The role of median fins in linear acceleration was one of the least explored issues in fish locomotor behaviors, but researchers from Beihang University and Harvard University have taken the first step in providing a quantified answer. In a study published online in the leading robotic academic journal Soft Robotics on April 10, the team led by Prof. Wen Li from the Biomechanics and Soft Robotics lab and the School of Mechanical Engineering and Automation used an undulatory robotic fish with soft fluidic elastomer actuated morphing median fins to mimic live fish, and they measured an increase in swim speed and a reduction in side forces as the median fins erected up. Dr. Valentina Di Sato and Prof. George Lauder from Harvard University conducted the live fish experiments and analysis. Graduate students Ren Ziyu (now a PhD student at Max Planck Institute, Germany), and Hu Kainan, Yuan Tao from Beihang University performed the design, fabrication and testing of the soft robotic fish model.
Fig. 1 (A) The biomimetic robotic ﬁsh with median ﬁns fabricated by the soft robotics lab (video). (B) The biomimetic soft dorsal fin at fully-erected (pneumatic pressure: 0 Kpa), half-erected (153 Kpa) and fully-fold-down (278 Kpa) states.
Based on the data of live fish provided by their Harvard collaborators, researchers from Beihang University made full use of their rich experiences in underwater soft robotics and fabricated spiny dorsal, soft dorsal, and soft anal ﬁns by multimaterial three-dimensional (3D) printing. The median fins were able to erect, fold and undulate from side to side (spiny ones can only erect and fold) using air-powered micro fluidic elastomeric soft actuators that imitate the corresponding erector/depressor muscles of live ﬁsh, as shown in Fig. 1(B). They were programmed to move according to ﬁve typical median ﬁn motion patterns observed in bony ﬁshes during swimming: all fins folded, spiny fin erected, soft fins half-erected, soft fins erected and all fins erected.The fins were fixed to an undulatory fish robotic model, which was connected to a towing system by a low-drag streamlined strut in head and a multi-axis ATI force transducer. During the experiments conducted at mid-depth in a tank, the model was first towed to reach a self-propelled speed as the thrust and drag force struck a balance over a single tail beat cycle. Next, the robotic ﬁsh model accelerated from initial zero velocity, and the mean axial forces at different rates of acceleration were calculated during the acceleration stage. A digital particle image velocimetry (DPIV) system, including the laser system, high-speed camera and particles, was adopted to visualize and measure the ﬂow ﬁeld.
Fig. 2 (A)Critical linear acceleration of the biorobotic ﬁsh under different median ﬁn states. (B) The average peak-to-peak side forces on the biorobotic ﬁsh under different median ﬁn states.
The experiment results show that the soft dorsal and anal ﬁns enhanced both the linear acceleration and the steady swimming speed. Compared with folded fins, fully erected soft ﬁns could produce at most a 32.5% increase in the linear acceleration of the robotic ﬁsh (Fig. 2(A)). Also, when the soft dorsal and anal ﬁns were fully erected, the self-propelled swimming speed (Usps) increased by 3.04% over that of the folded fin state, and the maximum increase in Usps (7.29%) was measured when the soft fins were half-erected. In both cases, erected spiny dorsal fins exerted little influence.
Apart from the speed, there was a probably counterintuitive discovery: erecting the soft dorsal and anal ﬁns could reduce up to 24.8% of the magnitude of the side force on the robotic swimmer (Fig. 2 (B)). A further analysis of the wake flows (Fig. 3) found that during both linear acceleration and steady swimming, the lateral force produced by the dorsal and anal ﬁns counteracted the caudal ﬁn force, because the motion of these two groups of ﬁns was largely out of phase and the two forces were oriented to opposite sides. The effect of the oscillatory forces produced by the tail was reduced by the dorsal and anal fin forces, and the fish was able to maintain its lateral stability better.
Fig. 3 Wake ﬂows generated by the soft dorsal ﬁn and caudal ﬁn of a bluegill sunﬁsh (A and B) and the robotic ﬁsh (C and D) during the linear acceleration.
The lightweight and compliant biomimetic dorsal/anal fins could resist fluid loading and quickly recover from external impacts in both the lateral and axial directions without damage, so the application of them to underwater robots was expected to improve the robots’ swimming performance in the complicated water environment. The researchers also planned to equip these highly adaptive fins with sensors that could help the fins sense the wake flow and then change their motion in response to external interferences, building them into an intelligent closed-loop system.
According to the study, morphological features of the biomimetic fin components, such as the size, shape, and mechanical stiffness, as well as the fin positions on the fish body, could be changed according to different research needs. Therefore, the modular and modiﬁable biorobotic system used in this study holds great potential as a scientiﬁc tool to further examine the locomotor functions of the diverse ﬁn morphologies, kinematic patterns and sensory feedback control of the complex and multifactorial swimming behaviors in live ﬁshes.
For more information:
The research article:
Biomechanics and Soft Robotics Lab:
Reported by Tan Lisha, Li Mingzhu and Xiong Ting
Written by Li Mingzhu
Designed by Yang Zhihan
Reviewed and Released by GEOOS
Special Thanks to the School of Mechanical Engineering and Automation and Biomechanics and Soft Robotics Lab
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