Vallon VMH3CS Mine Detector

[KingJL]

Eric,
On the torroid side of the board, there are some black cylindrical components... 6 at the top right just to the left and down from the yellow and orange wires and 4 more in the middle of the four MOSFETs. I believe these are diodes in a DO213 package... I have only seen this package used in schottky diodes. Can you discern any identifying markings? I believe the 8 MOSFETS comprise a complex bridge arrangement (still working on it). Is there a damping resistor in the distribution box where the yellow and orange wires are connected to the coax or is it somewhere on the pictured board. I am trying to deduce whether the complex bridge arrangement is leaving the coil disconnected from the damp for a portion of the flyback.

[Ferric Toes]

Eric,
On the torroid side of the board, there are some black cylindrical components... 6 at the top right just to the left and down from the yellow and orange wires and 4 more in the middle of the four MOSFETs. I believe these are diodes in a DO213 package... I have only seen this package used in schottky diodes. Can you discern any identifying markings? I believe the 8 MOSFETS comprise a complex bridge arrangement (still working on it). Is there a damping resistor in the distribution box where the yellow and orange wires are connected to the coax or is it somewhere on the pictured board. I am trying to deduce whether the complex bridge arrangement is leaving the coil disconnected from the damp for a portion of the flyback.

Hi Jim,
They are diodes, and after a hard job scraping off a bit of the conformal coating to connect to a test meter, the one to the left of the Mosfet at the top right of the board has a forward voltage of 0.488V. There is another size similar diode to the left of the blue and red wires and this has the same forward voltage. It is reverse connected across the supply wires and may be part of a reverse battery protection arrangement, the other part of which may be on the distribution board. If you put the batteries in wrong they do not connect anyway. The only marking I can see on the ones associated with the Mosfets is the figure 7. On the one by the supply wires it has 10 as well as a 7. I do not know what these numbers relate to. Also .488 Vf seems a bit high for a Schottky. There are 14 of the no.7 diodes on the underside of the Mosfet board, 12 of which seem to be associated with the toroidal transformers. Also a whole heap of other diodes.

No sign of any damping resistor anywhere. A testmeter across the yellow and orange wires reads 1.2Meg. The coil seems to have a floating ground as there is no connection to the yellow/green wire adjacent to the supply wires. This Y/G wire connects to the negative blue wire and the shielded box which houses the electronics.

Eric.

[Qiaozhi]

I had assumed that the HV kick was from capturing the HV flyback, but the TX voltage waveforms seem to refute this... capturing the HV flyback reduces the voltage significantly from the 250v that is indicated by the TX coil waveforms. Also capturing the flyback of 1 amp coli current will only kick start to about 750 ma. I suspect there is an addition HV being generated to kickstart at least 1 MOSFET in each TX cycle to 1A with the other MOSFET working off LV with total resistance set to limit current to 1A.

I think we're both arriving at the same conclusion. The Vallon does not appear to be attempting to recover energy from one flyback to drive the other, otherwise the flyback voltage would be greatly reduced. Also, the waveforms do not look consistent with the energy recovery process. Perhaps there's two poly-phase boost converters onboard, one for each polarity. Hence the 4 toroidal transformers. Using a higher voltage to stimulate the coil allows the energy to build up in a much shorter time period. If they were using an induction-balanced coil, I would hazard a guess that sampling was being done during TX turn-on, since it would allow for earlier sampling to occur. However, Eric says that this is not the case, as the coil is a mono. Therefore sampling must be occurring during TX-off, which is contrary to the use of a 1.5mH inductance coil.

I am trying to deduce whether the complex bridge arrangement is leaving the coil disconnected from the damp for a portion of the flyback.

Since the Vallon is designed to detect minimum metal mines, and hence low conductivity targets, the only possible way to sample early enough is to start the flyback with the damping resistor disconnected. If it's then connected at the point where flyback is at maximum (or at least close to it) the decay time can be massively reduced, allowing earlier sampling.

The coil seems to have a floating ground as there is no connection to the yellow/green wire adjacent to the supply wires.

As the Vallon uses bipolar pulsing, I would expect that to be the case.
Interestingly, the bipolar pulses are grouped relatively close together, followed by a longer period. This is either to allow for some complex signal processing to happen and/or sampling only occurs after the positive pulse. The negative pulse would then only be there to ensure the ground matrix and any targets do not become magnetized in the process. If it's the latter, any Earth field elimination would require a later sample as per a mono-polar transmitter.
Personally I imagine it's to allow sufficient time to do all the calculations, with the EF being eliminated by bipolar sampling.

