Allen Bradley 1746-BTM

Has anyone had any experience with the Allen Bradley 1746-BTM barrel temperature module. We started using them approximately 3 years ago with no problems. Lately however strange things have been happening that we can't put a finger on. On occaision, we had the temperatures on all four loops indicate negative temperatures on machine startup. If the operator notices this, and shuts the power back off and turns it on again, it goes away. If they don't notice, the barrel temperatures try to heat to setpoint and because the temperature stays negative it overheats.

This negative reading is the same reading I get if I simply disconnect the thermocouple from one of the loops. However the temperature will return to normal when I hook the TC back up. When they go negative on their own though, they stay there until power is cycled again. I don't believe it is code related as the BTM code is the Allen Bradley code downloaded from their site and has been unchanged by us. Also I don't think it is code related because I can't get the problem to go away by cycling the keyswitch on the 5/03 to program then back to run.

Where I am leading to is perhaps noise issues or shielding problems. I also have a 1746-FIO4V, a 1746-NI8 and a 1746-NT4. If I disable the NI8, I have yet to get this to occur again. But of course I can't run the machine without the NI8.

Also, I am now beginning to hear reports that this is starting to happen to the machines when they are running, not just when turned on. And it is not isolated to just our facility. While it was at one time, it is now starting to happen at four seperate locations separated by many states.

I was hoping that perhaps someone else has had this problem and has found a solution.

Thanks in advance.

ChuckM
I have several machines here with the 1746-BTM and have not had this problem... yet. We have had these machines between 2-4 years.
Have you tried replacing one of the BTM's with a new one just to see if there is any difference? If it happens with a new card then you need to look at the thermocouple connections. If this is an IMM then you probably know the biggest problem is all of the thermocouple and power wiring connections around the barrel. Do you have some type of plug where all of the thermocouples connect that could be corroded. Might have a bad conncetion somewhere. The BTM has an open thermocouple alarm that you should use to NOT let your heaters come on or turn them off if the temperature is negative.

ChuckM,

I don't know what is the problem. Sounds like the card is sensing all TC's open at once. You could sense the occurance and prevent damage--maybe even reset the card.

From the Manual:

If a loop input circuit becomes open (open wire) the loop can not
measure temperature. In automatic mode, the lack of temperature
feedback makes it impossible to control the temperature. To guard
against this condition, the BTM module provides TC break detection.
When a break is detected, the module responds in one of these ways:
· disables the loop
· forces CV to this (TC Break Control) value (word 4 for loop 1)
· forces the CV to the manual %–output value (O:e.8 for loop 1)
Once the thermocouple break has been repaired you must disable the
loop and then re-enable it (through the input image table O:e.0/0
loop 1).
For additional information, Refer to TC Break Response on page 3-2.

That is what is all the more bothering about this. While it appears that the TC has broken on all loops, the TC open circuit bits (N10:164/0, N10:165/0, N10:166/0 and N10:167/0) are not set. They will set if I disconnect the TC, but they are not when this problem happens.

As for connections, there are basically none to speak of except for the connection at the module and another at the thermocouple which is a terminal rather than a bayonet style TC plug. And this problem is as apt to happen on a machine while it is being tested in the shop after building as it is in one that has ran two years. And it will go away pretty much as it comes.

This is a strange problem.
If you cant find anything wrong with the connections or the BTM the first thing I would do while I am looking for the root cause of the problem is to add some code to the PLC that says if my temperature is less than 0 then set and alarm and dont allow the heating contactor to turn on at all. The barrell temperature will always be at least ambient plant temp. This will make the situation safer for everybody.
(Hot plastic is no fun to get on you).
Have you checked the AB knowledgebase and search for BTM's to see if they have any info there? Also talk to the machine vendor they might have seen this one before. I am going to look around today and see if I can find anything.

ChuckM's Post

"If I disable the NI8, I have yet to get this to occur again. But of course I can't run the machine without the NI8."

Click to expand...

Have you tried swapping the NI8 with a new one?

Today we all but ruled out wiring or shielding issues. A technician replaced all the shielded cables and thermocouple wires and shielded them in a fashin that I am as confident in as I would have to be if it was for my mother's life support system. After turning the machine on and off a half dozen times, the problem was back.

