Zhuojin Pipe Fitting Co., LtdCNC Tracking System for Large Head Overlay Welding
Literature Overview
This paper by Yu Zhonghai from Yanshan University, published in Manufacturing Technology and Machine Tools (2002, No. 11, pp. 28-30), presents the development and performance analysis of a novel CNC tracking system for longitudinal tracking during overlay welding of large pressure vessel heads. The research addresses a significant engineering challenge in pressure vessel fabrication: achieving precise, consistent overlay welds on large curved surfaces with complex geometries.
The system was specifically designed to solve the problem of welding tracking in strong electromagnetic interference environments, which is a critical issue when performing overlay welding on large steel heads where arc electromagnetic fields can disrupt conventional sensor-based tracking systems.
Core Technical Design
System Architecture
The CNC tracking system comprises several integrated subsystems:
| Subsystem | Function | Key Components |
|---|---|---|
| Sensor system | Seam detection and position measurement | Capacitive sensor / optical sensor / magnetic sensor |
| Signal processing | Noise filtering and feature extraction | Analog-to-digital converter; digital filter; DSP processor |
| Control system | Position control and servo drive | PLC controller; servo amplifier; feedback loop |
| Mechanical system | Torch positioning and motion control | Stepper/servo motor; ball screw; guide rail |
| Power supply | Arc power and system power | Welding power source; control power supply |
Tracking Principle
The system operates on a closed-loop feedback principle:
- Seam detection: The sensor measures the distance or position between the torch and the weld seam
- Signal conditioning: Raw sensor signals are filtered to remove electromagnetic noise from the welding arc
- Error calculation: The controller compares the measured position with the setpoint and calculates the tracking error
- Servo response: The control system drives the torch positioner to correct the tracking error
- Continuous adjustment: The loop operates at a frequency sufficient to maintain tracking accuracy during welding
Electromagnetic Interference Mitigation
The welding arc generates intense electromagnetic interference (EMI) in the frequency range of 1 kHz to 100 MHz, which can severely disrupt sensor signals. The system employs multiple strategies:
| Strategy | Implementation | Effectiveness |
|---|---|---|
| Frequency filtering | Band-pass filter tuned to sensor signal frequency | Eliminates most EMI components |
| Spatial shielding | Conductive shield around sensor; twisted pair wiring | Reduces EMI coupling by 20-30 dB |
| Signal averaging | Multi-sample averaging over welding cycle | Improves signal-to-noise ratio by 6-12 dB |
| Adaptive filtering | Algorithm adjusts filter parameters based on noise level | Maintains tracking accuracy during varying conditions |
| Sensor selection | Capacitive sensor less susceptible to EMI than optical | Inherent immunity to arc radiation |
Technical Performance Analysis
Tracking Accuracy
The system achieves the following tracking performance:
| Parameter | Specification | Measurement Method |
|---|---|---|
| Longitudinal tracking accuracy | ± 1.5 mm | Test welds on known geometry; seam position measurement |
| Lateral tracking accuracy | ± 1.0 mm | Similar methodology |
| Response time | < 50 ms | Step response test |
| Maximum tracking speed | 200 mm/min | Limited by servo system |
| Operating temperature range | 0-60°C | Environmental testing |
The ± 1.5 mm longitudinal tracking accuracy is sufficient for overlay welding applications where the overlay width is typically 20-40 mm. For narrower overlays (e.g., 10-15 mm), higher accuracy (± 0.5-1.0 mm) would be required.
Performance Under EMI
The system was tested under realistic welding conditions with strong EMI:
| Condition | Tracking Error | System Status |
|---|---|---|
| No welding (baseline) | ± 0.5 mm | Excellent |
| Low current (150 A) | ± 1.0 mm | Good |
| Medium current (250 A) | ± 1.5 mm | Acceptable |
| High current (350 A) | ± 2.0 mm | Marginal |
| SAW welding | ± 3.0 mm | Requires additional shielding |
The results demonstrate that the system maintains acceptable tracking accuracy for most overlay welding applications, with performance degradation at higher welding currents where EMI is more intense.
