A modified human shoulder model considering the migration of the humeral head from the scapular glenoid fossa for the design of shoulder exoskeletons

The wearable upper limb exoskeletons designed using the traditional shoulder model have a serious issue related to joint axis misalignment with the human shoulder. The misalignment is primarily due to the limited degrees of freedom model assumed for the shoulder. The glenohumeral joint in the conventional upper limb model is considered a single spherical joint, which, when considering the human anatomy, neglects the humeral head translation from the glenoid fossa of the scapula. This shoulder model cannot be used for designing the wearable upper limb exoskeletons, which can damage the shoulder joint due to interactive forces generated from the human-exoskeleton joint misalignment. This paper presents a modified, anatomically similar, human upper limb model where the single spherical joint of the glenohumeral joint is modified into two spherical joints connected by a prismatic joint. The kinematic and dynamic model of the proposed shoulder model is explained in detail. The elbow reachable space for this model has been calculated and compared with that of the existing models, and it is found to be closer to the actual workspace. The proposed model is simulated with a conventional exoskeleton attachment to show the effect of misalignment during humeral head translation and the benefits of using the proposed model in the design of the shoulder exoskeleton. An exoskeleton test bench that uses the modified shoulder model is also presented.