Cost Effective Remote IO Needed

[sparkie2]

I am looking for a cost effective solution for a small amount of remote IO (aprox 12 IO) at each of about 16 locations. The furthest remote IO location will be about 3500 feet from the master control point. The locations are out doors, and interconnecting wiring needs to be direct buried. Power for the remote IO equipment will be 110 VAC, which will need to be run in parallel with the remote IO fieldbus. The remote IO is slow to change, so response time need not be very fast. Wireless is not an option due to metal structures in the area that would interfere with the RF signals.

Will appreciate any recommendations that you would have, and/or experiences (pro or con) with any of the various fieldbus for remote IO.

 

[itsmoked]

Yikes 3500 feet, more than half a mile... Parallel 120Vac.. I'm not familiar with fieldbus. Are you using a FB module at the end of the line?

Look into carrier current so you don't actually run the field buss wires? Carrier current has to be AC noise resistant. One cable... Might work.

Are there FB fiber converters available?

Hope you don't need much power.

 

[danw2]

If your power is already available at the remote I/O location (meaning you don't have to trench to get power out wherever), you might consider investigating wireless.

The one watt, 900MHz, frequency hopping wireless is a different breed than earlier fixed frequency radio.

I had a vendor make an on-site survey although we were sure it wouldn't work. But it did, at 4,200 feet distance, with a suitable antenna. We used Phoenix Contact brand. Been working for a year now.

 

[sparkie2]

Thanks for your input. But, let me clarify a few items:
I wanted the term Fieldbus to be generic for the various fieldbus now available (Profibus DP, Ethernet, Remote I/O, etc.). In looking over some of the available Fieldbus remote IOs available, to obtain the distance I need, fiber may be required. Do to cost of running fiber, I wanted to know if any of the "twisted pair" fieldbus could do the job. If so, I would think it would be more cost effective.

I should also mention that at the furthest remote IO locations, local 110 VAC could be provided locally (i.e. it will not be necessary to run the 110 VAC 3500 feet).

 

[itsmoked]

Lets assess the potential noise.
What are you going to be powering out at these end points?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[sparkie2]

The largest load will be the operating of small linear actuator motors (<1.5A load) and only one will operate at a time. The rest of the IO will be indicating lights, and sensing of the position of the equipment with either limit switches or prox sensors.

 

[itsmoked]

sparkie2; The load sounds pretty noise free. As long as you are not using a brushed motor you should be okay.

But there are two other points to keep in mind.

1) Those are really, really, really, long runs of wire.
(3500ft).

If your power is really 120Vac not 110(rare) then typical allowed voltage drops at the other end should not exceed about 5%, or your equipment might malfunction when your supply voltage drops to 112V or something like that (your mileage may vary).

So you could drop about 5% of 120Vac = 6V
Now this means with a 1.5A draw: 6V/1.5A = 4 Ohms.

This means that the total round trip, 7000ft, of wire must have less than 4 Ohms of resistance or your voltage will drop dramatically. This means you would be running 8AWG for 4.5 Ohms or 6AWG for 2.8 Ohms!! $$$$$

Try to keep the two separate cables at least a foot apart with dirt between them to limit any coupling.

Options:
Place a battery at the endpoint. Run a very small trickle charger that draws say 0.2A,(allowing much smaller wire), to keep the battery topped up then at the battery use a small inverter to locally provide the 120Vac 1.5A (180Watts). This will work fine as long as the actuator is only commanded occasionally.

Or as danw2 correctly suggests(used frequently) would be a radio link using a highly directional antenna (yagi). This can be extremely reliable, using ISM band spread-spectrum. These work well for links many miles apart. You can use a solar panel and the inverter scheme mentioned above, powering both the radio and the actuator for a trench-less solution for both power and data.


2) If you use a trench for the data and get power locally you need to remember a few things:

You will now have a problem with the two system grounds being at different potentials. This can raise havoc with the data drivers and receivers on opposite ends of the runs.
This can be partially helped by actually bringing a ground wire back with the data lines to try to force both ends to the same potential. Or better by optically isolating the two ends of the data line from each other.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[ScottyUK]

Definitely run fibre. The cost of multimode fibre isn't so bad when compared against some of the semi-proprietary fieldbus cables, and the real costs are in the physical installation, regardsless of whether it is fibre or copper going in the trench. Fibre gives you excellent future-proofing because the bandwidth is high and there is plenty scope to piggyback other services onto the fibre in the future if you leave some spare cores, plus you have almost total immunity from noise and grounding problems.

