Festoon Cable with Shielded Twisted Pair (4-20mA encoder on moving equipment)

The platform is a PLC5/40 with RSLogix. I've been given the task of installing an encoder on a piece of moving equipment (linear motion ~45 meters/140 feet) for position indication. The equipment is driven by 9 600 volt 100hp motors on 3 seperate VFDs which utilize a rack and pinion type setup. I have a shaft/gear setup that is meshed with the rack and converting linear motion into rotational motion. I intend to couple my encoder (~250 turns/cycle) to this shaft.
The equipment is approximatley 100 meters (300 feet) away from the electrical building which conatins both the PLC5 processor and all the VFDs (and several DC drives). All of the power and contol cables are run in cable tray into two junction boxes (power & control). The festoon cable is run from these JBs to the moving equipment via a typical festoon assembly - a continuos beam with cable supports on rollers that move as the equipment moves. The cable that is currently being used is Super Vutron but it is all 600 volt power and 120 volt control. There are no instruments on this piece of equipment.

I have two concerns:

1) I have decided to use a multiturn AMCI absolute encoder with a 4-20 mA output and, therefore, need to source shielded, twisted pair festoon cable.......anybody have any ideas where I will be able to find such a beast.

2) I have two of these pieces of equipment running side by side so at any given time I can have 1800 hp + another 500 hp running in close proximity plus all sorts of other equipment sharing the same tray system. Is the 4-20 mA signal my best bet. I was looking into SSI but a tech that I have a great deal of respect for told me go the 4-20 mA route. I am concerned about signal integrity due to "noise". Any ideas or suggestions?

Thanks in advance.

I've got a 7.5 ton crane that has a Belden cable feeding a multi turn AB SSI encoder, running in the same festoon loops as my 480 V VFD lines to the motor. I haven't had a problem with them, the cable is a 4 pair twisted shielded cable, & I can get you the part number when I'm in office.

I'd really appreciate that information. I called Belden's 1-800 number but the tech support just kept telling me that Belden doesn't make a twisted/shielded pair in festoon cable. How long have you had it in place? Do you have any idea on how many times it moves/day. Thanks.

ptine said:

I'd really appreciate that information. I called Belden's 1-800 number but the tech support just kept telling me that Belden doesn't make a twisted/shielded pair in festoon cable. How long have you had it in place? Do you have any idea on how many times it moves/day. Thanks.

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It's not a festoon cable, & I've got 8 cranes in a building running 20 hours a day 5 days a week back & forth constantly, & it's been in service for over 2 years without problems. I put the cable zip tied to the festoons, & made sure they weren't the tightest rope in the bunch, that the festoon cable takes the brunt of the force from the motion.

At my former employer, we used TPC wire and cable as a source for specialty cabling that would handle twisting and other motion...They even made some special cables for us with the "high flex" features and specialized molded connectors...

http://www.tpcwire.com/

Alpha 6054C.

I would personally go with SSI or similar. Did the tech you refer to give you a reason why he thought 4-20 would be better? As you said, I think you open yourself up to more noise issues with the 4-20 solution.

Olflex also makes specialty cable for applications like yours.

Keith

PS: Sorry. Olflex is tha same as LappUSA.

The info I recieved from my tech friend, who is more of an electronics tech, was that the 4-20 mA was the most immune signal wrt "noise". I could use more input on this. As I said, my research brought me to the conclusion that SSI would be the way to go but he might have suggested the 4-20mA because he knows we have the IFE modules in stock and the SSI interfaces would have to be ordered and stocked. I was looking at the AMCI SSI interface for the PLC5, does anyone have any other manufacturer that produces a compatible SSI interface that they could recommend? Thanks