bluehash 1,581 Posted October 7, 2013 Share Posted October 7, 2013 @@mpymike Could you check your images for us Not sure what happened here. Quote Link to post Share on other sites
mpymike 18 Posted October 7, 2013 Share Posted October 7, 2013 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.) The large 50 turn 930uH coil bluehash 1 Quote Link to post Share on other sites
hlipka 11 Posted October 7, 2013 Share Posted October 7, 2013 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). Quote Link to post Share on other sites
mpymike 18 Posted October 8, 2013 Share Posted October 8, 2013 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 Quote Link to post Share on other sites
abecedarian 330 Posted October 8, 2013 Share Posted October 8, 2013 @@mpymike I wonder if being planar would have an affect on the coil, versus it being wound around a 'core' as you've done? Quote Link to post Share on other sites
mpymike 18 Posted October 8, 2013 Share Posted October 8, 2013 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. Quote Link to post Share on other sites
abecedarian 330 Posted October 9, 2013 Share Posted October 9, 2013 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. Quote Link to post Share on other sites
mpymike 18 Posted October 9, 2013 Share Posted October 9, 2013 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. Quote Link to post Share on other sites
abecedarian 330 Posted October 9, 2013 Share Posted October 9, 2013 @@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. Quote Link to post Share on other sites
hlipka 11 Posted October 14, 2013 Share Posted October 14, 2013 The question about the distance of sensing made it into TI''s LDC1000 FAQs: http://e2e.ti.com/blogs_/b/analogwire/archive/2013/10/11/inductive-sensing-top-three-ldc1000-questions.aspx?hootPostID=82de5e49190b64ee7efb9439a28c60e3 Quote Link to post Share on other sites
Derekisbusy 0 Posted May 22, 2015 Share Posted May 22, 2015 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. Quote Link to post Share on other sites
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.