Vallon VMH3CS Mine Detector

[KingJL]

... I dont understand it all...but learning!!

Some of it is not readily intuitive. I had to observe it to understand it. Work up some simulations based on the schematics and you can observe some of the details, One area that I didn't readily understand "why" was the pulse transformer MOSFET drive circuit. Traditional pulse transformer drive circuits involve significant drive current for the duration of the MOSFET drive signal (in fact for relatively long MOSFET drive signals, it becomes difficult to achieve sufficient drive throughout the duration). With the circuit employed by Vallon the pulse transformer drive only flows current for 1 usec at the beginning of the MOSFET drive signal and 1 usec at the trailing edge of the MOSFET drive signal regardless of MOSFET drive duration... SWEET! There are nuances of elegance in the pre-amp and integrators also.

[Mdtoday]

Certainly a very nice bit of engineering.

What ADC is being used KingJL?

cheers

Mdtoday

[KingJL]

What ADC is being used...

That was another surprise... INT1 and INT2 each feed a separate ADS1254 (INT3 is not digitized). The ADS1254 is a 24 bit ADC. Each ADS1254 is being clocked at 93.8 kHz (no that is not a misstatement) which results in a free running sample rate of just 244 Hz, which also results in the internal LP sinc filter 3 db point being at 49.654 Hz. Our units do not support the different filter settings available in some Vallons. I suspect that part of the key to the filter settings function, is a change in the clocking rate (and thus the sample rate) of the ADC's which would change the digital filtering parameters proportionally. But I do not know that for sure.

This is another example of the most appropriate technology being applied to the task at hand... Integration in the analog domain to allow digitizing a much lower clock speeds... clock speed means power dissipation... lots of it. Allowing ADC clocking a a much lower frequency means less power consumed (one of the military goals for a field mine detector). Also it decouples the time domains of the digital and analog processing. This allows the DSP processor to have reduced clocking needs. The DSP does not have to calculate/generate all of the timing signals... that task is relegated to an FPGA.

[Mdtoday]

I suspect that part of the key to the filter settings function, is a change in the clocking rate (and thus the sample rate) of the ADC's which would change the digital filtering parameters proportionally. But I do not know that for sure.

This is another example of the most appropriate technology being applied to the task at hand....

Thanks for the info, yes the ADS1254 is a good chip, I have used it in load cell and another ADS1282 in sensor (geophysics) designs, very capable.

It is always great to see product where, overall, you can appreciate the effort that the engineering team have put into it.

I'm looking forward to some more surprises with the Vallon, thanks for all your efforts and sharing.

cheers

Mdtoday

[Rocketdemon]

Bottom rest for Vallon VMH3CS

As Eric did I have added a bottom rest, sourced from a well known American device.

Drilled 4 holes and attached with cable ties.

[Carolina]

Filters

That was another surprise... INT1 and INT2 each feed a separate ADS1254 (INT3 is not digitized). The ADS1254 is a 24 bit ADC. Each ADS1254 is being clocked at 93.8 kHz (no that is not a misstatement) which results in a free running sample rate of just 244 Hz, which also results in the internal LP sinc filter 3 db point being at 49.654 Hz. Our units do not support the different filter settings available in some Vallons. I suspect that part of the key to the filter settings function, is a change in the clocking rate (and thus the sample rate) of the ADC's which would change the digital filtering parameters proportionally. But I do not know that for sure.

This is another example of the most appropriate technology being applied to the task at hand... Integration in the analog domain to allow digitizing a much lower clock speeds... clock speed means power dissipation... lots of it. Allowing ADC clocking a a much lower frequency means less power consumed (one of the military goals for a field mine detector). Also it decouples the time domains of the digital and analog processing. This allows the DSP processor to have reduced clocking needs. The DSP does not have to calculate/generate all of the timing signals... that task is relegated to an FPGA.

