PI simulation spreadsheet updated

[Anonymous]

I have updated the PI simulation spreadsheet to use engineering units (volts, micro seconds, micro Henrys).
It displays two target responses and each target has two time constants to simulate skin effect.
No ferrous targets yet.
It can be downloaded from the link below.
Robert

 

[Anonymous]

I had a slight error in the spreadsheet. I forgot to differentiate the target current to get the received voltage. So I was plotting target current. This makes the results completely wrong during the on time. I have updated the spreadsheet to correct this.
Robert

 

[Anonymous]

I have added another parameter to the spreadsheet to simulate the response of a ferrous target during the on-time.
This Excel spreadsheet can be downloaded from the site listed below which also has a description of the spreadsheet.
Robert

 

[Anonymous]

Any suggestions on how to add a realistic ground signal to the simulation?
Robert

 

[Anonymous]

How about a permeability signal about a hundred times as strong as the target signal? (Oh, yeah, I forgot, this is PI. Make it ten thousand times as strong.)
For graphing purposes, Robert, equal in strength would suffice.

 

[Anonymous]

Dave
I realize that the on-time ground signal can be 100's or 1000's of times stronger than the target signal, and I have one of Eric's log plots of an off-time ground signal, but I have no information about the relationship between the on-time and off-time ground signals.
The strength of the on-time signal is a good reason for not putting it in a simulation of a single pulse because the ground swamps out everything else. To do any practical discrimination of targets in the ground from on-time signals would require some motion filtering, and that is a completely different simulation. But it would be nice to be able to show the effect that the off-time ground signal has on the total off-time received signal (target + ground). I am under the impression that the off-time ground signal is not as strong as the on-time.
Robert

 

[Anonymous]

Hi Robert,
Do you mean the ON time ground signal as seen on my coaxial coil and PPD1 setup? The unbalance caused by the material's permiability is many times greater than the viscosity signal in the OFF time. This is one of the problems with this unit; it is very sensitive to ground permiability. It is also the reason I used an active system to balance the coil. Once having located a target on the straight channel, you could place the coil on the ground just to one side, and the electronics would re-balance the coil back to zero. You could then scrub the coil over the target and get a more reliable reading.
Eric.

 

[Anonymous]

Eric
I meant the on-time ground signal from any balanced coil on any detector will be much greater than the signal from deep targets. To get good information about the target, the ground signal will have to be subtracted off either by doing a DC balance nearby, or by continuously tracking the ground signals with a slow low pass filter, or by filtering with a faster low pass/band pass filter.
It was my understanding that the viscosity signal was much weaker than the on-time ground signal, but never having seen a live PI, I do not have a very good feel for the numbers.
Robert

 

[Anonymous]

Robert:
As high as 10% viscosity/permeability is uncommon but not rare. A target should be detectable in the presence of a permeability signal at least several hundred times as great as the target signal if VLF, much higher in PI-- except when everything gets messed up by the viscosity signal, which mimics metal.
--Dave J.

 

[Anonymous]

Before anyone beats me to it, yes I know that figure 8 coils cancel a lot of the ground signal and DD's cancel more than coaxial. I wish that I had thought of that before pushing the post button.
Robert