Inductive Proximty Switches Sensing Aluminum?

[motoboy]

I was working with some Osiprox Proximity sensors from Telemecanique and surprised to find out that they can detect aluminum (although at only about half the distance of steel). This flys in the face of my understanding of how an indutive sensor works. That being you basically have a transformer with no core and the sensed object becomes the core and induces a detectable voltage in the seconday windings. Since only magnetic materials can conduct flux (and be an effective core) how can an indutive sensor sense aluminum???

Please point me in the right direction if my logic is off.

 

[itsmoked]

I am not sure how your sensor is working but!

You can induce magnetic fields into non-feric metals quite easily that can then be detected. You know that moving current is associated with B-fields(magnetic)?

Eddy currents.

So if the sensor puts out a magnetic field and looks for changes in it it will be able to detect conductive things.

 

[jraef]

itsmoked hit it on the head.

Eddy currents. Click on this link for an article explaining it.

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[VEBill]

There was a nice demo on TV the other day about using eddy currents for the brakes on a 'land speed record' jet car.

The demo was a slab of aluminum laying on the table, the guy waves (motion is important) a powerful magnet over it (not touching it), and the Al slabs starts sliding around the table.

For the jet car, the magnets are brought up close to the spinning Al disk, the disk gets hot and the car slows down.

Q: "how can an indutive sensor sense aluminum???"

Is the aluminum MOVING when it is being detected ?

 

[analogkid2digitalman]

Hand on demo's of how non-ferrous and thus presumed to be non-magnetic materials (aluminum, copper, stainless steel in various grades) actually are affected by a field.

If you have some older (large) hard drive magnets laying about, place one on a sheet (1/4" thick works well) of aluminum, and then tilt the sheet vertical. You will find that the magnet falls (actually slides) downward quite slowly.

Or hold the magnet in your hand closely over the sheet and wave it back and forth. You will feel the eddy current induced resistance against your hand. Eventually, the sheet will warm up slightly. The faster the relative motion, the more resistance.

The killer demonstration is to get some large (1 or 2 feet long) finned aluminum heatsinks placed flat to flat with a gap of an inch or two between them. Drop the hard drive magnet between them for a very slow not-so-free fall.

Wheels within wheels / In a spiral array
A pattern so grand / And complex
Time after time / We lose sight of the way
Our causes can't see / Their effects.

 

[itsmoked]

ow, ow, och, uch, ow.

analogkid2digitalman that sounds like fun plus+!

Why does in need to be expensive big heat sinks? Wouldn't it work wih any 1/8"-1/4" Al? If you say yes, I might try it.

 

[analogkid2digitalman]

itsmoked:

This is just to wierd.

I was looking up this link and reviewing it just as our IT guy came up to me bearing a gift- a trashed hard drive and wanted to know if I wanted the magnets out of it! And your animated icon is what he typically does to the drives to make sure no data ever comes off of it.

http://www.electricstuff.co.uk/neodym.html

Fins seem to help per the author, you could play around with some sheet.

Have fun!

Wheels within wheels / In a spiral array
A pattern so grand / And complex
Time after time / We lose sight of the way
Our causes can't see / Their effects.

 

[Skogsgurra]

I think that you need this mental picture: The aluminum sheet is equivalent to a shorted turn in a transformer. You have no problem imaging what that does to the losses in the magnetic circuit, do you?

That is how non-magnetic targets are detected by proximity sensors.

Gunnar Englund

http://www.gke.org

 

[itsmoked]

Okay... Your trusty field reporter is back.

After throwing away probably 300 HDDs in my life Half of which were busted the other half obsolete.. I had to search high and low and finally murder a perfectly good 3.8G 5400rpm Seagate! (I need to keep my head down since Seagate is only a couple of blocks from here.)

I presumed the magnets in question are associated with the head servo system so I had to get inside the disk housing. I discovered that those absurd little screws of which there are about 18 are a complex star pattern drive... So complex that you can use an appropriate flat blade screwdriver with ease! LOLOLOL Just jam it into opposing star rays.

Once the pristine innards are visible I took said screw driver and moved it around the servo stuff finding only a weak B-field.... Never-the-less I proceeded to remove the top C shape flat plate.. It was just set on top of two steel shoulder pins!! Once it was removed its strength became apparent jumping onto everything in sight. I then unscrewed the shoulder pins to get to the mate. The magnets are on
complex steel plates. Inspection showed that they seemed to be located by little raised bumps leading me to suspect they were only held by the B-field. I couldn't budge them.. So I proceeded to try to gab one with vicegrips. While setting the vgrips one edge failed shattering about 3% of one face. This was strange because the pieces didn't go anywhere!! They stayed stuck the magnet.

