Thrust Analysis On A Single-Drive Robotic Fish With An Elastic Joint

This work simplified tuna’s swimming mode, then designed a single-drive robotic fish propulsion mechanism which including an elastic joint, and established the dynamics model of the mechanism. The thrust, resistance, resistance power on different peduncle oscillation parameters, and torsional stiffness of the caudal fin joint was simulated. The average thrust, maximum resistance and the average power grow with the increasing of the oscillating amplitude and the frequency. When the torsional stiffness of the caudal fin joint becomes larger, the thrust decreases, the resistance and the average power increase. The simulation results proved that the mechanism can generate thrust in water and it may be used as a robot fish propulsion mechanism. INTRODUCTION During millions of years of evolution, fishes have developed optimum body structures and appropriate swimming modes shaped by various environments. Their advantages in efficiency, low noise and maneuverability can potentially compensate the disadvantages of traditional propellers. Therefore, researchers have focused on the fish-mimicking mechanisms, with which the underwater vehicles can be propelled, target at developing a high performance robot fish. Fishes’ swimming modes can basically be categorized into Body and/or Caudal Fin (BCF) locomotion and Median and/ or Paired Fin (MPF) locomotion (Sfakiotakis et al. 1999). Around 85% of fishes adopt BCF locomotion, swim by bending their bodies into a backward-moving wave which extends into its caudal fin (VIDELER, JJ .1993). According to the wavelength and the amplitude enveloped of the wave, the BCF mode is divided into four types: Anguilliform, Subcarangiform, Carangiform and Thunniform (C. C. Lindsey, 1978). The main difference of the first three types is the characteristics of the body wave which produces the propulsive movement. By thunniform, the front part of the fish body can be basically regarded as rigid, propulsive movement is limited to the 1/3 rear part of the body, especially the caudal peduncle and caudal fin. The caudal fin generates more than 90% propulsion. This mode is suitable for a long-time and high-speed cruise. The fastest marine animals such as yellow-fin tunas represent thunniform. Tunas’ swimming speed can be up to 20 knots, and the propulsive efficiency can be as high as 80% or more (Tong Binggang 2000). Therefore, to mimic thunniform swimming is a hot spot in bionic robotic fish researches. Stix in MIT carried out research on bionic robotic fish. By developing the first complete bionic robotic fish Robotuna (Stix, G. 1994). The length of Robotuna is about 1.2m. Japan's National Institute of Oceanography developed a thunniform bionic robotic fish, it’s length is Proceedings 27th European Conference on Modelling and Simulation ©ECMS Webjørn Rekdalsbakken, Robin T. Bye, Houxiang Zhang (Editors) ISBN: 978-0-9564944-6-7 / ISBN: 978-0-9564944-7-4 (CD) about 1m, and the speed is up to 0.97m/s (www.nmri.go.jp). In China, Beihang University(BUAA) ( Liang, J.et al. 2011.), Harbin Engineering University (Cheng Wei et al. 2004), Institute of Automation of Chinese Academy of Sciences(CAS) (Su, Z. et al. 2010) developed different prototypes of thunniform bionic robotic fishes. In order to mimic the oscillation of the caudal peduncle and caudal fin, these prototypes often use 2 or 3 motors as drivers. Both the structure and the control system are relatively complex. In reality, the fish's flexible bodies can deform under the muscular driving force and the external forces. This makes it possible to generate the desired movements with fewer and simpler drives. Xu (XU et al. 2008, XU et al. 2007) applied this mechanism to the bionic flapping wing aircraft and a robotic fish with MPF swimming mode. Both of these approaches featured a single drive with flexible wings or fins. In the following chapters, a single-drive bionic propulsive mechanism with an elastic joint was introduced. The propulsive mechanism was analyzed, and a simulation model was built. The relationship between thrust, resistance, power and the peduncle oscillation parameters, the torsional stiffness of the caudal fin joint was obtained by simulation. The simulation results proved the feasibility of such a propulsion mechanism.