Biologically inspired swimming robotic frog based on pneumatic soft actuators

Research on soft robots and swimming robots has been widely reported and demonstrated. However, none of these soft swimming robots can swim flexibly and efficiently using legs, just like a frog. This paper demonstrates a self-contained, untethered swimming robotic frog actuated by 12 pneumatic soft actuators, which can swim in the water for dozens of minutes by mimicking the paddling gait of the natural frog. We designed two kinds of pneumatic soft actuators as the joints on the robotic frog's legs, which allows the legs to be lighter and more compact. It is found that such soft actuators have great potential in developing amphibious bionic robots, because they are fast-responding, inherently watertight and simple in structural design. The kinematic analysis in swimming locomotion was conducted for the prototype robotic frog, and the locomotion trajectory of each leg was planned based on the analysis of the paddling gait of frogs. Combined with the deformation model of the soft actuators, the robotic frog's legs are controlled by coordinating the air pressure of each joint actuator. The robotic frog's body is compact and the total mass is 1.29 kg. Different paddling gaits were tested to investigate swimming performance. The results show that the robotic frog has agile swimming ability and high environmental adaptability. The robotic frog can swim forward more than 0.6 m (3.4 times the body length) in one paddling gait cycle(6 s), whose average swimming velocity is about 0.1 m s^-1, and the minimum turning radius is about 0.15 m (less than 1 body length).