Robust and versatile control of lower limb exoskeletons

In this thesis, multiple aspects of control of lower limb exoskeletons with series elastic actuators are investigated. The first part explores the user-device interaction with a focus on delivering optimal assistance to a broad user group. Therefore, force control of the series elastic actuators with an active trade-off between torque bandwidth, apparent impedance and unconditional interaction stability is proposed. Such a trade-off guarantees that the controlled device can achieve the desired assistance, which is transparent to user motions, and passive with respect to its interactions with the user in its environment. Passive and transparent interactionis generator takes user and device limitations into account to prevent injuries tos give the user full control over the device, as their intended motions are always directly reflected in the exoskeletons motions. Initial insights into the relationship of these trade-offs and their relevance for specific applications lead to application and device specific guidelines for torque controllers of series elastic actuators. These guidelines include insights into methods that improve transparency, such as disturbance observers, and their limitations when interaction passivity is also required.
The second part proposes a trajectory generator, an assistance solution for lower limb exoskeleton users with complete spinal cord injuries. This solution generates gait trajectories based on high level targets, like a foot position for stepping onto a tilted surface. At the same time to the user. By also considering user preferences as well as comfort, the solution enables user-centric assistance design. Examples of such considerations are step height and speed preferences. Such design, minimizes the gap between user expectations and device performance, which allows for full focus on walking with the exoskeleton.