Metal detection ckt needed.

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I have an application that requires that I sense a metal ball (approx 0.5" dia) inside a glass tube. We have been using inductive prox sensors but their cost and size has required that we revisit this design. Can anyone suggest a simple ckt or an ic that will perform this function. 12dc power and a TTL o/p would be ideal but we can work around any changes if needed.

PS. The tube is filled with a thick black liquid.

Thanks,

A

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Would Hall Effect switch with small permanent magnet work?

Here is how I see it working:

You would have a Hall Effect sensor set on one side of the pipe. On opposite side you would have a small permanent magnet that is too weak to trip the sensor. When the metal ball passes between them, the ball would acts as a ‘magnetic conductor’ extending the magnetic field of the permanent magnet, hence tripping the sensor.

Rob

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Rob,

I did consider this approach but I did not think it would be effective. The wall of the tube is about 0.050" thick. Have you worked with these devices before?

A

AVR Studio 4 Ver. 4.18 684
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Yes, I have worked with these switches long time ago, but not in that configuration so this is pure speculation on my part.
That’s why I said:

Quote:
Would Hall Effect switch with small permanent magnet work?

I think the key for this to work, is in choosing adequate magnet.
With time, I’m sure others will comment on this and if it is wrong they will tell us with vengeance. ;)

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I can get my hands on some good low cost magnets. So that will not be problem. Can anyone add to whether or not this concept is viable?

A

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It should be viable unless your fluid is a ferrous slurry. It also depends on non-ferrous tubing or pipe.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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What about a small sonar sensor with a narrow eyebeam. The ball will produce a sizable "PING" when it interrupts the field of view.

Jim

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If you've been using an inductive prox sensor, why not make an inductive prox sensor? Usually they're something like a colpitts oscillator with the sense coil and a frequency detector like a lm567. There's probably heaps of circuits on the interweb for such things.

A quick google for 'metal detector circuit' found this:

http://www.zen22142.zen.co.uk/Ci...

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I have given some thought to my ball sensing app. It occurs to me that the permanent magnet will not work in this case. The ball is in a precision id tube and the ball has a precision od. The space between the wall and the surface of the ball is 0.0025". The liquids are pumped around the system and there are several valves as well. I am concerned that any metal in the system will be held by the magnet and will cause errors in the ball drop time.
Kartman,
I have seen this circuit and I am sure it would work in my app. The problem is that on-semi has marked this part as end of life. I need to be able to get this part for at least the next 10 years.
We do use the 567 in a circuit that is set up as photo detector. It works very well. Is there an simple way to set this up as an inductive prox? I will have a look at this.

Thanks,

A

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IMO there is no need for a 567, assuming there is an AVR around.
Version A is the most simple one to implement (mechanically)
B is worth a try, if A prooves to be inadequate
Version C is IMO the best way to go.

If you need fast updates of the ball position, I suggest you use T0, and measure with Timer1 how long it takes for Timer0 to overflow. This method provides accurate measurements, with an update-frequency of appr. 1000Hz. The big trick is to wait for T0 to be high, and thento go low, start the measurement. A synchronous start, so to speak.

If the frequency deviation is large enough, you can get away with a more simple frequency measurement.

Nard

Edit: the frequency lies around 400 to 450 kHz

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Nard,
All the timing, counting and testing is already designed and tested with an ATMega640. The only issue I need to deal with is the sensing of the ball.

Thanks,

A

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Nard,

I had a look at your circuit. Is this something you have used before? Can you provide a brief description of how this works?

Thanks,

A

EDIT: I had time to have a look at the cicuit and it is clearly a simple oscilator that changes freq with the detection of the ball. For my application this may not work because the board is already in place and requires a low or high input with the ball detection. Thanks for the effort Nard.

A

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You're welcome.

Yes, I used it in a Theremin. Your analysis is correct: the frequency drops when the ball is in the coil, or between the sensing-coils (depends on the concept)

For a solution with discrete components: take the output of the TLC372 to a LM567 Tone-decoder, or a LM565 PLL.
Big disadvantage of HW-based solutions is their need for trimming/adjusting/calibrating in the production process.
If implementing in the Mega640-software is not an option, I'd rather go for a small AVR after the TLC. You can make software that auto-calibrates so there will be no more need for adjusting. Another advantage is the flexibility: if parameters change (tube, liquid, ball-size), the detector is SW-upgradable, unless of course it is within cal-parameters.

Whatever the solution will be in the end: it all comes with a price.

Nard

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Nard,
This is what we currently use for object detection. I will try and modify it for inductive pickup.
A

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Man, just go to a junk yard and get a hall or VR sensor from a car. :)

-Chris

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Chris,
I would do just that if I only need to make a couple of these but with a little luck I could be making a 1000 a year.

A

AVR Studio 4 Ver. 4.18 684
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andrew99 wrote:
Chris,
I would do just that if I only need to make a couple of these but with a little luck I could be making a 1000 a year.

A

Ok. :) I just figured you needed a cheap and quick solution for testing purposes.

-Chris