PI sensitivity Problem

[xpcam]

Hi There,

Im currently building an Pulse Induction Metal Detector but i have some iritating problems and im almost giving the whole up becourse i dont know what im doing wrong. First of all. Ive got 12 volt input that is going directly to my coil 36 turns then it goes to my mosfet (IRFS31N20DTRL) and the mosfet is driving by an IR2110 as a low driver. The mosfet driver is controlled by my arduino with a 250uS on state and a 300 off state. From both poles of the coil ill go to 2 anti paralel diodes in this case i use 2 schottky diodes one output is going to the ground and the other to my plus gate of the opamp. The opamp that i use now is an LM1458 i control the opamp by using 2 pots one is the factor 2.2m and an offset of 220k. (Multi (20 turn Pots)). I also used a damping resistor 680R.


If im going to pulse and im looking at my oscilloscope and the signal is stable. Im trimming my pots and and can set the sensetivity. Only the thing now. If im going to puls over a pan cake coil of ID 6" and a OD of 8" (36 Turns) +/- 400uH and i set my pots to the most sensitive. I only get my one Euro at 2 cm. I tried with diffrent mosfets and the result is near the same. (the IRFS31.. still the best by testing) A gold ring with a diameter of 1.5 cm and 7 grams is detected at 2 cm. A iron screw driver is detected at 4 cm. A Roman Bronze coin didnt detect at all.

I dont know what im doing wrong. I thougth PI detectors are also good with small items? But if a gold of 7 grams is detected only on 2 cm i think im doing realy something wrong.

And i have seen some boys detecting a one euro coin at 40 cm? The Amp on my Reg. Power Supply is giving on 12v 0.7A if i drive the fet. on 8.5v it is 0.5A and at 15v 1.2A

I also tried several op amps and dont get good results.

Strange.

Sombody experiance the same results or?! am i the only one!?

Thanks.

Christian
The Netherlands

Mosfets i tried:

IRFS31N20DTRL (200v 31A) (the best in my tests)
IRF710 (400v 2A)
IRF730 (400v 5.5A)
STD5NK40ZT4 (400v 3A) (working super but getting hot! D2Pak)
STP3NB100 (1000v 3A)

[Qiaozhi]

Please attach a schematic of your design, and perhaps that will give us a clue as to the problem.

[xpcam]

I hope this is better :-) Sorry for my drawing i dont have a program to draw in only for making print layouts.
http://imageshack.com/a/img537/7464/kbU5qC.jpg

[Qiaozhi]

I hope this is better :-) Sorry for my drawing i dont have a program to draw in only for making print layouts.
http://imageshack.com/a/img537/7464/kbU5qC.jpg

If you are using the oscilloscope to look at the preamp output as an indication of the presence of a target, this may be the issue. After sampling the signal from the preamp, it is normal to amplify this sampled signal further to increase sensitivity. Typically, you can detect a coin several inches away from the coil, even though you will see no change on the oscilloscope by monitoring the preamp.

[DOOLEY]

xpcam,

on a brief look of your schematic I notice your trying to amplify a signal
that will be higher than the 12v rail using non-inverting op-amp arrangement.

the signal more than likely is outside the "window" of the op-amp.

either power the op-amp with at least 3v more than the expected signal (12v +/- 0.7v),
or........
tweek the + input signal down a bit by using a high value resistor to earth on the + input.

your 2.2 meg pot is between the output of the op-amp and the + !!! input,
needs to be between the output and the - input.
then, lets say via a 1k to the top rail.

have a look at various other schematics on this site for popular op-amp
arrangements for Pi detectors.

[Beenthereuk]

Yes it looks like you got your + and - in the wrong places. The op-amp should work if its input voltage range can reach the upper rail because of the voltage drop through the 470 ohm and and the 220 Ohm resistor. However, such a setup is not ideal so it is preferable to raise its rail by a few volts or use a blocking capacitor to bias the op-amp midway.

Either way, as Qiaozhi said, the preamp alone does not give sufficient gain to see the detection of a coin properly. You are lucky you managed 2cm with that alone.
One possible technique after preamplification is to sample the signal at a given time after it decays and pass that into an integrator, which will produce a ramp and a final DC voltage, and this is one way of adding further amplification. This final DC level, if held and displayed on a scope, does have the sensitivity to detect a coin at a few inches.

Alternatively, use a high speed comparator and a high speed clock on your microcontroller to detect the signal level at a fixed time after flyback and hence time the decay accurately. Sample it 10 to 100 times and use the average value as an indicator for the presence of metal. This is how the Pikini detector works http://users.telenet.be/willaert/MD/...pickini_en/V3/

[xpcam]

Thanks! Ill change my electronics today and make a comperater with sound output (i hope) This is what im going to do. Ill let the opamp working on 5v and good settings for the opamp resistors. Ill give it a try to get my mcu comperator to work. If i can get it to work and i get a one euro coin at +/- 35 cm ill make a copy of my layout and send them to you all 3 (Qiaozhi, DOOLEY and Beenthereuk) incl all SMD parts.

