Communications Over 5 kilometres

I have been asked to have a look at a job where they wish to control a diesel engine powered pump on a barge. There will be monitoring and control involved.

I was looking at using an Omron CJ1M PLC with a couple of comms cards, 1 input and 1 output card. Set up Modbus RTU comms to the Cummins engine, Modbus RTU comms to another barge and a small SCADA system to monitor and control.

The barge will only have the engine start batteries on board and I was looking at using a 24VDC powered PLC off the start batteries with a switchmode power supply between the batteries and the PLC.

Next problem - the barges could be up to 5 kilometres apart. Probably not good for line of site wireless - may have to shoot off a satellite.

Anyone have any suggestions/ideas/recommendations?

Any help would be much appreciated.

I'd give your mates at Elpro a call. It is a company in OZ that has done me well. You should be able to get 5km using a 900 mhz Modbus unit without issues. I think 12-20km without repeaters is possible. They are 24 vdc powered. www.elprotech.com/

With regard to wireless:

I've seen 900Mhz unlicensed wireless similar to Elpro at that distance over dry land done successfully. Water has an effect on 900Mhz, so be sure that's taken into account, if you pursue wireless.

With regard to wireless, are the barges always docked in the same location? Or are they docked in different locations? Or are they cruising? (Can directional antennas be fixed in one direction and work?

If docked,
Is there cabling from a dock to the barge? (could a stationary dock mounted mast/antenna be used, then comm cable for final link?)

Can a barge have a mast for an antenna to get some elevation for signal propagation (or do barges have to fit under bridges and the height limit is X meters above water line?)

Dan

You could go even lower than 900MHz. IIRC, 148MHz is unregulated. I've worked with a 148MHz system that gave a 20km range over those distances, but that was over the ocean. Lucky for you, that's your application. The lower the frequency, the greater the penetration and the further the signal will travel. The downside is that you can't fit as much information into the signal.

Also, with wireless, you're into RF voodoo hell, and things like rain, fog, birds, waves, sunspots, &etc. will introduce random noise into your signals. I've actually worked with a system that beamed a microwave signal across a helicopter landing pad. They wanted to know why the phones seemed to go out whenever a helicopter took off or landed.

Look into encoding and decoding the signals as well. With error correction, you can have a badly degraded signal and still get decent data. I wrote a Viterbi encoder / decoder that let the system recover from an 18% corruption rate. It was in C and a little intricate. I'm not sure how I'd implement it in ladder logic, but it would be possible.

Satellite will be expensive, but certainly doable. It will cost about $1 for an 8-byte message.

Maybe a cell phone system would work, if it's near a major center with good coverage.

Bob
At sea on open water the horizon is 12 miles away if I recall. On open water 5 miles between barges should be no problem for line of sight equipment. Now if you have a mountain in between them,,,, well I guess it wont work.

It depends on how high you are. If you use meters, then the distance to the horizon is

distance in km = sqrt( 13 * h )

where h is your height in meters.

So to get that 20km / 12 mile distance, the antenna would have to be 30m / 100ft above sea level.

I found the calclation on wikihow, so the usual caveats apply:
http://www.wikihow.com/Calculate-the-Distance-to-the-Horizon

I have contacted Elprotech and they have suggested a server for the job. Apparently these things an be programmed for change of state transmission. They also know about the job which is a big help.

Thank you all.