Proximity sensor application

I know little about proxys, the only time I have used them they just attached to a counter.
I want to have a 3' vertical track that a standard 5/8" washer slides up and
down. A wire will pass through the washer. As the wire is pulled on, the washer will go up the track and near the top it will pass a proxy that will activate a relay that will turn on a 3-phase gearmotor to feed the wire. (Actually, the relay will trigger a magnetic starter) As wire is fed, the washer will slide to the bottom and pass by another proxy that will turn off the relay and kill the feed gearmotor.
I know I need shielded induction type proxys but I don't know: AC, DC, PNP, NPN, 2-wire, 3-wire, 4-wire or any of the variables. I don't know what extraneous electronics is needed. Is there a simple/cheap way to do this? Of course, I can do this with limit switches but I hope the proxys will last longer and be more reliable. I have to build 9 of these feeders so cost is a big issue.
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Tom Gardner wrote:

Hmm - I found this http://www.hallsensors.de/CYD3144.pdf which might provide a starting point, if you can magnetize the washer.
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Morris Dovey
DeSoto Solar
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1. the sensors won't directly control the mag starter, you will need a SSR or relay for that. 2. depending on how you elect to control starter, you can choose the type of output from sensor. NPN with a pull up to VCC is what I would prefer - that could drive a small relay, put diode across relay coil of course. 3. detecting the washer reliably would be something you'd want to test - you don't say what size washer or how fast it's going, etc - 4. this sounds like an applicaiton for an interrupting optical sensor rather than magnetic - you may wish to consider alternatives

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William Noble wrote: (top posting fixed)

This sounds like the sort of design task that one wants to work backwards from. In other words, choose a motor sized to do it's job, then choose a magnetic actuator (relay) to turn it on, then select a solid-state relay that'll run the magnetic actuator, then finally you'll know what output on your proximity sensor.
Usually industrial sensors like this all work the same on the "front end", so they'll all sense the same. The differences come on the "back end" where they talk to your relay -- so that's where you'll need to pay attention.
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Tim Wescott
Wescott Design Services
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Tom Gardner wrote:

Consider using a cheap phototransistor and LED. You should be able to do it for about a buck per sensor.
See www.digikey.com for infrared emitters and sensors.
Or locate some surplus photo interrupters somewhere: http://www.makingthings.com/teleo/teleo/cookbook/ir_interruptor.htm
--Winston
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PNP, NPN are generally faster. AC is slower but you can use control voltage that I bet in your case is 120vac.
How big is the gear motor as in hp?
3 pha makes me think it is big which gets you into a prox, smaller relay and a contactor.
Wes
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Didn't read it critically enough. Look into latching relays. A latching relay has a set side and an unset side.
Wes -- "Additionally as a security officer, I carry a gun to protect government officials but my life isn't worth protecting at home in their eyes." Dick Anthony Heller
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On Sat, 16 Aug 2008 11:34:19 -0400, "Tom Gardner"

You'll need a shielded prox if you want the face flush with a metallic mount. For a given sensor diameter, the unshielded will have a longer range.
If you're simply turning a relay on and off, a 2-wire AC prox will be easiest to hook up. They're capable of switching up to 300mA, depending on the specific unit. The biggest downside of an AC prox is response time -- it's limited by line frequency and is somewhat unpredictable for the same reason. Small 3-wire DC proxs are very fast and repeatable and usually a better choice if there's a PLC or other controller in the scheme.
How is the target approaching the sensor? What do you need for repeatability? If it's passing across the face rather than approaching head-on, repeatability will suffer if the distance form the target to the prox face is not controlled.
An optical sensor is another thing to consider. The Banner convergent beam sensors work well for positioning and aren't as fussy about target range as a prox.
Automation Direct is by far the cheapest source for proxs I've found.
--
Ned Simmons

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I assume that the environment is dirty as hell, so optical isn't a good idea.
If one of these switches fails (either open or closed), who gets hurt, and what stops the drama?
I assume that the vertical track is made of steel. This will make a prox unhappy, unless there is a wide enough clear area.
Mechanical limit switches can be made to have very long lifetimes.
There are lots of kinds of "standard 5/8 washer". What is this, what is it made of, and is the same washer used until it wears out, or are washers consumed, becoming part of the product?
Joe Gwinn
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<snip>

That's my thought too!

I once had a de-spooler run all weekend...the BIGGEST Brillo-Pad you ever saw! It was 20 feet in diameter made of .017" x .117" flat oil-tempered wire. It took 2 guys 3 hours to remove it.

The washer can be anything, I'm just thinking the 5 pound box full I got from my supplier when I called and said "...and a box of 5/8" washers.
The more I think about it, good mechanical limit switches with cat whiskers sound better.
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"If one of these switches fails (either open or closed), who gets hurt, and what stops the drama?"
No problem here as you choose the mode/contact relationship so in the failure mode the choice above will turn the motor (and other critical motions) "OFF."
dennis in nca
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