Zhuojin Pipe Fitting Co., LtdModeling and Simulation of the Hydraulic-Electromechanical System for Internal High-Pressure Forming of Pipe Fittings
Literature Overview
The paper by Han Xinyu, Deng Xiujian, and Liu Li, published in the journal Computer Simulation (Vol. 27, No. 1, 2010, pp. 292–296), addresses a critical engineering challenge in internal high-pressure forming (IHF) of pipe fittings: the position synchronization control of asymmetric side cylinders in the hydraulic press. Internal high-pressure forming is a widely adopted process for manufacturing butt-weld fittings such as elbows, tees, and reducers from pipe blanks, particularly for materials that are difficult to form by conventional bending or forging methods. The authors originate from the School of Mechanical Engineering, Shanghai Jiao Tong University, and their work bridges theoretical modeling with experimental validation.
Core Technical Problem: Asymmetric Cylinder Dynamics
The fundamental difficulty identified in this study is that the side cylinders in an IHF hydraulic press typically adopt a valve-controlled asymmetric cylinder design. This structural asymmetry means that the effective piston areas differ between the extending and retracting strokes, leading to inherently different dynamic characteristics in the two directions. For a standard asymmetric cylinder, the forward (extending) force is proportional to the large-diameter area, while the return (retracting) force is proportional to the annular area (large diameter minus rod diameter). This difference causes unequal fluid flow requirements, pressure build-up rates, and load response times in each direction. When two such cylinders must operate in synchronization to form a fitting blank uniformly, any mismatch in their dynamic response results in asymmetric forming loads, which directly translate into dimensional inaccuracies, wall-thickness variations, and potential material defects in the finished fitting.
The engineering consequences of poor synchronization are significant. In production environments, asymmetric forming forces can cause:
- Non-uniform wall-thickness distribution, particularly at the bend apex and springback regions of elbows.
- Residual stress asymmetry that accelerates fatigue crack initiation in pressure vessels and piping systems.
- Increased post-forming correction requirements, reducing throughput and increasing scrap rates.
Mathematical Modeling and Simulation Approach
The authors developed a comprehensive mathematical model of the side cylinder position synchronization control system. The model incorporates the dynamics of the hydraulic system, including fluid compressibility, valve dynamics, cylinder inertia, and load characteristics. The simulation was performed using SIMULINK, which provides a powerful environment for modeling coupled nonlinear systems.
Key Modeling Parameters
| Parameter | Description | Typical Range |
|---|---|---|
| Cylinder bore diameter | Large-diameter effective area | 80–200 mm |
| Rod diameter | Reduces return-stroke effective area | 40–100 mm |
| Supply pressure | Maximum system pressure | 10–30 MPa |
| Valve response time | Directional control valve dynamics | 10–50 ms |
| Synchronization tolerance | Maximum allowable position deviation | ±0.5–2.0 mm |
| Fluid bulk modulus | Effective compressibility | 0.8–1.5 GPa |
The mathematical model captures the transfer functions of the valve-controlled cylinder system, including the nonlinearities introduced by the asymmetric areas. The governing equations for the forward and return strokes differ due to the area ratio, and the synchronization error dynamics are derived as a coupled differential equation system.
Three Synchronization Control Strategies
The paper proposes and compares three synchronization control strategies:
Strategy 1: Equal-Mode Control
In this approach, both cylinders receive identical command signals, and their positions are compared at discrete intervals. The control system adjusts the valve opening proportionally to the position error. This is the simplest strategy but does not account for the inherent asymmetry in the cylinder dynamics. The response is adequate for low-precision applications but exhibits larger synchronization errors during rapid load changes.
Strategy 2: Master-Slave Control
One cylinder is designated as the master, and the other as the slave. The slave cylinder's position is continuously adjusted to follow the master's trajectory. This strategy exploits the fact that the master cylinder can be controlled independently with full authority, while the slave compensates for any deviation. The synchronization accuracy is higher than the equal-mode approach, but the slave cylinder may experience delayed response during high-speed operations.
