magnetism physics problem

Some might call these prox sensors, but the term "prox" usually connotes an ECKO or Eddy Current Killed Oscillator as have been made by Turck, Omron, Schild (Switzerland), Balluff and a host of others. A big difference is that a prox has a precisely repeatable sensing distance while hall sensors typically don't -- in fact, the ATS642SLH and others like it auto-adjust to sense "something" while autocompensating for eccentricity and sensing distance.

In a big high-speed textile machine for example, you want it to "fire" the shuttle at exactly the right point in the machine's cycle every time.

Attractive force is B^2 * A * (units conversion factors) where B is flux density (gauss, tesla, etc)in the air gap between magnet and attracted pole, and A is air gap area. If an 0.5" dia button magnet exerts force of 0.1 lbf the flux density in the gap is 2210 gauss. Note that if you spread that flux over 1 square inch (a 1/2" x 2" pole piece) the flux density goes down to 433 gauss.

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.

Response time of this sensor is on the order of 10 milliseconds. At

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

Karl, We use a module from Turck, it's a standard module, IIRC

As you know, I'm really struggling with this one. The vendor does indeed state that maximum response time can be up to 10 MS, depending on conditions. It infers to me that magnet flux density, air gap, etc. effects response time. But they also say it can run at up to10K hertz frequency. I found out that this type sensor is what Hardinge uses on its CNC lathes - works for them - worth a shot for me.

I had this idea while working in the trees: Hook a 12 volt output up to an ignition coil and then an auto stobe light looking at the spindle. I'll have computer fire the output as soon as it sees and processes the prox. signal. I'll be able to visually see how many degrees of variation I have. idea any good?

Response to post above (Anthony): This is an index mark for lathe threading when needed for accuracy. It will also be a watchdog timer for VFD trips on the spindle (higher speeds, lower accuracy)

Karl

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

"Karl Townsend" wrote in news:ieBSh.19933$ snipped-for-privacy@newsread4.news.pas.earthlink.net:

They used to be available in the auto parts stores, as replacement pickups for aftermarket ignition systems.

You could use one of the fiber-optic sensors from Banner, Balluff, Keyence, and others. I'm not much on Keyence unless there is no other choice, they are quite proud of thier stuff ($$ wise), and on top of that the salesman don't want to show you anything but the latest and greatest with the highest sticker, you'll play hell getting them to even admit they have something cheaper that will do the same job. I'm not sure what the rest of the world gets for pricing from the other two, we have a direct worldwide corporate account from both, and as such get a considerable discount.

The Banner Q45 series with fiber cables has a response time of 2 milliseconds on, 1 millisecond off in opposed mode, in reflective mode, 2 milliseconds each way. I use tons of these things, they are as reliable as the sun, and in opposed mode, almost infallable. You can switch them from Light on to Dark on in just a few seconds (flip a switch under the cover). The ITS fibers will take a beating and still work properly.

This is a start...

Ned

I think first order of bidness is to determine the responsivity of whatever is receiving the sensor signal.

what am I missing here - a simple coil with a ferrite core mounted within a few thousandths of the magnet will respond well up into the Khz range, probably 40 Khz (note, off the top of the head guesses can be wrong) - an op amp for signal conditioning and shaping can then drive standard TTL logic which has a typical MINIMUM pulse width of 8 ns or better - use the TTL logic to do whatever it is you want to do. You can buy a coil with an amplifier, or you can find them already wound in all sorts of things, like radios, PC power supplies, etc.

Is the probem one of not wanting to design any electronics? Is the problem EMI or other concerns? if the environment is clean, an optical interrupter will easily meet your needs - look at rotary encoders, if you want to know exactly where the thing is. you can read out angular position to a fraction of a degree pretty easily at a few thousand RPM with a rotary encoder.

Response-time doesn't mean minimum pulse width. It is just the time the signal lags behind the event. You need to find the maximum frequency.

Nick

Ahh. Now I understand. This is useful to know for the future. I should do some research about these eddy current sensors.

These sensors use a high-frequency alternating magnetic field for sensing, but the field strength is far too weak to cause collection of ferrous swarf. They can work reliably in situations and environments far too dirty for optical sensors. Trip point can be reliable and repeatable to within thousanths of an inch of axial distance.

Don Foreman wrote in news: snipped-for-privacy@4ax.com:

In addition, you can get them with very tough faces, Turck even makes some now with a metal face. They have them designed for about any environment you can think of. Nothing beats those Banner Q45 opposed fiber optic sensors though, when you really, positively have to know whether something is there or not, such as in automation part detection.

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

Ned Simmons

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.

I've been away from this for some years now, but last I looked the guys at Banner were incredibly responsive and easy to work with. They did some contract manufacturing of elex for me back in the day and they did it well. The chief engineer had a model train setup that had to be seen to appreciate. Seriously neat! The founder and CEO at the time was into Lotus racecars, fun guy. I think his name was Bob, maybe Bob Fayfield but that's a scrape from the memory banks.