PLCs Versus Other Types of Controls

A PLC is not the only choice for controlling a process. Sticking with only basic relays may be of a benefit depending upon your application. Yet, on the other hand, a computer might be the way to go. The PLC vs. PC debate has been going on for a long time. More often though it doesn't come down to an "either or" situation but involves a mix of technologies.

PLC vs. Relay

When I first started programming PLCs it was still questionable if a PLC was necessary over just relay control. With PLC prices going down, size shrinking, and performance of PLCs improving over the years this has become less of a battle. Yet the designer has to ask themselves if a PLC is really overkill for their application. Some questions should be asked.

• Is there a need for flexibility in control logic changes? Will there be frequent control logic changes? Will there be a need for rapid modification?
  A lot companies believe they will never change a design but more often then not ideas and goals do change and modifications will need to be made. Do you want to do that in hardware (relays) or software (PLC)?
• Must similar control logic be used on different machines?
  It's so much easier to download a program then build another panel.
• Is there a need for future growth?
  A PLC can easily accept a new module in a slot or get an expansion base.
• Is there a need for high reliability?
  PLCs are seen as more robust over individual components.
• Is downtime a concern?
  Any change or troubleshooting on a relay system means the system might have to go offline. Changes in a PLC can often be made online with no downtime.
• Are space requirements important?
  Based on the number of relays a PLC can be a real space saver.
• Are increased capability and output required?
  PLCs can be faster then their mechanical counterparts.
• Are there data collection and communications required?
  Only possible with a PLC or computer.
• What are the overall costs?
  There's a certain price point comparison but in these days it's very low in favor of a PLC.

PLC vs. Dedicated Controller

A dedicated controller is a single instrument that is dedicated to controlling one parameter such as a PID controller measuring a temperature for heating control. They have the advantages of an all in one package, typically with display and buttons. This can be a very good thing to use in simple applications. A PLC these days can compete price wise and functionally with these controllers especially if you more then one controller is needed. PLCs offer a greater degree of flexibility too because the can be programmed to handle all sorts of different scenarios.

PLC vs. PC (Personal Computers)

The PLC vs. PC debate has been going on for years and I'm not going to attempt to give the definitive answer. They both have their pros and cons. What often happens is that the two are used for their strengths in different parts of the factory.

	PLC	PC
Environment	The PLC was specifically designed for harsh conditions with electrical noise, magnetic fields, vibration, extreme temperatures or humidity.	Common PCs are not designed for harsh environments. Industrial PCs are available but cost more.
Ease of Use	By design PLCs are friendlier to technicians since they are in ladder logic and have easy connections.	Operating systems like Windows are common. Connecting I/O to the PC is not always as easy.
Flexibility	PLCs in rack form are easy to exchange and add parts. They are designed for modularity and expansion.	Typical PCs are limited by the number of cards they can accommodate and are not easily expandable.
Speed	PLCs execute a single program in sequential order. The have better ability to handle events in real time.	PCs, by design, are meant to handle simultaneous tasks. They have difficulty handling real time events.
Reliability	A PLC never crashes over long periods of time. ("Never" may not be the right word but its close enough to be true.)	A PC locking up and crashing is frequent.
Programming languages	Languages are typically fixed to ladder logic, function block or structured text.	A PC is very flexible and powerful in what to use for programming.
Data management	Memory is limited in its ability to store a lot of data.	This is where the PC excels because of it's hard drive. Any long term data storage, history and trending is best done on a PC.
Cost	Just too hard to compare pricing with so many variables like I/O counts, hardware needed, programming software, etc.

Hybrids of PLC/PCs are common now (e.g. WinPLC). This type of hardware tries to mix the two platforms using the strengths of both. So the CPU might be able to run Windows CE or Linux in a rack that can accept common I/O modules.

Off the Shelf vs. Build Your Own

For some manufacturers the choice these days is coming down to buy a PLC or make your own. The benefits of PLCs have become so widely known that manufacturers looking to cut cost can engineer their own solutions and build them more cost effectively. An example of this is the Divelbiss "PLC on a Chip" with the accompanying EZ LADDER programming software.