GROUND CONTROL PI CIRCUIT

[daverave]

can anyone tell me what does the ground control circuit look like on a pi detector....i wondered if it was an adjustable second delay sample as used to cancell earth's magnetic field.

[bernte_one]

maybe the advanced version of hemmerhead pi could give you an answer (crossbow classic pi)

[Qiaozhi]

can anyone tell me what does the ground control circuit look like on a pi detector....i wondered if it was an adjustable second delay sample as used to cancell earth's magnetic field.

Please see attached extract from Inside the METAL DETECTOR - Published September 2012

[kt315]

my request to Eric is still in issue. make a book, Eric. no time more will be for that, chance is not given two times, when you would want it will be absent.

[daverave]

Please see attached extract from Inside the METAL DETECTOR - Published September 2012

thanks for the diagram...can you tell me if this second sample is separate to the earths magnetic sample ??? are there in fact 3 different samples ???

[daverave]

my request to Eric is still in issue. make a book, Eric. no time more will be for that, chance is not given two times, when you would want it will be absent.

hi kt315
is eric still involved in pi detector production or retired ??

[metal mania]

Please see attached extract from Inside the METAL DETECTOR - Published September 2012

very good book and great plug buy it guys Ive learnt a lot from it cant wait till the next one

[Qiaozhi]

thanks for the diagram...can you tell me if this second sample is separate to the earths magnetic sample ??? are there in fact 3 different samples ???

This is correct for a standard unipolar transmitter. The Earth field will need to be eliminated using a third sample, which increases the complexity even further. However, using a bipolar transmitter automatically takes care of the Earth field problem, and the third sample is not required.

[daverave]

This is correct for a standard unipolar transmitter. The Earth field will need to be eliminated using a third sample, which increases the complexity even further. However, using a bipolar transmitter automatically takes care of the Earth field problem, and the third sample is not required.

bipolar is new to me...i guess this is 2 mosfet drive to the coil....can somebody just use 3 samples ???

[Qiaozhi]

bipolar is new to me...i guess this is 2 mosfet drive to the coil....can somebody just use 3 samples ???

With S1 = main sample, S2 = GB sample, and S3 = EF sample, then:

Without ground balance the equation is simply x = A1(S1 - S3), where A1 is the gain of the main channel.

With ground balance the equation becomes x = A1(S1 - S3) - A2(S2 - S3), where A2 is the gain of the GB channel, and A2 > A1.

The result is that you have to perform 3 subtractions instead of 1, and adjusting the gain A2 allows you to achieve ground balance. Note, since the GB sample is amplified to match the amplitude of the main sample, it will contain more of the Earth field signal, and hence why the EF sample also needs the same amplification. In addition, any change made by the user to the main sample delay (or sample width) will require the detector to be rebalanced. One final gotcha, is that x will be positive on side of the ground balance point and negative on the other, which needs to be taken account of when processing the target signal.

An alternative method to amplifying the result of subtracting S3 from S2, is to widen the GB and EF sample pulse widths to increase the gain. However, the variation of gain with sample pulse width is non-linear, making this method even more difficult to implement.

Using a bipolar transmitter is much easier and provides better results.

[daverave]

With S1 = main sample, S2 = GB sample, and S3 = EF sample, then:

Without ground balance the equation is simply x = A1(S1 - S3), where A1 is the gain of the main channel.

With ground balance the equation becomes x = A1(S1 - S3) - A2(S2 - S3), where A2 is the gain of the GB channel, and A2 > A1.

The result is that you have to perform 3 subtractions instead of 1, and adjusting the gain A2 allows you to achieve ground balance. Note, since the GB sample is amplified to match the amplitude of the main sample, it will contain more of the Earth field signal, and hence why the EF sample also needs the same amplification. In addition, any change made by the user to the main sample delay (or sample width) will require the detector to be rebalanced. One final gotcha, is that x will be positive on side of the ground balance point and negative on the other, which needs to be taken account of when processing the target signal.

An alternative method to amplifying the result of subtracting S3 from S2, is to widen the GB and EF sample pulse widths to increase the gain. However, the variation of gain with sample pulse width is non-linear, making this method even more difficult to implement.

Using a bipolar transmitter is much easier and provides better results.

can you show me what a bipolar transmitter looks like

[Qiaozhi]

can you show me what a bipolar transmitter looks like

A typical implementation is an H-bridge:
http://en.wikipedia.org/wiki/H_bridge

[Silver Dollar]

Have a look here; http://www.geotech1.com/forums/showt...ghlight=fisher

The Fisher Impulse used Bipolar pulsing.

[Waikiki_Sweep]

can you show me what a bipolar transmitter looks like

Also you can replace one side of the bridge with two electrolytic capacitors.
Take a look how Fisher CZ Quicksilver TX build. It transmits square wave 5 kHz.

[mickstv]

Also you can replace one side of the bridge with two electrolytic capacitors.
Take a look how Fisher CZ Quicksilver TX build. It transmits square wave 5 kHz.

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The problem is it isn't PI the Fisher CZ is a VLF.