Saskia Baltrusch

125 Chapter 5 Spinal Structure Concept Since it is challenging even for skilled people to correctly align an exoskeleton [33], we decided to strive for a design, where only a minimal amount of initial alignment is required. Correct alignment even with single hinge joints, such as the elbow, are a challenge [18]. Therefore, correct alignment with all five spherical joints of the lumbar spine is arduous. However, since good results can be expected by compensating for misalignment, this approach was chosen here. Additional degrees of freedom can be added in many forms. One that at the same time allows to store energy, is the use of flexible materials (Figure 3A). A long and slim flexible structure can be bent in two directions and deformed under torsion. However, even to compensate for flexion and extension in the sagittal plane, at least two additional compensating degrees of freedom are required [18]. One to account for the length change, and one for correct alignment with the back. The same holds for the lateral bending and axial rotation. he choice of the position of the additional degrees of freedom used to provide compensation is relatively arbitrary, but attention should be payed to singular positions of the used mechanism [15]. However, the choice of a mechanism can have practical implications [16]. Choosing a center of rotation of an exoskeleton joint, to be located relatively far away from the corresponding human joint, will require a big compensatory effort, compared to almost coinciding joints. A flexible structure is used as a compensatory joint and energy storage a the same time. A combination with a linear joint along the flexible structure and an additional spherical joint was found to compensate for a large part of the misalignments in an iterative approach (Figure 4). 5