Design of a PLC-Based Automatic Machine for Roll Shell Surfacing

Literature Overview

This 2014 paper by Liang Guangrui, Mo Shenghan, and Liao Zhenfei, published in the journal "Welding & Cutting" (Volume 44, Issue 3, pages 45-47), describes the design and implementation of a PLC-controlled automatic surfacing machine for the repair of sugar cane press rolls. The work was supported by the Small and Medium-sized Enterprise Technology Innovation Fund (Project No. 13C26214504844). The paper is classified under TG409 (welding equipment) and TP273 (PLC control systems), reflecting its interdisciplinary nature spanning welding technology and industrial automation.

Problem Statement and Technical Background

Sugar cane press rolls in sugar mills are subjected to severe abrasive wear from the continuous crushing of cane stalks. The roll shells, typically made of medium-carbon steel, experience significant material loss over time, leading to reduced pressing efficiency, increased energy consumption, and shortened service life. Additionally, worn roll surfaces become difficult to re-install into the press frame, causing operational delays.

The authors identified three primary challenges in roll surfacing repair:

Uniformity of surfacing layer: Manual surfacing welding produces inconsistent layer thickness and hardness distribution.
Surface preparation: The roll shell surface requires texturing (pitting) to improve the bonding strength of the surfacing layer.
Automation: The repetitive nature of roll surfacing makes it an ideal candidate for automation, but the curved geometry and large diameter of the rolls pose challenges for conventional welding automation.

System Design and Control Architecture

The automatic surfacing machine described in this paper integrates several key subsystems:

Subsystem	Function	Key Components
Roll handling	Positioning and rotation of the roll	Hydraulic clamping, motor-driven rotation
Surfacing welding	Deposition of wear-resistant layer	Welding torch, wire feed, gas shielding
Surface preparation	Pitting/texturing of roll surface	Gouging tool or shot blasting
Control system	Process automation and monitoring	PLC, HMI, sensors
Drive system	Synchronized motion of roll and torch	Servo motors, gearboxes

The PLC control system is the heart of the automatic machine. The authors employed a sequential function chart (SFC) approach to program the control logic, which is a structured programming method recommended by IEC 61131-3. The SFC-based control allows for clear definition of process steps, transitions, and interlocks, making the system easier to debug and maintain.

The typical control sequence for a roll surfacing operation is as follows:

Roll loading and clamping
Surface inspection and cleaning
Surface pitting/texturing pass
Surfacing welding pass 1 (base layer)
Surfacing welding pass 2 (wear layer)
Post-weld inspection
Roll unloading

Each step is controlled by the PLC with appropriate sensor feedback and safety interlocks. The synchronized motion between the roll rotation and the welding torch travel is critical for achieving uniform surfacing layer thickness. This is accomplished through servo motor control with encoder feedback, ensuring precise positioning at all times.

Surfacing Process Parameters and Performance

The surfacing process parameters employed in this system are designed to achieve a thick, uniform, wear-resistant layer on the roll shell surface. Based on the reported performance improvements, the key parameters can be inferred as follows:

Parameter	Typical Value
Surfacing process	FCAW (Flux-Cored Arc Welding) or SAW (Submerged Arc Welding)
Surfacing wire	Hardfacing alloy wire (high-carbon or alloyed)
Shielding gas	CO2 or Ar/CO2 mixture (if FCAW)
Welding current	200-400 A
Arc voltage	20-30 V
Travel speed	200-400 mm/min
Surfacing layer thickness	3-8 mm total
Number of passes	2-4 passes

The reported performance improvements are remarkable:

Hardness increase: 76% improvement in roll shell hardness compared to the un-surfaced condition.
Wear reduction: Wear volume reduced to 9.9% of the un-surfaced condition, representing a 90.1% reduction in wear.

These results demonstrate the effectiveness of the automated surfacing process in extending roll service life. The consistency and repeatability of the automated process are key factors in achieving such significant improvements over manual methods.

Engineering Practice and Implementation Considerations

The implementation of an automatic surfacing machine requires careful consideration of several practical factors:

Roll diameter and length: The machine must accommodate the range of roll sizes used in the sugar mill. Modular design or adjustable configurations are necessary.
Roll surface condition: The degree of existing wear varies from roll to roll. The surface preparation step must be adaptable to different initial conditions.
Safety: The combination of rotating machinery, welding arcs, and high-temperature surfaces requires comprehensive safety measures, including guarding, emergency stop systems, and fume extraction.
Quality control: In-process monitoring of welding parameters and post-weld inspection are essential to ensure consistent surfacing quality.

The PLC-based control system provides several advantages over purely mechanical or relay-based control:

Flexibility: Process parameters can be easily adjusted through the HMI interface without hardware changes.
Diagnostics: Built-in diagnostic functions allow for rapid fault identification and troubleshooting.
Data logging: Process data can be recorded for quality traceability and process optimization.
Scalability: Additional functions, such as automated inspection or integrated cleaning, can be added through software updates.

Study Insights and Reflections

This paper exemplifies the successful integration of welding technology and industrial automation to solve a practical manufacturing problem. The use of SFC-based PLC programming is particularly noteworthy, as it provides a structured and maintainable approach to control system design. The reported performance improvements are compelling and demonstrate the economic viability of automated surfacing for repetitive, high-volume applications.

One area where I would suggest further development is the integration of real-time process monitoring and feedback control. Current automated surfacing systems typically rely on set-point control of welding parameters, without adjusting for variations in base material condition or surfacing layer thickness. The incorporation of sensors for measuring surfacing layer thickness, arc stability, and torch-to-workpiece distance could enable closed-loop control, further improving consistency and quality.

Additionally, the surface pitting/texturing step, while essential for bonding strength, could potentially be automated more effectively. The current approach likely involves a mechanical gouging tool, which may produce inconsistent pit depths. An alternative approach using controlled shot peening or plasma texturing could provide more uniform surface preparation.

Reference Value and Outlook

This work provides a valuable case study in the automation of welding repair processes. The methodology of identifying a repetitive, labour-intensive welding task and developing a dedicated automated solution is directly transferable to other industrial applications, including pipe fitting surfacing, turbine blade repair, and mining equipment refurbishment. The PLC control architecture described in this paper serves as a practical template for similar automation projects. The significant performance improvements reported in terms of hardness and wear resistance validate the technical approach and provide a strong business case for investment in automated surfacing equipment. As industries continue to seek cost reduction and productivity improvement, automated welding repair systems will play an increasingly important role in maintaining the service life of critical equipment.