Organizations: School of Automation, Guandong University of Technology, Guangzhou, China · School of Electromechanical Engineering, Guangdong University of Technology, Guangzhou, China · Department of Electronic and Computer Engineering, Hong Kong University of Science and Technology, Kowloon, Hong Kong, China
In flood disaster rescue scenarios, partially submerged buildings prevent aerial robots from accessing lower levels, limiting mission effectiveness. To address this challenge, this paper presents Wukong-Omni, a novel multimode robot capable of operating across land, air, and underwater using a unified propulsion system. The system is enabled by an innovative mechanical design that allows motor reuse and improves thrust generation. Efficiency and peak thrust are enhanced through simulation and tank-based optimization. Experimental results show a 100 percent improvement in propulsion efficiency and a 150 percent increase in maximum thrust compared with direct installation methods. Dynamic models for the three operating domains are developed, and a unified cross-domain control framework is proposed. Comprehensive experiments validate stable locomotion and smooth transition across domains. Outdoor experiments further demonstrate robustness and adaptability in real-world environments.
Figures & tables
References
Type
Land ability
Water ability
Propulsion number
Air endurance
Water hover endurance
[ 5 ]
Coaxial tricopter
Active differential
Surface
6
18 mins
None
[ 22 ]
Quadrotor
Passive rolling
Surface
4
None
None
[ 13 ]
Coaxial Quadrotor
Passive rolling
Underwater
8
None
None
Proposed
Tilt Quadrotor
Active ackermann
Underwater vector
4
5 mins
1-2h
Table 1: Comparison of Structure and Performance of Representative prototypes
Figure 1: Several representative cross-domain and motor reuse structure prototypes. A. SHOALBOT wheel-propeller prototype ( [ 16 ] ). B. Coaxial tricopter based prototype ( [ 5 ] ). C. Two-wheeled based prototype ( [ 22 ] ). D. Caged based prototype ( [ 13 ] ). E. Dipper’s motor reuse structure prototype ( [ 18 ] ). F. TJ-FlyingFish’s gearbox structure prototype ( [ 14 ] ).
Figure 2: Cross-Domain Operation Scenarios in Flood Rescue Missions. A . Taking off from the rescue boat. B . Swimming to the operation area. C . Flying to the operation area. D . Searching for accessible entry points at the operation area. E . Performing search and rescue tasks. F . Swimming back to the rescue boat via the water surface. G . Flying back to the rescue boat. H . Diving to access submerged entrances. I . Entering through open entrances via flight or rolling.
Figure 3: A .The prototype of Wukong-Omni. B .The diagram of Wukong-Omni’s various modes and the transitions.
Figure 4: All-in-One Propulsion Unit operation schematic. A. The Unit structure composition. B. The aerial mode. C. The aquatic and land mode. D. The exploded view of the developed unit. E. The front view and size of the unit. F. The gearbox structure details of the unit. G. The overall structure of the unit.
Parameter
unit
Value
Diameter
mm
180
Pitch ratio
-
0.71
Disk ratio
-
0.3056
Blade-number
-
3
Meanline type
-
NACA a=0.8
Thickness type
-
NACA 66 (DTRC)
Table 3: Wheel-Propeller Initial Param Selection
Figure 5: Performance evaluation of propellers in (160 mm, 180 mm, 200 mm) diameters. A . The Schematic of the test bench and water tank. B . The thrust-rpm curves. C . The efficiency-rpm curves. D . The torque-rpm curves. E . The motor–gearbox efficiency-rpm curves. F . The simulated efficiency-diameters curves using OpenProp.
Equipment
Type
Force sensor
RUILIDE RDF-TM20
Torque sensor
RUILIDE RDN-97B
Encoder
blheli dshot Feedback
Driver
Tmotor velox-V45A-V2
Distribution Board
Holybro PM07
Controller
Pixhawk 4 (custom high rate feedback)
Table 4: Test bench Component Selection
Propeller size
Max thrust
Max efficiency
Max unit efficiency
(mm)
(N)
(N/W)
(Q ω /UI)
D160mm
41.2
0.337
0.46
D180mm
49.6
0.449
0.488
D200mm
41.5
0.507
0.47
Table 5: Performance Comparison of Different Propeller Sizes in Thrust and Efficiency
Figure 6: The cross-sectional and 3D views of the mesh during simulation.
