In context
In 2023, parallel robots were gaining traction in industrial applications requiring high stiffness and precision, but their complex closed-loop dynamics hindered the implementation of intuitive drag-teaching and human-robot collaboration. Most zero-force control research focused on serial robots, leaving parallel architectures underserved. This work addressed that gap for a 6-RUS parallel robot used in spray painting of complex automotive LED components.
What was reported
Researchers from Tiangong University proposed a free-force control method based on a generalized-coordinate Newton-Euler dynamics model. The approach introduced a dynamic friction model with a pre-sliding S-curve to handle static-to-dynamic transitions, and used the Savitzky-Golay algorithm to smooth motor torque noise. The expected motor torque was computed from the dynamics model, and the error between expected and actual torque was compensated in real time.
Experiments on a 6-RUS parallel robot platform with three payloads (30 N, 45 N, 60 N) showed that the error between predicted and actual motor torque remained below 8.25%. During identical teaching trajectories, the proposed method required only 2.2 N·m of end torque, compared to 3.4 N·m for a conventional gravity-and-friction compensation method.
The method simplified dynamic modeling by avoiding the 366-dimensional differential-algebraic equations typical of standard Newton-Euler formulations, and improved torque prediction accuracy during static, startup, and motion-reversal phases.
Why it mattered
This work demonstrated a practical path to implementing free-force drag teaching on parallel robots, which could expand their use in tasks requiring direct human guidance, such as spray painting, polishing, and assembly. The generalized-coordinate approach offers a computationally efficient alternative for real-time control, potentially enabling broader adoption of parallel robots in collaborative manufacturing environments.
“In the same teaching process, the end torque required by the proposed method is only 2.2 N·m, and 3.4 N·m is required by the free-force control method based on the compensation of gravity and friction torques.”
Source: 《机器人》期刊 (robot.sia.cn) · Published 2023-05-09 · “基于广义坐标形式动力学的6-RUS并联机器人零力控制”