Qiaozhi: Ánd yes, i was looking only on the oscilloscope (Tektronix tds210)
DOOLEY: Ill change the opamp setup (ahah thats why i blow up all smd opamps :- )

Ill do my best to change the setup and ill post it here again if i have made the changes.

Something Else: I have been trying for several years now and i have still a movie on my cellphone where i got a realy good indicator for ferro and non-ferro objecs. When it was ferro the puls was going to de top to the null line and if it was a non-ferro my pulse get wider and shifted to the left. Should i upload the movie?! But im not shure how i did it, i thougt i added some 100n's at the coil and the opamp input to output. Also i know the schematic was working on 5v only.

[xpcam]

Hi again,

I dont get it at work. I tried a opamp schematic from http://4hv.org/e107_files/public/135...r_shematik.jpg and its resulting in (see photo's). I think the problem is more the opamp. I never used dual opamps before only the single ones with off-set arangement but i want to use a dual one while i got those at most. I have tried it with two different opamps the TS942AID and the TEB1033DT and both gives the same results. I cant set the offset. I Also use now +5v for input.

May i ask if you can draw a opamp with offset arangement for me?! Im not that good with electronics.

Photo's:

http://imageshack.com/a/img538/8569/RCwToG.jpg
http://imageshack.com/a/img673/2397/aRRjup.jpg
http://imageshack.com/a/img911/3308/h3Eara.jpg
http://imageshack.com/a/img673/425/VilLgR.jpg
http://imageshack.com/a/img913/3016/dJdEaw.jpg

Thanks,

Christian

[chemelec]

You should be using Op-Amps with Dual +/- Supply Voltage, Like a TL081 (Single) or a TL082 (Dual).
That Singly Supply, 458 Op-Amp has a VERY POOR Frequency Response.

[xpcam]

Hi,

I dont know what im doing wrong, it just dont work with 5 volt. If tried with serveral opamps and it is all the same. I cant get the offset to work. I also have blowen up my -5 volt so thats out of the picture i only have +5v and +12v.

I have been working the whole day and no results. Im realy stuck now. And about the 1458 thats the only opamp that i have in DIP8. For some reason i blow all the smd opamps on 12v and on the 5v the offset does not work. Help!!!

The opamps that i have tried: (All SMD)

TS942AID
TEB1033DT
TL071
LM358
TLC272CD

[Qiaozhi]

I dont know what im doing wrong ... Help!!!

I would suggest that you take a look at some already-working PI designs to see how they're put together, such as MiniPulse Plus, Surf-PI, or Hammerhead.

Minipulse Plus

Hammerhead

http://www.geotech1.com/cgi-bin/pages/common/index.pl?page=metdet&file=projects.dat

and/or buy this book ->
Inside the METAL DETECTOR - Published September 2012

which (unfortunately) is currently out of stock in the UK. Although, in a few weeks, I should have about 10 copies available.

[xpcam]

Mayby i should. I looked at the schematics on the site and on some other websites. But i cannot find a working opamp with a sepperated offset. If finaly get a opamp working on 5 volts only the detection/measure is on 250uS so small that i need to get it wider but i dont know how to do that. If just some one could help me only with the opamp i would be saved. I tried mayby 50 opamp schematics today and now i got the first working. Also with a problem if i set the offset low then the opamp reacts also to my hand. I now use the TEB1033DT and the TLC272CD is working also. Ill draw the schematic tomorrow mayby i need to adjust some things.

Christian

[xpcam]

Schematic of what i have now. On the left bottom corner is what i expected but i get the output as on the left. I dont have ever worked with 12v and opamps. Im a noob on analog elektronics. Im more from the digital side.

Photo: http://imageshack.com/a/img661/8121/EQoY5d.jpg

Christian

[Qiaozhi]

Schematic of what i have now. On the left bottom corner is what i expected but i get the output as on the left. I dont have ever worked with 12v and opamps. Im a noob on analog elektronics. Im more from the digital side.

Photo: http://imageshack.com/a/img661/8121/EQoY5d.jpg

Christian

Before we even discuss the opamp configuration, you need to correct the wiring to the coil.

1) You do not have a damping resistor across the coil.
2) There are two back-to-back diodes across the coil that are effectively "shorting" it out.
3) The 1k resistor should be between the coil and the diodes, which are there to protect the input of the opamp from the high voltage flyback.

[xpcam]

Before we even discuss the opamp configuration, you need to correct the wiring to the coil.

1) You do not have a damping resistor across the coil.
2) There are two back-to-back diodes across the coil that are effectively "shorting" it out.
3) The 1k resistor should be between the coil and the diodes, which are there to protect the input of the opamp from the high voltage flyback.

Done! http://imageshack.com/a/img909/9527/FtXQ9k.jpg