Dynamic Performance of a Bio-inspired Uuv: Effects of Fin Gaits and Orientation

The analysis of a proposed fin configuration change to a four-fin unmanned underwater vehicle (UUV) is described in this paper. Based on unsteady flow computations and experimental fin measurements, the forces and moments produced by the fins are evaluated for two fin configurations. As a result of this study, a change is made to a design that enables improved control symmetry in thrust production, higher reverse thrust and yaw moment production in hover, and higher forward thrust production at one knot forward speed. Vehicle responses to heading and depth commands are presented to validate improved vehicle performance. This fin configuration change provides an added benefit of eliminating vehicle forward thrust when all four fins use the same gait – a set of rib and stroke motions. This allows us to propose and analyze a new lift producing fin gait which is also described in this paper. The new gait, combined with previously used lift producing fin motions, provides improved vehicle lift in hover. Open-loop vehicle depth response to two different lift gaits is compared to validate improved performance enabled by the new gait. Introduction Current operational unmanned underwater vehicles (UUVs) excel at many critical tasks including deeply submerged and high-endurance operations, performing high-speed and large-radius maneuvers. However, the traditional propellerdriven vehicles performing these missions have not demonstrated the same levels of operational success in cluttered, near-shore environments where precise positioning and small-radius maneuvers are required in the presence of waves and alternating currents. Researchers have therefore studied the fin force production mechanisms employed by various fish species in their attempts to understand how these organisms achieve high maneuverability and control authority in difficult environments [1][2]. Within fish swimming, articulation of the pectoral fins has been shown to produce forces and moments ideal for high-maneuverability in low-speed and hovering operations [3]. Several investigators have developed and adapted passively deforming robotic pectoral fins onto UUVs [4][5][6][7], whereas others have pursued the development of active control deformation pectoral fins [8][9][10]. To enable unmanned vehicle missions in nearshore underwater environments, we have studied the swimming mechanisms of a particular coral reef fish, the Bird wrasse (Gomphosus varius). Inspired by the pectoral fin of this species, we have designed a robotic fin based on computational fluid dynamics (CFD) and experimental studies of the forces and moments generated by the fin flapping motions. The resulting robotic fin uses active curvature control through actuation of individual ribs to produce desired propulsive forces (Figure 1). Fig. 1. Actively controlled curvature robotic pectoral fin mounted on custom designed UUV hull. Our current work focuses on the integration of the robotic fin onto a UUV platform. After studying the performance of a two-fin vehicle [11][12], a four-fin vehicle configuration was designed and built to enable more precise control over the vehicle dynamics. Within the vehicle design, further CFD studies have helped identify fin-fin and fin-body interactions that are accounted for in vehicle models [13]. The scope of this paper is to analyze the vehicle configuration and fin kinematics with the goal of improving vehicle dynamic performance. First, we compare the maneuvering capabilities of the current vehicle configuration [14] with those enabled by a newly designed alternate configuration. While both configurations include four flapping fins, the orientation of these fins differ which impacts force generation and control authority. Second, we model vehicle lift generation and validate this model through experimental and computational results. We also introduce a new actively controlled fin stroke for improved lift generation over the current lift generating stroke. Vehicle Fin Orientation Initial fin configuration on the four-fin vehicle was designed to achieve maximum forward thrust capability based on computational and experimental results of an individual fin [9][15]. These results showed that a fin gait – a set of fin kinematics defined by rib deflections and fin stroke timehistories – can be designed to produce higher magnitude positive thrust than another gait can be designed to produce negative thrust, as defined in Figure 1. This positive thrust production propels the vehicle in the direction of the longer fin edge. Based on these studies, the original four-fin vehicle design incorporated all fins pointed in the same direction with the longer edge of the fin facing the front of the vehicle in a ‘traditional’ configuration (Figure 2a). However, because of the asymmetry in forward and reverse thrust generation capability inherent in this ‘traditional’ fin configuration, control authority over vehicle yaw was low. Computational studies were carried out to determine how a 180° change in the orientation of the two rear fins on the vehicle – improving fore-aft vehicle symmetry – would affect force production capability [13] (Figure 2b). While it was expected this ‘flipped’ configuration would allow for improved turning and reverse motion performance, it also showed improvement in thrust generation at a one knot forward speed. The Because computational studies showed there was no apparent downside to this design change, the hardware configuration was amended to allow for an experimental validation.