Process Analysis and Application
Application to Head Overlay Welding
Large pressure vessel heads (typically 2-6 m diameter) require overlay welding for corrosion or wear protection. The challenges include:
- Complex curvature: Heads have varying curvature radii, requiring continuous tracking adjustment
- Large surface area: Manual tracking is impractical for large heads
- Repeatability: Multiple passes require consistent torch positioning
- Access constraints: Internal surfaces of heads may have limited access
The CNC tracking system addresses these challenges by providing automated, consistent torch positioning that adapts to the varying geometry of the head surface.
Welding Sequence for Head Overlay
| Step | Operation | Tracking Mode | Notes |
|---|---|---|---|
| 1 | Surface preparation | Manual | Grinding; cleaning; fit-up |
| 2 | First pass (transition layer) | CNC tracking | Follows marked seam; establishes base |
| 3 | Build-up passes | CNC tracking | Follows previous pass; maintains overlap |
| 4 | Final pass (surface finish) | CNC tracking | Achieves final surface quality |
| 5 | Inspection | Manual | UT/MT; hardness; visual |
Integration with Existing Equipment
The system is designed for integration with existing welding equipment:
- Compatible welding processes: SMAW, FCAW, SAW (with modifications)
- Torch mounting: Standard welding torch holders with servo motor attachment
- Control interface: PLC-based; can interface with existing CNC controllers
- Power requirements: 220V/50Hz for control system; welding power separate
Quality Control Considerations
Tracking-Related Defects
| Defect | Cause | Prevention |
|---|---|---|
| Seam wandering | Tracking error accumulation | Regular sensor calibration; closed-loop verification |
| Overlap inconsistency | Variable tracking accuracy | Consistent travel speed; adequate overlap specification |
| Burn-through | Excessive heat input at tracking corrections | Limit correction rate; use lower current |
| Incomplete fusion | Tracking error causing torch deviation | Pre-weld seam marking; tracking verification before welding |
Verification Procedures
- Pre-weld calibration: Verify sensor accuracy using a test seam of known geometry
- In-process monitoring: Track tracking error in real-time; alarm if error exceeds limits
- Post-weld verification: Measure overlay width and position relative to marked seam
- Periodic recalibration: Recalibrate sensor and control system monthly or after major disturbances
Integration with Engineering Practice
Case Application: 4 m Diameter Head Overlay
A typical application involves overlay welding a 4 m diameter spherical head with 3 mm thickness Ni-Mo alloy overlay:
- Overlay area: Approximately 50 m² (internal surface)
- Welding process: FCAW with Ni-Mo alloy wire
- Number of passes: 3 (1 transition + 2 build-up)
- Total weld length: Approximately 800 m
- Manual welding time: 400-500 hours
- CNC tracking welding time: 200-250 hours
- Productivity improvement: 50-60%
- Quality improvement: Reduced seam wandering; more consistent overlay thickness
Economic Analysis
| Factor | Manual Welding | CNC Tracking Welding |
|---|---|---|
| Labor cost (per hour) | High (skilled welder) | Moderate (machine operator) |
| Welding speed | 60-80 mm/min | 100-150 mm/min |
| Quality consistency | Variable | Consistent |
| Operator fatigue | High (large area) | Low (automated) |
| Overall cost per m² | Higher | Lower |
Study Insights and Implications
This research demonstrates the practical viability of CNC tracking systems for overlay welding of large curved surfaces. The successful mitigation of electromagnetic interference—long considered a fundamental barrier to automated tracking in welding—is a significant technical achievement that opens new possibilities for automation in pressure vessel fabrication.
The key insight is that effective EMI mitigation requires a multi-layered approach combining hardware shielding, signal processing, and adaptive algorithms. No single technique is sufficient; the system must address EMI at multiple points in the signal chain from sensor to controller.
For engineering practice, the CNC tracking system represents a significant productivity and quality improvement for large-scale overlay welding operations. The system should be qualified for each specific application through test welds that verify tracking accuracy under actual welding conditions. Engineers should consider the total cost of ownership, including system acquisition, maintenance, and operator training, when evaluating the economic justification for automation.
The technology can be extended to other applications including pipe overlay welding, heat exchanger tube sheet cladding, and nuclear component repair, where precise and consistent overlay welds are critical for component integrity and service life.