Several years ago we installed fibre rather than copper for an data interconnector across our (large) site. At the time I only needed four cores, but because of the enormous installation costs compared with the cost of the cable my group installed a 24-core fibre. That interconnector has only three spare pairs now because of jobs which were never even dreamed of at the time of installation. The long term cost savings made because of our foresight run to many tens of thousands of pounds, and many projects have been possible which could not have been justifed if the cost of a data connection had to be factored in.


----------------------------------

I don't suffer from insanity. I enjoy it...

 

[sparkie2]

ScottyUK:
Thanks so much for your input. Was hoping that fiber would not be necessary. I was hoping that possibly Profibus DP would be OK (at 9kb it lists that each segment can be 1200 meters, but maybe this only works in the lab and not the real world).

Itsmoked:
Also, appreciate your comments as well. Will look into providing more of the power at each location locally, rather than running power from the central control point.

 

[xyzz]

What means slow change?
If one data exchange per 2-3 sec is OK, I would simply write my own serial protocol.
Using 110V signals each way and power neutral as a signal ground, you will end up with only two extra conductors in the cable.

 

[Ashereng]

I wanted the term Fieldbus to be generic for the various fieldbus now available (Profibus DP, Ethernet, Remote I/O, etc.).

Please don't. Please use the correct words and meanings - that is one of the good characteristics of this forum. We say what we mean, and we say it correctly.

Fieldbus has specific meaning, and is different than the other protocols and solutions you mention. Remote I/O is not the same as Fieldbus. Neither is Ethernet.

 

[nbucska]

Sparkie:
I would use off the shelf wireless, as high frequency as
possible with good directional receiving antenna and
good error correction -- it would be much cheaper than
digging miles of trenches.

 

[ScottyUK]

Hi nbucska,

How do you get around this:

Wireless is not an option due to metal structures in the area that would interfere with the RF signals.

Raising the frequency just makes it worse. Point-point microwave would have been my first suggestion, but COTS products tend to be dedicated to the major networking protocols rather than whatever the control system uses to access its remote I/O and it requires line-of-sight. But you know that already!

----------------------------------

I don't suffer from insanity. I enjoy it...

 

[itsmoked]

So there's metal structures.. Are they Faraday cages specifically rated for the ISM band hopping frequencies and surrounding the transmitter end points?

If they aren't solid steel then the signals can go thru them. If they are solid walls just use the right ones as reflectors.

danw2 and I talking about radios with 15 mile range connecting at 1/2 a mile. It will work.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Skogsgurra]

I think that xyzz (Electrical) has a very valid point.

And, you know what? I wouldn't even care to run any wires or fibre or use and radio. Power line communication is a very real and workable standard solution.

Talk to Echelon (not the bad guys, the good ones). Or try Westermo - they have loads of niceties for short-haul on/off and status signalling.

Gunnar Englund

http://www.gke.org

 

[Skogsgurra]

Short addendum: RS485 is known to work very well over much longer distances. Some standard CRC and retransmitting takes care of the reliability things.

Gunnar Englund

http://www.gke.org

 

[nbucska]

Scotty:
If the structures are stationary, you can find a reflected
signal which is strong enough considering itsmoked's
observation.

Spread-spectrum connection is not so sensitive for
the metall structures.

I think it is worth to invest some work to avoid
digging miles of trenches...

 

[itsmoked]

I think Scotty has a backhoe and likes diggin.

 

[sparkie2]

skogsgurra:
Today I was looking around and I found that GE's Fanuc GENIUS I/O is rated at 7500 feet, and about 6000 feet with a direct burried Belden twisted cable. Not sure but I think it uses some form of RS485. Looks intriguing, but not sure of cost yet. Will take a look at what
Echelon has to offer too.

Ashereng: Sorry about using fieldbus to group all the different lan types together, but I was lead astray by the following comparison chart:

http://www.synergetic.com/compare.htm

If you look at this document, you can see how I got this impression.

 

[ScottyUK]

What I really want is crane and wrecking ball, or a tank and a big supply of ammo... so many yes-men attitudes to be realigned, so little time!

----------------------------------

I don't suffer from insanity. I enjoy it...