PLC communication over power cable

I am being asked to maintain 24 conductor brush/ring assembly for a piece of mobile equipment that is over 30 years old. The old GE static logic was upgraded to Modicon 984 685e about 10 years ago. I have suggested removing the entire communications cable/rings/brushes and finding a wireless modem option. Preferably Modbus+ but could get away with Modbus. I am getting nowhere with this suggestion because of previous attempts to utilize wireless modems in this area (inside large metal building). I have heard of modems using the power cable to piggyback the communications. This has sparked some interest. Anyone have experience with industrial duty power cable modems for PLC com? Would be similar technology that household 'X10' devices use. Using 575V 3phase 200 amp feed to this particular machine.

Thanks,
Brian

Brian,

About 15 years ago I did a project with Square D Symax over a power conductor modem. It had some sporadic problems with harmonics with another machine we finally resolved and it worked fine after that. The one we used was 110v and we communicated over 480vac via a stepdown transformer to the modem. But that was 15 years ago and for the life of me I can't remember the manufacturer.

Two years ago I did a similar project for a customer that required extensive communications to a rotary turret. The solution I found that was cheap and successful was from a company call MercoTac in California. They manufacture mercury slip rings with little to no noise and a fairly high amperage rating on a couple of the pins.
The slip ring was like $150 for 6 poles. Three were used for AB RIO at 230kbaud and the rest provided power to the onboard panel. I never once (as far as I know to this date) have had a single com glitch to the remote. They may be worth investigating for your application.

Dave

mercury slip rings

Thanks Dave,

The Mercotac website is very interesting. I had never heard of mercury sliprings before but I do lead a rather sheltered existence. That would definately solve the problem of dirt and wear in the copper rings/brushes but equally difficult and expensive to maintain are the constant torque motor, reducer, chain drive, 500' control cable that would have to be kept in place. Too bad there wasn't a way to replace the power cable reel too (just kidding). But it seems to be built more ruggedly and presents less problems. Still interested in power conductor modem if anyone has any experience.

Brian.

SATEL has wireless modules for Profibus.

SICK has optical datatransmission for Profibus, Interbus, DH+ and RIO.

Also try "slip ring" on google.

In how many directions do the equipment move ?
Does it rotate ?

Jesper,
I believe a radio modem solution exists, but the resistance is high because of earlier attempts by others. Sometimes there are 'political' hurdles to overcome along with all the others. Satel looks promising if I can start down that path. Optical datatransmission? Searched SICK website without luck. What is the basic technology. If it's communicating optically over distance than forget it. The dust hanging in the air makes it impossible to see your feet at times. I really want to get away from the slip rings all together for control. Although if thats impossible the mercury look a lot better than what I currently have to work with. The machine is a Bulk granular material (cement clinker) reclaimer that operates on rails moving in a staright line over about 1000' with cables terminated in center (500' of cable) They are wound onto cable reels using Rueland constant torque motors that are extremely expensive to buy (I think the last we got for $9000). Two on the power and one on the control reel. This is one of 5 stackers and reclaimers that use similar power and control supply schemes. If I can get a good solution on one then the others are bound to follow.

Brian.

Brian,

Check out www.data-linc.com for some wireless solutions.

We are using Modpac Plus radio modems for remote water reservoir communications with our water treatment plant.
You can eithe use a licenced radio frequency or spread specrum.
Works very well here.

Daryl

OK, now I get it. It is a linear movement, but because of the reels you are using slip rings at present.

Just tried google with "Profibus Slip rings". There was an example of someone using standard slip rings with Profibus at 12 MBaud (that would have been limited to 100 meters). There wasnt much detail though.

Sicks optical datatransmission is called "ISD 300", search for that.
The max transmission distance is 200 meters. So it seems out of reach for your application.

What I would do: Ask Satel (or another wireless supplier) to lend you a set. Then make a test with datacommunication only. Let it run for a month and see how it goes.

There are several solutions out there, some very common, like wireless ethernet, or wireless 'serial cables'. I personally am not particularly fond of optical, as it is rapidly degraded by dirt or things passing through the transmission beam.

I have to stress though, before you actually implement ANY solution, get your hands on some demo units and TEST them over at least several weeks, on the actual equipment. Put a little brick PLC on the machine frame and another PLC outside, and run high-speed heartbeats back and forth with error counters on them.

I've had many situations where things 'looked good', but really were not. Either a crane moved by, or someone with a walkie-talkie keyed the mic and disrupted communications...

And of course, keep HARD WIRED safety circuits through the slip rings.

Ok I found it

Here is the new improved version of the unit we used..

http://www.data-linc.com/powerline.htm

It can communicate over a 120vac line.....we used dedicated stepdown transformers to utilize the 480 power wiring on the cranes...

Good luck,

Dave

Thanks

Daryl,

We tried Modpac spread spectrum radio modems in this application. (another technician) He experienced communication failures and pronounced it a lost cause. We have Modpac working in a loader/crusher application without a problem. I would like to reassess the Modpac solution but I have no support for that. Good to know you have some experience with them though.

Jesper, rdrast,

We all carry two-way radios so that may be part of the problem. A test would definately be in order with the low confidence level we seem to have. Running in parallel with the old control so that when (not if) the brushes fail the operators can opt to switch over and save me a call-in at 2 a.m. I figure after a few months (years?) I will rip out the control cable reel and nobody will ever miss it.

Dave,

Thanks, looks like the ticket, if I'm right this type of communication should be immune from interference from other RF sources.

Great forum!

Brian.