Thermocouple signal spiltter - got such thing?

Hi guys,

One of our customers has 6 thermocouple signal going into a B&R PLC thermocouple module. But for a new process, these thermocouple signals need to be send to another Siemens PLC for monitoring purpose. I think we cannot just tap the signal from the B&R PLC. Do we have something like an analog spiltter for thermocouple purpose?

I do not want to use comm/network because the B&R program is password protected. The cost could be higher using networking because we need to buy comm module for B&R and Siemens + programming as well. Basically I just need that 6 thermocouple signal, nothing more nothing less.

Hope you guys can enlighten me !

Thermocouples output a millivolt signal. In theory you could parallel the Siemens PLC to the B&R and it should work.

Issues become the input impedance of both controllers, and a real possiblity of noise on the signal. Distance between the B&R and Siemens units could be a factor, longer run, more possible noise.

Make sure you use thermocouple extension wire and if possible use a shielded cable to help with the noise issue.

Hope this will help.

It's probably an 80% chance that just parallelling the inputs will work. The 20% chance that that wont work is a result of loading on the TC itself, and the possibility that one (or both) of the T/C input modules have oddball inputs (such as one side grounded, or poor isolation from channel to channel due to multiplexers before the A/D converter, 'common-mode' connection of a cold-junction compensation device, etc).

If it doesn't work, two things that pop immediately to mind... first, just swap the existing thermocouples out with dual-element T/C's if possible. Second, you might have to go into a signal conditioner with the T/C, and feed both PLC's with a voltage or current-loop signal.

I agree with RDrast's 80% evaluation. In some circumstances, thermocouple break detection circuits in the receiver devices will conflict with one another.

If the thermocouples are grounded, there is more likelihood of difficulty for the reasons cited. If ungrounded better likelihood of success.

Heat treaters have routinely paralleled a thermocouple for years. One to a controller, one to a recorder.

For the trivial cost of thermocouple wire it's worth the effort to hook it up and try it.

Dan

If it doesn't work, two things that pop immediately to mind... first, just swap the existing thermocouples out with dual-element T/C's if possible.

Click to expand...

Great suggestion, as in many industries, it is normal practice to install dual thermocouples (two thermocouples mounted in the same thermowell, with two sets of connection blocks in the head). One of the thermocouples serves as a backup. If the first fails, operation can continue when the wires are swapped to the other set of terminals.

If you have dual T/C's, you can simply run another set of wires to the new PLC. However, the dual T/C's may not read exactly the same temperatures, but within a degree or two.

Yes, it is common to run T/C's to two different devices, except be aware of the possible pitfalls.

An interesting multi-channel unit from India:

http://www.canopusinstruments.com/dc_signal_splitter_module.html

That splitter is the cats meaow I tried to parallel 2 4-20ma sometime ago,It did not work as it should have,troubles with distance and really just getting the same results from the T/C to both devices was impossible.
Hope it helps

rdrast said:

If it doesn't work, two things that pop immediately to mind... first, just swap the existing thermocouples out with dual-element T/C's if possible..

Click to expand...

If paralleling the TC inputs doesn't work, then this will probably be by far the least expensive option, unless you are using an exotic metal TC such as type R, S or W.

The distance between the 2 plcs are about 20 - 25 meters away. I do not know we can just tap from the source for TC.

By changing the TC to dual element from single elements now, it's another cost. Even if it's a dual elements, after the cable travelling for 20 - 25meters away, there is a possibility of voltage drop or interferance or others resistance...

To play safe, I think it's better not to tap directly from the T/C source, who knows I might fall in the 20% category.

So, the best way (not the cheapest) is still using network? Something like get an Ethernet module at each of the PLC and let them pull the data over....can?

Thx guys for highlighting all the possiblility/risk involved.

Hi Flyers,
You can install an analog output card in the first PLC and an analog input card in the second PLC and work with 4-20mA instead of TC's. This solution is cheaper than ethernet.

Yes, if you have an existing network, it is very low cost to transport the information over the 25 meters. Depending upon the purpose of monitoring the data, it may work fine.

The other choice of doing dual thermocouples, analog retransmissions, or signal conditioning transmissions has better redundancy if the data is important. But that is more hardware cost and installation cost.

The distance between the 2 plcs are about 20 - 25 meters away. I do not know we can just tap from the source for TC.

Click to expand...

25 meters is no problem to run a thermocouple tap.

By changing the TC to dual element from single elements now, it's another cost. Even if it's a dual elements, after the cable travelling for 20 - 25meters away, there is a possibility of voltage drop or interferance or others resistance...

Click to expand...

The voltage drop will depend on your wire size. Use larger wire and it will be small. You may use shielded cable to eliminate the interferance.

I like Nonuke's suggestion about re-transmitting the analog temperature value. Normally that would be the cheap solution. Did you say that you could not access or change the exisiting PLC program? If so, then that will not help you either.

Time isn't always of essence, but sometimes it plays into the equation.

25 meters of TC wire is about $30. If you have a thermocouple input card, you can roll out the T/C wire, connect it and configure it in what, maybe an hour? And you know right away whether it works or not. Soa test is $30 and an hour's time.

If you have to ask about networking two PLCs, you've got a learning curve ahead of you for learning what that's all about. I'm guessing that the time involved in the learning curve is going to be signifiantly more than an hour or so to test the paralleling a TC.

Dan

An even cheaper way to retransmit the TC from PLC to PLC is with a 16 point DO card in the sending PLC and a 16 point DI card in the receiving PLC. Copy your temperature from N to O in the sending PLC and put it on the wires to the receiver. Well, you also need another DO and DI on each side for TX/RX; and if you want to get fancy, you need say another 3 outputs at the sender (and three corresponding inputs at the receiver) to tell which of 8 temperatures is being transmitted. If you didn't want to get fancy, you could just count at each end as you sent the values, or you could intersperse each temperature with an identifer before transmission.

I did something like this years ago to communicate analog data between an AB SLC and a GE PLC. It's more accurate than using the analog modules (which is why I did it) and it worked quite well. As a bonus, the LEDs flashing like mad on the DI/DO modules is really impressive!