# Design and Implementation of a Multi-Function Gripper for Grasping General Objects
## 2 Architecture Description of the Multi-Function Gripper
### Two-Finger Underactuated Gripper and Integrated Suction System  
Even though the robotic finger shown in Figure 3a can be fully actuated to achieve high dexterity, in the current study, we focus on developing an underactuated type that can realize parallel and shape-adaptive grasping. Similar to the two-phalanx underactuated robotic finger shown in Figure 1, one torsion spring and one mechanical stopper are installed between the proximal and intermediate phalanxes to realize an underactuated finger, as shown in Figure 3b. The torsion spring is used to prevent free motion between the proximal and intermediate phalanxes. The proximal and intermediate phalanxes are passively coupled with each other by the torsion spring and mechanical stopper. There exist two independent motions in this finger, i.e., the open-close motion of the finger and distal-phalanx orientation adjustment. Two motors are required to control this underactuated finger. One motor rotating link \(L_{\text{AD}}\) is used to control the open-close motion of the finger, and the other one rotating link \(L_{\text{AE}}\) is used to control the orientation of the distal phalanx. Thus, the open-close motion and distal-phalanx orientation adjustment are decoupled from each other. The grasping sequences of the parallel and shape-adaptive grasping are shown in Figure 4a,b, respectively. For the parallel grasping shown in Figure 4a, when independent link \(L_{\text{AD}}\) is actuated and if no external contact occurs at the proximal phalanx, the proximal and intermediate phalanxes move together as a single rigid body (from phase I to phase III) because the torsion spring prevents a relative free motion between them. In general, preloading of the torsion spring is required to prevent any undesired motion due to the gravity and inertia effects during the open-close motion. Figure 3: (**a**) Architecture of the three-DOF (degrees of freedom) linkage-driven robotic finger. (**b**) Using torsion spring and mechanical stopper to realize the underactuated finger. For the shape-adaptive grasping shown in Figure 4b, if no external contact occurs at the proximal phalanx, activating independent link \(L_{\text{AD}}\) generates a free open-close motion (from phase I to phase II), similar to that of the parallel grasping. Starting from phase II, physical contact occurs between the object to be grasped and the proximal phalanx. In this case, the proximal phalanx stops the movement, and the intermediate phalanx continues to move against the torsion spring until it makes contact with the object (from phase II to phase III). Thus, the torsion-spring stiffness should be designed as small as possible, but sufficiently big to prevent undesired motion during the free open-close motion. Furthermore, from phase III to phase IV, we can see that the orientation of the distal phalanx can be adjusted by activating independent link \(L_{\text{AE}}\) to add one more contact with the object. Actively adjusting the orientation of the distal phalanx allows this robotic finger to perform multiple grasping tasks. Controlling orientation of the distal phalanx is a special feature that other contemporary grippers have not had. It is noted that after preforming the shape adaptive grasping and releasing the object, stored load in the torsion spring will force the proximal and intermediate phalanxes to go back to their original configuration. The proposed robotic finger can be used to design multi-finger robotic hands/grippers. In this work, we use the underactuated-type robotic finger to design a two-finger underactuated gripper, as shown in Figure 5. The kinematic parameters of the underactuated gripper are listed in Table 1. The two fingers of this underactuated gripper are independently operated. Furthermore, for each underactuated finger, two actuators are required to control its open-close motion and the orientation of the distal phalanx. For each motor, a worm gear is used as a non-back-drivable transmission mechanism to ensure grasping.