Leveraging simulation to determine a single airbag is safer and more reliable than a multi-airbag system

Getting wheelchair users back on their feet

Millions of people are confined to wheelchairs due to illness or injury. Despite some advancements in wheelchair technology, the lack of significant progress since its invention still falls short in providing individuals with the ability to stand and walk. First, there is the social aspect of being at a different height from those around you. Second, the prolonged sitting position associated with wheelchair use can lead to health issues, such as reduced bone density, osteoporosis, muscle atrophy, pressure sores, spasms, changes in blood pressure, joint problems and even cardiovascular conditions.

Nicolas Simon has several family members who suffer from Charcot-Marie-Tooth, a degenerative condition that in its advanced stages often requires patients to use a wheelchair.

With no known cure, Simon wanted to provide an alternative. So in 2012 he founded Wandercraft with the aim of developing an exoskeleton that would allow people disabled below the waist to walk again.

The company has built and implemented the Atalante X in rehabilitation settings in hospitals, but it wants to expand that vision beyond a healthcare setting. “We want to give people more autonomy and to be able to use these exoskeletons in the real world, not only in a controlled environment with a doctor or physiotherapist,” says Fabien Expert, chief technology officer (CTO) of Wandercraft. “In the United States alone, we estimate there are 300,000 people with spinal cord injuries that could benefit from the exoskeleton in its current form. As we adapt the design in future versions, we hope to make it suitable for even more people by extending it to other pathologies, stroke rehabilitation and multiple sclerosis.”

To achieve this goal, Wandercraft adopted Siemens Digital Industries Software’s Simcenter™ Madymo™ software. Simcenter Madymo, which was developed primarily for the automotive industry, is used to develop better occupant and pedestrian safety solutions faster. Simcenter Madymo is part of the Siemens Xcelerator business platform of software, hardware and services.

Mitigating the risk of further injury

Taking the device to the streets is a big step. “Safety is absolutely critical,” explains Expert. “The people we are helping can get around by themselves in a wheelchair. The exoskeleton allows them to stand and walk, but we need to mitigate the risk of further injury. For instance, if they were to fall and suffer fractures or head injuries, this would put them in an even worse position than before.”

With the exoskeleton already proving functional, Wandercraft needed to adapt it to protect users so that they didn’t fear the possibility of an accident that would cause additional long-term injuries.

Personal airbag system

Wandercraft was inspired by airbags used in vehicles as they are designed to cushion impact on humans and minimize injuries. The exoskeleton is designed so the center of mass of the system is on the back, so if there is a power failure an imbalance occurs, the person using it would naturally fall backwards. This means they could install the airbag on the back to protect the user.

However, ensuring the effectiveness of the airbag required considerable analysis and assessment. It needed to be easy to add to the exoskeleton without incumbering the user, but also provide enough protection to significantly reduce the risk of injury in the event of a fall.

“We first had to understand if it was even feasible,” says Expert. “We must be able to detect that a fall is happening and then deploy the airbag within a half second. It was important to determine whether multiple airbags or just one would be better, and to get the sizing correct so that it provided enough protection without adding too much weight to the exoskeleton.”

Building physical prototypes to test this would have been a very time-consuming process because each airbag had to be made by hand. Using physical dummies would also not give full data on potential injuries to users. Wandercraft needed a faster solution that could fully replicate the human body and predict how well the airbag protected it during a fall.

Combining FEA with multibody simulation

Initially, Wandercraft used a finite element analysis (FEA) simulation tool, but this wasn’t sufficient to give them the data they needed. “We had no way of accurately modeling the patient to understand what injuries might be incurred,” explains Maxime Beck, head of mechanical engineering. “We had a separate multibody simulation tool, but we needed a solution that would combine both.”

To help them, Wandercraft reached out to the University of Strasbourg. “The university introduced us to Simcenter Madymo,” says Beck. “We could measure acceleration and angular speed, but we didn’t know how to use that to predict the impact on the patient. The University of Strasbourg showed us how to create the simulation with Simcenter Madymo and use its human body models to measure the effect on the user.

“The fact that Simcenter Madymo has been used for safety simulations in vehicles really helped as it has lots of models for how an airbag should perform. With Simcenter Madymo, we were able to match up the simulation results with physical test results, which gave us confidence to continue with it. Then we could optimize with each iteration without having to create a new prototype every time we changed the design.”

Simplified design and shorter development time

One of the most important outcomes from the simulation was to determine how many airbags should be used – primarily to maximize safety but also to make the device as cost-effective as possible. "Simulation allowed us to experiment with multiple airbags, but we found this didn’t add any more protection for the user,” says Beck “Each airbag needs its own gas and trigger mechanism, so the more you have, the more complex the setup is. Knowing that one large airbag gave as much protection as two or three smaller ones meant that we could reduce the complexity, making the whole unit easier and cheaper to manufacture.”

In fact, having just one airbag is not only more cost-effective but safer too. A multi-airbag system relies on each airbag to trigger at exactly the right moment. If one fails, then it’s the same as having no protection at all. The more complex a system is, the more chance there is of failure. So, by having just one airbag and one trigger the system immediately became more reliable.

Expert says that using Simcenter Madymo saved significant engineering development time, too: “The physical prototypes took one engineer three days to make each time. And it’s such a unique process that we only had one person with the necessary skill set. Without simulation, we would have had to wait this long between each iteration to test our theories. It would have taken so much time to reach the optimal design that it simply wouldn’t have been practical.

“Once the prototype is built it takes another full day to set up the test, but we can configure a simulation with any parameters we want in just a couple of hours.”

Regulations, improvements and new features

Now that Wandercraft is confident in the safety of its exoskeleton, the company is carrying out further testing to pass the necessary regulations. “We hope to achieve full regulatory clearance before the end of 2025,” says Expert. “Then we’ll be able to get it to market and we’ll see people using our exoskeleton in everyday life.”

But that is only the start. Thanks to simulation that made the first device possible, Wandercraft will continue to make use of it as they improve future products. “We’ve achieved the first target of getting patients out of wheelchairs,” says Expert. “But the aim is to give them more. We know they will demand more once they begin to experience their new freedom, whether it’s freedom they haven’t had since an injury or freedom they’ve never had due to being born with a certain condition. We intend to work on additional features to meet that demand. “Thanks to Simcenter Madymo, we’ll always be able to ensure that devices with these new features will be safe for the users.”