PLC to PLC remote control of mobile crawler

HI Folks, first time poster.

I am working to reserect an overgrown NU-NU (for those of you without rugrats, it is the cleanup machine on the preschool kid's show TeleTubbies.) It is a remote operated vehicle that drives around and sucks up the mud and muck off of the bottom of a concrete lined pond.

Anyway, the existing design uses a 250 foot long, 3" diameter umbilical cable that includes 2 480V motor cables, 40 to 50 control conductors, 4 optical fibers, and 2 garden hoses. The muck pump discharge is a separate 4" hose.

Umbilical management is difficult already, and now I understand that the length is about to quadruple! I believe it is time to multiplex everything over fiber and place the whole thing on a large reel.

My thought is to replace the discrete control lines and motor wiring in the umbilical with a custom cable made up of 2 optical fibers and 4 number 8 AWG conductors. I would use a PLC in the crawler to operate the hydraulic valves, monitor pressures, flows, liquid levels, and temperatures, interface to a locating system, and control a 10 HP and 15 HP motor. The crawler PLC would receive commands from another PLC in the control trailer. The trailer PLC would accept the digital and analog inputs from our standard control system, convert them to a robust communication format which would be sent over the fiber to the crawler. The trailer PLC would also convert return information back to analog and digital outputs for process monitoring.

Use a couple of VFDs in the crawler, in place of the motor starters in the control trailer, and now I am down to a power and control umbilical cable.

Am I making this more complicated than it has to be? Am I adding more failure points? I trust PLCs more than PC's, and this would reduce the number of "upgrade" relate problems. I can use industrial slip ring assemblies for the power, and if I convert the communications to light after it goes thru the slip rings, I avoid the need for optical fiber rotating unions.

Thanks for any help and/or opinions on the feasability of this.

Larry

Have you considered using wireless remote I/O for your control?

Sounds like a good plan, Larry. Are you currently running ALL those conductors through slip rings? Wow!...

If I was being picky, the only 'overkill' item I see is the fiber links, but that's just me. Do you really need to go with fiber optic?...

Since the entire control system will now be in the crawler, just be sure to design it to survive in that environment.

beerchug

-Eric

P.S. Welcome aboard PLCS.net!... :site:

I spent quite a few years with mining trucks and loaders, and also locomotives, plus a couple with the telephone company. I can not see how the fiber optic cable will hold up. I think either a data line or rf link would be best, with an auto stop if the machine mounted plc lost signal. Also, slip rings aren't real condusive to data lines. 480 and high current dc can arc over pretty good, but data is a little puny, phone company used to add a voltage (48vdc) to dry lines (no current) in some areas to keep resistance from building up and killing the signal.

regards.....casey

Thanks for all of these replies.

As far as wireless control, the crawler is going to be submerged up to 12 feet under the surface of muddy water. In addition, at the locations where we will operate, all RF sources have to be registered and approved. Since this is a mobile system, I would prefer not to go though that, at every job site.

The reasons I am leaning towards using optical fibers are as follows:
1) EMI resistance. Since I am looking at 1000’ of closely coupled conductors, EMI is a very definite concern. Especially if I use VFDs for the motors.
2) Bandwidth. Tentatively I need duplex RS-485 for the VFDs, 0-10V analog output signals for the proportional hydraulics, switched 24V outputs for solenoid valves, analog inputs for hydraulic pressure, flow, and temperatures, cooling system temperatures and flows. Other signals may include video, camera controls, external lighting, and location signals.
3) Umbilical size and weight. If I have 4 optical fibers plus 4 #8 AWG conductors, that is about a 1 inch diameter cable. That would be manageable if it was spooled on a five-foot diameter reel. Mount the copper to optical converters on the side of the spool and run electrical signals thru the slip rings. 480V power on power slip rings and communications thru little slip rings.

Communications
What do you folks suggest? We will be operating in real time, that is, the crawler driver is in the trailer and is driving by a map, some remote video feeds, and instrumentation displays. Top speed is probably 15 feet/minute. I am looking for something that is point to point or limited multi-drop. Probably 1 fiber to the crawler and 1 or 2 fibers back. Perhaps, control information to and from the crawler on 1 fiber, and 2 fibers back with video. Preferably something that does not need a computer to act as an interface. Is there such a thing off the shelf that allows 2 PLCs to be their own network?

Thanks

Larry

The only thing I would add is some risk analysis.
Look at each of your components and:
List all possible methods of failure (of course including water leaks)
List their probabilities of occurring
List the consequences if they occurred
Design to reduce risk.

By the sound of it, you may need to work out how to recover it if it fails in the middle of the pond. However, this is probably no different to what happens now, so this may already be covered.

For your two PLCs networking, ethernet may be OK.

I would also build myself a test rig to test your interfacing of the control signal, this could help to find any problems before you re-build your machine.

Hope this helps,

Doug