# The Development of a Soft Robot Hand with Pin-Array Structure
## 1 Introduction  
As one of the mediums for robots to interact with the outside world, robot hands have an important role and certain value for the research. Traditional robot hands can be divided into three main types: industrial grippers, dexterous robot hands, and underactuated robot hands. 1. Industrial grippers are designed for the purpose of simplicity, safety, and low cost. Usually, there are several parts that can move relative to each other, such as MPG gripper [1] and MPZ gripper [2] developed by the SCHUNK. In addition, some industrial grippers use electromagnets and suction cups to achieve the function of grasping. However, this kind of gripper has the disadvantage of a single function, and a particular type of gripper can only be used on a particular type of production line. 2. The dexterous robot hands are designed to mimic the structure of the human hand and are designed to have multiple fingers. [PERSON] [3] defines the dexterous robot hand like a robot hand with more than 3 fingers and more than 9 degrees of freedoms (DOFs). Common dexteroushands, such as Utah/MIT hand [4], Gifu hand [5; 6; 7], and shadow hand [8], mimic the structure of the human hand and can perform complex operations, but they bring complex drive systems and control systems, as well as high costs. 3. In order to solve the high-cost problem of the dexterous hand, the underactuated robot hand is proposed. The underdrive means that the number of driving sources is smaller than the DOFs of the robot hand. From the existing research, the underactuated robot hand can be roughly divided into two parts, one is with obvious finger structure and the other is without obvious finger structure. This paper mainly studies on the underactuated robot hand. Underactuated robot hands with distinct fingers structure include both rigid and flexible types. The rigid underactuated robot hands mainly use the combination of mechanical structure and reins or springs to achieve underdrive, such as SDM robot hand [9], FRH-4 hand [10], the hand developed by Shanghai Jiaotong University [11], TH-3R hand [12], and PASA hand [13]. The flexible underactuated robot hands tend to have more flexible adaptability, such as the soft hand developed by [PERSON] [14], OS Hand [15], Soft Grippers [16] developed by [PERSON] et al., and Gripper [17] developed by [PERSON] et al. These multi-finger underactuated hands have a good effect on reducing costs and reducing the difficulty of the control system. Only from the perspective of achieving more reliable performance of self-adaptive crawling, some underactuated hands without obvious finger structure have been proposed, such as the universal gripper based on the jamming of granular material [18], the universal gripper using mrc fluid [19], the SSA gripper [20], and the FlexShapeGripper developed by FESTO [21]. This type of underactuated hand has reliable gripping performance, low cost, and simple control, and has excellent performance. Therefore, this paper aims to develop a new type of underactuated robot hand. [PERSON] proposed a universal gripper named Omnigripper [22], as shown in Figure 1. The Omnigripper consists of a number of slidable pins that can slide up and down. These pins are divided into two sets, one is the left set, and the other is the right set. When grasping objects, the two sets of pins adapt to the shape of the object from the vertical direction, and then the two sets of pins move toward each other along a line, which provides a clamping force for gripping. The gripping method of omnigripper has an excellent self-adaptive ability, but since the pins move only along a straight line, the omnigripper can only provide a grasping force from one direction, and cannot provide a grasping force when grabbing certain objects. For example, in Figure 2, the long object A, and the object C covered by only one set of pins cannot be crawled by omnigripper. Figure 1: The Omnigripper.