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juani_c

TI releasing something "game changing" on Sept 16th

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Here are some piccies that mysteriously got dropped from my previous post.

 

The EVM split with 0.1" header pins added. (Attaching the header 

on the contoller part was quite a pain as the pads lifted and the

tracks joining the pads broke. But I managed to repair it.)  

 

post-16401-0-96586900-1381156757_thumb.jpg
 

The large 50 turn 930uH coil

 

post-16401-0-54280700-1381157083_thumb.jpg
post-16401-0-66185700-1381156770_thumb.jpg

 

 

 

post-16401-0-96586900-1381156757_thumb.jpg

post-16401-0-66185700-1381156770_thumb.jpg

post-16401-0-54280700-1381157083_thumb.jpg

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If you see noise or interference, you can try to increase the "response time" setting (or reduce the data rate). As far as I understand it, the former should increase the oversampling (though its not really explained in the data sheet).

 

According to the data sheet, the lowest frequency is 5kHz, so it should be possible to set this in the GUI to about 20% below the resonant frequency you measured (about 22kHz).

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I played around with the response time and the min sensor frequency settings but neither helped very much.

All my testing was indoors so I was surrounded by 60Hz mains interference, and I made no attempt

to shield the coil.

 

The datasheet doesn't explain in detail how the Proximity value is calculated, but I suspect that

the loop gain of the oscillator is controlled by the Rp_min and Rp_max register values.

 

My guess is that during operation the loop gain is alternately set by the Rp_min and Rp_max values.

When the LC tank is driven with Rp_min the amplitude of oscillation will grow in amplitude

until it reaches the threshold voltage, this is the LDC Configuration Amplitude setting (4v). It then

switches to use the Rp_max  value and the amplitude of oscillation starts

to reduce until the amplitude crosses the lower threshold. This cycle continues switching

between lower and upper thresholds. The time it takes for the oscillation to grow or decay

depends respectively on how close the Rp of the LC tank is to Rp_min and Rp_max values.  

The ratio of Rp_max time to Rp_min time  is used to calculate the LC tank Rp value using

the datasheet formula

 

RP = (RPMAX

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A planar inductor with the same number of turns is about half the Inductance 

 

http://www.66pacific.com/calculators/coil_calc.aspx

 

I was aiming for the maximum air-wound inductance to

give maximum sensitivity to nearby metal objects.

That's how conventional metal detector coils seem

to be designed. 

 

I'm not much of an EE so was only asking. Part of me thought having more 'metal' out there, spread across a larger area might make a difference. You know- more is better, right? ;-)

 

I did a search and it seems some detectors use planar / spiral windings with some multi-layered, and others wind the coil toroid-like around a ferrous ring, and others do as you did.

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Coil design for metal detectors is a big subject. As you say there are lots of designs.

 

I came across this site for some general guidelines on coil size vs detection

 

http://www.lammertbies.nl/electronics/PI_metal_detector.html

 

He describes coils for Pulse Induction type of detector but I think it applies 

equally to all detectors.

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@@mpymike - Thank you for the link.

As I said, I'm not an electrical engineer. It does make sense that there is some "break even" point where the current and voltage in the coil exceed what the supporting circuitry can handle, which then limits the ability of the coil to detect 'objects'.

 

I learn more every day. Now, if I could just get the left half of my brain to talk to the right half... I might get somewhere. ;)

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Unfortunately the coil picks up a lot of interference which you can see as ripple when looking at the

envelope of the voltage across the coil. This results in considerable variation in

the frequency count  data and in the Rp proximity data. I adjusted the Fmin, Rpmin, Rpmax

register settings and the filter cap value so I was sure that the circuit was operating

correctly as per the datasheet.

 

I think the EMI you saw was caused by static energy on the plastic jug. Were you able to get the interference filtered out? Also you could try to do the same thing with a piece of cardboard cut out in a circle shape then wrap the coil around the edges. 

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