Propeller Application: Inductive Sensor made easy

Beau Schwabe

Here is a unique way of using the Propeller to read an inductor as a sensing device.
Unlike the previous Coil Demo that I posted in the object exchange section, this version uses
far less parts (Only 3 resistors, and a Diode .... Plus the L and C components necessary to make this work.)



Note: Sensor.bmp AND Sensor.zip are simulated graphic images that are not the
graphic output of this demo. Refer to them from the 'text' description within 'Inductor.spin'
for an explanation on how this design works.

The "Green" line is the returned ADC value
The "Yellow" line indicates the position where the reported resonate frequency of the LC circuit is (read text in 'Inductor.spin')

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

Post Edited (Beau Schwabe (Parallax)) : 7/6/2006 12:33:11 AM GMT

Rob7

Cool,

Rob7

Oliver H. Bailey

Beau,
With a few more components you could have a 125 Khz RFID reader and writer.

Oliver

Beau Schwabe

Oliver H. Bailey,

I thought of that, but it wasn't quite the direction that I was going at the time... The Inductive sensor above is a result of waiting
for back-ordered parts from another supplier, and seeing what I could do with the components I did have available while I was
waiting. There is another approach to the above inductor application that closer resembles the operation and theory of an
LVDT or RVDT that I would like to try, so you might see another object for that.

The back-ordered parts actually came in on Friday, so I will be a little busy with that. <- more cool stuff you can do with the Propeller.

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

R Baggett

Was the ADC object you used special? ADC.SigmaDelta() starts a cog with the assembly ADC code that runs forever.. It seems that there is an attempt to start an infinite number of cogs in the inductor.DisplayInductorValue routine where the ADC.SigmaDelta(@FTemp) is found inside the loop! Also many more times in inductor.FindResonateFrequency where it is also inside a loop.

The ADC.spin in your archive here is the same as the one in the library..

(Or am I missing something profound here?)

Thanks much!

RA Baggett

Beau Schwabe

R Baggett,

I think you may be correct... When I get a moment I will look further into it, but at first glance you may be right. The ADC object should only be started once.

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

parsko

Beau,

When you blast your LC circuit with a (0V to 3.3V) square wave from the Prop, it produces an AC wave on the output (point B in your circuit).

Does this AC wave go positive and negative (aka, -10V to +10V) or does it stay only positive (0 to +10V)? (All values chosen arbitrarily)

You push the output through a diode prior to any further circuitry. If the output at B is (+/-10V), does this diode chop off the (0V to -10V) part of the output?

You then "clip" the circuit with the voltage divider (from 6.6V-0.6 (diode) divided by 2 ==> 3V) so that the prop can read the signal via software. When you "read" this signal, are you "reading" the 1.65V threshhold?

-Luke

Beau Schwabe

Luke,

"Does this AC wave go positive and negative (aka, -10V to +10V)"· - Yes
·
"(All values chosen arbitrarily)" - For L and C yes, but the 100K resistors were chosen to limit the current to within a safe range for the internal I/O diodes to safely handle.
·
·
"You push the output through a diode prior to any further circuitry. If the output at B is (+/-10V), does this diode chop off the (0V to -10V) part of the output?" - Yes
·
·
"You then "clip" the circuit with the voltage divider..."· - The internal diodes that are built into the I/O actually do the clipping.
·
"...When you "read" this signal, are you "reading" the 1.65V threshold?" - Yes, but through the function of a·Sigma delta ADC.· But remember, since we are cutting half of the wave out (the 0V to -10V) not only does the ADC measure magnitude, but now because of the nature of the Sigma-Delta ADC, the width of the pulse also becomes a factoring component that just happens to be beneficial to this application.


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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

parsko

Thanks Beau! That gave me much to think about. I still need to think about the clipping thing more, but I think experimenting will yield me answers.

One more question:

What kind of capacitor is safe to use for an LC circuit?

The circuit I'll be building will look for a low resonant frequency (~1-10kHz). Given the dimensions of the coil I'll be winding, the math calls for a 1uF capacitor. These seem to be harder to find in the non-polarized (metal-film/ceramic disc/etc...) style, but easy to find in a polarized (electrolytic) style.

My instinct tells me that I need a non-polarized capacitor, is this true?

I was planning on using:
THIS ONE

Thanks again,

-Luke

Beau Schwabe

"What kind of capacitor is safe to use for an LC circuit?" - safe? I think this would depend on voltages involved... something Non-polar for sure.


That RS cap should be fine.... remember though your 'Q' factor goes way down with higher L and C values and especially if your Coil has an Iron core. With the lower Q, you probably will not get a great deal of gain. Do you have a specific frequency in mind that you are looking at?


A 250uH coil is not that large for 10kHz, but 25mH coils can start to get bulky for 1kHz. Is there a way that you can hetero-dyne and use a higher detection frequency with a smaller Coil and Capacitor? This would also give you a higher 'Q'.

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

Ale

Beau, your implementations are normally top-notch but this time I've to make a small comment: nothing in function of nothing cannot give a curve.