Opinion: on tis rung

TheWaterboy

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Envision following simple rungs

Code:
Input [XIC]-------Valve Open STATUS [XIO]-------Valve Open COMMAND [OTE]
Input [XIC]-------------------------------------Valve Open COMMAND [OTE]

The intent in the first rung appears to be an effort to save wear of the output module and the interposing relay.

The attached device is not a lamp or other device that needs to have its input held to maintain position so that's out of the equation.

My counter argument would be that it is more difficult to troubleshoot when facing the PLC since you cant tell what outputs are set.

How common is this practice and is it worth doing? I would like to stop doing this if its just an old school technique that is no longer necessary but if there is some validity to it I would like to hear about it.

Thanks in Advance
 
You did not say what the output is controlling, if it is a solenoid its very common practice to deenergize the solenoid after the valve is fully open or closed. Solenoids last a lot longer.

I'am not sure what you a arguing for or against.
 
I would prefer not to use the extra bit in the code, I would also prefer to see the output bit lit on the module when the output should be expected. Its easier for electrician to troubleshoot that way
Having said that, in the case you cite of a solenoid that could maintain position once it reached it, and not doing so would continue to heat the coil unnecessarily, I can see the point.
I have none of those, all the solenoids I have ever used are single acting returned by spring so I don't have that option and they just sit there and get hot.

As I said the only benefit I can see is to reduce heating of the coils on the small din rail relays that the modules control. I'm not sure that's necessary. If others have seen a different benefit, i.e. the modules last much longer, that's what I'm looking for.

I should mention we are an AB shop so that's the output modules I would be using.
 
Ok I will play, if for no other reason than to possibly learn something different than the way my mind puts this together.

If these are two separate rungs and the input and output commands are the same then what would be the point in doing this at all. (I don't see it)


1) the first rung is already opening your valve on the condition that the valve is not already open and hitting its "Full open limit switch" which would be supplying feedback for the "Valve Open Status".

2) Would the 2nd rung not be over riding the first rung and making the output True no matter what the condition of the "Valve Open Status" instruction on Rung #1. (Double Coil Syndrome)

It would make since to me if the "INPUT" was from a HMI Push Button and your second rung was something like this:

Valve Open Status [XIC] ------------------------------Input [OTU]

I just don't see the logic in the second rung the way it is in your example at all but would love to learn something new if I am missing the point or seeing this wrong.

BCS
 
The 2 rungs are 2 options for the same operation.

Rung 1 will clear the output bit once the OTE device has reached end of travel
Rung 2 will maintain the output bit for as long as the input is set.

Discounting the effect on the actual device being driven which is a variable, the question relates to the effect on the PLC's output module and interposing relays which will be held on differently between the two options with the benefit of fewer bits in the PLC and easier readability of the PLC face for a casual electrician unfamiliar with the logic inside the PLC.
 
The logic on rung 1 I would say is for a motorized valve that stops motion when open.
Rung 2 is a standard off/on
Two differant jobs, so your question is you don't want to use the input?????
 
Well, IF you are using a valve that is Powered in both directions then I would prefer to use Rung #1.
You don't have to keep the output, relay, and coil energized, and you can use the feed-back "Valve Open Status" as indication that the valve actually did open and remained open.

However if you are using power to open and spring return to close valves then you probably have little choice but use Rung #2.

I guess that you could still have feedback added on another rung (when using example rung 2) for valve full open indication if you want and if it is available, which is always nice to have, but it is not saving you on any code as you want.

I personally like to have the feedback if it is available.

BCS
 
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Like they said, it all depends on what kind of valve actuator you're controlling. A lot of times on double acting valves we'll hold the open or close output on for a few seconds more after the limit switch comes in, just to make sure the valve gets on the mechanical stop, because they're never going to be exact.
 
Like they said, it all depends on what kind of valve actuator you're controlling. A lot of times on double acting valves we'll hold the open or close output on for a few seconds more after the limit switch comes in, just to make sure the valve gets on the mechanical stop, because they're never going to be exact.

Even more instructions, but a great addition for a trouble free application. :)

BCS
 
Good points, but my primary interest is in the control from the PLC up to but not including the device being controlled.

Forget what device you are trying to control, the variety of things that can be controlled and how they behave in their environment negates any chance of a hard and fast answer.

If it helps, in this case you can imagine the device being controlled only needs a pulse to do its job and its intelligent enough to handle the rest itself, like a button in an passenger elevator. There's no point in leaving the output energized (or floor button pushed), but is there any harm? The benefit to maintenance in this case is that by just looking at the PLC status indications you can see which floor the elevator SHOULD be going to, but if only the floor status input feedback is indicated, you wont know where its going till it gets there.

So just consider the two components in question, the output module of the PLC (solid state or relay) and the coil of the tiny low power din rail relay you use to protect your module from the outside world.

Considering ONLY those two components, how would you answer the question?
 
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Considering only the PLC output and the interposing relay:
An energized output will generate a small amount of heat at the PLC output module, so the first control rung minimizes the heat generated. The interposing relay's life will depend on its total on time and the number of times it switches. The number of times it switches is not affected by either control rung, so once again the first rung minimizes the on time.

Outside of your constraints, the first control rung tells the technician something about the nature of the device being controlled. It shows that once the action expected as a result of the output being energized has taken place, there is no longer a need for the output to remain energized, so the valve is not spring return to the opposite state. It is more likely a dual solenoid or possibly spring return to a blocked center position. It could also be a motorized valve. It also tells the troubleshooter that there are additional control rungs associated with the output, since the rung only controls action in one direction. With the second control method, there might be another rung to control operation in the opposite direction, but simply turning off the valve open command could be enough to command the valve to close.

In summary, the first control method minimizes heat, maximizes component life and answers more related questions than the second.
 
A lot of times on double acting valves we'll hold the open or close output on for a few seconds more after the limit switch comes in, just to make sure the valve gets on the mechanical stop, because they're never going to be exact.

Isn't there another open/close switch on valve at series with PLC outputs, so travel stops before machanical stop, even if plc output remains on?
 

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