Seat of the pants? Not when it comes to simulating occupants in cars

Sept. 23, 2010
Calculated Sitting Man in Research (Casimir) is a 3D human-body model that uses finite-element analysis (FEA) to assess car-seat designs.

The human body is an extraordinarily complex mechanism, so creating a realistic 3D computer model is daunting. However, the pursuit of more-comfortable car seats spurred engineers on the Seating Comfort and Biomechanics team at Wölfel in Höchberg, Germany, to further develop Calculated Sitting Man in Research (Casimir), a 3D human-body model that uses finite-element analysis (FEA) to assess car-seat designs.

Seat comfort depends on many factors including seat materials, vibration from the vehicle, sitting position when driving, length of time spent driving, and the seat’s structure and shape. Previously, car-seat comfort was taken for granted, but today the simple act of a customer getting behind the wheel at a dealer can be a make-or-break moment in a potential car sale.

Comfort can even become a health and safety issue, especially for those who spend a lot of time driving, such as airline pilots, traveling salesmen, construction workers, and bus and taxi drivers. In a recent survey of U. S. and Swedish drivers, 50% of the respondents said they suffered from low-back pain. To address this issue, standards have been put in place. For instance, heavy-equipment operator safety in the EU is now regulated by the 2002/44/EC standards, which quantify daily exposure limits to vibration. The standards are intended to reduce serious health problems such as circulatory disorders and musculoskeletal damage.

Seat-design basics such as shape, adjustability, and the right balance of firm and forgiving foam are important when considering comfort and health. So are hand-arm and whole-body vibration that vehicle occupants feel.

A method was needed to quantify seat comfort and make possible objective and comparative assessment. Also necessary was the capability to predict how the seat and the human body would interact early in the design stage, before manufacturing expensive seat prototypes. Casimir along with FEA software lets Wölfel and its collaborators and customers perform such analysis.

Casimir was developed by the Darmstadt University of Technology and the Federal Institute of Occupational Health and Safety in Germany in the mid-1990s and was originally used to compute forces on the spine. Early Casimir models were fairly abstract, using lumped masses connected together to represent the body. Subsequent models increased in complexity. Currently, the model contains volumetric parts, representing bodily tissue, which can interact with each other and the seat itself. The model’s posture can even mimic a human’s, varying from erect to normal to slouching, an important feature in evaluating low-back pain.

Seating-comfort analysis

Running simulations with Casimir necessitates tasks such as selecting materials for nonlinear and frequency-dependent representations of seat foam, trim, and padding and human tissue; performing contact analysis in the interaction zones between the metallic seat frame and foam and between seat upholstery and the human body; and accounting for large deformations in the seat foam and human tissue. To handle these multiple requirements, Wölfel selected Abaqus FEA software from Dassault Systèmes Simulia.

In evaluating seating comfort, first comes building a model of the seat. Wölfel engineers have developed an extremely accurate seat representation that defines key properties. These include the material parameters of upholstery materials and the static and dynamic stiffness and damping values of structural joints, such as the recliner and seat rail.

Once the seat model is defined, a typical comfort analysis involves a static seating analysis, which quantifies how the body touches the seat under gravity. The most important aspect of this analysis is the seat-pressure distribution. This calculation helps designers evaluate the fit of body parts with seat cushions, the ergonomic position of the body, and the “meat to metal,” or the distance between the seat frame and the skin.

Next comes dynamic simulation. It computes how the human body in combination with the seat moves up, down, and sideways when the vehicle is in motion. The simulation illustrates how the seat magnifies the vibrations that pass from the road and engine through the chassis to the seat and eventually to the driver and passengers.

For static and dynamic analyses, Wölfel engineers use Abaqus Standard, which is based on an implicit solver scheme. The static analysis uses a nonlinear solver and the dynamic analysis a steady-state solver in the frequency domain. Four CPUs with about 64-Gbytes total RAM and the Linux 64-bit O/S powered the calculations.

