Soft grippers with soft fluidic bending actuators acting as fingers have emerged as a promising technology for safe and delicate pick-and-place including foods, pharmaceuticals, and logistics applications. With conventional soft grippers, however, the object size that can be gripped is limited by the initial configuration (Figure 9A). To widen the gripper opening, prestressed or bidirectional soft fluidic actuators were developed,\cite{Wang2017a} but the problem where the grippers should only approach the object vertically to avoid collisions still remains, which may reduce the work efficiency and increase the burden of the manipulator control and operation.
We constructed a dual-origami two-finger gripping unit where each finger at both sides is a six-module dual-origami fluidic soft fluidic bending actuator. The initial configuration was designed to be parallel to the objects to achieve a wide gripper opening (Figure 9B). The total width of the two-finger unit was relatively small compared to conventional soft robotic finger, but the finger unit could grip larger objects, even objects larger than its initial size. In addition, because the compactness greatly reduced the risk of collision, the finger unit could approach to objects via various paths (vertical, horizontal, and diagonal from bottom to top, Figure 9C).
The motion and closed configuration were pre-programmed via FEA simulation and prototyping for fast and versatile grip. When all strain-limiting layers were built with the same parameters that prefer deployment over bending (Design II of Figure 9D), the closed configuration was achieved at 100 kPa with a 93 mm gap. On the other hand, when the modules at the bottom were designed to be bent greatly (Design I of Figure 9D), the gripping unit formed a narrow-closed configuration with a 65 mm gap at 40 kPa. Therefore, Design I can be considered as the suitable unit that allows for fast grip with sufficient contact area during power grip (grip that wraps around an object).
The gripping force of Design I gripping unit was measured (experimental setup is shown in Figure 10 and explained in the Experimental Section). We tested gripping three PLA cylinders of different diameters, 25 mm, 50 mm and 75 mm. While the gripping unit was holding the cylinders, the cylinders were pulled at a uniform speed (3.3 mm/s). As shown in Figure 9E, the peak force appeared during power grip, and then a relatively uniform force was measured during pinch grip, and finally the cylinder was completed pulled out. At 220 kPa, the peak force of 3.15 kgf, 1.53 kgf, and 1.25 kgf were measured for 75 mm, 50 mm, and 25 mm cylinders, respectively (Figure 9F). The gripping unit is lightweight (56.8 g), and the payload-to-weight ratio exceeds 55. We also tested for repeatability of the gripping unit. After 1000-cycles at 40 kPa, the bending angle of the gripping unit was change by 8.19% compared to the 200-cycles (Figure 11).