Near Time-Optimal Hybrid Motion Planning for Timber Cranes

TL;DR

This work tackles time-efficient, collision-free motion planning for hydraulically actuated timber cranes with passive joints. It extends VP-STO to incorporate pump flow rate constraints (PFRC-VP-STO) and introduces a weighted signed distance collision cost for robustness, showing promise as a global planner. A hybrid framework combines PFRC-VP-STO with a gradient-based local planner that accounts for passive dynamics to achieve near time-optimal trajectories with sway damping. Numerical results demonstrate improved success rates and shorter, robust trajectories compared to traditional RRT* TOPP, and confirm real-time feasibility of the local planner. The approach enables safer, faster autonomous crane operation in complex environments and sets the stage for deployment on real machines.

Abstract

Efficient, collision-free motion planning is essential for automating large-scale manipulators like timber cranes. They come with unique challenges such as hydraulic actuation constraints and passive joints-factors that are seldom addressed by current motion planning methods. This paper introduces a novel approach for time-optimal, collision-free hybrid motion planning for a hydraulically actuated timber crane with passive joints. We enhance the via-point-based stochastic trajectory optimization (VP-STO) algorithm to include pump flow rate constraints and develop a novel collision cost formulation to improve robustness. The effectiveness of the enhanced VP-STO as an optimal single-query global planner is validated by comparison with an informed RRT* algorithm using a time-optimal path parameterization (TOPP). The overall hybrid motion planning is formed by combination with a gradient-based local planner that is designed to follow the global planner's reference and to systematically consider the passive joint dynamics for both collision avoidance and sway damping.

Figures (4)

• Figure 1: The actual timber crane considered in this work.
• Figure 2: Illustration of the kinematic chain of the timber crane, where $q_1,q_2,q_3,q_4,q_7$ and $q_8$ are actuated and $q_5$ and $q_6$ are passive joints.
• Figure 3: Environments used in the experiments: Environment 1 (top) and Environment 2 (bottom) with logs 1-3 (left to right)
• Figure 4: Numerical results for a sample trajectory: Proposed hybrid motion planner with PFRC VP-STO based global planner + iLQR based local planner (blue lines), reference of PFRC-VP-STO (green dashed line), PFRC VP-STO applied to the system with passive dynamics (\ref{['eq:ddqu_DynamicsSimplified']}) without local planner (gray line), pump flow rate limit (red dashed line).