Seating-comfort analyses using a “homogenized” version of Casimir, in which different tissues like fat and muscles are not differentiated, have shown good correlation with real measurements. The tool helps engineers zero-in on features, compare different designs, and rapidly optimize designs for both static and dynamic comfort.

Simulation minimizes the need for expensive physical prototypes. “Designing and building a single hardware seat prototype can cost €10,000 to €20,000 and take several days to weeks, but Abaqus model setups take less than a day,” says Alexander Siefert, assistant manager for Seating Comfort and Biomechanics at Wölfel. “Car-seat development always involves more than one design iteration, so the cost savings that simulation provides can be significant.”

Yet more realistic models and simulations

Casimir has already supported better designs and improved comfort, but Wölfel engineers thought the model could do even better. “So we added anatomical data to Casimir that let us make more accurate simulations,” says Siefert.

First, the engineers developed a continuum model of the thigh and buttocks area, which represented the fat, muscles, and skin separately. To make the model more realistic, they took data from the U. S. National Library of Medicine’s Visible Human Project (photographs of 600 cross sections at 1-mm-thick intervals, supplemented by CT and MRI images), assembled the 2D cross sections using SCAN-IP, and imported the digitized anatomy into the model. The result was more-lifelike 3D muscle volume detail. The engineers then meshed the model with the HyperMesh preprocessor, using linear tetrahedron elements with a typical size of about 20 mm and only 47K DOF for the buttock tissue, to shorten computation time.

Wölfel engineers also developed a discrete muscle model that more accurately represents the complex musculoskeletal behavior of the hip and thigh. Starting with a standard anatomy atlas as reference, they used spring and damper elements (similar to those used in automotive fatigue analyses) to simulate active muscle behavior. To define the muscle’s line of action, the engineers used strings. And around the joints, where the muscle action is most complex, they defined a muscle path (the shortest distance over the joint) and attached it by connector elements to the skeleton.

Applying the model in standard simulations showed that muscle activity influenced seating comfort. Flexing the thigh-buttock muscles caused a reduced rotation of the pelvis and a higher compression on the intervertebral discs of the lumbar spine. In addition, engineers established that these internal forces, which can cause low-back pain, were accentuated at the human body’s 5-Hz natural resonance, a frequency excited by normal driving conditions.

The analyses guided the engineers in how to improve seat designs. For example, when a driver pushes on the gas and brake pedals, the muscles of the leg flex and stiffen, which affects seat comfort. Passengers, on the other hand, are more passive and usually do not activate their leg muscles. This has let Wölfel use the original homogeneous model for passenger simulations and the enhanced model for driver simulations. Enhanced simulations provide more-accurate guidelines for improving driver-seat designs, such as adding length to the seat to better-support the thighs.

Human-body models not just for car seats

To create a state-of-the-art FEA human model takes actual biological data. CT scans use X-rays to collect data in 3D slices, which are then reassembled digitally. MRI uses a rotating magnetic field to align hydrogen atoms in the body’s water molecules and yields higher-contrast images than CT scans for soft tissues such as the brain, muscles, and cardiovascular system. Both technologies continue to improve in technique and resolution, yielding greater detail and accuracy. The Visible Human Project at the U. S. National Library of Medicine and the Digital Human Project, a proposed open-source, billion-dollar initiative, represent efforts to catalog and share extensive anatomical data derived from these technologies.

While still in its infancy, biomedical simulation using models of body parts is increasing rapidly. Difficulties arise because human skeletal and tissue geometries are complex and unique to each individual. In addition, the problems are highly nonlinear and material behavior is difficult to characterize using conventional tools and methods.

The potential benefits of biomedical simulation are enormous. Examples might include virtual design of prosthetics and implants, patient-specific implants, and enhanced flow efficiency in medical devices. As digital representations of the body become increasingly realistic, simulation is providing a more-powerful tool for a widening variety of medical applications.

© 2010 Penton Media, Inc.

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