Please explain

[Unregistered]

I didnt know if should post here or on Hammerhead anyway.
Can someone experience tell me h ow the second sampling work in Hammerhead or any PI.
How it cancel Interference in simple language.
Also explain how ground cancil work.. i read that u take one channel gold and other channel ground and u subtract it .. how does subtract work .. can someone show circuit and explain in simple language.
thanking u
george

[Unregistered]

74 people look my message but they connot explain ??

[Molzar]

George,

This is about as simple as I can describe the first/second sampling process.

When there is a metal object in the Coil (Electrical) Field the object becomes “charged” and creates it’s own Object (Electrical) Field. When you turn the “Coil Field” off (as quickly as possible) the “Object Field” remains present for some period of time after the “Coil Field” has disappeared. The object field will also disappear (decay) over time.

The first sample is taken right after the “Coil Field” has disappeared and the “Object Field” is at it’s strongest.
The second sample is taken later after the “Object Field” has become less or has disappeared.

Therefore, if there is a metal object in the “Coil Field” the first sample registers some value (voltage) from the “Object Field”. Then when the second sample is taken (at a later time) the “Object Field” has either diminished or disappeared. If you subtract the first sample from the second sample this will give you some value other than zero, and this is what is amplified and heard as a target on a PI detector.

If there were no metal object in the “Coil Field” both the first and second sample values would be zero (or the same value). If you subtract the first sample from the second sample this will give you zero and no response would be heard.

If you want to learn, you should read all the materials on this site. Also, if you want people to answer you should be patient and you should register, this gives you credibility in the forum.

Molzar

[Max]

I didnt know if should post here or on Hammerhead anyway.
Can someone experience tell me h ow the second sampling work in Hammerhead or any PI.
How it cancel Interference in simple language.
Also explain how ground cancil work.. i read that u take one channel gold and other channel ground and u subtract it .. how does subtract work .. can someone show circuit and explain in simple language.
thanking u
george

Hi,
maybe you didn't ask right people...

Many here knows about that.... but few read posts from unregistered users... anyway...

to make it easy: you have a differential integrator there and the "second pulse" is related to that part of circuit.

In older PI , without diff. integrator, you have just one sample pulse and not 2 as you have in hammerhead... cause hammerhead has diff. integrator made with op. amps. stuff.

Now... to make it more easy... the diff integrator thing is there cause you have to eliminate (ideally) or at least reduce effect of noise.

Noise is randomic component that can cause spot variations on just a single integration over a single sample at a time... that was a big problem with older PIs cause the behave erratic when spot noise happens there...

So why second sample ? SIMPLE... cause you take one just at few us after switchoff... to detect variations on evenlope due to target... and the second just after say 150-200us where you don't expect any target contribute to the signal.... and diff integration will actually subtract just noise components at each cycle...

So...if you diff. integrate signal at each cycle and use large time constants e.g. over 20 cycles...you'll get your thing more stable cause spot istantaneous noise components tend to average to zero (cause it's randomic stuff... there's a bit of statistic in that discussion about noise... but nevermind), where , instead, your useful signal doesn't average to zero but give consistent increase trend on signal when you pass coil over target.

So just add a bit of inertia and a good AC coupling and you'll get rock stable thing that way... but I think this is covered by HH documentation , isn't it ?

Now... to make even more simple stuff... you can also double/diff integrate using a sequence of e.g. alternate samples... BUT if you get 2 samples at each cycle of your PI you keep work going on... about another interesting thing... uhm... the "ferrictoes effect" !

I mean that someone used that approach to implement a rough kind of disc on his PI designs... you know... refinite people NEED THAT STUFF... expecially in australian goldfields with lot of red stuff.... you can use the later sampled pulse (that can also be the 3rd... etc) to get informations on e.g. iron related effects in detection: problem is that it's not reliable as VLF disc... unfortunately... but maybe better if you ask someone else about this.

Anyway... just my 5cents!

Kind regards,
Max