Transmitting linear 4-20mA d.c. signal over fibre optic cable

[[email protected]]

I have a signal conditioner which outputs a 4-20mA d.c. signal
linearly proportional to a measured input.

About 4 miles away is panel meter which displays the 4-20mA d.c.
signal. The amplifier is connected to the meter by a screened twisted
copper pair.

I have now been requested to decommission the copper wire and replace
it with a fibre optic cable.

What additional equipment would I need to send the 4-20mA signal over
a fibre optic cable, and can anyone recommend a bespoke product,
please?

(I'm guessing I'll need to digitise or frequency modulate the 4-20mA
and then demodulate at the meter end but if someone has a simpler
solution, please let me know)

There is power available at both the transmitter end and the panel
meter end.

Thanks

 

[Tim Wescott]

I have a signal conditioner which outputs a 4-20mA d.c. signal
linearly proportional to a measured input.

About 4 miles away is panel meter which displays the 4-20mA d.c.
signal. The amplifier is connected to the meter by a screened twisted
copper pair.

I have now been requested to decommission the copper wire and replace
it with a fibre optic cable.

What additional equipment would I need to send the 4-20mA signal over
a fibre optic cable, and can anyone recommend a bespoke product,
please?

(I'm guessing I'll need to digitise or frequency modulate the 4-20mA
and then demodulate at the meter end but if someone has a simpler
solution, please let me know)

There is power available at both the transmitter end and the panel
meter end.

Thanks

There has _got_ to be a way to do this with off-the-shelf equipment. I'd
web search on "20mA" and "fiber" and see what pops up. Failing that, I'd
post on sci.engr.control -- most of that group is experienced industrial
control guys, so you'll not only get a reply from someone who knows of a
box, you'll get a reply from someone whose used a few different brands and
can tell you where the bodies are buried.

--
Tim Wescott
Control systems and communications consulting
http://www.wescottdesign.com

Need to learn how to apply control theory in your embedded system?
"Applied Control Theory for Embedded Systems" by Tim Wescott
Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html

 

[Ecnerwal]

What additional equipment would I need to send the 4-20mA signal over
a fibre optic cable, and can anyone recommend a bespoke product,
please?

Here's one, probably massive overkill, but cheaper than engineering a
new version.

http://lascomm.com/video/lm170.htm

Try "current loop" and "fiber optic" for search terms to find others.

 

[Don Bowey]

I have a signal conditioner which outputs a 4-20mA d.c. signal
linearly proportional to a measured input.

About 4 miles away is panel meter which displays the 4-20mA d.c.
signal. The amplifier is connected to the meter by a screened twisted
copper pair.

I have now been requested to decommission the copper wire and replace
it with a fibre optic cable.

What additional equipment would I need to send the 4-20mA signal over
a fibre optic cable, and can anyone recommend a bespoke product,
please?

(I'm guessing I'll need to digitise or frequency modulate the 4-20mA
and then demodulate at the meter end but if someone has a simpler
solution, please let me know)

There is power available at both the transmitter end and the panel
meter end.

Thanks

I assume(?) there is no fiber mux you can get a channel assignment in.

If you must use a whole fiber capacity for the signal channel, look at the
Black Box catalog. I recall a RS/EIA 232 to fiber, converter for only a
couple hundred $ per end, then you will need a converter to RS232 from
4-20mA for each end. They are likely off-the-shelf too.

 

[SCADA]

I have a signal conditioner which outputs a 4-20mA d.c. signal
linearly proportional to a measured input.

About 4 miles away is panel meter which displays the 4-20mA d.c.
signal. The amplifier is connected to the meter by a screened twisted
copper pair.

I have now been requested to decommission the copper wire and replace
it with a fibre optic cable.

What additional equipment would I need to send the 4-20mA signal over
a fibre optic cable, and can anyone recommend a bespoke product,
please?

(I'm guessing I'll need to digitise or frequency modulate the 4-20mA
and then demodulate at the meter end but if someone has a simpler
solution, please let me know)

There is power available at both the transmitter end and the panel
meter end.

Thanks

You could use a PLC (Programable Logic Controller) at both ends. The input
card would be an analog 4-20ma . The other end could be whatever you want to
display.

 

[Jamie]

I have a signal conditioner which outputs a 4-20mA d.c. signal
linearly proportional to a measured input.

About 4 miles away is panel meter which displays the 4-20mA d.c.
signal. The amplifier is connected to the meter by a screened twisted
copper pair.

