Undesired Constraint Forces in non-ergonomic wearable Exoskeletons

xoskeleton robots are currently being developed in many research lab’s for the rehabilitation of patients suffering from injuries to the nervous system. A multitude of exoskeletons for interaction with the human limb are proposed for rehabilitation training, ranging from external, end-point based devices to wearable full limb exoskeletons [1]. Their ability to smoothly interact with the human subject is crucial for successful application in physical therapy. It is important that devices interacting closely with a human limb are intrinsically safe, comfortable and are able to exploit the full range of natural motion for movement training. The two main aspects that need good consideration are the implementation of the actuation and motor control, as well as the intrinsic mechanical and kinematic design of their structure. Still a couple of years ago, such rehabilitation exoskeletons were mostly equipped with motor controllers that dictate movement to the patient. While this seemed to suffice during the childhood of such devices, now, researchers want more freedom to implement assistive therapy protocols as well. In patient assist therapies, the rehabilitation devices actuation supports, but does not impose, the natural movement of the patient during training. Sophisticated controller concepts based on impedance and admittance control are currently being implemented in such rehabilitation devices. Hybrid controllers in rehabilitation exoskeletons provide already significantly better safety for the user than the earlier position controlled devices. Thus, from an actuation and motor control point of view, safety and user comfort criteria can be satisfied. An area, however, to which too little attention is paid, is the appropriate kinematic design of the rehabilitation device structure. If the kinematic setting of a rehabilitation robot is not well matched to the patient, undesired interaction forces can be created during motion, even if no actuation is provided at all. Those constraint forces can be large in magnitude and provide a safety hazard as well as discomfort to the user. It is interesting to note that such forces, stemming mostly from misalignments between the device’s axis of motion and the human limb, can usually not be compensated by the device’s actuators. Kinematic mismatches between the Lokomat leg orthosis and patient legs, for instance, were already shown to be responsible for injuries and discomfort. This was reported in [2] and [3]. Furthermore, it was shown in [4] that kinematic mismatch between an orthosis and a patient can alter the natural muscle activation patterns. This is counterproductive during physical therapy and could also lead to injury.