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PLC-based controls for a material handling system[/caption]
PC-based controls were all the rage a decade or so ago. I saw many systems delivered with PC-based controls touting advantages ranging from less cost to more functionality. From my perspective, it seems as though the tide has turned in recent years back to PLCs.
After having been through the implementation of controls systems
using both approaches, let’s take a minute to reflect—from my perspective—on the actual pros and cons of each.
PC-based controls are programmed on a Windows computer and used to communicate with and monitor an entire material handling system or specific material handling equipment.
- Not tied to any one hardware platform
- Fast communication between controls programming and equipment
- Supports more programming languages
- Difficult to upgrade as new versions of Windows are released
- Difficult to implement system changes down the road because it is hard to find controls engineers familiar with the platform (You tend to be locked into using the OEM as they have the “specialists” necessary for implementation.)
- Long-term product support, which ties back to the various version of Windows that the PC-based controls must support
A programmable logic controller (PLC) is a small computer, often located inside a larger electrical control panel, programmed to run an entire system or specific equipment. Unlike a typical computer, a PLC can only be used to handle programming language for machine automation.
- Durability: Industrially hardened and extremely stable (no Windows limitations)
- Reliability: In over 20 years, I personally have only seen one PLC fail, and it was bad out of the box.
- Long life: Life cycle of a PLC is 20+ years
- Familiarity: Thousands of controls engineers are familiar with how to program the primary PLC manufacturers (Allen-Bradley, Siemens, etc.).
- Maintainability: It is much easier for electricians and controls technicians to troubleshoot systems using PLCs. They tend to be intimidated by PC-based controls. The ladder logic used in PLCs is familiar to them.
- Easy integration: PLC manufacturers make their branded hardware easy to integrate with their PLCs, which also means lots of built-in features.
- Perceived to be expensive: You must take true life cycle costs into consideration, though.
- Limited IO options: You tend to be tied to the PLC manufacturer
From my perspective, the vast majority of applications are better suited for PLC implementations. There certainly could be projects where PC-based control would be advantageous, but the disadvantages need to be seriously considered. To me, the single biggest issue is uptime. If a computer running a PC- based controller crashes, and you don’t have a backup ready to go, you could be down for days. The worst case scenario with a PLC is you have to run across town, buy a PLC, download the project onto the PLC, and you are up and running in a matter of minutes.
I’m sure my view is somewhat skewed because I have been burned a couple of times by PC-based controls implementations. The reality, though, is the experiences I have had are real world examples of the disadvantages of PC-based controls and, rightfully, should mold my view.
In any project, all options should be on the table, but for my money (and risk aversion), I would recommend a PLC system unless there is an extremely compelling reason to do otherwise.
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