4.2. Adjustments in Practice Suppos

4.2. Adjustments in Practice Suppose the misalignment errors e1 and e2 do not change during the upper limb movement. Their values are relatively small and unknown to the user, which are quite common in the application of the exoskeleton as the human-machine joints are aligned as perfectly as possible but without guarantee. We propose a method to adjust the PGCUs to achieve good performances. 1) Tunning of PGCU 1 First, we select an off-the-shelf linear compression spring whose stiffness, ku, ots, is as close as possible to the nominal stiffness value, ku, nomial, calculated by Eq. (21.1) with misalignment errors e1 and e2 set to zeros. ku, ots is usually not equal to ku, nomial from the fact that stiffnesses of available off-the-shelf springs are discrete. Next, we tune the crank radius of PGCU 1, namely, ru, to simultaneously compensate the spring stiffness deviation and the errors e1 and e2 by observing whether the upper part is in a static state. Note in an implementation, the length adjustment of ru may be realized by the lead screw and nut mechanism with the self-locking performance which facilitates the tunning process. 2) Tunning of PGCU 2 We select an off-the-shelf linear compression spring whose stiffness, kf, ots, is as close as possible to the nominal stiffness value, kf, nomial, calculated by Eq. (22) with l5 equal to l4. Then, turn the crank radius rf to make the unbalanced torque as small as possible.