# Design and Analysis of a Novel Variable Stiffness Joint Based on Leaf Springs
## 3 Overall Structural Design of VSJ
The reconfiguration mechanism includes a lower end cover (Part 22) that matches the support column, a screw end cover (Part 18) which is used to restrict the radial movement of the screw, a screw (Part 21) parallel to the axial direction of the support column, and a leaf spring seat (Part 20) fitted onto the screw. The outer side of the leaf spring seat is fixed to the leaf spring end (Part 17), while there are matching sliders and slides between the inner side of the leaf spring seat and the support column. The free end of the leaf spring is clamped between the two column heads of the double fork body. In order to adjust the quantity of leaf springs engaged in the transmission process, the screw (Part 21) can be manually (or automatically, if necessary) individually rotated. This action induces the corresponding leaf spring seat to traverse along the axial direction of the VSJ, thereby facilitating the vertical displacement of the corresponding leaf spring. When \(L>h\), the leaf spring completely disengages from the double fork body of the slider. At this point, the leaf spring does not participate in the stiffness adjustment. The adaptability of the VSJ to the current working condition is enhanced by the discrete adjustment of the stiffness range of the VSJ and its load capacity. Under normal operation conditions, the input torque is transmitted from the drive motor to the output frame in the order shown in Figure 10a as follows: 1--drive motor \(\rightarrow\) 2--timing belt drive \(\rightarrow\) 3--drive disc \(\rightarrow\) 4--main drive frame \(\rightarrow\) 5--slider \(\rightarrow\) 6--leaf spring \(\rightarrow\) 7--output disc. At the initial stage of the joint's operation, the compliance of the joint is achieved via the bending of the leaf spring under the action of the load. The bending of the leaf spring absorbs the energy fluctuations of the load, thus preventing sudden load changes from impacting the joint. Simultaneously, the joint driving process becomes more stable, which also serves as a form of protection for the robot joint. During the cyclic movement of the motion, the compliance of the joint has the potential to repeatedly utilize the energy stored in the spring deformation, which contributes to a more energy-efficient operation. Figure 12: CAD model of stiffness/load reconfiguration mechanism: 15—Slider; 17—Leaf spring; 18—Screw end cover; 19—Support column; 20—Leaf spring seat; 21—Screw; 22—Lower end cover. Moreover, in order to perceive external forces within a certain range and obtain feedback of key control variables, the sensing system of the proposed VSJ includes three encoders. An encoder ring is installed on the outer side of the drive end disk, which is used to measure the relative rotation between the drive end disk and the shell, while reader \(1\) is installed at the lower end of the shell. Another encoder is installed at the tail of the stiffness adjustment motor, which is used to measure the relative rotation angle between the stiffness adjustment disk and the transmission disk. This angle is used to calculate the effective length of the leaf spring during the stiffness adjustment process, based on the kinematic relationships. An encoder ring is installed on the circumferential surface of the output end disk, and reader \(2\) is set on the end face of the shell, which is then used to measure the relative rotation between the output end disk and the shell. Based on the difference between the data of the two readers and the calibrated mechanical zero point, the elastic deviation angle of the entire VSJ can be obtained. Combined with the established stiffness model (refer to Equation (9)), the magnitude of external forces or load forces can be perceived to a certain extent. The design of the VSJ concerns future exoskeleton robot scenarios. By referring to the existing literature and combining the above simulation process attempts, the design parameters of the VSJ shown in Table 2 are finally obtained.