# Hybrid Locomotion Evaluation for a Novel Amphibious Spherical Robot  
[PERSON] 1  
1 Key Laboratory of Convergence Medical Engineering System and Healthcare Technology,  
1 the Ministry of Industry and Information Technology, School of Life Science,  
Beijing Institute of Technology, Beijing 100081, China; [EMAIL_ADDRESS] (H.X. ); [EMAIL_ADDRESS] (Y.H. ); [EMAIL_ADDRESS] (S.S.); [EMAIL_ADDRESS] (Z.C. ); [EMAIL_ADDRESS] (X.H.) 1  
[PERSON] 1  
1 Key Laboratory of Convergence Medical Engineering System and Healthcare Technology,  
1 the Ministry of Industry and Information Technology,  
developed a multi-legged underwater-walking robot (CR200) [9; 10] to survey underwater structures and shipwrecks off the coast of the Korean Peninsula; the robot is slow because it features legged propulsion. In 2013, [PERSON] designed a bio-amphibious salamander robot (Salamandra robotic II) [11] that both walked and swam; the robot has four legs and an active spine allowing anguilliform swimming in water and on-ground walking with various gaits, featuring body-limb coordination and bodily undulation. Earlier, in 2006, [PERSON] et al. created an amphibious snake robot termed AmphiBot II [12] that crawled and swam using a central-pattern generator. In 2009, [PERSON] developed a wheel-propeller-leg-integrated amphibious robot [13] that crawled on land, swam underwater (at certain depths), and crept on the ocean floor. In 2016, [PERSON] developed the amphibious robot AmphiHex-I [14] which walked on rough terrain, maneuvered underwater, and used specially designed transformable flipper legs to traverse soft muddy or sandy substrates in littoral areas between land and water. All of these amphibious robots have particular characteristics and advantages. In terms of on-land locomotion, legged robots [15; 16; 17; 18] deal better with uneven terrain, but are slow, whereas wheeled robots [19; 20] move rapidly but cope poorly with rough ground; serpentined robots [21; 22] (inspired by snakes) work well on flat terrain but it is difficult to control the direction and speed of motion. In water, robots with screw propellers are more stable and mobile than those with oscillatory and undulatory capacities. Earlier, inspired by amphibious turtles, the amphibious spherical robot I (ASR-I) [23; 24] that could move both on land and underwater was constructed. The ASR-I diameter (including its four legs) was only 25 cm. To improve stability while retaining precision, we built the ASR-II [25; 26; 27; 28] using 3-D printing. This robot could walk at up to 8.5 cm/s (0.34 the body length) and climb a slope of 5\({}^{\circ}\). To improve locomotion on land, [PERSON] added a propulsion mechanism featuring active wheels [29]. The direct current (DC) motors driving the wheels increased the speed. An active wheel-legged robot [30] with 6 legs was developed to get better actions in unknown environment with reinforcement learning. The robot used values of external force measured on the robot's legs as states and rewards, and it can adapt to different terrains in real time using on-line learning. However, propulsion system redundancy was evident, and the active wheels (with DC motors) increased the size of the robot, causing problems during walking. The passive wheel-legged robot named Roller-walker [31; 32; 33; 34] with two locomotion modes was early proposed. Roller-walker can transform into sole mode by rotating the ankle roll joint with the passive wheel on each leg. Using the transformation mechanism, the robot can switch between the quadruped walking and roller-skating on smooth flat terrain. The transformation of the actuation modes can be realized by one degree of freedom of each leg. The mechanical legs were redesigned to feature four passive wheels [35], allowing the amphibious robot to move rapidly on smooth flat ground. The robot exhibited both roller-skating and walking locomotion. Compared with active-wheeled robots, passive wheel-legged robots are of lower weight, consume less energy, and are smaller.