King JL : My Vallon offers six filters. Can you give me some instances that each might be used for? They are as follows

1: No filter, same as your VALLONS software version
2: 6 Hz Displacement between alarm and object at 1 m/s search speed 10 cm
3: 4 Hz Displacement between alarm and object at 1 m/s search speed 15 cm
4: 2 Hz Displacement between alarm and object at 1 m/s search speed 25 cm
5: 1 Hz Displacement between alarm and object at 1 m/s search speed 30 cm
6: 0.5 Hz Displacement between alarm and object at 1 m/s search speed 40 cm

Thank you
Carolina

[KingJL]

King JL : My Vallon offers six filters. Can you give me some instances that each might be used for? They are as follows

1: No filter, same as your VALLONS software version
2: 6 Hz Displacement between alarm and object at 1 m/s search speed 10 cm
3: 4 Hz Displacement between alarm and object at 1 m/s search speed 15 cm
4: 2 Hz Displacement between alarm and object at 1 m/s search speed 25 cm
5: 1 Hz Displacement between alarm and object at 1 m/s search speed 30 cm
6: 0.5 Hz Displacement between alarm and object at 1 m/s search speed 40 cm

Thank you
Carolina

Carolina,
I do not know. I believe their purpose is to effectively reduce sensitivity to fast signal responses (which is what most electromagnetic interference is). I believe this would reduce the sensitivity for small targets also, and require you to slow down the search speed accordingly. The discussion of the settings and displacement are basically giving the delay from initial detection to the alarm which is equivalent to the group delay of the filter (when swinging at 1 m/s: 10 cm=1/10 second, 15 cm=1/7 second, 25 cm=1/4 second, 30 cm=1/3 second, 40 cm=a little less than 1/2 second). You compensate by slowing down the search speed.

With no filter you are at 49Hz which is the base set by the integrators and the sinc filter of the ADC's. This would tend to make them very susceptible to mains (50-60 Hz) interference. The Filter settings would reduce this susceptibility.

All this is conjecture as I have now means of testing this.

[kingswood]

Thats interesting.
Dunno if any of this helps, but ages ago I found a study that said the VMH3CS has 19% loss of sensitivity at slow speed (0.1 m/s) and 3% loss at 1 m/s. The optimal speed is 0.5-0.6 m/s.

[Ferric Toes]

I have a Vallon with the filter options mentioned by Carolina but have not yet done any real field tests with it. In my opinion, the standard Vallon without the options is a little too fast in that for coins and rings it tunes out too quickly. I would think that settings of 2 or 3 would be better, particularly for the more deeply buried targets. Also, as KingJL states, interference rejection would be better.

Additional coils are available for the Vallon (at huge expense) for UXO searches. These are 30cm and 60cm diameter and the slower response would certainly be better when using these.

I have noticed, that in the 'mineral' setting of the standard Vallon the filtering seems slower. Perhaps KingJL could confirm this.

Eric.

[Carolina]

Thank you all for your quick response. Eric, good to see you back.

[Rocketdemon]

If my Vallon had the filter options installed/available how would they be accessed?

[KingJL]

If my Vallon had the filter options installed/available how would they be accessed?

From the Vallon manual(ver 1_19... ver 1_14 does not address filter):
3.5. Software filter setting
Filters may be activated with different bandwidths to reduce the sensitivity to electromagnetic
interference. The filters are set as follows:
1. Press " C ";
2. Press the keys "+" and "-" the following combination: + - ++ - +++ ---
3. Press " C ";
4. The filter setting is now active. Use the "+" and "-" can be one of 6 filters are selected (see below).
An LED indicates the selected filter.
5. Press " C "; to confirm your selection.
3.5.1. Remove number
1. LED1: no filter
2. LED2: 6 Hz
This is the default, which is automatically activated when turned on. The offset between alarm and object at
a scanning speed of 1 m / s is about 10 cm
3. LED3: 4 Hz
The offset between alarm and object at a scanning speed of 1 m / s is about 15 cm. LED 14
flashes when this filter is activated.
4. LED4: 2 Hz
The offset between alarm and object at a scanning speed of 1 m / s is about 25 cm. LED 13
flashes when this filter is activated.
5. LED5: 1 Hz
The offset between alarm and object at a scanning speed of 1 m / s is about 30 cm. LED 12
flashes when this filter is activated.
6. led6: 0.5 Hz
The offset between alarm and object at a scanning speed of 1 m / s is about 40 cm. LED 11
flashes when this filter is activated.
The filter setting is not saved when you turn off the detector and must be renewed each time the detector is turned on.

[qute1]

Outstanding Kingjl thanks for that, if only we could find the rumoured yet illusive discrimination the vallon supposedly has.

[KingJL]

Outstanding Kingjl thanks for that, if only we could find the rumoured yet illusive discrimination the vallon supposedly has.

I am not aware of any claim for discrimination!

[Altra]

This model claims ferrous/non ferrous ID?

http://www.vallon.de/pdf/VMXC1-1_leaflet_05_2013.pdf