Figure 7: Performance evaluation of propellers in different structure (D180mm, wheel+propeller, wheel-propeller fusion)
Rotation speed
Simulation results
Experimental results
Error
(RPM)
(N)
(N)
(%)
200
2.53
2.605
2.9
300
7.24
6.4
11.6
400
13.07
11.1
15.07
500
20.33
17.72
12.83
600
28.40
24.1
15.64
Table 6: Comparison of Simulated and Experimental wheel-propeller fusion Thrust Results at Varying Rotation Speeds
Propeller
Max thrust
Max efficiency
Max speed
Max drag
type
(N)
(N/W)
(RPM)
torque(NM)
D180mm
49.6
0.449
734
-
W+P
12.1
0.209
441
0.603
fusion
36.5
0.399
710
0.06
Table 7: Tank test Performance Comparison of Different Propeller shapes in Thrust and Efficiency
Figure 8: The Verification of the simulation results in the water tank.
Figure 9: A-C .Comparison of hydrodynamic characteristics for different wheel–propeller tilt angle sets. D .Tank test results of three wheel–propeller tilt angle.
Figure 10: Simulation results of the thrust and efficiency in five different agnle at different rotate-speed and five different width at different speed.
Figure 11: Simulation results of the thrust produced by the five different agnle at different rotate-speed.
Propeller
Max
Test 600r
Simulation
Error
Efficiency
type
thrust(N)
thrust(N)
thrust(N)
(%)
(N/W)
D180mm
49.6
24.98
29.39
15.01
0.449
30deg
38.0
24.10
28.40
15.14
0.399
0deg
28.6
18.12
21.21
14.56
0.338
-30deg
20.3
15.87
18.82
15.67
0.310
Table 8: Tank test Performance Comparison of Different wheel angles in Thrust and Efficiency
Figure 12: Performance evaluation of air-propellers in different diameter and blades number. A . The thrust-rpm curves. B . The efficiency-rpm curves. C . The torque-rpm curves.
Width
Max
Max
300r
500r
700r
(mm)
thrust(N)
torque(NM)
Eff(N/W)
Eff(N/W)
Eff(N/W)
12
45.6
1.18
1.16
0.72
0.52
20
44.4
1.19
1.13
0.69
0.50
28
37.0
0.90
1.21
0.76
0.56
36
40.4
1.16
1.042
0.65
0.48
44
40.8
1.27
0.96
0.60
0.44
Table 9: Simulation Performance Comparison of Different width in Thrust and Efficiency
Propeller
Max thrust
Max efficiency
Valid efficiency(6000 r/min)
type
(N)
(N/W)
(N/W)
T9045-3
14.8
0.1674
0.0869
T9045-2
12.9
0.0849
0.0791
T1050-3
18.2
0.1599
0.0882
T1070-2
13.9
0.1519
0.0786
Table 10: Performance Comparison of Different Propeller size in Thrust and Efficiency
Figure 13: All-in-one propulsion unit performance evaluation experiments. A . Evaluation of aerial propeller interference on underwater mode. B . Evaluation of one-Way bearing impact on aerial mode. C . Evaluation of wheel–propeller interference with aerial inflow.
Name
Weight
Aquatic Unit Efficiency
Aerial Unit Efficiency
Aerial Efficiency(N/W)
Aquatic Efficiency(N/W)
Max Aquatic Thrust/Weight Ratio
Air
Land
Water
[ 14 ]
122g
52.5%
63.3%
0.073
0.265
32N/122g = 0.262
✓
✗
✓
[ 24 ]
28g
46.0%
50.0%
0.031
0.172
9.5N/28g = 0.339
✓
✗
✓
[ 23 ]
19.3g
NA
12.3%(simu)
NA
0.4(simu)
4.8N/19.3g = 0.249(simu)
✓
✗
✓
[ 16 ]
≥ 500g
NA
NA
NA
NA
55N/500g = 0.11
✗
✓
✓
[ 1 ]
130.4g
NA
NA
NA
0.18
12N/130.4g = 0.092
✓
✗
✓
[ 3 ]
83.4g
NA
NA
NA
0.27
15N/83.4g = 0.180
✗
✗
✓
Table 11: Comparison of the Performance of Representative Drive Unit
Figure 14: The prototype of Wukong-Omni and the labels of its main components.
