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Power rails/Electro-mechanical problem

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Snowman

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Hi,

I have a sensor array that moves on a linear track that requires power, gnd, and 3 data lines. So far I've been using ribbon cable to supply this but the cable often gets tangled, inhibits motion by providing elastic resistance, and is generally unreliable physically (prone to tearing).

I was wondering if anyone could come up with a novel way of providing power to this sensor array that is extremely cheap yet is not a hack made from household materials.

I have already considered using a spool for the wire and using inductor coils, both these options are expensive. The only cheap option I see is power rails. Literally, 5 parallel physical rails that the sensor head somehow makes contact with in its motion. I have concerns only with price of components, mechanical stability and complexity, and electrical noise from this design.

If anyone has done this with a robot , could provide me with a link or some pictures of a successfull power rail, or has some new ideas...I'd be much appreciative.

Thanks,
Avital
 
What I have done in the past is use high strand count test lead wire bundled into a mesh sheath. Something like 65 / 36 stranded 18 Gauge wire. 65/36 means 65 strands of 36 gauge wire, making an 18 gauge conductor. Belden #8897 for example.

The bundle is then allowed to lay in a tray along the axis of motion, and simply come back onto itself or layout in its full length as the sensor moves back and forth. The tray can be a length of Panduit wire channel, or something similar.

There a specific cables made just for this application, but they tend to be expensive, or hard to get in short lengths.

Using power rails tends to be a bit of a nightmare for low level signals, and as with anything mechanical, requires maintenance. Reliable rail systems, and precision slip rings, are not cheap.

Belden makes a high flex cable for this type of use, it's #7403a or a full shielded version #7403as.


If you have the room, you can hang the wire down from above, midway along the length of travel, then allow it to pendulum back and forth with the sensors movement. Coiled cord works well for this since it can take up some of its own slack and help prevent tangling.
 
I think you should definitly look into a different wiring solution. seems like there is quite a bit that could go wrong with the rail setup.

What kind of power requirements are required to travel through this wire?

I found a bunch of these cables that look like these when I took apart a bunch of old elctric typewriters.

**broken link removed**


It's called flat-flex wire. It's very flexible and should solve your problem.
I think it's kinda expensive though.

If you want, I saved all of the circuit boards and cables, so I could mail you a cable and a couple of connectors for it. (as long as it doesnt have to be really long.)

It's pretty common too. If you have an old scanner or something around, you could salvage it from there..
 
Thanks for the quick replies!

I sense an aversion to power rails from both of you, which makes me happy because, although cheap, is very prone to mechanical problems as you've pointed out.

I didn't mention in the original post that this sensor-head rides on a short 1 foot track above a small rectangular platform, where it interacts with a stationary piece of plastic. Imagine a rectangular 6cmX30cmX3cm(widthXlengthXheight) box where the sensor-head rides on a rail on the top of the box and interacts with a fixed piece of plastic in the center of the bottom of the box. We're talking about TTL power levels here, small currents (5-10mA).

I will look into the Belden cables in more detail. I've tried the "pendulum" idea horizontally: I feed the wires from the sensor head to a loose joint fixed midway and offside the track. The joint moves side to side as the sensors move back and forth. Then the wires from the joint can be fixed and fed out of the box via a jack. This doesn't work all that well...I'll keep trying to perfect this as it seems as the simplest thing to do.

jrz126, thank you for the offer to send me those flex cables. I looked up prices for such cables online and they're pretty steep, I need a solution that's in the range of .50c :) ie: really cheap

I'll keep checking here and posting any results...
Thank you all for your input!
Avital
 
I distinctly recall seeing plastic strips with conductive material applied to the surface - possibly on printer heads or something similar. Each end was inserted into a socket so that the strip had the shape of a "U". Someone must make that stuff. You might also make you own using a plastic strip (or metal, like feeler gage stock) and adhering the fine but flexible wire that is used in microphone cables.

In all cases the plastic strip(or metal) is made to move without exceeding any of it's limits. The strip acts to carry the wires or conductors along a predictable path.
 
I think you are talking about the flat flex cable that I mentioned
 
The 'plastic' that I am describing is definitely not the cable you described. It's most similar to a very thin, clear circuit board with conductive foil. Unlike the cable the plastic strip has enough body to maintain a shape but is still flexible. If someone asked me to make my own I'd sandwich some fine wire (like Litz wire) between two or more layers of 35 mm film - or the mylar sheets that one might use to make transparencies on a copier.
 
