acoustic prototyping. Based on the results presented in this paper, we will perform an in situ TPA for a high amount of motors in order to find the blocked force and some statistics values like scattering. The last validation step consists of an onboard validation of the calculated forces. The forces will be transferred to the numerical model after finishing the preprocessing. 6. ACKNOWLEDGEMENTS

We gratefully acknowledge Hi-Lex Europe GmbH for providing the numerical models as well as the test rig and support. 7. REFERENCES

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