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PLC Considerations

When selecting a PLC or similar control engine there are many questions:

1. How much I/O?
2. What type of I/O?
3. What type of control logic -- simple ON/OFF or is there PID and data analysis?
4. What type of data is monitored and captured?
5. Are there recipes (databases) involved?
6. Is there an operator interface involved?
7. Are there special communication interfaces required? For example, flow meters, scales, thermocouples, or other signals that are not a regular discrete or analog signal.
8. Does the application require links to an external network, database, or some type of MES system?
9. Does the application require motion control, bar coding, machine vision, etc?

PLC and / or Computer

Sometimes we do not use a typical PLC for the control engine. Typically what we do is:

1. If the application is small (less than 50 I/O), no databases (only a few choices), and simplistic HMI then use a PLC.
2. If the application is small, slow (response time greater than 50 milliseconds) and requires computer functionality (machine vision, networking, databases, multiple axis motion control, etc) we prefer to do the entire application in Visual Basic (VB).
3. If there are large amounts of I/O (over 100) or you need fast, real-time response, then you will probably appreciate the PLC handling your real-time and direct I/O tasks and letting the computer handle the non-real-time tasks (such as HMI, databases, etc). There are a lot of gray areas in between.

Although today’s Pentium III running Windows NT or 2000 at 1 GHz with 512 KB RAM is very fast in comparison to technology only three years ago, it is still nice, in large systems, to use a PLC to help segment the system functionality. You can write subroutines to segment functionality -- you can also segment using different controllers.

Frequently Asked Questions (FAQs)

1. What about soft PLCs? Soft PLCs are where the PLC is actually software that resides on a computer. Although this is way that the industry is headed for the future, as with all new technology, we would recommend that your first test of new technology not be a critical application.
2. What brand of PLC is the best? It depends. J If you have a plant full of GE PLCs then it does not make sense to change to AB. If you have a plant full of some manufacturer that is no longer in business and you have to switch anyways then it may be time to reevaluate. If you have small applications that can be linked by computer networks then an AutomationDirect PLC may be fine. If you have large processes (thousands of I/O) requiring integration of drives and redundancy then you may want to consider AB or Siemens.
3. How can you program so many different PLCs? Most good PLC programmers, after learning three different PLC programming languages, can program most any PLC since the basic functions are the same. In fact there is an International PLC programming standard (IEC 61131-3) that your best PLC manufacturer’s are adopting.

To learn more

There is a lot you can learn about PLCs.  For example, there are books written on PLCs and some of these books do not seem complete.  If you want to learn more, there are many good resources.  As always try searching the best source of information -- the internet.

 

Other references:

• PLCMentor.com
• PLCS.net
• The History of the PLC by Dick Morley
• Any large on-line book store such as Amazon or Barnes & Noble will carry books on PLCs
• Instrument Society of America
• Industrial text and Video good books and videos.

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