Structural Dynamics Characterization of the Vehicle Seat for NVH Performance Analysis Mohammad Fard , Pushpakumar Sugirtharaj and Reddy Sekhar RMIT University, Australia
Email: [email protected]
Abstract Most of the researches about the transmission of the vibration to the vehicle seat had little or no focus on the structural modal characteristics of the seat while these characteristics can have large effects on the transmission of the vibration to the vehicle occupant. To realize these structural effects the main modal characteristics of a front seat of sedan car haven been characterized by performing modal tests. The experimental results showed that seat has some major mode shapes at frequencies below 80 Hz. Among these major modes, the seat torsion mode was focused as its resonance frequency was near the selected car body torsion mode. The interaction of this seat mode with the car body, here BIW (bodyin-white), torsion mode was evident by a large seat vibration level in the frequency response of the seat to the front main member of the car BIW. To reduce this interaction, the seat structure needs to be considerably changed. However, such changes are only possible in the early design phase. To predict and optimize the seat structural dynamics, in early design phase where there is no enough detailed design data, CAE beam concept models for BIW and the seat were developed. The developed BIW beam model was consisted of the following main members of the engine compartment, the floor members, body side members, roof members, as well as the trunk members. For development of seat CAE concept beam model we have used similar approach as it was used for above mentioned BIW. CAE NVH analyses of the concept beam model of the body and seat (BIW+ front seat) have shown similar results with the corresponding advanced CAE model results. The concept CAE model results similarly indicate that if the seat resonance frequency and the body resonance frequency are close, it may become a cause of large seat vibration. It is shown that these beam concept models can be used to predict and optimize the seat vibration. The proposed method can be used as an important tool to improve the NVH performance of the seat vibration in early design phase.
Introduction The dynamic response behaviour of the automotive seat structure is affected by the transmitted vibration from excitation sources of the vehicle such as the road and power train. These sources transmit the vibration to the automotive seat and human body through the car body structure. Most of studies about automotive seat vibration [5, 6, 7, 8] consider seat as a rigid structure and focus on the response of seated human body to the vibration. Earlier experimental study shows that seat structure exhibit more discomfort levels at some certain frequency ranges. Therefore, these studies indicate that there are some major mode shapes of the seat structure, at certain frequency ranges, that must be characterized. In other words, it is necessary to do further research as the seat structural behaviours
may have great effects on the vibration characteristic of the seat and therefore ride comfort. To characterize the seat structural dynamics, it is required to realize the possible seat mode shapes and corresponding resonance frequencies.
The vibration of the vehicle seat is also highly related to the structural dynamic interaction of the vehicle body with its mounted seat. This interaction optimization is related to how to control seat resonance frequencies away from vehicle structural resonance frequencies. In other words, when the vehicle seat resonance frequencies matches with the vehicle body resonance frequencies, it is likely to have a poor ride comfort. Nevertheless, this structure optimization may be very difficult and costly if they have not been characterized in early vehicle design phase. Furthermore, the main focus must be on seat dynamics as changing the vehicle body fundamental mode shapes or its resonance frequencies is quite difficult in comparison with that of the seat structure.
To optimize the structural dynamics of the seat and its interaction with vehicle body, it is necessary to develop an advanced CAE model which can simulate the structural dynamics behaviour of the system. However, in the early vehicle design phase, as there is no enough design data to develop advanced CAE model and to predict the dynamic behaviour of the vehicle system, it is necessary to develop concept CAE models based on the design information and predecessor vehicle data. For predicting the fundamental structural dynamic behaviour of the vehicle, simple beam concept CAE model is quite practical and useful. Note that special care must be taken for developing concept beam models, at which these concept beam models must show an acceptable correlations with corresponding predecessor advanced CAE models. It worth mentioning that nowadays the vehicle advanced CAE models have a very little discrepancy with the corresponding physical vehicle test results.
This paper, introduce a method to predict the seat vibration level which is caused by the vehicle external inputs. It also shows how the interaction of the vehicle body and seat dynamic modes can be the cause of high level of the seat vibration. To this end, the vehicle seat and BIW (body-in-white) structural-modes have been characterized using modal testing and advanced CAE analysis. By considering these test and advanced CAE results, the concept beam CAE models were then developed for the both seat and BIW. Hence, it is shown that the concept beam models can be used to predict and therefore optimize the seat vibration level in early vehicle design phase.
Method Experimental Method Experimental modal analysis  was performed to characterize the major mode shapes and corresponding resonant frequencies of the front seat structure of a selected sedan car. The test setup, as shown in the Figure 1, was consisted of the seat structure, fixture and excitation mechanism with the required hardware and software. LMS Test Lab was used for modal data acquisition and analysis. The seat structure was suspended using bungee ropes to
obtain its modal characteristics in free condition. This type of suspending allows the seat system to be nearly free in six axes.
Figure 1: Seat modal test set up (front view and side view)
An electromagnetic shaker (Agilent Technologies, Baird, 2010) was used as a source of excitation. This electromagnetic shaker works on the principle of electromagnetic induction – a force is induced by a current carrying coil placed in a magnetic field. The shaker is connected to the load mass with the help of a stinger.
A sweep sine excitation signal between 0 to 100Hz was used for this experiment. In this test ten tri-axial accelerometers (Figure2) were used to measure the response of the structure in all the three axes. The mounting of the accelerometer was done using adhesive wax.
Development of NVH CAE Model To analytically investigate the structural dynamics behaviour of the automotive body structure and its interaction with the seat, we have developed a concept CAE model for the selected sedan car BIW and its seat structure. The BIW and the seat structure concept models were developed from their corresponding advanced CAE models. These concept models were developed using only beam elements. For numerical analysis, beam FEM models are extensively used in the Automotive
Industry. According to literature (Giancarlo Genta et al., 2009), beam models are developed and utilized in the early vehicle design phases. This is mainly for the upfront prediction of the critical performance of the vehicle structure as well as characterizing its fundamental mechanisms for improving the NVH performance.
BIW Concept Beam Modeling A major source of knowledge to develop the BIW beam model was its corresponding advanced CAE FEM model. The design of the BIW beam model needed to cover the main members of the BIW
advanced model which they have large influences on the fundamental modal characteristics (