Central Pattern Generator (CPG)-Based Locomotion Control and Hydrodynamic Experiments of Synergistical Interaction between Pectoral Fins and Caudal Fin for Boxfish-like Robot

Abstract

Locomotion control of synergistical interaction between fins has been one of the key problems in the field of robotic fish research owing to its contribution to improving and enhancing swimming performance. In this paper, the coordinated locomotion control of the boxfish-like robot with pectoral and caudal fins is studied, and the effects of different control parameters on the propulsion performance are quantitatively analyzed by using hydrodynamic experiments. First, an untethered boxfish-like robot with two pectoral fins and one caudal fin was designed. Second, a central pattern generator (CPG)-based controller is used to coordinate the motions of the pectoral and caudal fins to realize the bionic locomotion of the boxfish-like robot. Finally, extensive hydrodynamic experiments are conducted to explore the effects of different CPG parameters on the propulsion performance under the synergistic interaction of pectoral and caudal fins. Results show that the amplitude and frequency significantly affect the propulsion performance, and the propulsion ability is the best when the frequency is 1 Hz. Different phase lags and offset angles between twisting and flapping of the pectoral fin can generate positive and reverse forces, which realize the forward, backward, and pitching swimming by adjusting these parameters. This paper reveals for the first time the effects of different CPG parameters on the propulsion performance in the case of the synergistic interaction between the pectoral fins and the caudal fin using hydrodynamic experimental methods, which sheds light on the optimization of the design and control parameters of the robotic fish.

Keywords: CPG; hydrodynamic analysis; motion control; multifin synergy; robotic fish.

Figure 1

Shape characteristics and actuation mechanism of the bionic boxfish-like robot. (a) Swimming instability and maneuverability of boxfish [30]; (b) Designed virtual prototype of the boxfish-like robot.

Figure 2

Actual prototype of boxfish-like robot. (a) Front view; (b) Side view.

Figure 3

Designed and actual illustration of the pectoral fin and caudal fin of the prototype. (a) Shape characteristic parameters of the pectoral and caudal fins; (b) Shape characteristics of the actual pectoral and caudal fin.

Figure 4

Control system of designed boxfish-like robot prototype. (a) Hardware system structure; (b) Designed artificial CPG network.

Figure 5

Schematic diagram of the hydrodynamic experimental platform.

Figure 6

Experimental pool and 3-axis Force Measurement System. (a) Schematic diagram of the swimming pool; (b) Actual swimming pool and 3-axis force measuring device.

Figure 7

Effects of the amplitudes between the two caudal joints on propulsive performance. (a) Propulsive force; (b) Lateral force.

Figure 8

T Effects of the phase lags between the two caudal joints on propulsive performance for two amplitudes. (a) Amplitudes of the two caudal joints are $R_3={15}^{°}$, $R_6={15}^{°}$; (b) Amplitudes of the two caudal joints are $R_3={15}^{°}$, $R_6={20}^{°}$.

Figure 9

Effects of the frequency on propulsive performance for two amplitudes and phase lags. (a) Amplitude and phase lags are $R_3={15}^{°}$, $R_6={15}^{°}$, $\Delta {\phi }_{36}={270}^{°}$; (b) Amplitude and phase lags are $R_3={15}^{°}$, $R_6={15}^{°}$, $\Delta {\phi }_{36}={290}^{°}$; (c) Amplitude and phase lags are $R_3={15}^{°}$, $R_6={20}^{°}$, $\Delta {\phi }_{36}={270}^{°}$; (d) Amplitude and phase lags are $R_3={15}^{°}$, $R_6={20}^{°}$,$\Delta {\phi }_{36}={290}^{°}$.

Figure 10

Effects of the amplitudes of twisting and flapping on propulsion performance. (a) Average propulsive force; (b) Average lift force.

Figure 11

Effects of the offset angles between pectoral fin twisting and flapping on propulsion performance. (a) Average propulsive force; (b) Average lift force.

Figure 12

Effects of the phase lag between twisting and flapping on propulsion performance. (a) Average propulsive force; (b) Average lift force.

Figure 13

Effect of the frequency on propulsion performance in lift-based mode.

Figure 14

Effects of the frequency on propulsion performance in drag-based mode. (a) $X_4={40}^{°}$, $X_5={30}^{°}$; (b) $X_4=-{40}^{°}$, $X_5=-{30}^{°}$.

Figure 15

Effects of the time asymmetric flapping on propulsion performance. (a) Frequency; (b) Time asymmetric coefficient.

Figure 16

Effects of the phase lags on propulsion performance under the synergistical interaction between pectoral fins and caudal fin. (a) Average propulsive force; (b) Average lift force.

Figure 17

Effects of the frequency on propulsion performance under the synergistical interaction between pectoral fins and caudal fin.

Figure 18

Measuring method of swimming speed. (a) Experimental scene of speed test; (b) Analysis of swimming speed.

Figure 19

Relationship between forward swimming speed and frequency in robotic fish.