Zhuojin Pipe Fitting Co., LtdAutomatic Control Method for Grinding Roller Open-Arc Surfacing System
Literature Overview
The paper by Li Zhenying, Dai Liping, Hou Ming, and He Qiong from Beijing Information Science and Technology University, published in Welding Journal (2008, Vol. 29, No. 4, pp. 82–84), presents an automatic control method for the open-arc surfacing of grinding rollers used in coal mills. The authors address the challenge of automating the surfacing process on a rotating cylindrical workpiece with a curved surface, developing coordinate formulas for the motion system and a trajectory control method based on spherical surface approximation of the roller surface. The study includes error analysis using MATLAB, demonstrating that the proposed control method meets the surfacing process requirements.
Core Technical Content and Control Methodology
The grinding roller is a critical component in coal grinding mills used in power plants and cement kilns. The roller surface is subjected to severe abrasion from the coal and grinding elements, and periodic resurfacing is required to restore the original dimensions and wear-resistant properties. The open-arc surfacing process (as opposed to submerged arc or plasma arc) is chosen for its flexibility and ability to deposit thick layers of wear-resistant material.
The fundamental challenge in automating the surfacing of a grinding roller is the need to maintain a constant torch-to-workpiece distance and angle as the roller rotates and the torch traverses along its length. The roller surface is not a simple cylinder but may have a crowned or concave profile, and the wear pattern may be non-uniform, requiring the torch trajectory to be adjusted accordingly.
Coordinate System and Motion Formulas
The authors establish a coordinate system for the surfacing machine's motion system. The grinding roller is mounted on a rotating axis, and the surfacing torch is positioned on a gantry or carriage system that can move in multiple directions. The key motion parameters include:
- Rotational axis: The roller rotates about its central axis.
- Longitudinal axis: The torch moves along the length of the roller.
- Radial axis: The torch moves radially to maintain a constant distance from the roller surface.
- Angular axis: The torch orientation may need to be adjusted to maintain a constant angle relative to the roller surface.
The authors derive coordinate formulas for the motion system based on the mechanical structure of the surfacing machine. These formulas relate the desired torch position and orientation to the actual motor positions and speeds, accounting for the kinematics of the machine.
Spherical Surface Approximation and Trajectory Control
A key innovation in this paper is the approximation of the roller surface as a spherical surface. While the grinding roller is nominally cylindrical, the actual surface profile may be crowned or have a complex curvature. By approximating the surface as a sphere, the authors simplify the trajectory calculation while maintaining sufficient accuracy for the surfacing process.
The surfacing process is divided into two phases:
- Same-arc segment surfacing: The torch moves along a continuous arc on the roller surface, depositing a uniform layer of surfacing material. The motion control formula for this phase ensures that the torch maintains a constant distance and angle relative to the spherical surface.
- Transition arc segment surfacing: When the torch moves from one surfacing pass to the next, it must traverse a transition path that connects the end of one arc to the beginning of the next. The motion control formula for this phase ensures a smooth transition without abrupt changes in speed or direction.
Error Analysis and MATLAB Simulation
The authors derive error formulas for the control trajectory and use MATLAB to analyze the accuracy of the proposed control method. The error analysis likely includes:
- Position error: The deviation of the actual torch position from the desired position, which depends on the accuracy of the coordinate transformation and the precision of the motion system.
- Speed error: The deviation of the actual torch speed from the desired speed, which affects the deposition rate and weld geometry.
- Angular error: The deviation of the actual torch angle from the desired angle, which affects the arc stability and weld quality.
The MATLAB simulation results demonstrate that the proposed control method achieves the required accuracy for the surfacing process, with errors within acceptable limits for the application.
Process Analysis and Engineering Implications
The automation of grinding roller surfacing offers several advantages over manual surfacing:
| Aspect | Manual Surfacing | Automated Surfacing |
|---|---|---|
| Consistency | Operator-dependent, variable | Consistent, repeatable |
| Deposition rate | Lower, variable | Higher, controlled |
| Layer thickness uniformity | Poor | Excellent |
| Operator exposure to hazards | High (fumes, UV, heat) | Low (remote operation) |
| Productivity | Lower | Higher |
| Cost per unit area | Higher | Lower (at scale) |
The open-arc surfacing process is particularly suitable for automated systems because it does not require a flux or a submerged arc configuration, which would complicate the mechanical design of the surfacing machine. The open arc allows for direct visual monitoring of the weld pool and easy adjustment of the torch position and angle.
