detection of a metal part near an inductive loop

meereck · Joined: 09 Nov 2006 Posts: 173

hello all,
i would like to detect a metal part near an inductive loop.
There are for sure several principles, one of them is to measure a resonance frequency. Unfortunately, I haven't found any scheme which would be helpful.
Can I ask you for any suggestions on that? I do not insist on using that principle.

Thanks in advance,
regards meereck

Guest · Posted: Tue Jan 08, 2008 2:30 pm

Like you said there are many ways to get to a solution here.

Do a Google search for: 'Metal Detector"

A simple one I designed many years ago is here,

edn.com/archives/1997/121897/26di_02.htm

HTH - Steve H.

SherpaDoug · Joined: 07 Sep 2003 Posts: 1640 Location: Cape Cod Mass USA

The simplest to interface to a uP is to build the loop into an oscillator, then measure the frequency of the oscillation with the PIC.
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meereck · Joined: 09 Nov 2006 Posts: 173

John P · Joined: 17 Sep 2003 Posts: 331

Well, in theory you don't need a resonating circuit.

Connect one side of the inductive loop to a port pin, and the other side of the loop to another port pin set up as an input and also to a resistor, with the other side of the resistor to ground. Zero a timer, and start it running, and at the same time (doesn't need to be exact), set the port pin high.

Then what will happen is that current will flow from the port pin through the coil and resistor, with all the voltage drop initially across the inductor. Over time, the voltage drop will shift to being mostly across the resistor; this is an LR circuit that's in all the textbooks. You use the second port pin to monitor the voltage across the resistor, and when it makes the transition from "low" to "high" you trigger an interrupt and read the value of the timer. The higher the inductance, the longer this time interval will be. When the transition occurs, set the output pin low again. But be aware that current will continue to flow because you have an inductive circuit, and if you don't put a diode in to handle it, the port pin protection diode will conduct, and that might not be good. A Schottky diode oriented with its anode to ground would deal with this.

If you want to use more current through the coil than the processor pin can give, I don't see why the circuit would function any differently if you had a transistor to switch the current. Also, you could use an A/D input instead of just the low-high transition at the input pin, but it would be reading a varying voltage.

Note that I only said it was theoretically workable! I guarantee that this idea is worth every penny of what you've paid.

meereck · Joined: 09 Nov 2006 Posts: 173

your solution makes sense, thanks for it. There is just one drawback which i didnt mention. I need to measure the inductance as fast as possible, which in the case of your principle cannot work since there is a time delay.

cheers

SherpaDoug · Joined: 07 Sep 2003 Posts: 1640 Location: Cape Cod Mass USA

RLScott · Joined: 10 Jul 2007 Posts: 465

inductive_dave · Guest

Do a google search as suggested but type in "Pulse Induction metal detectors", this is basically a coil of wire fired with a square wave pulse, as the pulse switches off you get a decay curve, any metal near the coil alters this decay curve, this is probably a bit more complex than you want but their are pleanty of PI detector circuits available on the web for you to look at as well as explanations, there are a few simpler ones which you can adapt to include a PIC micro.

John P · Joined: 17 Sep 2003 Posts: 331

You could describe my design as a "pulse induction metal detector" too. But I'm puzzled why a design using an oscillator would be fast enough and a pulse-based one wouldn't. Doesn't the oscillator need time to stabilize to a particular frequency and doesn't the processor need time to read it? I'd have thought that pulses produced by the processor and measured using one of its on-chip timers would be the simplest and most direct way of measuring the circuit's characteristics.

necati · Joined: 12 Sep 2003 Posts: 37 Location: istanbul

http://sjeffroy.free.fr/DetectPI/detectpi.html

SherpaDoug · Joined: 07 Sep 2003 Posts: 1640 Location: Cape Cod Mass USA

I have never used anything like the pulse induction detectors described, and they may work well and quickly. They do seem more complex than the simple op-amp oscillator and PIC frequency counter I described.

With the oscillator if you count frequency at the zero crossings you don't have to wait for the amplitude to stabilize. Actually you can monitor either amplitude or frequency to detect an anomaly. Generally it is easier for a uP to measure time than voltage so I use frequency. If your PIC Xtal is fast enough, or you use an external high speed counter, you can measure frequency by period in a single oscillator cycle.

Please tell us which method you try and how well it works.
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RLScott · Joined: 10 Jul 2007 Posts: 465

SherpaDoug · Joined: 07 Sep 2003 Posts: 1640 Location: Cape Cod Mass USA

Tell us more about your application. Are you just detecting the presence of an object? Do you want to know the size or type of metal? What type of metal are you expecting? It makes a difference if you are detecting a car stopped at a red light, or a speeding bullet.
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The search for better is endless. Instead simply find very good and get the job done.

John P · Joined: 17 Sep 2003 Posts: 331

I read the stuff at RLScott's link, and--wow, those things can be as complicated as you want them to be! But I doubt if the original question was aimed at anything like that.