I struggled to remove these pieces that were about 1mm square, about 4 of them. Regrouping instead I next grabbed the plate that the magnet was on, at both ends, with vgrips and bent it into a bow away from the magnet. This showed that the magnets are glued down with a very small bit of white adhesive which fails. Next you stuff a screw driver into the new gap and the magnet comes off with ease.

I stuck them together which makes a stack about 0.25" by 0.8" by 1.5".

I then hunted down a 50AMP power supply on a rack with narly heat sinks with about 3 inch fins projecting off the side. I droped the magnet stack between the fins and it.... (drum roll please).. fell half and inch down then went back deeper into the fins all the way to the back then came up half an inch and stopped!!!

Shock translated to chagrin as I discovered a steel screw head under the magnet holding the heatsink on...

I retrieved the magnet and shifted the test to the middle of the sink (no screws). Upon dropping the magnet it falls straight down the inter fin slot without hitting the sides at a speed of about 1.5"/sec really strange... amazing.

analogkid2digitalman thanks for that link. Nice.

So ends today's experiment. Class is excused.

 

[Bung]

Wow, sounds cool. My son has a stack of old HDD magnets kicking around somewhere - can't wait to get home to try it! But he didn't have any trouble extracting them, as far as I remember. Must be a different make.

Bung
Life is non-linear...

 

[Bung]

Actually, thinking about it a bit more, wouldn't the fact that aluminium is diamagnetic have some bearing on what is happening here?

Bung
Life is non-linear...

 

[Bung]

me again - aluminium isn't diamagnetic, it is paramagnetic. Shows you shouldn't rely on memories of high school physics over 1/3 of a centruy after the event!

Bung
Life is non-linear...

 

[Skogsgurra]

That's how a truly scientific mind works. It HAS to be real tough. There HAS to be vintage equipment around. Something HAS to be sacrificed.

I (not so scientifically inclined) stopped by the hobby shop and got me a few of those terribly strong little magnets. Sliding them down an aluminium sheet was a strange experience.

BTW, para or dia? Does it really matter? The deviation from 1.000 is to small to be detected in any course experiment like the ones we are talking about here. Or?

Gunnar Englund

http://www.gke.org

 

[amptramp]

From Turck Sensors Tech info:
"An inductive proximity sensor consists of a coil and ferrite core arrangement, an oscillator and detector circuit, and a solid-state output. The oscillator creates a high frequency field radiating from the coil in front of the sensor, centered around the axis of the coil. The ferrite core bundles and directs the electro-magnetic field to the front. When a metal object enters the high-frequency field, eddy currents are induced on the surface of the target. This results in a loss of energy in the oscillator circuit and, consequently, a smaller amplitude of oscillation. The detector circuit recognizes a specific change in amplitude and generates a signal which will turn the solid-state output “ON” or “OFF”. When the metal object leaves the sensing area, the oscillator regenerates, allowing the sensor to return to its normal state."

The principle is reffered to as "ecko" or Eddy Current Killed Oscillator.

 

[itsmoked]

This is the exact same thing as a grid dip coil system.

You have this oscillator in a box with a meter on it. You plug the appropriate colored coil into a socket on the box. When you move the box/coil/meter near a circuit that is tuned to a frequency in the chosen coil's range the oscillator will be loaded since it will start to drive the nearby circuit. Hence it is "dipped".

 

[analogkid2digitalman]

Sounds very similar to the principle that metal detectors work on. Mine will easily detect aluminum trash (cans, tabs, foil) but has discriminators to reject such items if desired.

Wheels within wheels / In a spiral array
A pattern so grand / And complex
Time after time / We lose sight of the way
Our causes can't see / Their effects.

 

[Higgler]

Pretend you are moving the magnet very quickly across the aluminum, instead of the aluminum moving across the magnet.

Sounds like a motor with permanent magnets.

Does the proximity sensor provide a detection output for static conditions, or only when the aluminum is moving?

kch

 

[itsmoked]

Since the field is in oscillation the the aluminum doesn't have to be in motion.

 

[wagnerc]

Think about it this way. Any conductor impinging on a changing magnetic field will have a current induced in it. An electric motor could just as easily be made of aluminum wire as copper. This leads me to an interesting idea. Would the metal detector trip if u had a large long closed tube of salt water? It conducts so u should be able to induce a current in it. That would be hilarious to get pulled aside due to a "massive metalic signal" only to find a hose of water.

 

[itsmoked]

lololo.. "Full cavity search for this prankster!"

Sounds like we need an experiment done.