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PC-Based Controls vs. PLC-Based Controls for Machine Automation

 
[caption id="attachment_8029" align="alignright" width="250"]PLC-based controls for material handling system 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

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. Advantages
  • Not tied to any one hardware platform
  • Fast communication between controls programming and equipment
  • Supports more programming languages
Disadvantages
  • 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

PLC-Based Controls

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. Advantages
  • 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.
Disadvantages
  • 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.

Author: Ron Daggett

Comments

Robby says:
8/28/2018 10:08 AM

Ron, I agree with everything you said, but one key factor about why PC solutions were used back in the day was the use of subroutines and parameters. From an OEM perspective the code re-usability of PC controls was unmatched by PLCs until V16 of RSLogix 5k introduced AOIs. However, PLCs have caught up and in many cases surpassed those features that were once only available in a PC based control solution and now PLCs are the only way to go. Unless, as Nate mentioned, you are gathering data at the plant level with a data logger such as Historian or a high level plant wide SCADA. Hope you and the team at Bastian are all doing well! Take Care Robby

nate says:
8/28/2018 10:08 AM

Great way to sum it up. Pretty much mirrors what I have seen. I think PC control is going away at the machine level. PLC's are doing everything you want, and what people want are complex MES that gather data and display dashboards. that is PC territory.

Ron Daggett says:
8/28/2018 10:08 AM

Is this Mr. Flowers???? Robby – I agree with you completely. The advent of the AOI has been a key reason why PLCs have become so powerful.

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