I had a minute and found this - gets close to what I am describing:

**broken link removed**

'flex circuits' is one way of describing it.
 
stevez said:
I had a minute and found this - gets close to what I am describing:

**broken link removed**

But not flexible enough - I know exactly what 'jrz126' is talking about, you see it in typewriters, printers, scanners, CD players - all doing a similar job, but it comes as an assembly, not as a coil you can cut shorter, presumably you have to order it made and terminated in the lengths you require?.

Presumably it's also not a cheap option, certainly replacement ones aren't cheap!.
 
I've spent most of the moring emailing companies making these cables, most are unwilling to give prices and insist on making only production runs. However, on digikey I typed in "flex cable" and found several reasonable lengths of cable which you *can* cut down to size from a spool.

For example: http://ca.digikey.com/scripts/DkSearch/dksus.dll?Detail?Ref=184971&Row=204380&Site=CA

However, for some reason, I can't open the webpage of Parlex and even if I could I'm not sure that this cable is what I'm looking for - judging from Nigel's comment. Anyone have any experience with this specific cable/company or can tell with some degree of certainty that these cables would fit the purpose? (I know, slightly vague question but...educated guesses are welcome :)

Thanks again for all your input :)
 
I just remembered that I have a longer piece of this cable which has wide conductors in it. They are ~1/8 wide, so you might be able to solder to them. I'll take a look when get done with browsing the new meeages on this board, I mean get done with "work"
 
Along the lines of what has been mentioned, you could get an old dot-matrix printer, preferable a wide carriage one, and use the printhead cable from it. I have an older epson printer at the office that has been going for nearly 10 years, and the flat ribbon in it has never failed. This thing has taken a whipping too, going from a flat run to a complete horizontal "U" shape hundreds of thousands of times ( likely more) as the print head goes back and forth. A computer surplus place would probably have one you could get your hands on. It would have the slide-and-lock connectors also. Maybe Active Surplus on Queen St., downtown Toronto, or somewhere similar would have one.

This is, of course, only viable for a one-off, not a production run of a product.
 
Check a goodwill store too. I did some volunteer work at one that was run by the city mission, for awhile and I was in the electronics department. They get a lot of old junk that people donate because they think it is still good and they would feel guilty about throwing it away. Let's just say it's a good thing I was a volunteer, because I spent more time taking stuff apart than actual work. (although I did help a little bit by taking home some of the junkier items)

If it's anything like the one I was at, they should have a bunch of them somewhere.

aproximatly how long do you need it to be?
 
Zevon,

I think I've resolved to using the flex cable almost everyone has mentioned - this forum and other web research shows that it is designed for my purpose. Any other ideas seem to simply exceed the price/complexity expectations I have.

Also, I called Active Surplus earlier today and they do not carry the cable. However, the person I spoke to didn't really seem to care or really know what I was asking for, so it is likely that I'll find some old printer or something that has a piece of flex cable for testing laying around. I'll probably make it down there tomorrow or on the weekend. I have also ordered some flex cable off digikey, I'll see how it works. Will make a comment on this thread once I test the cable.

Thanks for the input :)
Avital
 
What you are looking for can be found by looking for "flexible circuit cable". This is the type used to connect moving print heads, etc for an extremely high number of cycles.

You will find many mfg, I have no doubt you will be able to find someone with various cable lengths in a usable termination (some do use solder) in stock. There's got to be some decent DINs at least.

The rails are difficult to make, noisy (as you noted), and extremely unreliable.
 
Yes, Active can be that way when you call there. If you go down to the store, they have moved upstairs, above where they were, very easy to walk past. It's pretty much organized now inside, not the treasure hunt of piles of stuff like a scrap yard. I have gone there for nearly 20 years and spent many a hour poking around the aisles like a kid in a candy store.

But I digress............. :D

There are a few places along Queen that may have what you need. The one thing I have found about this type cable is that the ones that simply have exposed conductors at the end, are more durable than the ones with pins pressed into the strip. The exposed strip is inserted into a socket similar to a card edge socket, and held in place with a sliding lock. Active may have a roll of the stuff and not know it. They have alot of stuff they don't realize, LOL.

Digikey carries a Parlex cable, 6 conductor, 18" long, for about 5 dollars US, part number HF06U-18ND page 49 of the online catalog. They carry other Parlex cables too.
 
I found an old dot matrix printer and bashed it open, found some flex cable. I cut it to size and even managed to cut the connector, it's a bit flakey but that's due to my workmanship, it certainly seem to be ideal for this type of motion/design. I will order some cables and connectors from digikey as Zevon pointed out. I'm now pretty confident this solves the problem.

Thank you everyone for your helpful input :)
Regards,
Avital
 
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