Design and Control of a Multimodal Legged-Aerial Robot Through Separate Control Contraction Metrics
Authors: Kaushik Venkatesh Krishnamurthy, Eric Sihite, Alireza Ramezani, Morteza Gharib
Organizations: Authors are with the Silicon Synapse Labs, Department of Electrical and Computer Engineering, Northeastern University, Boston, USA. · Authors are with Center for Autonomus Systems and Technologies, Caltech, Pasadena, CA.
The integration of dynamic legged locomotion and aerial mobility offers versatility that is deterred by challenges in payload distribution, actuation, and the contrasting requirements of both domains. In this paper, we present the design and operation of a multimodal quadrupedal aerial-legged robot. The presence of legged aerial systems also allows for thruster-assisted legged locomotion, which we make possible with a unified control framework using Control Contraction Metrics (CCM). We search for separate domain-specific contraction metrics for walking and flight independently offline and unify them using an online energy-based Control Lyapunov Function Quadratic Program (CLF QP). This enables optimal, cross-domain control allocation, which we show using simulations. We also show experimental results that validate the mechanical viability of our hardware and the paradigm of thruster-assisted trotting. The stability of said experiments motivates the deployment of the control framework for multimodal operations.
Figures & tables
Fig. 1: The legged-aerial quadrupedal robot used in this study. The robot has three joint actuator for each leg, and four thrusters for aerial locomotion and thruster-assisted walking.
Fig. 2: Section view of actuator assembly, showing the harmonic drive, stator, and rotor embedded in the additively manufactured actuator housing.
Property
Value
Units
Mass
12
kg
Height
0.6
m
Width
1
m
Length
1.5
m
Joint Actuator Motor
T-Motor MN4004 Antigravity
-
Motor KV
270
kV
TABLE I: Robot properties
Fig. 3: CAD model rendering of the robot, closeup view of all subsystems and components including custom-design actuators, thruster, power electronics, joints, and onboard computers.
Fig. 4: Planar VTOL and legged models used for obtaining contraction metrics. We separate the contraction metrics for the the legged and aerial systems.
Fig. 5: A. Snapshots of Matlab simulation showing thruster-assisted trotting using the min-norm controller with the unified contraction constraints. B. Zoomed in view of a couple of steps, showing the relative direction and magnitude of front and rear thruster forces and contact forces. Along with trajectories of positions, velocity, pitch, phase plots, contact forces, and constraint satisfaction.
Fig. 6: Snapshots from experimental results. The legged phase involves 2-point contact trotting while a human operator provides thruster inputs to. During the flight phase, the flight operator provides thruster inputs through the onboard CubeOrange flight controller. The robot lands again and continues trotting stably.
Fig. 7: Joint positions and velocities reference and tracking during thruster-assisted trotting experiments
Fig. 8: Euler angles in the thruster assisted trotting.
Fig. 9: Snapshots of the robot flying and landing on an obstacle.
This paper presents the hardware implementation of a quadratic program-based controller on Harpy: thruster-assisted bipedal robot to perform thruster-assisted trotting. This is inspired by wing-assisted locomotion observed in birds such as the chukar partridge, which employ thrust vectoring to manipulate the ground reaction force. The proposed controller formulates the thruster-assisted spring-loaded inverted pendulum dynamics as a condensed QP-based model predictive control problem, enabling simultaneous regulation of ground contact force and active thrust vectoring at 500 Hz. A decoupled control architecture is used, where the robot's sagittal dynamics are controlled using QP-MPC while the frontal dynamics is regulated using a cascade orientation stabilization controller. Two experiments were conducted with different forward reference velocities of 0 m/s and 0.1 m/s. While the instantaneous velocity oscillates, the mean velocities can track their respective reference. Additionally, joint tracking error remains within 1 deg while tracking the desired leg length given by the leg trajectory controller and desired swing trajectory given by Raibert's heuristic. The QP-MPC problem is consistently solved with 0.18ms, which is well within the 2ms computation budget while satisfying the GRF friction cone constraint.
Shreyansh Pitroda, Eric Sihite, Alireza Ramezani +1
The authors are with SiliconeSynapse Labs, the Department of Electrical Engineering, Northeastern University, USA. · The authors are with the Department of Aerospace Engineering, California Institute of Technology, USA.
We present a novel and minimalistic design of an aerial-terrestrial robot DASH: Ducted Aerial Spring Hopper. The goal is to enable both aerial and ground locomotion capabilities on a unified mobile robot that is mechanically-minimalistic, locomotion-versatile, and energy-efficient. We propose an organic integration of ducted fan co-axial body with a springy leg at the bottom for realization. The ducted fan module provides thrust-vectoring as the main actuation for agile flying; when it is combined with the light-weight spring leg, the robot realizes highly efficient ground hopping with energy circulation. Moreover, to realize optimal locomotion with two modes, we employ a contact-implicit model predictive controller to automatically choose locomotion modes and actuation. We successfully validated the design and control of DASH through a range of tasks, including periodic hopping, aerial flight, and mode-free locomotion with autonomous mode transitions during obstacle traversal.
Ryan Gomes Paiva, Conrad Ho, Jiarong Kang +3
University of Wisconsin–Madison, WI, USA. · Shanghai Innovation Institute (SII), Shanghai, 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.
Yufan Liu, Rixi Yu, Junjie Li +7
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