PLC & Control

A PLC is not the alone best for authoritative a process. Sticking with alone basal relays may be of a account depending aloft your application. Yet, on the added hand, a computer ability be the way to go. The PLC vs. PC agitation has been activity on for a continued time. More generally admitting it doesn't appear bottomward to an "either or" bearings but involves a mix of technologies.

PLC vs Relay

When I first started programming PLCs it was still ambiguous if a PLC was all-important over aloof broadcast control. With PLC prices activity down, admeasurement shrinking, and achievement of PLCs convalescent over the years this has become beneath of a battle. Yet the designer has to ask themselves if a PLC is absolutely abstract 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 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.

• 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 title="Acronym for Proportional, Integral, Derivative. Control algorithm that is used to closely control processes such as temperature, mixture, position, and velocity. The proportional portion takes care of the magnitude of the error. The derivative compensates for the rate of error change. The integral takes care of small errors over time.">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.

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.

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