Key Process Parameters
The following process parameters are critical for the automated surfacing of grinding rollers:
| Parameter | Typical Value | Control Method |
|---|---|---|
| Wire feed speed | 5–15 m/min | Constant wire feed (CWF) or constant voltage (CV) control |
| Welding current | 200–400 A | Power source control |
| Welding voltage | 22–30 V | Arc voltage feedback |
| Travel speed | 100–300 mm/min | Servo motor control |
| Torch-to-workpiece distance | 5–15 mm | Capacitive sensing or vision system |
| Shielding gas flow rate | 15–25 L/min | Mass flow controller |
| Roller rotation speed | 0.5–5 rpm | Servo motor control |
Integration with Quality Control
The automated surfacing system should incorporate quality control measures at multiple levels:
- In-process monitoring: Real-time monitoring of welding current, voltage, and travel speed to detect process deviations.
- Post-weld inspection: Dimensional measurement of the surfacing layer thickness, hardness profiling, and non-destructive testing (MT or PT) for surface defects.
- Feedback control: Use of inspection results to adjust process parameters for subsequent surfacing operations, implementing a PDCA cycle for continuous improvement.
Defect Analysis and Countermeasures
The primary defects in automated grinding roller surfacing include:
| Defect Type | Cause | Countermeasure |
|---|---|---|
| Uneven layer thickness | Inaccurate trajectory control | Improve coordinate transformation accuracy, calibrate motion system |
| Lack of fusion | Insufficient heat input or travel speed too high | Increase current, reduce travel speed |
| Excessive penetration | Excessive heat input | Reduce current, increase travel speed |
| Porosity | Inadequate shielding gas coverage | Increase gas flow rate, improve torch design |
| Cracking | High residual stress, incompatible consumables | Preheat, use low-hydrogen consumables, post-weld stress relief |
The trajectory control method proposed in this paper directly addresses the issue of uneven layer thickness, which is the most common defect in automated surfacing. By approximating the roller surface as a sphere and deriving precise motion control formulas, the authors ensure that the torch maintains a consistent distance and angle throughout the surfacing process, resulting in a uniform deposition rate and layer thickness.
Integration with Engineering Practice
The automated surfacing of grinding rollers is applicable in power plants, cement kilns, and other industries where coal grinding mills are used. The key engineering considerations include:
- Machine design: The surfacing machine must be designed to accommodate the specific dimensions and profile of the grinding roller, with sufficient range of motion and precision to maintain the required torch position and angle.
- Process optimization: The welding parameters must be optimized for the specific consumable and base material combination, taking into account the required layer thickness, hardness, and wear resistance.
- Quality assurance: A comprehensive quality assurance program must be implemented, including pre-weld inspection of the roller surface, in-process monitoring, and post-weld testing.
- Maintenance and calibration: The motion system must be regularly calibrated to ensure accurate trajectory control, and the torch must be maintained to prevent wear and drift.
Key Questions and Reflections
The paper presents a well-developed control method for automated grinding roller surfacing, but several questions remain. First, the paper does not address the effect of the surfacing layer on the roller dynamics. A thick surfacing layer can change the mass distribution of the roller, potentially affecting the balance and vibration characteristics of the grinding mill. This is a critical consideration in practice, and the surfacing process must be designed to maintain the roller balance within acceptable limits.
Second, the paper does not discuss the selection of the surfacing consumable. The choice of consumable is critical for achieving the required wear resistance, hardness, and compatibility with the base material. Common consumables for grinding roller surfacing include high-chromium alloy wires, tungsten carbide-containing wires, and stainless steel wires, each with different properties and applications.
Third, the error analysis using MATLAB provides a theoretical assessment of the control accuracy, but practical validation through actual surfacing trials would be valuable. The theoretical error may not account for all the factors that affect the actual torch position, such as mechanical backlash, thermal expansion of the machine structure, and wear of the motion components.
Study Insights and Implications
This paper demonstrates the feasibility of automating the surfacing of grinding rollers using a spherical surface approximation and precise motion control. The key insight is that the complexity of the roller surface can be managed through appropriate mathematical modeling and control theory, enabling consistent and repeatable surfacing results. The use of MATLAB for error analysis is a practical approach that can be adopted by other engineers developing similar automated surfacing systems.
For fellow engineers, the key takeaways are: (1) automated surfacing requires careful attention to trajectory control and motion system design; (2) surface approximation methods can simplify the control problem while maintaining sufficient accuracy; and (3) error analysis is essential for validating the control method and ensuring process reliability. The paper provides a solid foundation for the development of automated surfacing systems for other cylindrical or curved workpieces.