I have now been requested to decommission the copper wire and replace
it with a fibre optic cable.

What additional equipment would I need to send the 4-20mA signal over
a fibre optic cable, and can anyone recommend a bespoke product,
please?

(I'm guessing I'll need to digitise or frequency modulate the 4-20mA
and then demodulate at the meter end but if someone has a simpler
solution, please let me know)

There is power available at both the transmitter end and the panel
meter end.

Thanks

http://www.addvid.co.za/html/ll120.htm

That, or, if you want to make something your self.
got out one of those isolator boards and output the
signal to a fiber transmitter LED that connects on the end of the
fiber..
the other end would use another isolator board that will connect to
the output side of the circuit as the receiver..

We use fiber connectors that fit on a PCB's how ever, this will
use the whole fiber just for that one job..

 

[John Larkin]

http://www.addvid.co.za/html/ll120.htm

Any idea what that costs?

John

 

[Jamie]

Any idea what that costs?

John

I think that one was around 350 or so, Also, that particular unit
operates on 220v. it was just an example.

http://atkinsonelectronics.com/product_pdfs/overview.pdf
Look at the link near the bottom, they have fiber trunk
transmitters/receivers that will except TTL, 485 etc..
I didn't see a 4..20 but with that above, I think you can use a
something to adapt.

You know, this isn't rocket science..
Get your self a fiber Transmitter coupler and some fiber
on a roll with the prep kit.. make up a PWM to transpose the
signal on each end.

I did that just screwing around with something.. It wasn't
4..20 but could be used to transpose what you need.
I used a low power fiber transmitter diode coupler with a cheap
receiving photo diode coupler at the other end. Something like you
fine in audio systems.

 

[JosephKK]

[email protected] [email protected] posted to
sci.electronics.design:

I have a signal conditioner which outputs a 4-20mA d.c. signal
linearly proportional to a measured input.

About 4 miles away is panel meter which displays the 4-20mA d.c.
signal. The amplifier is connected to the meter by a screened
twisted copper pair.

I have now been requested to decommission the copper wire and
replace it with a fibre optic cable.

What additional equipment would I need to send the 4-20mA signal
over a fibre optic cable, and can anyone recommend a bespoke
product, please?

(I'm guessing I'll need to digitise or frequency modulate the 4-20mA
and then demodulate at the meter end but if someone has a simpler
solution, please let me know)

There is power available at both the transmitter end and the panel
meter end.

Thanks

Convert the current loop to digital and transmit that. Check SCADA
manufacturers for the equipment.

 

[[email protected]]

http://www.addvid.co.za/html/ll120.htm

Thanks, Jamie. That's exactly what I'm looking for.

Strange. A simple application but this product seems thin on the
ground. I guess single channel requirements must be relatively rare.

I'll pop over to the control forum to see if anybody knows any
equivalents.

Thanks again

 

[Ecnerwal]

Strange. A simple application but this product seems thin on the
ground. I guess single channel requirements must be relatively rare.

Think of it this way: the fiber can carry a huge amount of data - and
it's not much harder for it to carry many channels (drive electronics
wise) than to carry one - so the default mode is going to be to provide
the ability to replace many wire loops with one fiber, where some users
might end up with excess capacity, rather than to have single channel
devices where a lot of end-user money would have to go to excess fiber
if they had more than one wire loop.

 

[Jamie]

Thanks, Jamie. That's exactly what I'm looking for.

Strange. A simple application but this product seems thin on the
ground. I guess single channel requirements must be relatively rare.

I'll pop over to the control forum to see if anybody knows any
equivalents.

Thanks again

Well, they might be rare how ever, I used a simple fiber optic
transmitter and receiver PCB mounted. PWM modulated the transmitter
and then do the simple duty cycle receive at the other end to translate
that to a voltage or current in your case.
You may want to look into a trunk line transceiver set up. That will
allow you to expand later on the different types of information you can
convey.
I remember making a simple trunk transmitter and receiver years back
when I wanted to experiment a bit with it on my own. I used a what I
call TWM (Time width modulation).
A short pulse would indicate a reset for the string of channels and
place the index back to 0.
if the pulse past the resync window time, the remainder was used as a
time window to generate a 0..100% reference. etc..
I used a PIC chip to do both ends and had the options of digital or
ADC output..
16 channels on that one. only limited with the speed and IO lines I
was getting with the old version PIC.
today, speed is cheap.
how ever, It only did one way on each fiber..