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PCs and PLCs: A Powerful Combination
CNC machining benefits from PLC operation linked to PC control, networking
- By Eamonn Walsh
- August 5, 2014
- Article
- Automation and Software
Advanced CNC machines are enabling the fabrication of large, intricate parts with complexity and tolerances that were not achievable even just a few years ago.
Improvements in CAD/CAM analysis, in combination with planning and machine setup, have also created additional and aggressive expectations by designers and fabrication shops. As usual, technological advancement has widened the goal posts of possibility.
It’s not unusual for multiaxis machining operations on a single workpiece to require many unattended hours, with each step carefully analyzed, calculated, and choreographed in advance. It’s also a world in which a single wrong move or mistaken instruction cannot be undone and can ruin an expensive project.
The long-established PLC is a good fit as the controller for a CNC machine because it can handle the demands of tight tool control and feedback, while also functioning amid the harsh environment of the shop floor. Implementing the individual fabrication steps with the tight tolerances and intricate motions for defined precision shapes and precise dimensions is a task well-suited to a PLC.
However, the PLC is not the appropriate controller for analyzing and preparing the machine’s operation or for providing the long, carefully timed set of instructions to the CNC machine—a process which may span hours. It is limited in both computation capability and sequencing ability. Furthermore, it is limited in its ability to provide work-in-process updates or production-related data.
Adding Computing Power
PCs working in conjunction with PLCs are the best practice for those tasks. With their computational power, PCs provide the necessary calculationintensive capabilities for CAD/CAM applications to work out the complex dimensional analysis needed for the various cuts, machining paths, critical angles, and their overall production sequencing. In addition, PCs and their multifaceted networking options can easily link to other PCs to provide Ethernet connectivity and other standard I/O ports. They also support legacy interfaces such as RS-232 and support the myriad formats and protocols required by the diverse world of PLC-based CNC machines.
Using PCs for analysis, interfacing, and networking in conjunction with basic PLC also provides management benefits. They can provide the numerous, increasingly mandated regulatory and traceability reports. They also can link multiple test or production stations, factory inventory status, customer information, and other production considerations.
Pulling PLCs and PCs Together
The PLC has its well-established role at the production line, while the PC provides higher-level access to an almost unlimited array of application software for advanced data analysis to support production planning, tracking, traceability, and scheduling. Adding the resources of PCs and their networks to a PLC test or production system takes advantage of the benefits of each while avoiding their traditional weaknesses.
Using CAD/CAM application tools, the PC can work out machining specifics and then feed the CNC a long sequence of precise instructions— which must often be via a proprietary CNC protocol. The PC also enables further optimization, including improved yields, margins, and profits, as well as improved regulatory compliance.
PC-based analysis and connectivity give management insight into production flow, along with a broad view of the process shifts that affect output, such as operational and maintenance issues. As a result, management is better equipped to anticipate problems and deal with them proactively, rather than just being reactive.
There’s a hardware benefit as well. By adding data analysis as well as connectivity via the PC and network, management can harness the power of PC/network components in terms of creating systems of higher performance and lower cost. Moreover, it brings flexibility in connecting to, viewing, and querying PC-friendly networks using smartphones and tablets, so the possibilities become dramatic and wide-ranging. The right modules can bridge the divide.
Computing Legacy
Still, there’s a legitimate concern which can’t be ignored: namely, the legacy issues and need for ongoing compatibility with the installed base of PLCs running CNC machines and their unique I/O. Replacing these is simply not a viable option in existing facilities, nor for new installations. The problem that often arises is integrating these two approaches, supporting installed legacy hardware and software as needed, and remaining open to future requirements.
Fortunately, some multilevel approaches merge the PLC and PC worlds, along with multiple networking and interoperability concerns, plus legacy factors. In the right combinations, these integrate the vertical path, from transducer I/O and its basic interfaces to factory protocols and advanced network compatibility. They also encompass the broad horizontal span of system-level gateways, databases, development tools, and high-level network access.
A new highly integrated digital networking module for industrial machine control demonstrates how these advanced building blocks have evolved to bridge the gap between PC and PLC worlds.
These modules combine a dual-port Ethernet switch, an Ethernet to RS- 232 serial interface, and an Ethernet to 4-channel digital I/O in a compact three-in-one system that saves the cost of deploying three separate units.
Modules can be configured, monitored, and controlled using a Windows ® COM port driver, which enables legacy software support by providing an interface to the Windows platform. Seamlessly networking the older serial modules extends their life for many years, keeping costs low and investment return high.
These modules are compatible with industry-standard PC packages such as LabVIEW®, MATLAB®, and Agilent VEE® so users can continue to exploit the benefits of their existing development and process-control system.
They can also deliver data directly to smartphones and tablets for Android ™ and Apple iPhone® and iPad®, and any other device that has a web browser.
Further, it’s important to look ahead while not obsoleting the installed base; even the venerable RS-232 interface still has an important place in the PLC/CNC world. That’s why there are adapter modules that provide connectivity and protocol compatibility between well-established serial interface standards and the Ethernet port. This allows the PC-based system to provide operating management and supervision all the way down to the PLC level, if desired.
The Road Ahead
As PCs increase their role with respect to PLC-based test and production, modules such as these can support plant managers and manufacturers who stay with PLCs for basic control, but who also want to have access to the systemwide, analysis-focused, and networked advantages which PCs can bring—especially when integrated with PLCs where appropriate. Selecting a module with PC and network interfaces and compatibility allows them to keep their options open for the future.
The current direction of the industrial markets is convergent with the PLC market, shifting to increasing PCbased, high-level involvement. This is a consequence of the pervasiveness of the PC and its associated applications. Whereas PLCs are proprietary systems with so-called “open” standards that actually differ from manufacturer to manufacturer, PCs, with their high-level networks and application packages, fit the needs of advanced insight, broad management, interconnection, and data sharing.
When properly configured and deployed, an architecture which builds on PLC-based test and production, and enhances it with PC power, offers genuine synergy and significant benefits from top to bottom. It allows management to acquire more and better product-test data, then analyze, use, track, report, archive, and retrieve it, all resulting in improved insight and performance.
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