Inspiration

1) Omnidirectional Swimming: Certain species of stingray are unique among fish due to their ability to perform ‘disc starts’, a method of rapid escape that allows them to move omnidirectionally away from danger [2]. This behavior arises from their highly symmetric, flattened pectoral fins (together called the disc). In normal locomotion, these undulate along the anterior-posterior direction to produce forward thrust, with undulation beginning at the anterior margin and propagating backwards. Due to the symmetry of the fins, this undulation can begin at multiple points along the disc, resulting in different directions of motion (Fig. 1). However, while they are in theory capable of continuous motion in multiple directions, during sideways or backwards starts stingrays turn immediately after the initial escape stroke. This indicates a clear preference for forward swimming, which can be explained not only by visibility (the eyes are in the front of the head) but also by increased swimming efficiency in the forwards direction.

2) Thrust Generation: Swimming in fish is characterized along two main axes: the part of the fish that produces movement, either the body and/or caudal fins (BCF) or median and pectoral fins (MPF); and the number of waveforms that pass over the surface of propulsion, a continuum ranging from fully oscillatory (<0.5 waves) to fully undulatory (>1 wave). Undulatory swimming in fish that use their pectoral fins for propulsion is termed rajiform locomotion [3], which is distinct from the oscillatory mobuliform pattern seen in pelagic species such as manta rays [4]. The mechanism by which undulatory swimming causes forward thrust is primarily drag-based. In this method of locomotion, the propulsive surface (in this case, the stingray’s pectoral fins) generates a force that pushes the water backwards. The reaction force then serves to propel the fish forwards. However, stingrays do not rely solely on undulation to produce forward thrust. In particular, [5] found that thrust enhancement is achieved through two additional properties related to the shape of the stingray body. At both slow and fast swimming speeds (from 1.5 disc-lengths/sec to 2.5 disclengths/sec), a low-pressure region develops in front of the stingray due to a horseshoe vortex that occurs at the relatively high curvature area around the nose. Additionally, at high speeds, a leading-edge vortex forms on the superior portion of the wing, which provides additional thrust.

A number of researchers have produced similarly inspired robots with varying degrees of biomimicry. These range from an entirely soft, tissue-engineered miniature ray [6] to a knifefish-inspired floating rigid robot with a single undulatory fin attached [7], along with [8], [9], [10], [11], and [12]. Additionally, others have developed kinematic and/or computational fluid-based analyses of undulatory locomotion, without building a physical system. Of the examples with published maximum velocity, forward speed normalized by body length ranged from 0.18 disc-lengths/sec to 0.35 body-lengths/sec: significantly slower than the ‘slow’ swimming speed seen in the biology literature. The majority were either bio-inspired solely in that they mimicked an undulatory behavior (e.g., a solid rectangular body with two undulating pectoral fins in [8], or a perfectly symmetric disc with a single encircling fin in [12]) or they did not attempt to mimic an omnidirectional behavior, choosing instead to limit the number of actuators and locomotion complexity ([10], [11]). [9] did incorporate hydrodynamicity in their robot, but the largely cylindrical shape of the central body caused issues with excessive roll.