Resonance confusion

[Sim Joo Khai]

A piece of specially-designed metal strip assembly resonates at 58 kHz. If I
have an LC tank of resonant frequency 58 kHz too and I move the material
near to the tank, will the potential difference across the tank increase or
decrease?

Please help me with this question. I strongly assert that it is NOT a
homework or whatever crap question. It is for the design of an EAS
tag-activated air conditioning access controller.

Thanks in advance.

Sim Joo Khai
[email protected] (Delete the obvious)
Singapore

 

[John Popelish]

A piece of specially-designed metal strip assembly resonates at 58 kHz. If I
have an LC tank of resonant frequency 58 kHz too and I move the material
near to the tank, will the potential difference across the tank increase or
decrease?

Not much of either, I think. The metal movement will alter either the
total inductance or capacitance or both of the tank, but those changes
will go through a complete cycle once every electrical cycle, so will
have little net effect on the fundamental frequency. I think the
strongest effect will be that the second harmonic of the tank
frequency will be increased.

 

[Scott Stephens]

A piece of specially-designed metal strip assembly resonates at 58 kHz. If I
have an LC tank of resonant frequency 58 kHz too and I move the material
near to the tank, will the potential difference across the tank increase or
decrease?

I would expect an initial decrease in the sense tank, but then, due to
inevitable tunning differences, there would be beats as the energy
sloshes between the tanks. To make matters worse, there will be doppler
effects according to relative velocity.

So why not just put a detector, differentiator/high-pass filter and
amplifier on the sense tank to detect changes in power level?

Please help me with this question. I strongly assert that it is NOT a
homework or whatever crap question. It is for the design of an EAS
tag-activated air conditioning access controller.

I hope whomever has the magic tag doesn't try to go shopping, lets he
set off shoplifting detectors wherever he goes =)

Scott

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**********************************

DIY Piezo-Gyro, PCB Drill Bot & More Soon!

http://home.comcast.net/~scottxs/

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[Jim Thompson]

I agree with JP... it might go either way, but not a lot. But why ask
on a newsgroup, when you could try it in a jiffy?

If the coil were not resonant, and had a fairly low reactance at 58K,
I'd expect its impedance to increase if your resonant strip came near.

John

Sounds like a homework problem ;-)

But if I really wanted to pick up a signal from a resonant metal strip
I'd make the strip part of the tank *capacitance*.

...Jim Thompson

 

[R.Legg]

A piece of specially-designed metal strip assembly resonates at 58 kHz. If I
have an LC tank of resonant frequency 58 kHz too and I move the material
near to the tank, will the potential difference across the tank increase or
decrease?

You don't make explicit what the intended coupling mechanism is.

There is no first-order coupling mechanism between an isolated
mechanical part and an isolated electrical tank circuit.

Does tank current flow in the mechanically resonant part?

Does the tank circuit L have an open magnetic field?

Is the mechanical part composed of a magnetic material, or even
conductive material? You say metal, so perhaps we could assume the
latter.

What is the basis for active resonance in the mechanical part?

RL

 

[Tom Bruhns]

A piece of specially-designed metal strip assembly resonates at 58 kHz. If I
have an LC tank of resonant frequency 58 kHz too and I move the material
near to the tank, will the potential difference across the tank increase or
decrease?

Please help me with this question. I strongly assert that it is NOT a
homework or whatever crap question. It is for the design of an EAS
tag-activated air conditioning access controller.

I don't think you have given enough information. As I see it, you
have two resonators (one electrical, one mechanical) which may or may
not be coupled. If they are coupled, I suppose it is through the
magnetic field. In addition, metal in the vicinity of the electrical
resonator will likely change its resonant frequency and lower its Q.

I can imagine in one case, with the mechanical resonator excited and
magnetic and the electrical resonator not excited except through the
mechanical one that the potential across the electrical resonator
would increase with increased coupling to the mechanical resonator.

I can imagine in another case, with the electrical resonator excited
and the mechanical resonator loaded (low Q) that as you brought the
mechanical resonator close to the electrical one, it would remove
energy from the electrical one and possibly decrease the potential
across it, unless it was excited from a voltage source. It would be
rather like a "grid dip meter".

