Unwieldy Object Delivery with Nonholonomic Mobile Base: A Stable Pushing Approach

TL;DR

This paper introduces a stable pushing method for nonholonomic mobile robots to manipulate unwieldy objects, utilizing a linear motion constraint and nonlinear model predictive control to improve efficiency and ease of implementation.

Contribution

It demonstrates that line contact is essential for stable pushing with nonholonomic robots and simplifies the pushing constraints into a linear form for effective NMPC planning.

Findings

Reduced robot travel distance by 23.8%

Decreased operation time by 77.4%

Fewer hyperparameters and decision variables than LTV MPC

Abstract

This paper addresses the problem of pushing manipulation with nonholonomic mobile robots. Pushing is a fundamental skill that enables robots to move unwieldy objects that cannot be grasped. We propose a stable pushing method that maintains stiff contact between the robot and the object to avoid consuming repositioning actions. We prove that a line contact, rather than a single point contact, is necessary for nonholonomic robots to achieve stable pushing. We also show that the stable pushing constraint and the nonholonomic constraint of the robot can be simplified as a concise linear motion constraint. Then the pushing planning problem can be formulated as a constrained optimization problem using nonlinear model predictive control (NMPC). According to the experiments, our NMPC-based planner outperforms a reactive pushing strategy in terms of efficiency, reducing the robot's traveled distance by 23.8\% and time by 77.4\%. Furthermore, our method requires four fewer hyperparameters and decision variables than the Linear Time-Varying (LTV) MPC approach, making it easier to implement. Real-world experiments are carried out to validate the proposed method with two differential-drive robots, Husky and Boxer, under different friction conditions.