What is the advantage of PLC compared to "HARD WIRING"???

 

1. The amount of wiring can be reduced by PLC.
2. Simple to maniplate the Input and output logic control.
3. Faster to troubleshoot the faulty components(Sensors).
4. The control box will be simple for PLC.
5. Additional control tast or modification of system is easier by PLC.
6. System Upgrading faster by PLC.
7. And many more.

 

There are plenty of advantages. Most applications require alot less relays and other timing and switching devices, saving money and space. If a proficient programmer is sort it could also save construction time. If a PLC is used in a process control application, changes can be made to the software if the hardware requirements change, without the need for switchboard and/or hardware changes/replacement. PLC's are relativley cheap and can easily save you money, even on a small project such as controling a variable frequency drive for a booster pump.

 

I will list a few advantages of plc vs hard wire
1) less panel space
2) if you need to add a timer,relay pid loop,ect
with a plc you program these items (you have to hard wire the i/o but, you save on the components that would have to buy relay,timers pid controllers,ect.
3) one thing you need to consider is how much and what type of i/o your machine needs.do this before you pick your plc.this saves you from problems and add expense later.
i hope this helps one last thing who ever will be
programming your plc,will need training on the software. this a initial expense but, will save you problems down the road.

 

PLC's are more flexible, more powerful, smaller, cheaper, easier to troubleshoot, faster to implement, easier to document, and have features (like communication) that cannot be accomplished by hard wiring.

There's always a downside, of course. With hard wiring, you can figure out what's going with a multimeter. With a PLC, you can't put a multimeter on the legs of the processor and get any meaningful information. Also, you have to understand (and maintain) the PLC's programming software. But even very simple systems (two inputs, a timer, and an output) can now be done faster, cheaper, and easier using a nano-PLC.

-James Ingraham
Sage Automation, Inc.

 

Safeties should always be hard wired. Conditions relevant to the function (valve B always opens after pump A starts, system C only runs when system D is on) are better served as hard wired. I deal with customers who believe the PLC WILL crash. Some things just dont need to be taking up processor space anyway.

 

Have a look in the "Topics" link and you will find all the information you require.

 

Simply speaking, hard wiring (= designing the
circuit to do the task) is very hard to modify
or adjust. On the other hand, redoing a PLC
program is also becoming very hard to adjust,
because of the lack of college courses in the U.S.
With the improvements in embedded micro- controllers, I'd prefer using microcontrollers, programmed in C/C++.

 

9. Mechanical timers are hard to set.

 

> Safeties should always be hard wired.

Believe it or not, even that is no longer entirely true. Some safety systems are getting so complex that hardwiring them is no longer a simple affair.

For example, in the old days when all I/O was brought to a single cabinet, inhibiting a large pump from starting when a downstream valve was closed was a straight forward affair.

These days, however, that instrument wiring may not even exist. It may be a fiber cable for reliability. In older systems all sorts of things from grounding problems, induced currents, and even a lightning strike could take out instrument cables, fiber is mostly immune.

Now, how are you going to set up that valve safety? The answer is, in the PLC. Yes, we still get an uncomfortable feeling when we do things like this. But the alternative may not be much better. We're trading one set of problems for another.

Welcome to the brave new world.

 

About six miles of wire for a medium sized job.

Regards,
Ralph Mackiewicz
SISCO, Inc.

 

Sometimes its not a matter of what we used to do, but what is now economical. Used to be that pumps with seal water typically had the seal water valve hardwired into the motor starter circuit. This made sense since the valve would come on if the pump came on either manually or automatically.

Now with remote I/O, its often so much simpler to not have to run a wire all the way back to the MCC. Just hook it to the PLC I/O thats already near the valve. The PLC knows when the pump is running (from the MCC feedback) and turns on the valve.

The downside to this is that maybe the PLC will some day be shut down and someone will have to run the pump manually and forget to open the valve.

We now have to think what is the extra cost of hardwiring versus the potential downside of doing it through the PLC.

There are other issues. Typically hardwiring interlocks requires running more or larger conduits. This adds to the clutter in plants, and takes up space that could go towards more productive use.

Often hardwiring stuff leads to additional wiring connections, and devices that are not easily monitored for failure. The extra devices and terminations are places where additional failures can occur. And they can be spread out over a much wider physical area, making debugging problems with them a much longer process than if PLC I/O was used.

I have come to the conclusion that the best answer general answer is to put it in the PLC. This gives you the maximum flexibility in operation and the easiest means of debugging problems. There are a few cases where this is not appropriate (such as running lights in MCC buckets and some safety circuits) but for most cases, PLC control is the way to do it.

At least IMHO.

 

On January 29, 2003 10:38 am, Jake Brodsky wrote: <clip>
> Believe it or not, even that is no longer entirely true. Some safety
> systems are getting so complex that hardwiring them is no longer a
> simple affair.
<clip>

Leuze sells a programmable safety relay (Siemens sells the same device under a private label). It uses a safety rated version of ASI to monitor guard switches, e-stop buttons, light curtains, etc. There are up to two independent relay outputs. Jokab has an equivalent product as well. I haven't used either, so I can't say how well they actually work.

The Leuze/Siemens version monitors the ASI bus to see the safety inputs. There is a special safety "profile" for ASI to allow this. What this means is that you can monitor each guard door or e-stop button independently without having banks of conventional safety relays or masses of interlocking wiring.

A programmable system should be able to provide improved safety which is more closely tailored to the application needs without being overly complex to wire up.

--

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Michael Griffin
London, Ont. Canada
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If that is the case....just what do you use your PLC for if not for such things as you stated........which to me are interlocks not safeties. And without getting into a long winded debate with you about what is and isn't safety..........the arguement can be made that every valve, limit switch etc. is a safety............I will argue and agree both to that statement.....but if your definition and example are true then I have to rip out every PLC I have and re-hard wire everything......

My feeling is that everything requires inteligence to be safe, and some things need to be hard wired.......(e-stops) in case of processor failure but then I will argue "what if the e-stop button fails", so you say put in 2, and I say "what if they fail" and you say , put in 3.......so on and so forth.........

There are many, many instances of "not" hard wired examples you quoted all over the world.......

Dave Ferguson

 

The way I read the post, he meant that operational interlocking is fine to be done by a PLC, but industry standard is that anything with the potential to cause death, disfigurement, bad hair day, etc., MUST have a hardwired safety, which out community has in the past judged to be more fail safe than a programmable device. This is not to say it cannot or should not also be safed through the PLC, just that the PLC is not viewed as a substitute for hard wired safeties. If you think your E-Stops are not reliable enough, buy better ones, or do a dual loop system with a safety relay.

With the new breeds of "safety PLC's" built like a safety relay, this consensus may change. Also, no one ever said that the hard wired system was truly fail safe, just safer than a PLC (or at least, the PLC's that existed when the consensus was formed).