Why not simply do an experiment in the configuration you have in mind
and see what happens? Develop your model from measurements you make
on your experiment.

Cheers,
Tom

 

[Paul Burridge]

A piece of specially-designed metal strip assembly resonates at 58 kHz. If I
have an LC tank of resonant frequency 58 kHz too and I move the material
near to the tank, will the potential difference across the tank increase or
decrease?

If you're talking about one of these store security devices, then I
would imagine it *draws* energy from the tank (which forms part of the
visible loops one walks through) thereby *lowering* the potential
across the tank in the same manner as a high-impedance grid dip
oscillator functions to show resonance.
Caveat: I am not an Engineer so this is just a hunch.

 

[Jim Thompson]

If you're talking about one of these store security devices, then I
would imagine it *draws* energy from the tank (which forms part of the
visible loops one walks through) thereby *lowering* the potential
across the tank in the same manner as a high-impedance grid dip
oscillator functions to show resonance.
Caveat: I am not an Engineer so this is just a hunch.

The cheap tags are nothing but a loop plus a diode. Emersed in a
field they produce second harmonic which is detected.

...Jim Thompson

 

[Marc H.Popek]

thats the ticket if you want some vibrational resonant gain for sure.

 

[Paul Burridge]

The cheap tags are nothing but a loop plus a diode. Emersed in a
field they produce second harmonic which is detected.

Are you sure, Jim? That implies UHF and I was under the impression
that these devices worked at about 9Mhz.

 

[Jonathan Kirwan]

Are you sure, Jim? That implies UHF and I was under the impression
that these devices worked at about 9Mhz.

I don't know about the frequency, but I believe Jim is correct.
Try taking a diode hooked to a wire hoop through one. Worked
for me a while back. Also, it makes sense that they'd need
"real cheap" units to apply to the tens of thousands of items
which go through the doors of some shops. Can't be complicated.
And most folks don't go carrying diode-loops as jewelry. So it
mades sense.

Jon

 

[Jonathan Kirwan]

Sorry, but I'm more than a little unclear about this configuration. In
what way "hooked" to a diode? And what diameter loop?

I've also encountered a security tag that appears to be nothing more
than a thin strip of metal around one inch long. How the heck that
system works I've no idea!

It's a capacitance diode with a tuned dipole, I gather. Take a
look at one of those very carefully and peal the parts away from
each other. When I do so, I've gotten a pair of foils. Not
just one.

Anyway, read:


http://www.rfid-handbook.de/downloads/E2E_chapter03-rfid-handbook.pdf

I think this will help some. Figure 3.5, for example.

(In my case, I just tried a bunch of different small loops.
Some seemed to work, some didn't. I was just curious, but not
willing to spend a lot of my friend's time, so that was all I
did.)

Jon

 

[John Larkin]

I've also encountered a security tag that appears to be nothing more
than a thin strip of metal around one inch long. How the heck that
system works I've no idea!

Some of these are a strip of permalloy or something, magnetized. These
are often used in books. Exposed to an AC magnetic field at the door,
they generate a second harmonic field which the gadget picks up. They
are deactivated at the checkout by an AC demagnetizer. A video tape
will often set off the detector by accident.

John

 

[Paul Burridge]

Some of these are a strip of permalloy or something, magnetized. These
are often used in books. Exposed to an AC magnetic field at the door,
they generate a second harmonic field which the gadget picks up. They
are deactivated at the checkout by an AC demagnetizer. A video tape
will often set off the detector by accident.

That explains why a lot of shoplifters these days are taking
aluminium/aloominum foil into shops with them. They presumably wrap
the stolen goods in the foil and thereby 'foil' this security measure,
if you'll excuse the pun.

 

[Ben Bradley]

In sci.electronics.design, Paul Burridge

That explains why a lot of shoplifters these days are taking
aluminium/aloominum foil into shops with them. They presumably wrap
the stolen goods in the foil and thereby 'foil' this security measure,
if you'll excuse the pun.

Oh, so a lot of plain old, ordinary-looking kooks are actually
shoplifters in disguise:

http://zapatopi.net/afdb.html