magnetism physics problem

I'm trying to make an index marker for my CNC lathe spindle. I need one signal per revolution and it should be "on" for about 1/4 of the time on my
three inch diameter shaft.
I'll be using a hall effect proximity switch by Cherry Corp. http://www.cherrycorp.com/english/sensors/mp1005.htm This sensor is on whenever the magnetic field is between 60 and 300 gauss right in front of the sensor, south pole only. <FWIW, I found the units: 1 Gauss= .1 (gram*amp)/(sec*sec)>
OK, I'm trying to figure out how to make a permanent bar magnet. I'll use a piece of cold rolled steel 1/2" thick by 2" wide. Then cut a 1 1/2" radius on both ends. This can be mounted to the end of the aluminum 3" timing wheel. The prox switch would then be mounted so the end of the magnet bar lines up with the end of the prox. switch.
I'll make this bar magnetic by imbedding 1/8" diameter permanent magnets in it. McMaster Carr rates the force of one 4" magnet at 0.4 lbs. (I'm planning on cutting them up - maybe 1" long?)
OK, I have NO CLUE how many little magnets to imbed and where.
Ideas?
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suggest using two little magnets, one for the start of the 1/4 rotation and one for the end, and create the logic electronically -
message

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Karl Townsend wrote:

I'd suggest not cutting up magnets: the steel is hard, & the heat & vibration of cutting will likely weaken them. Try to find smaller ones that will (possibly 2 or 3 together) fit.
At work, we make camshaft sensors for engines with a bar magnet parallel to the shaft, fitted in a hole in an aluminium wheel. The Hall sensor then faces one side of the wheel. This needs no extra steel pole-pieces: a common, small bar magnet does the job directly.
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Karl Townsend wrote:

If you need an on signal for a full quarter turn, the following might be easier. Have 1 magnet on shaft and mount 2 sensors on frame, 1/4 turn apart. Feed one sensor to the R input and the other to the S input of an RS flip flop. (See eg http://www.phys.ualberta.ca/~gingrich/phys395/notes/node136.html ) This would be simpler than the already-suggested 2-magnets approach. Make an RS or not(RS) FF, using Q or -Q output as appropriate, from a 74LS02 or 74LS00 (or equivalent in 74 AHC, ALS, HCT, etc. family)
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Karl;
You're making it harder than it is. Use an inductive proximity sensor instead of a hall sensor. An inductive proximity sensor will detect the presence of a steel index tab directly without the need of magnets.
Cobalt tool steels will magnatize directly. No need to embed magnets. Just hit em with the field from a coil.
starbolin
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Reply to several points here.
I need hall effect because its high speed. I also need large size magnetic area because I want to see how fast I can run the lathe and still pick up the signal. I'm hoping for 5000 rpm - this size target would energize the prox for 3 milliseconds. Cutting machinable magnets in two is no big deal. Sticking the pieces in holes and bonding should be a piece of cake.
My question really boils down to: How Do you convert Gauss to lbs. force on a permanent magnet? I guess I'll do it the old fashioned way, order a bunch of magnets and start trying different combinations.
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Karl Townsend wrote:

I think a single magnet will work fine. A single magnet is used on a bunch of machinery spindles that I have worked on. These sensors are used to get a 0 position for tool changes and orentation of the spindle for boring tool positioning. You will have to condition the pulse using a zero crossing detector or similar circuit.
John
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ok, the guy who recommended two sensors has the right idea - on many cars there is a crankshaft position sensor - these cars operate at well over 7,000 RPM, the sensor is a magnet and a coil, there is an allen screw in the flywheel which comes withiin 1/2 mm of the coil - 5000 RPM is not fast, particularly if you use a small coil to pick up the voltage spike as the magnet comes by - a high pass and a schmidt trigger ought to condition the pulse well enough, there are also pulse conditioning ICs. And you can use optical methods well above 100,000 RPM.

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High speed shouldn't have anything to do with it. We use a standard Balluff BES 12mm dia prox sensor and a 12mm w x 12 mm wide metal tab on the spindle. (Spindle speed = 5000 RPM). The high speed stuff is needed on the end the sensor connects to, the sensor will detect the tab. The switching occurs faster than the PLC scan time, so a module is needed between the sensor and the PLC. This would be the case no matter what type of sensor is used.
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Anthony wrote:

Possibly the likes of a LM1815 adaptive variable reluctance sensor amplifier would form a good basis for processing the signal from the inductive sensor.

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THANKS for the input. This sensor operates at 1/10th the speed of the one I was looking at, so I ruled this style out. Maybe I should reconsider. I'm making a large target because of the scan time and response time issues. Seems simpler than an electronic module.
Karl
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Or an optical one with reflective tape-you can make the pulse width as long as you want then.
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On Mon, 09 Apr 2007 12:42:38 GMT, "Karl Townsend"

I would continue to rule it out. You can trick a slow sensor into responding by giving it a longer-duration stimulus -- but precision and repeatability will still be questionable. It's like making an index line with chalk instead of a scribe.
Prox sensors, AKA ECKO (eddy current killed oscillator) are very rugged and have very repeatable sense distance (with a given target), but they are not quick.
Dave is right that whatever is responding to the sensor must also have acceptable speed. If you have milliseconds of uncertainty due to scan time, then your indexing will be ragged no matter what sensor is used.
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snipped-for-privacy@NOSPAMgoldengate.net says...

I got the, perhaps mistaken, impression that indexing accuracy wasn't very critical. If it is, I'd suggest avoiding Hall sensors as well. An optical slot sensor and suitable flag will be much less sensitive to target alignment than either of the other technologies, and very fast and cheap to boot. Downside is you need to keep stray objects away from the slot.
Re the response time of inductive prox sensors, P&F, Turck, et al quote switching frequencies from 3 to 5 kHz for mini to small (3 to 8mm diameter) sensors.
Ned Simmons
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wrote:

Opto can definitely be faster but Hall could work too. Allegro ATS625LSG shows considerably better than 0.1 degree accuracy for signature (index) tooth rising edge at 2000 RPM -- and it has its own magnet.
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snipped-for-privacy@NOSPAMgoldengate.net says...

Very cute. Please remind me of those next time I'm trying to stuff 10 pounds of sensor into a 5# sack.
Ned Simmons
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wrote:

If that was meant to be a slam, I don't get it. I merely recount what I see on Allegro's spec sheet.
I would also choose opto here, and I think that's how Karl will proceed.
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snipped-for-privacy@NOSPAMgoldengate.net says...

Other than the tongue-in-cheek request to remind me of the Allegro sensors when I need them, I was sincere. The tubular shape and lead orientation of normal prox sensors often makes them difficult to fit into a design, prox's in flat packages are an expensive compromise, and I wasn't aware of self-contained zero-speed Hall sensors.
Karl emailed me that you were helping him choose an appropriate device.
Ned
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wrote:

I think he's going to go with opto. I agree with you that opto is the best choice where indexing repeatability is an issue. There's no such thing as a "sharp edge" with magnetic flux, while .010" of shutter motion can completely interrupt a beam of light. That's about 0.4 deg of rotation with a 3" dia disc.
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Ned,
I'd like to tke a look at an "Optical slot sensor" to see what it is. And more importantly, if I can find a way to mount one. Do you happen to know a vendor and part number. A tech. data sheet would be great, also.
<()> An apple a day keeps the doctor away. Two apples a day gets the doctor's OK. Five a day makes you a fruit grower like me.
Karl Townsend
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