# Design and Implementation of a Multi-Function Gripper for Grasping General Objects
## 2 Architecture Description of the Multi-Function Gripper  
In this section, the architecture of the proposed three-DOF robotic finger and the two-fingered underactuated gripper is presented in detail. The suction system and its integration into the gripper are introduced.  
As we mentioned earlier, underactuation can be achieved through using passive elements such as springs and mechanical stoppers. First, a demonstration of the closing sequence of a two-phalanx robotic finger is shown in Figure 1 to clearly understand the working principle of underactuation. This finger is actuated by the lower link indicated by the arrow. This finger has two phalanxes, whereas only one actuator is used for control. The spring and mechanical stopper are used to constrain the relative motion between the two phalanxes. First, before the proximal phalanx makes physical contact with the object to be grasped, the whole finger moves as a single rigid body, as shown in Figure 1a,b. Second, when the proximal phalanx makes physical contact with the object, it stops its movement. In this case, the actuation toque overcomes the preloading of the spring, and the distal phalanx continues to rotate [as shown in Figure 1c] relative to the proximal phalanx until it also makes physical contact with the object [as shown in Figure 1d]. It is noted that the finger closing sequence is automatically generated by continuous actuation of the lower link, as indicated by the arrow.  
Similarly, the closing sequence of the traditional three-phalanx robotic finger is shown in Figure 2, which is an underactuated finger that can be controlled using only one motor. Robotiq three-finger adaptive gripper [21] is based on the modification of this principle. The springs and mechanical stoppers are installed at the pivot joint between each phalanx. Actuating the lower link (indicated by the arrow) can automatically generate this closing sequence. The three phalanxes will make contact with the object in sequence. However, this underactuated robotic finger can only be used to perform shape-adaptive grasping. In a real world scenario, we expect the robotic finger to have multiple grasping modes such as parallel and shape-adaptive grasping. In this case, special design architecture of the three-phalanx robotic finger needs to be developed. Moreover, sometimes, we need to fully actuate a linkage-driven robotic finger to achieve high dexterity. With respect to the three-phalanx robotic finger shown in Figure 2, we cannot install all actuators at the base. Floating actuators will generate large moving inertia, and the size of the floating actuator is confined to the mechanical dimension of the finger.  
Thus, the initial motivation of this research is to propose a new three-phalanx robotic finger that can achieve multiple grasping modes. To fully actuate this three-phalanx robotic finger, another design goal is to install all the actuators at the base. Furthermore, the motion of the distal phalanx is expected to be decoupled from the proximal and intermediate phalanxes to reduce control complexity and increase dexterity. Finally, a suction-gripping system is integrated to grasp objects in cluttered, narrow spaces.  
Figure 1: The closing sequence of a two-phalanx underactuated robotic finger.  
Figure 2: The closing sequence of a three-phalanx robotic finger.