Fine-tuning car seat comfort

Feeling tired after a long car ride is no accident. Although the driver is seated the entire time, navigating busy city streets, curvy country roads, or even slow-moving commuter traffic puts them in an almost constant state of motion, pushing and releasing the vehicle’s pedals to accelerate, brake, or clutch.

Legs and feet continuously change position. As the ankle flexes, the lower leg extends and retracts, and the muscles in the thigh and buttocks contract and relax. Since the thigh muscles are in constant use, seat designs – especially the front of the pad that supports the thigh – are important in driver comfort.

“Even holding the accelerator pedal at a desired position requires constant muscle activation,” says Alexander Siefert, manager of seating comfort and biomechanics at Wölfel Group, a German company specializing in engineering services and related testing systems for the design and development of car seats. “This alters the stiffness of the muscles involved. It also has a significant effect on seat-pressure distribution and stress/strain values within the tissue, which are important measures of seat comfort.”
 

Seat simulations with motion

Engineers on Wölfel’s seating comfort and biomechanics team developed the calculated sitting man in research (CASIMIR) finite-element model to help set current German occupational and health standards (vibration and shock) for working drivers of vehicles such as trucks, taxis, buses, and construction equipment.

The original version of the model simulated the impact that various foams and seat materials had on human muscles. However, to get a better picture of how to make seats more comfortable, engineers needed to better understand pressures on muscles as they moved during typical vehicle operations.

Working with Abaqus, the finite-element analysis (FEA) tool in Dassault Systèmes’ Simulia software, Wölfel engineers enhanced CASIMIR to include detailed muscular models that make their digital-driving simulations even closer approximations of real-world conditions.

“We have high confidence in our Abaqus analyses because of the software’s advanced non-linear and contact capabilities,” Siefert says. “Its material models for seat foam and human tissue are extremely useful when conducting human-body simulations for car seats.”

Wölfel engineers recently coupled two models to better simulate seating impact on muscles:

• A volumetric model representing passive nonlinear muscle behavior
• A filamentary model representing the active muscle force required to either maintain posture or make the movements, such as pedal operation, that are necessary for driving
   

By coupling the two models, engineers could see that passive volume stiffens when the filamentary model is activated.

Pursuing this strategy, the team performed seat-comfort simulations that accounted for muscle activation. In one study, they validated muscle activity in the abdomen and back for use in upright-seating-posture studies. In another, they analyzed the thigh and buttocks and began to understand the importance of pedal operation on seat comfort.
 

Testing simulation methods

To further prove the concept and benefits of coupling the volumetric and filamentary muscle models, the team decided first to simulate an imaginary muscle outside of their CASIMIR software.

After setting up simulations of different loads on the volumetric model and different states of muscle contraction on the filamentary model, the engineers used the embedded element option in Abaqus to generate a kinematic relationship between the two.
 

In an experimental setup designed to demonstrate the real-world veracity of this coupled scenario, engineers lowered five different weights onto a sample calf muscle (from a rat) to mimic muscle contraction and measured the results. The team observed that the upper muscle volume in the calf lifted up in both the coupled simulation and the real-muscle experiment.

“We’re still fine-tuning our simulations so that they’ll even more closely correspond to measurements,” Siefert notes. “But after our study validated the coupled-model method in the isolated muscle, we wanted to show it would work in the full body-model.”

To prepare the full-body model for a similar coupled analysis, the separate thigh and buttocks models needed to be further enhanced. High-contrast photos from the U.S. National Library of Medicine’s (NLM) Visible Human project were especially useful in achieving more accurate muscle volumes. Whole-body MRI scans in the prone and supine positions (from another source) provided additional detail. In the volumetric thigh model, smaller muscles were assembled into one volume to simplify the calculation.

For the filamentary model, the team first focused on the hamstrings (flexors), since that is the muscle group that contacts the seat and is activated during driving.

CASIMIR’s thigh and buttocks simulations were then placed on a cube of foam representative of a car seat. The model was loaded for its own weight, as well as with the hamstrings activated. When the team compared calculations from this setup with actual measurements from test subjects, the gravity-loaded scenario results were in close agreement (there was lateral expansion of the full thigh and movement between the muscle volumes). For the muscle contraction case, the vertical displacement (lifting up) of the leg also matched expectations.
 

Full-body simulation model

Finally, it was time to try out CASIMIR’s full-body model on a seat-pressure distribution scenario with muscle activation. The team explored a number of different loads including gravity and then knee flexion with resulting heel forces. Simulation and test results were in close enough agreement to validate coupling of the muscle models in future full-body-model simulations.

Now that CASIMIR is capable of simulating not only passive reactions of tissue to external forces but also active muscle contractions, the team can offer Wölfel automotive customers sophisticated seat-design and driving-comfort guidelines.

The engineers’ research benefits not only drivers, but the bottom line of car-seat suppliers and manufacturers everywhere.

“Experimental seat-comfort studies have traditionally required many subjects and the testing of several hardware prototypes, all of which can be time consuming and expensive,” says Andreas Nuber, Wölfel assistant manager for research and development. “Simulating comfort using FEA greatly simplifies the process. It’s objective, reproducible, and cost-effective. If we eliminate just a single hardware prototype during the design process, the savings can be as large as $69,000.”

Wölfel Group
www.wolfel.de

Dassault Systèmes
www.3ds.com
IMTS 2014 booth #E-3125,