Strategy 3: Hybrid Equal-Mode Plus Master-Slave Control
This strategy combines the advantages of both approaches. The equal-mode control provides a baseline synchronization, while the master-slave mechanism corrects residual errors. The hybrid approach was found to offer the best overall performance in terms of both response speed and accuracy.
Comparative Analysis of Strategies
| Control Strategy | Synchronization Accuracy | Response Speed | Implementation Complexity | Suitability for Asymmetric Cylinders |
|---|---|---|---|---|
| Equal-Mode | Moderate (±1.5–3.0 mm) | Fast | Low | Limited |
| Master-Slave | Good (±0.5–1.5 mm) | Moderate | Medium | Good |
| Hybrid (Equal + Master-Slave) | Excellent (±0.3–1.0 mm) | Fast | Medium-High | Excellent |
Experimental Validation and Results
The experimental results confirmed the feasibility and effectiveness of the proposed control strategies. The displacement and pressure systems both operated normally under the implemented control logic. The hybrid strategy demonstrated the tightest synchronization tolerance, with position deviations consistently within ±1.0 mm during the forming cycle. The pressure response was stable, and no oscillation or instability was observed during the forming process.
Experimental Test Conditions
| Parameter | Value |
|---|---|
| Fitting type | 90° elbow, DN100 |
| Pipe material | 20# carbon steel |
| Wall thickness | 4.0 mm |
| Forming pressure | 25 MPa |
| Forming temperature | Room temperature (cold forming) |
| Cycle time | 12 s |
| Measured synchronization error | ±0.8 mm (hybrid strategy) |
Engineering Practice Integration
From a practical standpoint, this research has direct implications for the design and commissioning of IHF production lines. The following engineering considerations should be emphasized:
- Cylinder selection: When specifying asymmetric cylinders for IHF presses, the area ratio should be carefully evaluated to ensure that the return-stroke force is adequate for the forming load while maintaining acceptable synchronization dynamics.
- Control system tuning: The PID or adaptive control parameters must be tuned separately for the forward and return strokes due to the different effective areas. A single set of parameters is insufficient for both directions.
- Pressure compensation: The control system should incorporate pressure feedback to compensate for load variations during the forming cycle, particularly as the blank work-hardens and the required forming force increases.
- Tooling alignment: Mechanical alignment of the die set and the cylinder mounting interfaces must be verified to minimize parasitic loads that can degrade synchronization performance.
Key Technical Insights and Reflections
The study demonstrates that the synchronization problem in asymmetric hydraulic cylinders is not merely a control theory issue but is deeply rooted in the mechanical and hydraulic design of the system. The authors' approach of combining mathematical modeling with simulation and experimental validation is exemplary for engineering research. The hybrid control strategy, while more complex to implement, offers a practical solution that balances accuracy and responsiveness.
One observation that merits further discussion is the role of fluid compressibility in the synchronization dynamics. The authors model the fluid as having a finite bulk modulus, which is physically correct, but in high-pressure forming applications where pressures exceed 25 MPa, the effective bulk modulus decreases due to dissolved gas evolution and temperature effects. This can lead to unexpected synchronization errors that are not captured by the baseline model. Future work should consider incorporating temperature-dependent fluid properties and gas evolution effects into the simulation.
Another important consideration is the interaction between the synchronization control and the forming process itself. As the pipe blank deforms, the load distribution changes continuously, and the required forming pressure may vary along the cylinder stroke. The control system must be robust enough to handle these load variations without sacrificing synchronization accuracy. The authors' experimental results suggest that the hybrid strategy provides sufficient robustness, but a systematic study of the control performance under varying material properties and forming conditions would strengthen the practical applicability of the findings.
Study Insights and Implications
This paper provides a valuable reference for engineers involved in the design and optimization of IHF production equipment. The key takeaway is that synchronization control of asymmetric cylinders requires a holistic approach that integrates mechanical design, hydraulic system modeling, and control strategy selection. The three strategies proposed offer a practical menu of options, with the hybrid approach being the recommended choice for high-precision forming applications. Engineers should also note that the simulation results, while validated experimentally, are specific to the test conditions; extrapolation to different fitting geometries, materials, and production speeds requires additional verification.