Maintaining the Level of a Payload carried by Multi-Robot System on Irregular Surface

TL;DR

The paper addresses maintaining a payload's orientation while transporting it with a multi-robot team over unknown, uneven terrain. It couples a piston-based leveling mechanism with an open-loop height computation and a closed-loop PID controller within a loosely coupled leader-follower formation, underpinned by skid-steer kinematics and a ball-socket piston interface. Key contributions include a B·θ mapping from payload angles to per-robot piston adjustments, a per-piston PID control scheme, and validation in Gazebo/ROS across challenging terrains. Results show the payload remains effectively level (near zero roll/pitch) and piston heights converge to a mean length, demonstrating practical viability. Future enhancements target robustness and terrain-aware planning via ahead-surface sensing.

Abstract

In this paper, we introduce a multi robot payload transport system to carry payloads through an environment of unknown and uneven inclinations while maintaining the desired orientation of the payload. For this task, we used custom built robots with a linear actuator (pistons) mounted on top of each robot. The system continuously monitors the payload's orientation and computes the required piston height of each robot to maintain the desired orientation of the payload. In this work, we propose an open loop controller coupled with a closed loop PID controller to achieve the goal. As our modelling makes no assumptions on the type of terrain, the system can work on any unknown and uneven terrains and inclinations. We showcase the efficacy of our proposed controller by testing it on various simulated environments with varied and complex terrains.

Figures (14)

• Figure 1: Robots with Linear Actuator carrying a payload on inclined plane where the black line shows orientation when both robots have same piston length while the blue line shows desired orientation of the payload.
• Figure 2: Piston model showing rod's end and spherical ball joint which we propose as an interface between the piston and the piston plate.
• Figure 3: Angle $\phi$ created between piston rod and the plate. $\phi$ can be manipulated by changing the rod length of different robots.
• Figure 4: Block diagram representing the open loop control system that utilises the roll and the pitch angle of the payload to achieve required piston lengths through PID controller. $l_{ref}$ is reference piston height, $\Delta l_{ref}$ is difference in current and desired piston length and $l_{in}$ is input length feed to piston.
• Figure 5: Height of piston at Roll = $\ang{0}$ and Pitch = $\ang{0}$.