Robotics inspired biology—using bio-robotics to investigate fish high maneuverability
Flexible body and appendages allow fish performing extraordinary swimming capability with highly effective cost of transport. Why do these natural creatures possess such amazing ability? The robotic technology, was traditionally used for industrial manufacturing, can also be used to reveal the underlying mechanism of biomechanics.

Figure 1. Three main vortex rings generated during C-start process of a Bluegill Sunfish (source: Integrative and Comparative Biology)

One of the amazing unsteady locomotor behaviors exhibited by fishes is the escape response. Many fish species are able to bend their body into a C shape and kick out their bodies within 60ms, therefore to escape from predators. Dr. Li Wen, a faculty of School of Mechanical Engineering and Automation at Beihang University, collaborating with Dr. Chuck Witt from department of Mechanical Engineering and Aerospace at Princeton University, and George Lauder, a professor from Organismic and Evolutionary Biology at Harvard, investigated the C-start escape behavior of live fish using a fluid technology called Digital Particle Image Velocimetry (DPIV). They found that the bluegill sunfish generated three main vortex rings during C-start as shown in figure 1. To obtain in-depth insight of this mechanism, a flexible thin foil was fabricated as a simplified physical model. Applying heave and pitch motions at the center of mass, the thin foil can produce fish-like movement of C-start process underwater. By using optimal foil modulus and imposing moderate actuation, the flexible foil would generate very similar kinematics and wake structure to that of a live fish.

Figure 2. (a) The vortexes generated by three-dimension caudal fin motion of Bluegill Sunfish which enables high agility performance. (b) Best technical paper award of International Conference on Climbing and Walking Robots.

Fish swimming has often been simplified into the motions of a two-dimensional slice through the horizontal midline, in particular, the functions of fish tail have always been viewed as a propeller which can generate force in two-dimensions and primarily propel the fish forward only since 1920s. Prof. Li Wen and his graduate student Ziyu Ren revisited this scientific issue from a new angle. According to repeatable experiments of a robotic caudal fin model that can precisely replicate the three-dimension motion of a live Bluegill Sunfish caudal fin, the researchers found that the caudal fin can control magnitude of thrust force, lift force, torque and the jet flow according to multi-axis force and DPIV synchronization. The mechanism of the moving caudal fin can be regarded as a thrust-vectoring nozzle, with which fish can move in three-dimensional domain with thrust produced, sinking or rising in the water even while propelling forward.

The research paper on fish C-start has been published on Journal of Integrative and Comparative Biology [1]. Three-dimension movement of bio-inspired caudal fin has been awarded the best technical paper award of International Conference on Climbing and Walking Robots [2] and presented at IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS, Hamburg, Germany), a leading conference in the robotic field [3]. Above research would shed light on future high-mobility, flexible underwater robotics.

[1] Witt W C, Wen L, Lauder G V. Hydrodynamics of C-Start Escape Responses of Fish as Studied with Simple Physical Models[J]. Integrative and comparative biology, 2015: icv016.
[2] Ren Z, Zhu Q, Wang T and Wen L.* Bio-Robotic Model as a Scientific Tool for Experimentally Investigating Hydrodynamic Functions of Fish Caudal Fin[C]. Proceedings of the 18th International Conference on CLAWAR. 2015: 232-247. (Best technical paper award)
[3] Ren Z. Y, Wang T. M, Wen L.* Hydrodynamic functions of a robotic fish caudal fin: effects of kinematics and flow speed[C]. IEEE/RSJ International Conference on Intelligent Robots and Systems, Hamburg, 2015: 3882-3887.

Figure 1. Three main vortex rings generated during C-start process of a Bluegill Sunfish (source: Integrative and Comparative Biology)