Atalante, the exoskeleton designed and manufactured by Wandercraft is the first med-ical device making the walking-impaired dynamically walk again without crutches or walker. It can walk autonomously in a dynamic and stable way thanks to the Hybrid Zero Dynamics (HZD) algorithms, particularly developed with the collaboration of J.W. Grizzle [13]. Besides, the exoskeleton can stand up, sit down and turn around. To achieve these movements, the robot has 12 degrees of freedom, three for the hip, one for the knee and two for the ankle for each leg. A remote control and another keyboard on the back of the robot achieve switching between the robot states. The movement is then triggered with an inertial measurement unit. Each of these degrees of freedom is powered and actuated with a frameless motor and a gear reducer. The algorithms replicate the human motion so that the human gait is respected and the users not hurt.

Wandercraft now develops a fifth version. The exoskeleton is made of a composite back, where the user is attached with a safety vest. It is also attached to a gantry with a winch to prevent falls for the user safety. Each hip is composed of three pivoting links approaching the motion of the human joint. The thighs and the tibias are adjustable to fit a large number of patient. They also comprise two supports for the thigh and the knee of the user. In addition to the feet and the back, they are all the interfaces between the user and the device. These interfaces and the maximum torque provided by actuators are the basis of the load cases created to check the mechanical strength of the structure. They were implemented for the design of fourth version of the exoskeleton but the load cases were revised and improved for the fifth version. For instance, all the structural parts were tested in multi-body simulations for the first time. Moreover, data from sensors were retrieved from tests on the robot and clinical trials, in particular the force foot sensors. The data were interpreted to create completely new load cases based on the impact values.

After having defined load cases based on the functions, the torque of the actuators and the sensors data, they were implemented in finite element method simulations. The purpose was to verify if the strength of the structure complied with the safety factors defined by the international standards applied to the medical devices such as the IEC 60601-1 [10]. Most of the simulations consisted in static linear elastic analysis. The major issue in this kind of simulation was to manage the numerical singularities. Most of the time, they occurred in low stress area and a method has to be determined to explain that the stress values in this area shall not be considered. However, singularities can appear in the most loaded zone and then a change in design is required to bring the stress below an acceptable threshold. In the scope of linear simulations, a pre-analysis was performed on the composite back to monitor the calculation performed by the engineering firm handling the design on that specific part. The analysis started by estimating the properties of the laminate and then implement the evaluated values in finite element software. The part being a thin part compared to its maximum length, the back was modelled with shell elements based on an orthotropic behaviour of the material. The last part consists in explaining how non-linear simulations were required in specific occasion to determine the structure strength. Regarding the robot, it involves contact modelling in a screwed interface and elastoplastic behaviour on one of the most loaded area of the device.