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Coordinated motion control of a wire arc additive manufacturing robotic system for multi-directional building parts

Fernando Coutinho, Nicolas Lizarralde, Fernando Lizarralde

TL;DR

This work tackles the challenge of WAAM parts with non-flat geometries by enabling deposition directions to remain aligned with gravity across layers. It introduces an augmented Jacobian task-augmentation framework for a two-robot WAAM cell, treating deposition trajectory in the deposition frame and orientation in an inertial frame as a unified reference, with a secondary task enforcing gravity-aligned TCP orientation. The approach includes a damped least-squares solution to handle algorithmic singularities and shows equivalence with a constrained Jacobian formulation, supported by Lyapunov-based stability arguments. Experimental validation across inclined, curved, and intake-funnel geometries demonstrates robust online coordination between the manipulator and the positioning table, achieving small tracking errors and symmetric final parts.

Abstract

This work investigates the manufacturing of complex shapes parts with wire arc additive manufacturing (WAAM). In order to guarantee the integrity and quality of each deposited layer that composes the final piece, the deposition process is usually carried out in a flat position. However, for complex geometry parts with non-flat surfaces, this strategy causes unsupported overhangs and staircase effect, which contribute to a poor surface finishing. Generally, the build direction is not constant for every deposited section or layer in complex geometry parts. As a result, there is an additional concern to ensure the build direction is aligned with gravity, thus improving the quality of the final part. This paper proposes an algorithm to control the torch motion with respect to a deposition substrate as well as the torch orientation with respect to an inertial frame. The control scheme is based on task augmentation applied to an extended kinematic chain composed by two robots, which constitutes a coordinated control problem, and allows the deposition trajectory to be planned with respect to the deposition substrate coordinate frame while aligning each layer buildup direction with gravity (or any other direction defined for an inertial frame). Parts with complex geometry aspects have been produced in a WAAM cell composed by two robots (a manipulator with a welding torch and a positioning table holding the workpiece) in order to validate the proposed approach.

Coordinated motion control of a wire arc additive manufacturing robotic system for multi-directional building parts

TL;DR

This work tackles the challenge of WAAM parts with non-flat geometries by enabling deposition directions to remain aligned with gravity across layers. It introduces an augmented Jacobian task-augmentation framework for a two-robot WAAM cell, treating deposition trajectory in the deposition frame and orientation in an inertial frame as a unified reference, with a secondary task enforcing gravity-aligned TCP orientation. The approach includes a damped least-squares solution to handle algorithmic singularities and shows equivalence with a constrained Jacobian formulation, supported by Lyapunov-based stability arguments. Experimental validation across inclined, curved, and intake-funnel geometries demonstrates robust online coordination between the manipulator and the positioning table, achieving small tracking errors and symmetric final parts.

Abstract

This work investigates the manufacturing of complex shapes parts with wire arc additive manufacturing (WAAM). In order to guarantee the integrity and quality of each deposited layer that composes the final piece, the deposition process is usually carried out in a flat position. However, for complex geometry parts with non-flat surfaces, this strategy causes unsupported overhangs and staircase effect, which contribute to a poor surface finishing. Generally, the build direction is not constant for every deposited section or layer in complex geometry parts. As a result, there is an additional concern to ensure the build direction is aligned with gravity, thus improving the quality of the final part. This paper proposes an algorithm to control the torch motion with respect to a deposition substrate as well as the torch orientation with respect to an inertial frame. The control scheme is based on task augmentation applied to an extended kinematic chain composed by two robots, which constitutes a coordinated control problem, and allows the deposition trajectory to be planned with respect to the deposition substrate coordinate frame while aligning each layer buildup direction with gravity (or any other direction defined for an inertial frame). Parts with complex geometry aspects have been produced in a WAAM cell composed by two robots (a manipulator with a welding torch and a positioning table holding the workpiece) in order to validate the proposed approach.

Paper Structure

This paper contains 18 sections, 1 theorem, 52 equations, 15 figures, 1 table.

Table of Contents

  1. Introduction
  2. Problem overview and system description
  3. Differential kinematics model
  4. Task space augmentation
  5. Augmented tasks for WAAM redundant systems
  6. Control law
  7. Constrained kinematics control
  8. Constrained differential model
  9. Equivalence with the Augmented Jacobian
  10. Singularity Analysis
  11. Experimental results and analysis
  12. Experimental setup
  13. Inclined thin-wall
  14. Curved thin-wall
  15. Intake funnel
  16. ...and 3 more sections

Key Result

Theorem 1

Consider the closed-loop system described by (eq:errordynamics) Assume that the reference signal $p_{d}$ is piecewise continuous and uniformly bounded in norm, and $q_{d}$ the is the unit quaternion representation of $R_{d} \in SO(3)$. Then, under assumptions (A1) and (A2), and considering $\bm{\eta

Figures (15)

  • Figure 1: Multi-directional building example.
  • Figure 2: Build direction schematic.
  • Figure 3: WAAM cell schematic
  • Figure 4: Robotic system setup, consisting of a KUKA KR90 robotic arm with a welding torch and a KUKA KP2 positioning table
  • Figure 5: Position and orientation trajectory errors of the $5^{th}$, $10^{th}$ and $15^{th}$ layers of the inclined wall
  • ...and 10 more figures

Theorems & Definitions (2)

  • Theorem 1
  • proof