Analog Signals (4-20mA)

[ControlLogix]

Hi guys,

I know I shouldn't be asking this question because I should know the answer but whatever..I don't!

Background:- I have 20 injection moulding machines that I want to take signals from and run them back to a new central PLC. This PLC will be monitoring all 20 machines for downtime etc. as well as gathering information on operating temperatures and pressures for the process engineers so they can improve the performance of the machines.

My question is this:- what length of cable can I run safely with my 4-20mA signal i.e. no loss of quality in analog signal.

As I have said, a simple question that I should know the answer to!

Thanks,

Colm

 

[Ken Moore]

I don't know the answer either, but I currently have several analog inputs that are over 1000 feet away, and they work fine.

The longer connections have a lot of voltage drop, but not current.

Ken

 

[Me]

depends on the size of the 24v power supply really!!

 

[Fritz_frederix]

Hello,

The lenght depends on the power supply and the tickness of your cabeling (wiring), you can calculate the voltage drop and compensate it by using a power supply which can deliver voltage from let say 24 to 30V. (do a search a believe this question was posted a few weeks ago)

But as I was reading your text, why don't you make use of an industrial network. Profibus should be ideal for what you're trying to achieve.

Rudi

 

[FNMdeJong]

Depends also on your environment, EMC trouble!!!

 

[GregPLC]

EMI is going to play a large part over long distances. Also make sure you don't run the signal wires parallel to 3 phase or higher voltage conduits. This can induce a lot of noise, and contribute to the degredation of signal.

Other than that, you need to get the ohm/foot value of your wire, and do some calculations to find voltage drops, make sure you have sufficient voltage to satisfy your analog input card.

Greg

 

[Tom Jenkins]

There isn't really a limit on the length of the signal leads. Obviously, as previously mentioned, EMI and RFI can be a problem, but using separate conduit for 4-20 mA signals and using shielded twisted pair cable for wiring with one end of the shield properly grounded will usually prevent these problems. The real limit is that the wire resistance plus the load resistance can't exceed the maximum load allowed by the transmitter. For a typical 2-wore transmitter 750 Ohms is a common limit. If the load of the PLC analog input is 250 Ohm then the max resistance (both leads) would be 500 Ohms.

Some more info can be found here:

http://www.plctalk.net/qanda/showthread.php?s=&threadid=8504&highlight=wire+size

 

[ControlLogix]

Thanks

Guys,

Once again this forum has proved that help is always close at hand. Given that my signals are at the most 200ft apart, it looks like I will be ok.

I didn't want to put Remote I/O into each panel since I am only picking up on 3 signals from each machine and I couldn't justify the cost per machine regardless of how quick the payback would be! However, I should be able to get money to put one beefy PLC in there so now might be the time to branch out and try something other than Allen Bradley. In my plant, we are allowed AB, Modicon, Tele and Siemens and so I might give these Siemens a lash. Are they really horrible to program like everybody maintains or are these people just like me, who can't walk away from the simplicity of Allen-Bradleys...only time will tell ;-)

Take care all,

Until l8r,

Colm

 

[JesperMP]

I have for many 4-20 transmitters observed that the manuals state a "max load" of 500 ohm or 600 ohm. This is under the condition that the supply is 24VDC.

Actually, there is a minimal voltage at which the transmitter will operate. Typically this is 12V or 15V.
You can then calculate the the max permissible load for supply voltages different than 24VDC.

Rmax = (Vsupply - Vmin)/Imax

Example: Rmax = (30V - 15V) / 0.020 A = 750 ohm

edit:
Rmax includes cable resistance AND the input resistor in the analog input card. If further devices are added to the loop (meters, instruments, splitters), they have to be considered as well.

 

[rsdoran]

I am curious, do the injection machines have plc's in them now? If so why not network? At 200 feet you could easily do a network, place a central unit to gather data from the machine units...dont have to worry about analog wiring, just one network cable.

This next option may be cost prohibitive but in the long run may offer more options for the system(s). If they dont have plc's could you add small units like micrologix, DL05 or S7-200 to each and bring the signals into them then network the units to a master plc that collects the data?

I am not sure how many analog signals etc you will be pulling from the machines but sounds like a lot of wiring/labor involved. May want to do a cost analysis on labor time, downtime and materials to see what the cost differential would be and do a comparison of benefits.