# Design and Analysis of a Novel Variable Stiffness Joint Based on Leaf Springs
Joints that follow this design include CETs-VSA [8] and SPVSA [9], which are advantageous in terms of strong adaptive motion control and better simulations of the dynamic behavior of human joints. However, the coupled motion of the two motors results in complex controls [10]. In the case of the independent layout of VSJs, there are various methods to change the joint stiffness. Changing the spring preload is a strategy where the driver stiffness is regulated by altering the spring force received during deformation via the adjustment of the initial deformation of the spring. The mechanical adjustable compliant controllable equilibrium position actuator (MACCEPA) is a representative example [11]. Meanwhile, the floating spring joint (FSJ) [12], the bidirectional antagonistic floating spring actuator (BAFSA) [13], the lightweight variable stiffness actuator (LVSA) [14], and the reconfigurable variable stiffness actuator (RVSA) [15] are all based on this scheme. However, the regulation of the spring compression amount (determining the stiffness characteristics) depends on the cam profile curve. The cam profile curve is based on the stiffness adjustment mechanism of the cam device, which increases the complexity of stiffness calculations and control [16], while joint compliance is adjusted by changing the preload of the elastic element, which then results in high energy consumption and high resistance to adjustment [17]. In the case of a VSA, stiffness adjustment can also be achieved by introducing a variable lever mechanism between the elastic element and the load connection point. Specifically, the compression of the spring can be dynamically adjusted by changing the point of action of the elastic element on the lever, the load connection point [18], or the lever fulcrum [19], thereby adjusting the output stiffness of the driver. Based on this principle, researchers have proposed various VSMs, such as the variable stiffness actuator vsaUT-II [20], the pseudo-linear variable ratio lever variable stiffness actuator (PLVL-VSA) [21], a rotational joint actuator based on the mechanism of a flexible rack and gear (vsaFGR) [22], the variable stiffness actuator based on a rocker-linked epicyclic geartrain (REGT-VSA) [23], and the variable stiffness actuator based on a second-order lever mechanism [24], among others. In these VSMs, flexible rack mechanisms, planetary gear mechanisms, or linkages mechanisms are used to adjust the position of the lever fulcrum. However, the introduction of these mechanisms causes the entire joint to be too complex and bulky. The method of changing the lever fulcrum position via the use of an Archimedes spiral displacement mechanism alleviates this limitation [25]. In the above stiffness adjustment methods, the physical parameters of the flexible element itself are not changed during the adjustment process, while changing the effective length of the elastic element is a common practice to achieve stiffness adjustment. Typical examples include the parallel double-motor compliant VSJ [26], the mechanical rotary variable impedance actuator (MeRIA) [27], the vsaSDR designed in combination with Archimedes spiral [28], and the variable stiffness actuator based on a cam-leaf spring mechanism (CLSM-VSA) [29], among others. However, in most current designs, the adjustment of the effective length of the elastic element is often achieved using ball screws, gears, or linkages, which can often complicate the joint design, and can only provide a single stiffness range, which is not effective for certain application scenarios. Therefore, joints with reconfigurable capabilities have been developed, whereas the number of plate springs can be reconfigured in the joint LSVSJ [30] to achieve stiffness adjustments across different ranges. However, its stiffness adjustment depends on the manual operation and its load capacity, which is often quite low.