CLADDING TECHNOLOGY SHANXI CO., LTDInverse Design Method for Automatic Overlay Welding Robot Mechanism of Membrane-Type Water-Cooled Wall Boiler
Literature Overview and Engineering Challenge
This study, published in Transactions of the China Welding Institution in 2018 by researchers from Hohai University and Suzhou Hailu Heavy Industry Co., Ltd., presents an inverse design methodology for the kinematic mechanism of an automatic overlay welding robot system used for membrane-type water-cooled wall (WCB) boilers. Membrane-type water-cooled walls are critical components in supercritical and ultra-supercritical coal-fired power boilers, where they serve as both the primary heat transfer surface and the structural shell of the boiler furnace. The membrane tubes, typically made of ferritic-martensitic steels such as P91 or P92, require overlay welding of corrosion-resistant and high-temperature-resistant materials (e.g., 310SS, Inconel 625, or 309SS) on the furnace-facing surface to protect against slagging, ash deposition, and high-temperature oxidation.
The challenge of overlay welding membrane-type WCBs lies in the complex three-dimensional geometry of the membrane tubes, which include straight sections, bends, and connections to headers. The overlay must be applied uniformly across the entire furnace-facing surface with precise thickness control, while avoiding excessive heat input that could distort the thin-walled membrane tubes or degrade the mechanical properties of the base material. Traditional manual or semi-automatic overlay methods lack the precision and repeatability required for high-quality overlay on such complex geometries, making robotic automation a necessary solution.
Inverse Design Methodology
The inverse design approach reverses the conventional forward kinematics problem. In forward kinematics, the joint angles of the robot mechanism are specified, and the end-effector position and orientation are calculated. In inverse design, the desired end-effector trajectory (the overlay weld path) is specified first, and the required joint angles and mechanism configuration are then determined. This approach is particularly suitable for overlay welding applications where the weld path is dictated by the component geometry, and the robot mechanism must be designed to achieve that path with the required precision.
The methodology typically involves the following steps: defining the weld path geometry based on the WCB membrane tube layout, establishing the kinematic model of the proposed robot mechanism, performing inverse kinematic analysis to determine joint trajectories, evaluating the trajectory feasibility (reach, singularities, joint limits), and optimizing the mechanism parameters (link lengths, joint positions) to minimize errors and ensure smooth motion. The kinematic model is usually derived using Denavit-Hartenberg (DH) parameters or homogeneous transformation matrices, and the inverse kinematic solution may involve analytical methods, numerical iteration (Newton-Raphson), or optimization-based approaches.
| Design Parameter | Typical Specification | Rationale |
|---|---|---|
| Robot reach | 1.5–3.0 m | Covers membrane tube width and height |
| Position accuracy | ±0.5 mm | Ensures uniform overlay thickness |
| Orientation accuracy | ±0.5° | Maintains torch-to-surface angle |
| Maximum welding speed | 100–200 mm/min | Balances deposition rate and quality |
| Torch types | SAW, FCAW, or PTA | Depends on overlay material and thickness |
| Overlay thickness per pass | 1.5–3.0 mm | Controlled by wire diameter and travel speed |
| Total overlay thickness | 3–6 mm | Provides adequate corrosion resistance |
Process Integration and Control Considerations
The inverse design of the robot mechanism must be integrated with the welding process parameters to ensure that the overlay quality meets the required specifications. Key process parameters include heat input (controlled by voltage, current, and travel speed), wire feed rate, torch angle, and travel speed. The robot controller must synchronize the motion trajectory with the welding parameters to maintain a constant heat input and deposition rate throughout the weld path, even when the geometry changes (e.g., at bends or transitions between straight sections).
A critical consideration in the robotic overlay of membrane-type WCBs is the thermal management of the thin-walled tubes. The membrane tubes typically have a wall thickness of 4–6 mm, and excessive heat input can cause warping, distortion, or even burn-through. The robot system must incorporate features such as back-plate cooling, controlled interpass temperature monitoring, and possibly multi-pass strategies with lower heat input per pass. In practice, the overlay is often applied in 2–3 passes, with each pass depositing 1.5–2.0 mm of overlay material.
Quality Assurance and Field Implementation
The quality of the robotic overlay is verified through non-destructive testing (NDT) methods including ultrasonic testing (UT) for bond strength and internal defects, magnetic particle testing (MT) for surface cracks, and visual examination for surface quality. The overlay thickness is measured at multiple locations using ultrasonic thickness gauges, and the thickness uniformity is typically required to be within ±0.5 mm across the entire overlay area.
From an engineering practice perspective, the inverse design methodology offers several advantages over traditional robot programming approaches. It allows for systematic optimization of the mechanism design before physical fabrication, reducing the risk of design failures and costly modifications. It also facilitates the adaptation of the robot system to different WCB geometries by simply updating the weld path definition and re-running the inverse kinematic analysis. This flexibility is particularly valuable in power plant construction, where each boiler project may have slightly different membrane tube layouts.
A notable practical challenge is the calibration and verification of the robot system in the field. The theoretical kinematic model must be validated against actual measurements, and any discrepancies (due to mechanical tolerances, backlash, or deformation) must be compensated through calibration procedures. In field implementations, a coordinate system calibration step is performed before welding begins, using reference points on the WCB structure to establish the relationship between the robot coordinate system and the component coordinate system.
In conclusion, this research presents a systematic and rigorous approach to the design of robotic overlay welding systems for membrane-type water-cooled wall boilers. The inverse design methodology provides a powerful framework for optimizing the kinematic mechanism to meet the demanding requirements of high-quality overlay on complex geometries. The integration of kinematic design with welding process control and quality assurance creates a comprehensive engineering solution that is directly applicable to modern power plant construction.