Exp
Max thrust
Proportion
Max efficiency
Proportion
type
loss(N)
(%)
loss(N/W)
(%)
Test1
2.7
7.397
0.02
9.09
Test2
2.0
1.098
0.0126
9.77
Test3
1.5
8.33
0.0074
11.2
Table 12: Comparison of interference between air propeller and wheel-propeller in Thrust and Efficiency
Component
Type
Motor
T-MOTOR AT2321-KV950
Aerial propeller
GEMFAN1050X3
Aerial ESCs
velox-V45A-V2
Battery
Geshi LiPo 4S 5300mAh
Flight Controller
Pixhawk6C
Onboard Computer
Orin Nx-16Gb
Table 13: Prototype Component Selection
Figure 15: Model of Wukong-Omni with the labels on the coordinate systems and thrusters arrangement in air, land, water surface mode.
Figure 16: Electronic and hardware framework of Wukong-Omni.
Figure 17: Wukong-Omni’s integrated control framework structure.
Figure 18: Air-to-Ground Mode Transition Sequence of Wukong-Omni.
Figure 19: A . Multi-constraint analysis of wheel–propeller configuration and tilt angle. B . Illustration of main-arm rotation during the air-to-ground transition.
Symbol
Value
[Ixx,Iyy,Izz]
[1.44,1.48,2.13]×10−1kg⋅m2
[ma,V]
[4.4 kg ,3.21×10−3m3]
[La,Wa,Ha]
[44,22,20] cm
[ρa,ρw]
[1.29,1000]kg/m3
Kd,Cd
diag([0.9,0.5,0.8]),0.9
g
9.81m/s2
Table 14: Model Parameters
Figure 20: A . Complete water-air cross-domain motion loop. B . Experimental results showing attitude and power data during the transition. C . The robot hovers underwater after submerging. D . Trajectory of the water-to-air transition loop. E . Underwater depth-hold performance results.
Performance
Value
Performance
Value
Roll RMSE(deg)
1.54
Roll eˉ (deg)
1.50
Pitch RMSE(deg)
1.86
Pitch eˉ (deg)
1.37
Depth RMSE(m)
0.11
Depth eˉ (m)
0.07
Max Surface Power(W)
220
Stable Flight Power(W)
860
Table 15: Evaluation of robot Performance in water-air Transition
Performance
Value
Performance
Value
Roll RMSE(deg)
1.78
Roll eˉ (deg)
1.45
Pitch RMSE(deg)
2.19
Pitch eˉ (deg)
1.77
Yaw RMSE(deg)
7.40
Yaw eˉ (deg)
4.54
Max Land Power(W)
74.2
Max Flight Power(W)
1010
Table 16: Evaluation of robot Performance in land-air Transition
Figure 21: A . Complete air-land cross-domain motion loop. B . Experimental results showing attitude and power data during the transition. C . The robot mission path in field test. D . Trajectory of the air-land transition loop.
Figure 22: A . Wukong-Omni tracking a designated path in land mode. B . Wukong-Omni performing manual circling in land mode. C . Experimental results showing the trajectory, velocity, and yaw during tracking. D . Experimental results showing the trajectory, velocity, and yaw during the circling test.
Cruise Performance
Value
Manual Performance
Value
Yaw eˉ (deg)
0.08
Yaw eˉ (deg)
8.71
Yaw RMSE(deg)
0.53
Yaw RMSE(deg)
10.5
Speed(m/s)
1.0
Max Speed(m/s)
3.3
Stop Radius(m)
2.5
Max Power(W)
152
Average Power(W)
27.36
Average Power(W)
76
Table 17: Evaluation of Robot Performance In Land Mode
Figure 23: A . Schematic illustration of underwater lateral motion. B . Underwater roll attitude response and corresponding motor speed. C . Schematic illustration of underwater yaw maneuver. D . Underwater yaw rate response and tilt servo actuation data.
Figure 24: A . Schematic illustration of trajectory tracking in aerial mode. B . Experimental results of trajectory tracking. C . Experimental results of body velocity and motor rotational speed.
Figure 25: A . The Obstacles on the water surface and the coordinate system which they are traversed. B . Experimental results of the attitude and position during the traversed. C . Underwater crossing process pictures.
