TWIICE-2018 is an evolution of TWIICE-2016 and keeps its most important design points, that are the composite mechanical structure and the 4 actuated DoFs. In addition, the rolling sole of the foot compensates for the lack of a flexible ankle, and allows turning easily, thanks to the small area of contact with the floor. The control strategy also remains position-control, with operation modes to set the joints trajectory. Every step or other action is triggered by the fingers of the pilot, by the 4 buttons on the right crutch handle.
This simple design makes TWIICE lightweight and predictable. However, due to the lack of actuation at the ankle and the pre-defined trajectories, walking across unstructured terrain or an obstacle not handled by a specific control mode is difficult for the pilot. The contribution from the arms and the crutches is then increased, to ensure the stability.
TWIICE-2018 brings an improved mechanical design to increase the joints torque and the compactness. This was possible thanks to the custom integrated actuator, including the motor, gears and sensory electronics. Compliance was also added to the rolling feet.
The embedded controller was also extended with additional operating modes to address two new obstacles of the CYBATHLON: the “tilted path” (that was different in 2016) and the “rough terrain”. The gait trajectories were optimized by trial-and-error thanks to a desktop application allowing quick iterations. Manual control of the exoskeleton by the pilot using the buttons on the crutch also turned out to be a good strategy and was maintained in the new version. This method of control, in combination with predefined trajectories, has rendered the interaction between the pilot and the exoskeleton fairly simple, thanks to the predictability of its behavior. This, however, comes at the cost of inflexibility in the face of uncertainties in the environment. This problem could be mitigated by providing several locomotion modes, each with a trajectory designed for a specific type of terrain.
The main outcome is a better time to go through the CYBATHLON obstacles, compared to the 2016 edition.
It can be noted that the gait speed did not increase significantly. This is due to the gait trajectories being triggered step by step requiring a pause in the middle of the stride. Indeed, all the joints are fully stopped between each step, so time is lost to decelerate, wait for the user input, and accelerate again.
The main limitation of these results is that the performance of TWIICE was assessed with a single pilot only. Chosen for her athletic condition, her performance is not necessarily representative of most people affected by paraplegia. The performance indices exposed here should then be considered as the maximum achievable, not the typical one.
In the future, the controller of TWIICE will be improved to support a continuous gait, to increase the ambulation velocity. To allow walking in unstructured terrain, adding more gait trajectories is not sustainable because of the increased time to select them, and the increased risk of error. Terrain-awareness and dynamically-computed gait could be implemented if extra sensing is added to TWIICE, possibly using a LIDAR distance sensor.