# Design and Demonstration of a Flying-Squirrel-Inspired Jumping Robot with Two Modes
## 4 Design of Jumping Robot
### The Mechanism of Loading and Releasing of Spring  
During takeoff, the movement of the hind limbs are driven by the spring, and the energy storage and releasing of the spring are controlled by the loading and releasing mechanism. In order to meet the different requirements of the takeoff speed of the different jumping modes and considering the influence of the overall mass on the jumping performance, the loading and releasing mechanism of the spring should have the following three characteristics besides being able to completely release the stored energy in an instant: 1. The loading degree is controllable; 2. releasing time is controllable; and 3. loading and releasing are driven by the same actuator. Therefore, we designed a mechanism that can realize controllable loading and releasing of the spring, as shown in Figure 11. It is composed of the transmission gear, transmission shaft, cylindrical cam, spring plugs, one-way bearing, ratchet, pawl, winding pulley, restriction spring, and wires. The transmission gear and cylindrical cam are fixed on the transmission shaft. One end of the plugs is in contact with the curved profile of the cylindrical cam, and the other end is inserted into the through hole of the ratchet. The ratchet is mounted on the transmission shaft by the one-way bearing. The winding pulley can rotate freely around the transmission shaft, and its axial movement is constrained by the restriction spring. The wires are wound around  
Figure 10: 3D model of hind limbs of the robot. the winding pulley and connected to the end of the hind limbs. The overall structure is shown in Figure 11.  
It is assumed that the plugs are inserted into the circumferential distribution holes of the winding pulley under the extrusion of the cylindrical cam at the initial moment. When the transmission gear drives the cylindrical cam to rotate clockwise through the transmission shaft, the ratchet rotates in the same direction and at the same speed with the cylindrical cam under the action of the one-way bearing. Furthermore, the winding pulley also rotates clockwise driven by the plugs, thus winding the wires to compress the structure of the hind limbs, which realizes the loading of springs and storage of energy, as shown in Figure 11. When the transmission gear drives the cylindrical cam to rotate counterclockwise through the transmission shaft, the ratchet stays still due to the pawl, which causes the plugs to quickly withdraw from the hole in the pulley. In this case, the pulley loses its rotational constraint and instantly loosens the wires to release springs of hind limbs, as shown in Figure 11. Under the constraint of the restriction spring, continuing counterclockwise rotation of the cylindrical cam causes the plugs to be reinserted into the pulley, ready for the next loading and releasing. Different from the triggering mechanism of most miniature jumping robots, this completely independent design of loading and releasing allows the springs to be released in an arbitrary state of energy storage, which realizes the active control of takeoff speed. Figure 11: Composition and principle of the mechanism of loading and releasing of spring. (**a**) Overall structure; (**b**) loading; (**c**) releasing. In addition, the combination of one-way bearing and cylindrical cam makes it possible to meet the driving requirements of the mechanism only using a single motor. The transmission system is shown in Figure 12. The output shaft of the motor transfers the torque to the transmission gear through the two-stage reduction gears, which drives the transmission shaft to rotate clockwise or counterclockwise. The main parameters of each piece of gear are shown in Table 1 (see Appendix A).