Air Performance
value
Water Performance
value
Trajectory eˉ (m)
0.31
Roll eˉ (deg)
3.33
Trajectory RMSE(m)
0.35
Roll RMSE(deg)
6.56
Max Speed(m/s)
4.86
Max Speed(m/s)
1.2
Average Power(W)
942
Average Power(W)
128
Table 18: Evaluation of Robot Performance In Air And Water Mode
Figure 26: A . Illustration of the field mission execution. B–E . Wukong-Omni successfully performing outdoor tasks at an artificial lake, including water surface moving, underwater operation, ground locomotion, and air–water transitions.
Figure 27: A . Illustration of the mission execution. B . Outdoor experiment trajectory.
Triphibious robots capable of multi-domain motion and cross-domain transitions are promising to handle complex tasks across diverse environments. However, existing designs primarily focus on dual-mode platforms, and some designs suffer from high mechanical complexity or low propulsion efficiency, which limits their application. In this paper, we propose a novel triphibious robot capable of aerial, terrestrial, and aquatic motion, by a minimalist design combining a quadcopter structure with two passive wheels, without extra actuators. To address inefficiency of ground-support motion (moving on land/seabed) for quadcopter based designs, we introduce an eccentric Center of Gravity (CoG) design that inherently aligns thrust with motion, enhancing efficiency without specialized mechanical transformation designs. Furthermore, to address the drastic differences in motion control caused by different fluids (air and water), we develop a unified propulsion system based on Field-Oriented Control (FOC). This method resolves torque matching issues and enables precise, rapid bidirectional thrust across different mediums. Grounded in the perspective of living condition and ground support, we analyse the robot's dynamics and propose a Hybrid Nonlinear Model Predictive Control (HNMPC)-PID control system to ensure stable multi-domain motion and seamless transitions. Experimental results validate the robot's multi-domain motion and cross-mode transition capability, along with the efficiency and adaptability of the proposed propulsion system.
Xiangyu Li, Tiancheng Lai, Mingwei Lai +6
State Key Laboratory of Industrial Control Technology, Zhejiang University, Hangzhou 310027, China · Huzhou Institute of Zhejiang University, Huzhou 313000, China · Hainan Red Tower Cigarette Co., Ltd., Hainan 571100, China
Legged robots are versatile on land, but their use in underwater environments remains limited. Extending quadruped locomotion to water enables amphibious mobility with applications in inspection, environmental monitoring and disaster response. This paper presents the design, modeling, and experimental validation of a reproducible underwater quadruped robot. The robot is built around custom waterproof motor housings machined from polyoxymethylene plastic, which use off-the-shelf O-rings and dynamic shaft seals. A simplified model is derived to describe the dynamics of this underwater legged system, capturing how drag forces on spherical end effectors transmit torque to the floating base. Building on this model, a closed-loop attitude controller is developed using an error formulation defined on the special orthogonal group SO(3). The controller is evaluated both in simulation and experimentally in a water tank, where the robot tracks desired orientation setpoints in roll, pitch and yaw.
Davide Molinaroli, Mohit Singh, Kostas Alexis
Norwegian University of Science and Technology, Trondheim 7034, Norway
We present a palm-scale (65 mm, 34 g) swim-and-breach robot platform. Two vertically stacked propellers provide both propulsion and differential-thrust pitch control under a proportional-integral-derivative (PID) loop, and a tail rudder adds yaw control. The hull, evaluated by flow simulation, reduces the drag five-fold relative to an equivalent cuboid, and the propellers are optimized using B-series modeling validated by dynamometer measurements. The current robot swims at 13.9 body lengths per second and turns at 209 deg per second, corresponding to the upper limits reported for underwater robots. In free swimming, the pitch loop turns the body to any commanded nose-up pitch angle, and, with the rudder stabilizing the exit, the current robot leaps 1.6 body lengths high and 3.7 long in a seamless cruise-leap-cruise sequence. The platform can be used to build small-scale robots that cross barriers and dry gaps between pools for inspection in streams, flooded structures, and industrial systems.
Daehyun Choi, Ian Bergerson, Hengjia Zhu +2
BioFrontiers Institute, University of Colorado Boulder, Boulder, CO 80309, USA