Autonomous balance control of underactuated torque-controlled lower-limb exoskeletons for paraplegic users

Balance control is key to every motion humans make and posture humans hold. The pull of gravity, reaction forces to our motion, and external disturbances compromise balance and need to be compensated to prevent balance loss. Muscles work autonomously to achieve balance control while performing the desired motion, and this involuntary control is often taken for granted. However, people with complete spinal cord injury (cSCI) lack motor function below their lesion. Thus, they cannot stand, walk, or balance using their legs. The loss of autonomy leads to muscle atrophy and other secondary health complications, which include a toll on mental health. 

Lower-limb exoskeletons (LLEs) are a solution for people with cSCI to regain their ambulatory capabilities. These devices are wearable robots that apply moments at the joints of the user to drive their legs. However, with a couple of notable exceptions, LLEs require the user to apply the forces necessary to maintain balance with their arm muscles utilizing crutches, which is quite exhausting. Furthermore, users cannot use their hands to interact and perform daily life activities. These shortcomings limit the use and appeal of LLEs for cSCI subjects, and the inclusion of balance control in the LLE control can address them. 

Autonomous balance has been achieved in humanoid robots by applying forces and moments at the center of mass (CoM) of the robot to steer the linear and angular momentum in a controlled manner. This is called momentum-based control (MBC). MBC needs to be tailored to the controlled device, and the application of forces and moments at the CoM is non-trivial: the joints that the LLE drives, the limits of the joint moments, whole-body posture, contact configuration and friction at the contact interfaces influence the forces and moments that are produced at the CoM. 

In this thesis, we present a method to evaluate LLE design and suitability for control strategies and a controller architecture that successfully keeps the balance of a person with cSCI. The validation of this MBC and the acquired insight into its inner workings are great and promising steps toward restoring the autonomy of people with cSCI.