Automotive Electronics Sitting comfortably?


Hyundai has developed a dummy model to test seat vibrations virtually, as CIE finds out


D


uring long periods, more than two or three hours, of driving, a seat occupant feels an increase in muscle fatigue due to vibrations. Indeed, seat foam stiffness increases, which reduces insulation. Through an analysis of the seat transfer function, Hyundai Motor engineers have been able to improve seat design to decrease the discomfort originating from vibrations.


Seat occupant comfort is impacted by vibrations; this constraint must be taken into account during the seat design process. Therefore a reliable test has to be defined.


Hyundai Motor used a simulation system from ESI for checking the occupant’s posture in the seat, the body pressure and the foam hardness. This includes a graphical user interface, of which the upcoming version is called Virtual Seat. It is Windows-native, available for all Windows and Linux platform. Included is a mathematical solver for the same platforms, with parallel computations in distributed-memory processing or shared-memory processing, with single or double precision.


evaluate the impact of product use under normal or accidental conditions. ESI’s products fit into a single collaborative and open environment for end-to-end virtual prototyping. These are delivered using technologies such as immersive virtual reality to bring products to life in 3D. The company employs about 950 specialists worldwide covering more than 30 countries.


The challenge of this project, performed by Hyundai’s Body & Trim development team, was to develop a virtual dummy model and perform accurate vibration tests (see Figure. 1). With a fully validated dummy model, Hyundai can now perform virtual vibration testing with Virtual Seat earlier in the development process. By first sitting the dummy and taking into account foam compression and induced dynamic properties at each point of the seat, the virtual vibration test is accurate and can replace some of the physical prototypes. “Since seats contain lots of components, it's very difficult to find the factors that influence the dynamic comfort of the seat,” said Han Ji Won from the


Difficulties Working with the seat transfer function is a good way to avoid the subjectivity of tests performed with human volunteers. Additionally, it is very difficult to obtain a good repeatability of measurement with human tests, since each individual human body has a particular intrinsic non-linear response, and any change in position will change the dynamic response. A seat transfer function obtained with a dedicated vibration test dummy is a way to measure objectively the dynamic comfort of the seat. This vibration test dummy uses a moving mass and actuator system to have the similar characteristics to a human vibration response.


Development To perform vibrations tests virtually, a dummy model had to be developed. The geometry of the physical dummy was first captured and meshed, then a spring-mass system with multiple degrees of freedom was developed.


For each sub-component of the physical dummy, the transfer function was measured, and the finite element dummy model was tuned accordingly. The stiffness of the springs were adjusted from the natural frequency and the damping coefficients from the amplitude of the transfer function at the natural frequency.


The finite element dummy model alone was then validated by comparing for each of its components the accelerations during several vibration tests with the physical ones. The transfer function of each sub-component was also measured and validated.


Such comparisons were performed


with three dummy sizes: female fifth percentile, male 50th percentile and male 95th percentile.


Figure 1: Positioning of vibration tests in the seat system used by Hyundai Motor


Virtual Seat actually dates back to the 1990s, when it was known as Pam- Comfort following a collaboration between the ESI Group and Renault. It then evolved via other collaborations including Nissan and Hyundai.


ESI bills itself as an expert in virtual


product engineering, based on an integrated suite of coherent, industry- oriented applications. The aim is to replace physical prototypes by realistically simulating a product’s behaviour during testing, to fine-tune fabrication and assembly processes in accordance with desired product performance, and to


20 October 2013


Body & Trim development team. “We tried to figure this out using Pam-Comfort and reached our goal. This new way of working will help us save money and time effectively.”


The seat must be designed in such a way that:


• the transmissibility at the natural


frequency is reduced; • the natural frequency corresponds to


an area of less sensitivity for the human body; and


• the transmissibility in the 8 to 30Hz range, which is related to long-term driving, is reduced as much as possible.


Components in Electronics


Validation on industrial seat After having correctly defined the system modelling for the finite element dummy and compared its dynamic answers with the ones of the physical dummy, a validation on an industrial seat was initiated. As shown in Figure 2, the first step of this validation was the sitting of the dummy in the seat.


This stage was mandatory as it set the


right pre-loading conditions. It also determined the right dynamic properties, which are highly non-uniform in the foam and dependant of the foam compression. The second step was to simulate the vibrations test with the seated dummy vibration and these chained properties to obtain the transfer function (pink curve in Figure 2).


As shown in the graph (Figure 2), the model provided a good correlation with the physical prototyping (blue points in Figure 2).


The virtual vibration test is thus


predictive enough and can replace some of the physical prototypes.


Conclusion


Thanks to this fully validated dummy model, Hyundai performs virtual dynamic comfort testing with ESI's Virtual Seat earlier in the development process and can better arrive at the right seat design. The


Figure 2: Seat and dummy model


virtual vibration test is accurate and partly replaces real prototypes, thus allowing the car manufacturer to develop more comfortable seat designs in less time and at less cost. Virtual Seat also saves time and increases efficiency by using a single core model of the seat to support evaluation of several seat performance domains and to enable design team collaboration.


ESI Group | www.esi-group.com www.cieonline.co.uk


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