# Multi-Mode Compound Grasping Robot Finger Driven by Linkage
## 2 Design of MCG Finger
### Working Principle of MCG Finger  
In its initial state, the MCG finger is set up so that the first linkage makes contact with the first pushing block, the second linkage contacts the first limiting block, and the middle phalanx connects with the second limiting block, thanks to the force exerted by the first to the third springs. By coordinating the movement of the two motors, the MCG finger can switch between various composite grasping modes. Figure 8: Structure sketch of the MCG finger. Figure 7: Oblique view (**a**) and main view (**b**) of inner parts of MCG finger. During the initial stage, the first, second, and third springs will pre-stretch and create a contact angle \(\xi_{1-3}\) between the first linkage and the first pushing block, the second linkage and the first limiting block, and the middle phalanx and the second limiting block, respectively. As the first and second pushing blocks are connected to the first and second motors, respectively, this ensures that the MCG finger remains in a stable state prior to grasping. According to the structure of the MCG finger, the working principle is as follows (shown in Figures 9 and 10):  
Figure 10: Working principle of the MCG finger (P-GC, CO and D-GC mode). Figure 9: Working principle of the MCG finger (PA-S&D-SA mode). One of the characteristics of underactuated robot fingers is that they can grasp without complicated control. Therefore, when controlling the movement of MCG fingers, each motor will have three different output states: stop (speed is zero), rotate(speed is 0.5\(v\)), and rotate(speed is \(v\)). \(v\) is the speed of the motor at rated voltage, and correspondingly, 0.5\(v\) is the speed at half of the given rated voltage. For different grasping modes, different output speeds can be set for the two motors; details can be seen in the graph, which is shown in Table 1.  
In order to effectively switch between different grasping modes, the MCG finger relies on the contact conditions with the object and the desired outcome during the grasping process. For active modes (PA, CO, P-GC, and D-GC grasping modes), the MCG finger proactively adjusts its position and movement based on predefined commands, which are determined by the object's shape and the desired grip. On the other hand, the passive modes (M-SA and D-SA) are activated in response to contact with the object, allowing the finger to adapt to the object's shape for a more stable and secure grip. The contact points and the sequence in which the phalanges interact with the object determine the appropriate grasping mode. As the MCG finger encounters an object, the contact conditions are evaluated in real-time, allowing the system to adapt and choose the most suitable grasping mode. The specific grasping process is shown below (take the PA-M&D-SA mode, for example):  
The first motor rotates forward at speed \(v\), drives the first worm through the first reducer, the first worm wheel rotates, and the first transition shaft rotates to drive the first gear to rotate so that the first transmission wheel rotates forward, and the first pushing block rotates forward, pushing the first linkage to rotate around the proximal shaft, and then pushing the distal phalanx through the third linkage. Because the second motor is not started, the second driving wheel and the second pushing block are not moved. The second spring connects the second linkage with the second pushing block, which makes the second linkage maintain the initial posture relative to the base. Before the proximal phalanx and the middle phalanx do not contact the object, the quadrilateral ACFE maintains the parallelogram, so the phalanx FC remains parallel to the phalanx AE, that is, the distal phalanx keeps the initial posture unchanged relative to the base. Thus, the parallel clamping process is realized. When contacting objects, there are three possible results:  
1. The distal phalanx first contacts the object, the grasping is finished, and the parallel clamping function for the object is realized. 2.