Flow Switch Ladder Logic?

Describe your system and processes, and post your config file.
BrunDog
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Flow Switch Ladder Logic?

Post by BrunDog »

Gents,

I have a problem that I am trying to solve but having difficulty... figured I needed to ask the gururururus here...

I have a flow switch that detects flow through my RIMS tube. It correctly turns DIN4 on when there is flow, and DIN4 correctly turns off when there is not flow --> so the hardware is working. This is, of course, a safeguard to make sure I don't dry fire my tube element.

I want to put a safety in the software such that if the RIMS output (OUT2) will not turn on when called for in a Process if DIN4 is off. It was my understanding that ladder logic was the last set of conditions evaluated before affecting hardware.

So I wrote the following line in ladder:
---| |---------| |--------------------------O----
__OUT2____DIN4______________OUT2

It works in manual mode... meaning if I turn on OUT2, and the flow switch (DIN4) is not ON, then OUT2 turns off. The problem is this does not work in a process. In the process, the ladder is ignored, and the output stays in (both in software and in hardware).

Any ideas how to implement this?

TIA,

BD
JonW
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Re: Flow Switch Ladder Logic?

Post by JonW »

There's definitely something odd here. I replicated your setup and get the same results. The control hierarchy is supposed to be Ladder Logic overrides process control and process control overrides manual control.

Is the flow switch a dry contact type? If so, until we can figure out what is causing this, you could wire the flow switch inline with OUT2 as a safety and it will accomplish the same thing.
BrunDog
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Re: Flow Switch Ladder Logic?

Post by BrunDog »

Thanks for validating, Jon. I appreciate you taking time to test this.

The switch I have is not a dry contact, but I could have it drive a transistor/relay combo to inhibit the signal to the SSR. Unfortunately I designed and installed the hardware on faith of the documentation, which is ladder holds highest priority. Therefore, changing the hardware would take a good bit of effort.

I will manage without it right now but would like to hear how to resolve from ECC. I am surprised this hasn't popped up before, as this has to be a common application. Perhaps having an output as a relay and contact in the ladder is not allowed?

-BD
JonW
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Re: Flow Switch Ladder Logic?

Post by JonW »

BrunDog wrote: Perhaps having an output as a relay and contact in the ladder is not allowed?
That was actually the first thing I was thinking when i saw your setup, but it works as expected in manual mode, just not under process control. I sent a note to Brent to check it out and either he or I will get back with you on it.
brahn
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Re: Flow Switch Ladder Logic?

Post by brahn »

Ladder logic is currently being applied prior to output control, resulting in the ladder logic being overruled by the output control. Looking at the source, this appears to be in 3.x as well as the 4.0 beta releases but I've only confirmed the behavior in 4.0. This will be fixed in the final 4.0 release.
BrunDog
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Re: Flow Switch Ladder Logic?

Post by BrunDog »

Thanks guys! Gotta love this forum as a resource.

But, you know the question I am going to ask next...

... when will that release be, would you fathom?

-BD
brahn
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Re: Flow Switch Ladder Logic?

Post by brahn »

If you've been using 4.0 you know that it's pretty stable. What we're working on right now is documentation before we call it officially released. That's coming along really nicely. I'm hoping to announce the final release in September.

All of the issues that have been identified for completion in 4.0 are fixed (including this one!), I'm just not releasing the builds right now. If any of these items are major issues that are preventing people from using the BCS I'll consider releasing another non-production build to get that resolved.
BrunDog
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Re: Flow Switch Ladder Logic?

Post by BrunDog »

Understood, thanks!

-BD
BrunDog
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Re: Flow Switch Ladder Logic?

Post by BrunDog »

Confirmed this is FIXED with the new 4.0 release. I thank you... and someday my elements will thank you!

-BD
RiverCityBrewer
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Re: Flow Switch Ladder Logic?

Post by RiverCityBrewer »

I understand why this works the way it does now on 4.0 final, and I understand why it needs to be this way, but now I am lost as to what I need to do to make a manual switch work correctly.

I had a DIN asserting an output directly in ladder logic, and even when the din was open, it still allowed process control to cycle the output as needed. I know why this is bad and fully recognize the need for it to be this way.

My question is, now, how do I have my din assert the output (as needed) and also still allow process control to assert the output? I've tried a few iterations but usually end up with the output "sticking" closed.

My din in my question is an on/off switch, and my output is a pump.

Thanks,

Joe
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