Communications Over 5 kilometres

[BobB]

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.

[Ken Moore]

I have been using a wireless that claims to go much farther than 5 km. They are from you part of the world: http://www.elprotech.com/ Are you sure that wireless is out? I think wireless would be a viable option with the right setup, may need to install a tall antenna at the "base" station. The wireless units work on 24VDC, so that's not an issue.

[BobB]

Thanks Ken - I will check them out. New experiemce for me quite frankly.

[ECSI]

I am also using Elpro radio modems right now on a project involving Omron and modbus comms. They are very easy to set up and if line of sight is a problem you can use up to 5 repeaters. I tried a satellite phone connection to a CS1 a couple of years ago. We never did achieve a connection from CX-Programmer to the PLC so in the end we just gave up on it. It wasn't really needed anyway.

[paulengr]

I've had very good luck with radio modems from Aerocomm. You can order a pair for about $150 from Mouser electronics. Your choice of RS-485 or RS-232 interfacing. If you buy the "Ethernet" one, it's just an OEM'd Lantronix serial port server stuffed on top of the radio. That comes with all the advantages and limitations of the Lantronix implementation. Like it can't be a true Ethernet "bridge" if that's your intention. The range is 50+ km line of site. Over water, your biggest enemies are making sure that you have a reasonable antenna height so that "line of site" remains just that. The second enemy is going to be the fact that water is so conductive that the radio horizon is not what you think it should be (tends to be shorter than line of site...over land, the opposite happens). Either way, I recommend you go for a LOW frequency like 900 MHz. The higher the frequency, the higher the bandwidth...BUT, the shorter the RANGE. Every doubling of frequencies cuts your distance in half. You can easily communicate around the entire world over long wave (<100 Khz) although the antenna lengths get very large (>20-30 meters) for even a poor antenna. At the same time, the slower the communication rate, the longer the range. So go low and slow and you shouldn't have any trouble. It may take a couple rounds of tuning to find the right combination but it shouldn't be too hard. One final issue. Most of the radios out there at least in the U.S. are required by FCC unlicensed wireless rules to have either nonstandard antenna connectors or connectors that are soldered onto the board. Most radios come with "RP-SMA" which is a standard SMA connector except that the center pin is inverted (RP=reversed polarity). Once you know the type of antenna connector, it is fairly easy to find LNA's (low noise amplifiers aka preamplifiers) and connectors so that you can separate the radio from the antenna. Keep in mind that this is microwave stuff. You MUST be very precise when making up your connectors and be very good about following rules with regards to your ground planes and such. If you are at all concerned about what you are doing, make sure to have someone else do the work, even if it's just a matter of ordering a premade cable.

[BobB]

Elprotech have been in touch and have supplied a price and suggestions as to the best way to do the job. They have suggested a server that can be programmed for change of state transmissions. Just putting a price together now.

[Ken Moore]

When I did my wireless system, Elpro brought a couple of Demo units out to the site to "prove" that it worked. Just a couple of switches and lights, showed that a switch at the base location could turn on a light out in the field and a switch in the field operated a light in the control room. The field demo is what sold me, none of the other vendors offered a demo. I think the tech. support is decent, and the local rep is good to. No regrets. good luck. Ken

[Nathan]

You might also consider devices that work over your local cell phone network. These tend to be robust and fairly simple You can often get a secure broadband data connection cheaper than a satellite link. I can't speak to service in Australia, though.

[BobB]

Thanks Nathan but cell phones do not work in the area.