I did take the conditions that enable the loops by setting bits N10:180/1, N10:181/1, N10:182/1 and N10:183/1 out of an MCR and made it where those bits were set by OTLs when I wanted the loops on and resetting them with OTUs when I want them off. After repeatedly turning the machine on and off a bajillion more times the problem did not happen. However I've been down this road before where I thought something fixed it only to find that it comes back in a few days to a few weeks.

Chuck,

Interesting. I've been having similar problems across multiple machines and plant locations. Ours seems to be somewhat related to how quickly power is cycled. If it is too fast, it seems more likely that the module will power-up and report temperatures of negative 210 celsius. BTM is setting the valid config bit 3 in words 4-7 of input image table. BTM also sets the Heat TPO bit 6 found in words 8-11 and therefore causes the process to overheat. Recycling the power fixes the problem. I'm planning on getting a test stand set up next year to further try to diagnose and eliminate this problem. A/B tech support is telling me to not let my customer cycle the power too quickly.

Any additional info you would like to add?

We just gave up on them and started using NT8 thermocouple modules and our own pid loops. Machines with the BTMs are retrofitted with NT8s as they cause problems.

Thanks for the info. Not really what I wanted to hear.

Do you have any old BTM's you want to get rid of?

ChuckM said:

That is what is all the more bothering about this. While it appears that the TC has broken on all loops, the TC open circuit bits (N10:164/0, N10:165/0, N10:166/0 and N10:167/0) are not set. They will set if I disconnect the TC, but they are not when this problem happens.

As for connections, there are basically none to speak of except for the connection at the module and another at the thermocouple which is a terminal rather than a bayonet style TC plug. And this problem is as apt to happen on a machine while it is being tested in the shop after building as it is in one that has ran two years. And it will go away pretty much as it comes.

Click to expand...

Worked at an extrusion outfit in Seattle. Cabinets were not adequately filtered and all electrical components were covered in dust and gungy stuff. Extruders often had oil and water on them. TCs used were not just TC conductor but TC probes.

In another post there was mention of grounded vs ungrounded TCs.

This leads me to wonder how old are your TCs and are they breaking down and possibly breaking or partially breaking at the junction or grounding? Gotta admit it is puzzling if all channels (ie TCs) go goofy all at once.

What about that connection between the sense point and the AB input card?

What about plastic dust and who knows what else on the input card?
Dan Bentler

agarb said:

Do you have any old BTM's you want to get rid of?

Click to expand...

We have a big project going on at the moment, but soon we are going to be looking at ridding ourselves of some surplus. I'll shoot you a pm if we get around to this. I even think we have some new in the box BTMs.

leitmotif said:

This leads me to wonder how old are your TCs and are they breaking down and possibly breaking or partially breaking at the junction or grounding?

Click to expand...

These were new thermocouples and new BTMs. And there is little dust or contaminates of that nature in the panels.

I've had this problem also. Out of the blue, a card showed open circuits on every channel.

I've tried to troubleshoot this by swapping the card with the card beside it, but the problem stayed with the slot, not the card. OK, so I'm thinking the wiring terminal is flaky, maybe the cjc is funked up somehow. I order a new wiring terminal and install it. Same thing happens.

This problem was with a brand new extruder with 6 zones. Only the four zones connected to the one card gave this problem. This is a very intermittent problem, only occurring occasionally.

I have a slew of the 1746-BTM cards in operation, and this is the only recurring problem I've had. It seems to me at this point that the problem is either in the chassis, or is an interference problem.

This morning, I'm checking out and getting ready to tune an extruder that was installed to replace the extruder I mentioned above. I,m hooked up to the processor looking at the temperatures for the extruder, and lo and behold, the first four zones are all showing open circuit, then suddenly start reading correctly.

Still scratching my head over this. Anyone have any suggestions?

Just curious... Is the slot that you are not having problems from closer to the processor slot? In retrospect, one thing I wish I would have tried would be putting my BTM in slot 1.

I have 20 BTM's on 5/04's installed on 5 extruders, one being about 8 years old (from memory, it was one of the first BTM's out) and i have yet ta have any problems whatsoever with them ?

The machines run 24/7 as well.