Interlocking two Machines with Run signals

MoneyMike1987

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Oct 2016
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Oregon
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First things first, thank you for all of the help.

I have a strong background with PLC's, but mostly Compact Logix and Control Logix platforms.

I used to write programs for an OEM while I was a Controls Engineer at my previous job. I took a promotion that pulled me away from my programming and had me traveling 95% of the year, so I am afraid I am a little rusty since taking this new job, but its slowly coming back.

I am the company Lead Electrician as well as their programmer. Unfortunately, there is many different platforms in the plant. SLC 504/505, Micro Logix 1400's, Control Logix and Compact Logix. Unfortunately, I can't stream line them due to this being a family owned company on a big scale. They are minimalists which limits what I can do.

I advised them that on their new line we need to interlock the equipment to prevent product from piling up at the infeed of the down equipment. I did this in the past for a vision system on an upstream and downstream system, but they were both L73 CL Processors. Forgive me for being so out of the loop, but would messaging suffice?
 
What are the processors that you need to interlock? If they are Compact or Control Logix I would use Produced/Consumed Tags. That is what I use to exchange info between Logix Processors.
 
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What are the processors that you need to interlock? If they are Compact or Control Logix I would use Produced/Consumed Tags. That is what I use to exchange info between Logix Processors.
He is saying that he hs done it with newer equipment. Emote so I'm guessing he needs to do it with a SLC now. Personally I'd hardwire them incase of a comms glitch. But maybe that's just old school me. Lol. I guess it depends one what you have available to you already, it if there is comms between them but not spare wires then go that route. You also have to see how critical it is. I mean if they aren't currently setup it can't be that critical. Lol
 
If the equipment is all connected to a line controller or PLC over some form of comms/fieldbus then I would do it over that to save having to install additional hardware. If there's nothing in place currently linking the systems, and they are not more than 150m-200m from each other then I would just hardwire in some Infeed Permit signals through solid state relays. We use both hardwire and Ethernet permit signals for our machines.

If your line can accumulate, you can consider putting some outfeed high level sensors on each machine.
 
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If these machines are not presently interlocked, I would lean toward doing it with messages. It will be a big improvement over what you don't have now. You can do a lot more with a MSG command than you can with even a dozen wires/relays.

If you want it more bullet proof, or you need instantaneous response, then hard wire them.

It's a good idea if messaging, to include a heartbeat or something to prove the controller is actually running at the other end of the messages. Think through what you want to happen if a message errors, if a PLC is switched to program mode, how fast must the other machine react, which one needs to be the "master" etc.

I generally include a read and write message in the chosen "master" PLC, I try to use the same data addresses in each PLC to make it easier to follow, I always use at least ten words in a message even if I'm only using one bit.

Repeat the messages at one second intervals typically. You can do it much faster over a physical network, and your application may call for them to be faster.

Don't repeat the messages if you get an ER bit...better to halt the messages and take appropriate action than to over fill a buffer.

Keep an eye on the channel status at the number of connections. It should only use one additional connection in each PLC to add multiple messages to and from a single controller.

For a heartbeat, you can use a counter value that increments on each done bit as part of the message. In the receiving controller, you can echo that value back as part of the other message block (assuming you do a read and a write). I also have seen the free running clock copied into the message block which amounts to a pseudo-random number. if that heartbeat doesn't change for ... say five seconds ... assume the connection is lost or the other machine is down and act accordingly.
 
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If these machines are not presently interlocked, I would lean toward doing it with messages. It will be a big improvement over what you don't have now. You can do a lot more with a MSG command than you can with even a dozen wires/relays.


Thanks!

I was leaning towards messaging with integers. I just recently spoke to my old boss who was a NASA engineer. i haven't had the opportunity to talk to him in a while, but the messaging is spot on. Since this is such a minimal data transfer (Run Signal) messaging will work just fine. I am trying to streamline this company for the future. They are stuck in the 80's and 90's. The better off they can be, the easier my job will be (Until I leave). The goal is to set them up. I am running CAT 6 between switches and going to go from there.

Any issues you see from there?

Again, thank you so much
 
Okie,

Great response. I only had a brief time to talk to my buddy and messaging was the way to go, but you went above and beyond. Do you care to elaborate a bit? We never touched the heartbeat, but maybe you and I are at a difference as far as terms. I'm just trying to make my job and their job easier.

Thank you!
 
Heartbeat checks corresponding device is functional and active. in my opinion of you are using and fieldbus/ethernet solution its a must.

So there are a couple of ways easiest is to increment an integer on each scan and check for the change in value in the partner PLC. If value remains the same there is an issue.

Other ways is to use a bit that you can set in one PLC and reset in other. If state doesnt change you raise interlock/alarm.

Hearbeats are used generally to shut down systems safely in case communication goes down. You can take it a step further and monitor response times ect.

This can assist in complex networks with lots for devices
 
I forgot to mention that most of the equipment has different IP's. Some are 10.10, and some are 192.168 and they do not have the original files. This makes it difficult. I never got a chance to ask my buddy, but I can't get a lot of the text/tags with the program. Is there anyway around this? I've been shooting in the dark.
 
Heartbeat checks corresponding device is functional and active. in my opinion of you are using and fieldbus/ethernet solution its a must.

So there are a couple of ways easiest is to increment an integer on each scan and check for the change in value in the partner PLC. If value remains the same there is an issue.

Other ways is to use a bit that you can set in one PLC and reset in other. If state doesnt change you raise interlock/alarm.

Hearbeats are used generally to shut down systems safely in case communication goes down. You can take it a step further and monitor response times ect.

This can assist in complex networks with lots for devices


So right now, I have a Com issue between two pieces of equipment. I have implemented a clean mode on this so called equipment that allows them to bypass the run mode, due to com issues. They wanted a fast solution until I can come up with a permanent solution. I have 120ft of conduit with cat 5E cable running through it that I have to check. Before I start programming around this completely, I am going to pull the old cable and run new before I cross that out.

EDIT: I have already checked COM, and sometimes I can pull the data and other times I can't, so I am forced to check the old CAT5E cable
 

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