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

Written By: Ron Daggett

PLC-based controls for material handling system

PLC-based controls for a material handling system

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.

Ron is the Controls Manager at Bastian Solutions and has been with the company since 2007.

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12 comments

  1. nate says:

    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.

  2. Robby says:

    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

  3. Ron Daggett says:

    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.

  4. Robby says:

    Yep!

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  6. Sifat Ullah Khan says:

    Can we implement complex calculations (e.g. single and two dimensional table, steam tabe etc) in Siemens Step 7 PLC.

  7. JJ Chapman says:

    Industrial PCs are not desktop PCs, and I would never recommend that industry operate their automation on a standard home PC.

    Industrial PCs (IPCs), if you spend your money wisely, will give you 24/7 365 up-time, simply because of the build-quality of an IPC, compared to your desktop PC.

    If you look at areas such as vibration, EMI and contamination protection, dust and water protection (IP-rating) which are all specified in the datasheets (and it appears many people are reluctant to red datasheets), you will note what IPCs are designed for what environment.

    Lets not forget: PLCs gained popularity over switches and relays NOT because they were more robust, but because they offered lowered cost and more flexibility (and were more compact). The IPC is now doing the same thing to the PLC.

    Either way, I believe the author of this blog would do well to describe the PCs that he has seen crash, what make they are, so that readers can see whether these crashed PCs were intended for industrial use. He appears more than happy to name PLC manufacturers, i.e. Siemens et al., but it would be prudent to do the same for IPC manufacturers as well.

    As for system upgrades; IPCs can run on whatever platform you please. The need to upgrade an OS is simply to keep it compatible with the rest of the company’s OS model. PLCs aren’t compatible with the company’s OS model to begin with, so I don’t see this as a valid argument. IPCs will always give you more flexibility than a PLC – as well as the rugged performance (please read your datasheets).

    This article is out-dated anyway. I thought I was reading something from 1998

  8. Joe Ottenhof says:

    Unfortunately this perspective on PC based control sounds a bit like the execs at Blackberry when the iPhone was introduced. “Who would want such a device? Battery life is short, consumes the network resources, isn’t secure etc etc.” Less than 5 years later we can see what happened. The industry changed…completely. You simply cannot stop the integration of IT into Automation Technology. The Internet of Things is real folks and it will be part of your automation strategy going forward…it has to be! And the PC IS the platform for that integration…you really cannot argue that from a technical perspective. PC based control and its inherent, fundamental integration with Intel and Microsoft is why you must look at the PC. Deal with its “issues” and reap the benefits.

  9. ricardo says:

    hello,

    very informative discussion. You nail my question.

    Now, to be more specific:

    1. In order to control a hydraulic machine (think a hydraulic press, industrial scale) and a conveyor belt (variable speed), what type of control should I used? PLC or PC based?

    2. What Industrial PC’s would you recommend?

    3. For image analysis and response based on the result of the image analysis (flawed articles rejection)..what type of control should I used? PLC or PC based?

    thanks for any input,

    ricardo.

  10. Rob says:

    I think Ron used the old FloPro and Open Control software from Nematron. He clearly hasn’t used Steeplechase Visual Logic Controller, currently sold by Phoenix Contact. It successfully addresses every complaint Ron made regarding PC control. It is so easy to use that I have taught an accountant how to troubleshoot a complicated control system in about 5 – 10 minutes. You just have to be working with someone who is not PC-phobic (and most young people love their computers). Over the past 30+ years I have worked with about every PLC out there. I have created very complex control systems in a single PC in much less time than would have been necessary with a PLC. Many of these systems were so complex that when I implemented them with PLCs, they required multiple PLCs on the same machine and the programs were nearly impossible for a non-engineer to follow. Steeplechase’s flowcharting language made quick work of these projects, on a single PC. The control programs were very easy to follow. Virtually anyone could troubleshoot the machine.

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