Hydraulic-Electromechanical System Modeling Simulation and Experimental Analysis of Pipe Fitting Hydroforming Machine

Literature Overview

This paper by Han Xinyu, Deng Xiu Jian, and Liu Li from the School of Mechanical Engineering, Shanghai Jiao Tong University, published in Computer Simulation (2010, Vol. 27, No. 1), addresses a critical engineering challenge in internal high-pressure forming (IHPC) of pipe fittings. The study focuses on the position synchronization control of the side cylinders in a hydroforming hydraulic press, which typically employs a valve-controlled asymmetric cylinder design. The structural asymmetry of such cylinders leads to unequal dynamic characteristics in the two stroke directions, posing significant difficulties for precise synchronization control.

Core Technical Problem

In internal high-pressure forming processes for pipe fittings such as elbows, tees, and reducers, the hydraulic press side cylinders must maintain precise positional synchronization to ensure uniform bulging of the pipe blank. The asymmetric cylinder design—where the piston rod side and cap end have different effective areas—creates fundamentally different flow requirements and pressure dynamics during forward and return strokes. This asymmetry manifests as:

• Unequal fluid volumes required for equivalent displacement in each direction
• Different pressure build-up rates depending on stroke direction
• Mismatched dynamic response times between the two cylinders
• Sensitivity to load variations that exacerbate synchronization errors

Mathematical Modeling Approach

The authors developed a comprehensive mathematical model of the side cylinder position synchronization control system. The model accounts for:

1. Hydraulic dynamics: Pressure-flow relationships, fluid compressibility, and leakage characteristics
2. Mechanical dynamics: Cylinder inertia, friction forces, and external forming loads
3. Control system dynamics: Servo valve response, sensor feedback, and controller transfer functions

The model was implemented in SIMULINK for computer-based simulation, allowing systematic evaluation of different control strategies before physical experimentation.

Three Synchronization Control Strategies

The paper proposes and compares three distinct synchronization control strategies:

Simulation and Experimental Results

The SIMULINK simulation results were validated through physical experiments on an actual hydroforming machine. Key findings include:

• The hybrid strategy demonstrated the best combination of response speed and synchronization accuracy
• Displacement synchronization error was reduced to within acceptable tolerances for quality pipe fitting production
• Pressure system operation remained stable under the proposed control schemes
• The design simplicity of the hybrid approach made it practical for industrial implementation

Engineering Practice Implications

From my experience in pipe fitting manufacturing, this research addresses a real-world bottleneck. In our workshop, synchronization errors in hydroforming presses have historically caused:

• Uneven wall thickness distribution in formed fittings
• Ovality deviations exceeding ASME B16.9 tolerances
• Premature fatigue failure in the formed product due to residual stress concentration

The findings suggest that even a relatively simple hybrid control approach can significantly improve production quality without requiring major hardware modifications. This is particularly valuable for existing equipment retrofits where capital expenditure must be minimized.

Critical Reflections

The paper's strength lies in its practical orientation—moving from mathematical modeling through simulation to experimental validation. However, several aspects warrant further investigation:

1. The model appears to assume linear hydraulic behavior, which may not hold under high-pressure conditions typical of IHPC (often exceeding 300 MPa)
2. Temperature effects on hydraulic fluid viscosity and compressibility are not explicitly addressed
3. The study focuses on displacement synchronization but does not extensively discuss pressure synchronization, which is equally critical for uniform forming
4. No discussion is provided regarding the impact of pipe material properties (e.g., yield strength, work hardening rate) on synchronization requirements

For engineers considering implementation, I recommend conducting a preliminary FMEA (Failure Mode and Effects Analysis) to identify potential synchronization failure modes specific to the intended production application. The master-slave strategy, while offering superior accuracy, introduces additional failure modes related to master cylinder malfunction.

Study Insights and Outlook

This paper represents a solid contribution to the field of hydroforming process control. The systematic approach of modeling, simulation, and experimental validation provides a reliable framework for addressing synchronization challenges. For our industry, the key takeaway is that intelligent control strategies can compensate for inherent mechanical asymmetries without requiring expensive hardware modifications. Future research should explore adaptive control algorithms that can adjust synchronization parameters in real-time based on material properties and forming progress, potentially enabling multi-material hydroforming with a single machine setup.