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.

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sensor is on whenever the magnetic field is between 60 and 300 gauss right in front of the sensor, south pole only.

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?

Reply to
Karl Townsend
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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 - "Karl Townsend" wrote in message news:CgeSh.20890$ snipped-for-privacy@newsread1.news.pas.earthlink.net...

Reply to
Bill N

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.

Reply to
David R Brooks

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

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) 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)
Reply to
James Waldby

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

Reply to
starbolins

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.

Reply to
Karl Townsend

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

Reply to
John

Get one of these:

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a short curved 1/2 thick x 2" wide curved pole piece, stick the magnet to the back of it with the 1/2 x 2 dimensions matched up. You could also make a back piece for balance if you like, but you want the magnet close to one end. You will then have considerably more than

300 gauss within 1/8" or so of the face of the polepiece.

Rare earth magnets are rather different than ceramic or alnico because they have very high induction in very short lengths while having rather low permeability. The result is that they don't need external magnetic structure to exhibit high flux density (gauss) near the pole faces.

You may also have a gob of swarf on it before long if it isn't enclosed.

Reply to
Don Foreman

Why for a quarter of a turn? Who told you that?

Nick

Reply to
Nick Mueller

the part # for those wrenches is 91777-3VGA. 3/8--5/8. $9.99

Reply to
best wire

Reply to
Bill N

"Karl Townsend" wrote in news:raiSh.1127$ snipped-for-privacy@newsread3.news.pas.earthlink.net:

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.

Reply to
Anthony

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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Reply to
David Billington

How is using an already-made magnet harder than first _making_ one, then using it as you would have the bought version?

LLoyd

Reply to
Lloyd E. Sponenburgh

"Karl Townsend" wrote in message news:raiSh.1127$ snipped-for-privacy@newsread3.news.pas.earthlink.net...

Karl, it's simple if you do it empirically.

Grab a handful of odds-n-ends magnets. Hook up the sensor (usually, the small Hall Effect sensors' outputs will show conductance on a DVM with a diode scale, but you'll need an external power source to actually run the sensor).

Start bringing the various magnets' poles closer and closer to the sensor face until they trigger it, then move them slowly away. There should be some hysteresis. Of course, you may have to try twice to find the south pole.

The bottom line is that almost any off-the-shelf ferrite or alnico magnet you can find is strong enough to trigger the common Hall-Effect sensors.

I replaced the speedo pickup on a Dodge Neon with wheel-mounted (well... CVJ hub mounted) magnets and a small Hall sensor when the $6.00 plastic gear broke inside the tranny, and it was going to mean an $1100 job to replace it.

I just bought 9 little ferrite "button" magnets from Radio-we don't know nuthin' 'bout no stinkin' 'tronics!-Shack.

LLoyd

Reply to
Lloyd E. Sponenburgh

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

Reply to
Karl Townsend

A typical 8mm DC inductive prox sensor will respond more than quickly enough to pick up a 3ms target. $20-30 at Automation Direct will get you going. Smaller sensors have even faster response, at the expense of lower sensing range and higher $$.

If you do use Hall sensors, don't underestimate the forces on the magnets. The speed limitation on brushless motors is most often the bonding of the magnets in the rotor. I figure your magnets will be subject to over 1000 g's.

Ned Simmons

Reply to
Ned Simmons

Or an optical one with reflective tape-you can make the pulse width as long as you want then.

Reply to
Rick

On Mon, 09 Apr 2007 03:12:23 GMT, with neither quill nor qualm, "Karl Townsend" quickly quoth:

What does Cherry say about conversions factors? They should have that data handy.

--- Is it time for your medication or mine?

Reply to
Larry Jaques

If my memory serves me correctly, inductive prox sensors are just hall effect sensors with their own biasing magnet. We've used

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, ATS642LSH, with significant success as a gear tooth counter. Our particular application included an OEM PCB with an amplifier and a few other components, but this wouldn't be too hard to build. They produce as an output a pulse that corresponds to the gear tooth, roughly a 40-60% pulse width if I remember correctly. This would allow you to detect a simple steel bar or sheet metal tab. We used these with a 60 tooth gear spinning around 300 rpm, which would be 300 pulses per second, with less than 0.25% error (roughly 1 pulse a second). One tab on a 5000 rpm spindle would be 83 pulses per second, so you could probably get even higher accuracy. Hope this gives something helpful ww88

Reply to
woodworker88

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