MADYMO Helps Optimize Integrated Car Seat
"Coupling MADYMO and LS-DYNA3D was critical to the success of the seat."
JOSEPH FORMICOLA, HOFF AND ASSOCIATES
An approach to product development called Analytically Driven DesignSM, in which MADYMO®, human body modeling software from TNO, played a key role, recently helped reduce the design cycle for a new automobile seat by 40%. In this joint development effort between the engineering consulting firm Hoff and Associates and automotive supplier Johnson Controls Inc. (JCI), the seat was optimized for strength, manufacturability, and cost so that while it is stronger than its predecessor, the new seat costs less to produce.
As its name implies, Analytically Driven Design is based on making analysis results available to design engineers to guide their work. In this project, design engineers used a variety of analysis methods including finite-element analysis, mechanism analysis, and crash simulation with and without occupant, right from the start of the design cycle. Crash simulations with an occupant were done by coupling MADYMO with LS-DYNA3D® from Livermore Software Technology Corp.
"Coupling MADYMO and LS-DYNA3D was the most technically challenging aspect of the whole project, but one that was critical to the success of the seat," says Joseph Formicola, the Hoff Vice President in charge of the JCI project. "It had to be done — quickly, efficiently, and accurately. We just tuned our internal systems to be able to accomplish that."
Integrated Seat System
The seat that was being redesigned was an "integrated" car seat. This type of seat, currently found on some convertibles, luxury cars, and pickups, is an aesthetic and functional improvement over the traditional seat/seat belt configuration. In an integrated seat, the seat belts are mounted to the seat structure. In addition to making the seat area look neater, an integrated seat offers the benefits of improved belt comfort and convenience and protects drivers better in a roll-over.
Integrated seats also provide the vehicle manufacturer greater styling flexibility, particularly in the b pillar area. Unlike the traditional design where the seat belt is mounted on the b pillar, an integrated seat bears the entire load in a crash. Thus, one of the design challenges for the next-generation integrated seat was making sure it is strong enough to meet Federal Motor Vehicle Safety Standards. Another design challenge was getting the cost down. Some automobile industry observers believe that once the cost of integrated seats becomes competitive with conventional seats and seat belt systems, all cars will have integrated seats.
Engineers at Hoff and Associates, which has its headquarters in Ann Arbor, Michigan, worked closely with JCI, beginning with the modeling of individual components and sub-assemblies. They didn't wait until they had complete models to use analysis, however. Analyses were performed right from the beginning to evaluate rough design concepts. Analysis software used included: MacNeal-Schwendler's MSC/NASTRAN® for load management studies, PTC's Pro/MECHANICA Motion® for seat reclining mechanism analysis, and ABAQUS®, from Hibbit, Karlsson & Sorensen Inc., for non-linear buckling analysis.
After parts and sub-assemblies were combined into an entire seat assembly model, crash analysis was added to the project. Front and rear impacts were evaluated in accordance with various (U.S.) Federal Motor Vehicle Safety standards. Some of the simulations involved just the seat and were performed using LS-DYNA3D. Others included an occupant, supplied by MADYMO, and the integrated functioning of the two programs.
According to John Varellis, project engineer at Hoff and Associates, MADYMO was chosen for this project for many reasons: it is easy to use, it has a large database of highly biofidelic dummies, and it is fast and accurate. Most important, however, is MADYMO's ability to interact with other software such as LS-DYNA3D. Simulations of a sled test, for example, would not have been possible without the ability to run these two programs concurrently.
Varellis explains the coupled analysis process this way: "The seat structure, seat belts, and sled were modeled in LS-DYNA3D. Then we took a dummy from the MADYMO database and positioned it in the seat. HyperMesh® from Altair Computing Inc. was used to visualize and verify the right position of the dummy in the seat. When we ran the coupled analysis, LS-DYNA3D calculated the mechanical loads on the car seat and MADYMO calculated the accelerations on the dummy, while the two programs constantly exchanged information."
Head Injury Criteria (HIC) and chest gs, provided by MADYMO, were used to determine the survival or "death" of the dummy while the output from LS-DYNA3D allowed the evaluation of the structural integrity of the seat in the simulated sled test. LS-TAURUS was used for post-processing. It made it possible to inspect the dummy kinematics, verified the proper interaction between the dummy and the seat, and provided the seat belt forces, the retractor performance, and the energy balance in the seat structure. HyperMesh was also used in post-processing to provide the displacement and stress levels in the seat.
According to Varellis, the process of coupling LS-DYNA3D and MADYMO involved the creation of a MADYMO data file, an LS-DYNA3D data file and a coupling data file containing commands that would enable LS-DYNA3D to exchange information with MADYMO. The coupling file also contained all the contact definitions between the dummy with the toepan, the seat belt, the cushion pan, and the back of the seat. The first time coupling between MADYMO and LS-DYNA3D was attempted, it took about two weeks to set up a successful run. During that time Varellis was frequently in touch with TNO and Livermore support staffs. Having been through the coupling process, it would now take him a few hours to integrate the two programs and launch a new run.
The success of this endeavor has led Hoff and Associates to propose the use of coupled MADYMO LS-DYNA3D to several other companies. These include an aerospace seat manufacturer and a truck seat manufacturer; however, the process is applicable to any manufacturer who needs to predict how the human body will react in a new product design.