# Hybrid Locomotion Evaluation for a Novel Amphibious Spherical Robot
## 5 The Underwater Locomotion Experiments  
Due to the symmetrical spherical design, the robot has the same model for forward motion and backward motion. Hence experiments in forward motion were carried out to evaluate the performance of ASR-III underwater motions. Figure 22 shows the video sequence of the forward motion of ASR-III on the water surface of the pool with the size of 3 m \(\times\) 2 m \(\times\) 1 m (height \(\times\) width \(\times\) depth). In the pool, there were two vertical rulers (2 m) and one horizontal ruler (2.5 m). We assumed that the water was static. As depicted in Section 3, we adapted the forward motion strategy with four thrusters. The motor of thruster worked under the voltage of 20 V. Initially the robot was put at one side of the horizontal ruler, and then swam along the ruler to other side. We recorded the time and the displacement of forward motion, and then calculate the velocity. As shown in Figure 23, the red and blue curves indicate the displacement and the velocity of the robot varying with time. The robot speeded up to the maximum velocity (21.5 cm/s, 0.61 body/s) at about 6.5 s, and then the robot swam at the constant speed. From the start to the end, the average velocity of the robot was 18.1 cm/s (0.52 body/s). Furthermore, the mean power measured approximately at maximum velocity was 64.5 W, and the robot can cruise 2.5 h in the range of the radius of 1.5 km.  
Figure 21: The heading angle in waypoint trajectory experiment. The red and blue curves represent heading angle with closed loop PID and an open loop manner, respectively.  
Figure 20: The experimental waypoint trajectory. The red stars represent the waypoints. We show the four waypoints A (20, 20), B (130, 20), C (130, 90), and D (20, 90), and the reference trajectory (blue lines; a precise rectangle). The red and green curves are the trajectory of the robot with closed loop PID and an open loop manner as recorded by the vision localization system, respectively.  
The heading angle control experiment was conducted to evaluate the performance of ASR-III with the strategy of the left turning and right turning. The heading angle measured by the JY901 IMU module was regarded as the feedback for the closed-loop control. The experiment was carried out in the same pool. Figure 24 shows four snapshots of the heading angle control motion. The initial angle of the robot was set to \(0^{\circ}\), and the excepted heading angle was set to \(90^{\circ}\). As shown in Figure 25a, the robot can keep stable at \(16.5\) s.  
Figure 23: Experimental results for forward motion.  
Figure 22: Underwater forward motion on the water surface.  
Besides the forward motion and the heading angle control experiment, the depth control experiment was carried out to evaluate the performance of ASR-III with the strategy of floating and sinking. The depth values were collected by IIC bus from the pressure sensor (MS5803-01 BA module) installed under the middle plant, and regarded as the feedback for the closed-loop PID control. The robot started at a depth of 15 cm, and the expected depth was 32 cm. The results of depth control were illustrated in Figure 25b. The robot reached the expected depth after 20 s. In the steady state, the mean depth error was 0.56 cm.