Fish and robots swimming together: attraction towards the robot demands biomimetic locomotion

Abstract

The integration of biomimetic robots in a fish school may enable a better understanding of collective behaviour, offering a new experimental method to test group feedback in response to behavioural modulations of its 'engineered' member. Here, we analyse a robotic fish and individual golden shiners (Notemigonus crysoleucas) swimming together in a water tunnel at different flow velocities. We determine the positional preference of fish with respect to the robot, and we study the flow structure using a digital particle image velocimetry system. We find that biomimetic locomotion is a determinant of fish preference as fish are more attracted towards the robot when its tail is beating rather than when it is statically immersed in the water as a 'dummy'. At specific conditions, the fish hold station behind the robot, which may be due to the hydrodynamic advantage obtained by swimming in the robot's wake. This work makes a compelling case for the need of biomimetic locomotion in promoting robot-animal interactions and it strengthens the hypothesis that biomimetic robots can be used to study and modulate collective animal behaviour.

Figure 1.

Illustration of the robotic fish and golden shiner with scale: (a) computer-aided design of the robotic fish and (b) picture of a golden shiner. (Online version in colour.)

Figure 2.

Schematics of the experimental setups. (a) Behavioural experiment: the robotic fish and individual golden shiners swimming in a section of a Blazka-type water tunnel delimited by two plastic honeycombs. A camera placed above the tunnel records their swimming. (b) DPIV analysis: the robotic fish swimming in a section of a Blazka-type water tunnel delimited by two plastic honeycombs. A laser sheet oriented in the horizontal (x,y) plane illuminates the seeded water in correspondence with the compliant mylar caudal fin. A camera placed below the tunnel records the area illuminated by the laser.

Figure 3.

Picture from DPIV analysis illustrating the tail of the robotic fish and seed particles illuminated by the laser sheet. Superimposed dotted orange lines identify the region where DPIV analysis is performed and superimposed solid green lines define the region for which flow data are presented using displayed x- and y-coordinates. Superimposed red dot represents the centre of mass of the robot and dashed green lines define the tail rest position and a representative direction at the back of the robot for which the angle a is defined. (Online version in colour.)

Figure 4.

Top view of the water tunnel with schematic illustration of some of the experimental variables. CM_robot represents the centre of mass of the robotic fish. CM represents the centre of mass of the fish. A live fish is considered to interact with the robot whenever the x-coordinate of its CM is in a region (R; section in grey), which extends 4 BL from the x-coordinate of CM_robot. R is additionally partitioned into the front (R^ft) and the back (R^bk) regions. (Online version in colour.)

Figure 5.

Mean time spent within 4 BL to the robotic fish at the 12 different conditions (n = 6 at each condition) scaled with respect to the total acquisition time. Black empty circles represent the mean time spent when swimming in R () at each of the 12 conditions. Red empty triangles represent the mean time spent in R^ft () for each of the 12 conditions. (Online version in colour.)

Figure 6.

Examples of fish swimming with robotic fish at different conditions. The left column shows four snapshots from V1F2 at different time of the experiment. Fish swimming at this condition tend to hold station behind the robot. (See electronic supplementary material.) The right column shows four snapshots from V1F0 at different time of the experiment. Fish swimming at this condition swim randomly around the robotic fish. (Online version in colour.)

Figure 7.

TBF measured at different speeds. TBF increases as water velocity increases. Black empty circles represent TBF measured when swimming in R^bk at 14, 16 and 28 cm s⁻¹. Red empty triangles represent TBF measured when swimming in R^ft at these three different water velocities. (Online version in colour.)

Figure 8.

Relationship between Δf and τ^bk scaled with respect to the total acquisition time (linear regression: y = 21.9 + 41.8x, r² = 0.2, p < 0.05, n = 29). Red empty triangles identify individuals that hold station behind the robot. (Online version in colour.)

Figure 9.

Relationship between Δf_robot measured in both (a) front and (b) back position and τ^bk scaled with respect to the total acquisition time (linear regression, front position, y = 30.2 + 6x, r² = 0.13, p < 0.05, n = 29; back position, y = 64.2 + 15.3x, r² = 0.36, p < 0.01, n = 32). Red empty triangles identify individuals that hold station behind the robot. (Online version in colour.)

Figure 10.

Examples of traces of the tail movement of the robotic fish (black empty circles) and the fish (red empty triangles) when swimming together in the water tunnel at (a) 14 cm s⁻¹ with TBF_robot = 2 Hz (V1F2) and at (b) 16 cm s⁻¹ with TBF_robot = 3 Hz (V2F3). Solid lines represent sinusoidal fits of the tails' movements. (Online version in colour.)

Figure 11.

Velocity of the robot at the different conditions normalized with respect to the water velocity.

Figure 12.

Velocity intensity in the vicinity of the flapping robot tail averaged over the entire sequence of frames for all conditions. Note that the first row represents the case of TBF_robot = 0 Hz. The region of observation corresponds to the area identified in figure 3.

Figure 13.

Time-resolved vorticity field induced by the robot's tail beating for the conditions where some of the fish hold station at its back. Red structures represent counterclockwise vortices; blue structures represent clockwise vortices. The region of observation corresponds to the area identified in figure 3.