Questions: Probably been answered previously, but here goes..

[Anonymous]

Coil Output voltage on PI unit is measured in
milliwatts ? y/n
How many ? average
What effect does duration and interval of pulse have on the performance?
Ever tried varying pulse frequency, i.e. high,low,
interval, etc.? Thanks-- there's more after these
Richard

 

[Anonymous]

Hi Richard,
I'll try to answer your questions using a bit of phisics...(hope I remember all the stuff <IMG SRC="/forums/images/smile.gif" BORDER=0 ALT="">
all the differences of electric potential are ever misured in Volt (dimensionally...in phisic terms : l^2*m*t^-3*i^-1).
So if you think about a 'voltage' (dop) u'll use the volt (derivate) unit to describe its value.
But if you mean the power u'll use the watt (l^2*m*t^-3) or the BTU (british thermal unit) equivalent and their multiples (or also the old HP- horse power, that is 750 watt or so <IMG SRC="/forums/images/smile.gif" BORDER=0 ALT="">.
Another issues are the magnetic field force and the magnetic induction and the magnetic moment:
- the first is the oersted (Oe) , generally referred as H,scalar, dimensionally as i*l^-1 or in the SI measured as A/m
- the second is the vectorial induction (B) , dim. as m*t^2*i^-1 , in the SI measured in Tesla and Gauss
- the 3rd is the mag. moment , dim. i*l^2, vectorial, measured in A*m^2
From a PI point of view what's really matter are the last three params and the voltage spike is just a result of the mutual induction between the coil and the target mag. paths.
This can change A LOT from machine to machine, depending on the tx switch and supply and even more from the coil shape, size, ect.
For more informations take a look at the Corbyn's article (part one) on the geotechnology page.
For the time/pulse width ...generally speaking... if you use higher frequencies (fpr) e.g. some KHz u'll find (generally) a faster response on smaller targets...but it's just becouse of the integrator circuitry that is used in almost all PI design. (see the Eric post about for more details).
The pulse rate is critical becouse (see Corbyn's article again) u'll focus on small and well defined 'time constants' for the smaller targets and if your pi doesn't allow a right (small) delay, flyback drop it'll never see any small obj.
Hope this can help you,
Massimo

 

[Anonymous]

Hi Richard,
I will try to give you a very basic answer to your questions.
First, normally the battery voltage or possibly a regulated output from the battery is applied to the coil for a short duration (maybe 50 usec to 250 usec for typical PI's). The actual current could be limited just by the coil windings or have a resistor in series with the windings. It is the current that produces the coil field.
At the end of the pulse duration the current is shut off very quickly which causes the field in the coil to collapse. This is quite similar to what happens on an auto ignition coil.
The result of the sudden shut off of the current is there will be a high voltage spike generated that is normally limited by the FET transistor breakdown voltage or there could be zener type diodes in place to limit the voltage. Such voltage produced could be from 60V to 400 V or so and is dependent upon the limiting technique or FET used. This high voltage spike will decay very quickly in a few microseconds if properly damped.
Damping is done by placing the right value resistor across the coil so the voltage reduces as quickly as possible without any oscillation.
As a rule of thumb, one can measure the current in the coil during the pulse on cycle and the number of pulses per second to figure the pulse power used.
The actual number of pulses per second can vary widely also, from a few hundred per second to a few thousand. On some detectors, this is variable to minimize external noise problems. On other detectors the pulse width is increased as the delay is increased.
Generally, increasing the pulse rate allows for a faster response from a target plus increases the signal to noise ratio. This basically means one can move the coil a little faster over the ground and not be as likely to miss a target as well as have a clearer target signal.
As a general rule of thumb, increasing the current during the pulse on time will increase the depth capabilities if all other factors are equal.
Depth of detection of a buried target will be governed by the size of the coil, the size of the target as well as the energy into the coil during the pulse on time and the ground conditions.
The sensitivity to small low conductive targets will also be governed by the delay at which the signal sample is taken. The shorter the delay, the more sensitive to small low conductive objects such as gold or lead.
I hope this helps.
Reg

 

[Anonymous]

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