# The Enhanced Adaptive Grasping of a Soft Robotic Gripper Using Rigid Supports
## 1 Introduction  
A soft robotic gripper using a soft pneumatic actuator (SPA) is able to grasp irregularly shaped objects due to its excellent properties [1, 2]. It is effective to apply rigid structures to improve the loading ability of SPA. [PERSON] et al. integrated soft material and rigid structures to increase the lifting force [3]; [PERSON] et al. added a stiff rod inside each soft finger to change to a bending point [4]. Rigid structures can be used to improve the loading ability of SPA, but embedding rigid structures inside the soft fingers can easily puncture the soft finger and cause air leakage. Applying a rigid actuator outside the soft gripper is another way to improve the lifting force by providing an extra support force [5].  
High air pressure has higher requirements for equipment and higher expenses. [PERSON]'s gripper required 80 kPa to reach a lifting force of 13.72 N [6], and [PERSON] et al.'s gripper needed 80 kPa to reach a lifting force of 50.1 N [7]. They needed high incremental pressure to grasp heavy objects, which is also a security hazard when the air chamber cannot withstand such high pressure. Grasping heavy objects (about 5 kg) at lower incremental pressure (below 30 kPa) can save energy and improve production efficiency.  
The dimensions of objects typically govern the structure of designed grippers. When the grippers grasp large-sized objects, a large contact area can help deliver a wrapping grip, which favors the stability of grasping [8]. However, the grasping of large-size objects requires a long actuator [9]. When the actuator is too long, the deformation curvature of the actuator will be much greater than the diameter of the sphere [10], which decreases the contact area between the gripper and the surface of objects. Applying the skeleton layer outside the soft gripper well constrains the radial expansion and significantly enhances the bending motion [11].  
In addition, one common issue for traditional SPA is that the contact area between the soft fingers and the grasped objects is small, which affects the stability of the grasping [12]. [PERSON] et al. increased the contact area between the soft fingers and the grasped objects to significantly enhance the conformability of the gripper by using mechanical metamaterial [12]. Using soft silicone is another way to increase the contact area. In manyprevious studies [3; 4; 5; 6; 7], only one type of silicone was selected for the manufacturing of the soft fingers, which limits the performance of the soft gripper. The same type of silicone has the same expansion rate, which makes the bending curvature the same. Under this circumstance, the upper surface of some grabbed items like cubes will not be pressed by the fingers because the grasping curve is nearly a circle. On the contrary, beneficial intrinsic mechanical properties of silicones directly implied in the different parts of the soft gripper can improve the performance of the soft gripper [13]. Soft silicone is easier to deform and fit items but harder to support the item due to its low strength, while hard silicone has a stronger anti-extrusion property but poor deformation. By applying different types of silicone with specific hardness in specific parts of the soft fingers, we make use of the beneficial intrinsic mechanical properties of both soft silicone and hard silicone. The curvature of the grasping curve is variable because of the variable hardness of the silicone.  
In this paper, we design the rigid supports outside the soft fingers, which provide an inward torque, making the objects lifted mainly by the support force and limiting the growth of the deformation curvature to improve the grabbing ability. The main contributions of this paper are as follows: (1) the design and the principles of the rigid supports, (2) the fabrication and experimental verification of the soft gripper, and (3) the demonstration of the soft gripper.