Just a few thoughts.

[6666]

Found this in another thread
http://www.geotech1.com/forums/attac...7&d=1375976277

[Ferric Toes]

Found this in another thread
http://www.geotech1.com/forums/attac...7&d=1375976277

The VMH2 was an earlier detector introduced in 1999. Go to page 33. https://www.yumpu.com/en/document/view/10475611/vallon-vmh21-humanitarian-demining


Eric.

[Ferric Toes]

I think we're both arriving at the same conclusion. The Vallon does not appear to be attempting to recover energy from one flyback to drive the other, otherwise the flyback voltage would be greatly reduced. Also, the waveforms do not look consistent with the energy recovery process. Perhaps there's two poly-phase boost converters onboard, one for each polarity. Hence the 4 toroidal transformers. Using a higher voltage to stimulate the coil allows the energy to build up in a much shorter time period. If they were using an induction-balanced coil, I would hazard a guess that sampling was being done during TX turn-on, since it would allow for earlier sampling to occur. However, Eric says that this is not the case, as the coil is a mono. Therefore sampling must be occurring during TX-off, which is contrary to the use of a 1.5mH inductance coil.


Since the Vallon is designed to detect minimum metal mines, and hence low conductivity targets, the only possible way to sample early enough is to start the flyback with the damping resistor disconnected. If it's then connected at the point where flyback is at maximum (or at least close to it) the decay time can be massively reduced, allowing earlier sampling.

As the Vallon uses bipolar pulsing, I would expect that to be the case.
Interestingly, the bipolar pulses are grouped relatively close together, followed by a longer period. This is either to allow for some complex signal processing to happen and/or sampling only occurs after the positive pulse. The negative pulse would then only be there to ensure the ground matrix and any targets do not become magnetized in the process. If it's the latter, any Earth field elimination would require a later sample as per a mono-polar transmitter.
Personally I imagine it's to allow sufficient time to do all the calculations, with the EF being eliminated by bipolar sampling.

Just a few thoughts.

Hi George,
Here are some measurements done early this morning.

Without a coil plugged in, the +'ve and -'ve rectangular pulses have amplitudes of +9 and -9 volts. This drops to + and - 6.5V with the coil plugged in.

With coil, the high voltage spike peaks at 260V at the leading edge and 280V at the falling edge. The polarity reverses depending on the + or - of the pulse. The Mosfets are 400V types, so the HV is well below avalanche level.

All pulses are equi-distant and not grouped in pairs. From start to start of +'ve pulses is 1020uS and the same for -'ve pulses. Start of +'ve pulse to start of -'ve pulse is 510uS. Overall pulse rate is 1952 pps although this can be varied to prevent interference between detectors.

The main purpose of bi-polar pulses is to reduce to net field to zero so that there is no possibility of triggering magnetically detonated mines. The military and demining organisations will not accept mono-polar pulse detectors, as most hobby PI's are, for demining.

Eric.

[TH'r]

Vallon VMH3CS

Hello Eric; I an a relic hunter in the USA. I recently received one of those British surplus VMH3CS's from E-b. It is in good shape, tactile switch covers are good, has some scuff marks, arm rest is OK. The coil/lower shaft look new, not a scratch on them. Not good weather to get outside, so gave it an air test, on normal with sensitivity maxed. Got a little EMI here inside, but would detect a US nickel at 13 1/2 inches. I don't have any nuggets to test and used my wife's very thin and small baby ring. It is 10K and about the size of a pencil, won't read on my digital scales which start at one gram. It was detected at eight inches.

Hi All,


I have just obtained a Vallon VMH3CS mine detector and am surprised at how good the performance is. I've not done any detailed tests as yet, but it will detect coins at great distances in air and a 0.3gm gold nugget at 2-3 inches. This is without ground compensation. With compensation switched in (mineralisation on switch) it will cancel a substantial piece of Australian ironstone. The machine is fantastically rugged and runs off three D size batteries, so it feels a bit heavy, but is reasonably well balanced. The search coil measures 1.5mH and is 3 ohms resistance. TX pulses are bipolar and 50uS wide. It is not immediately apparent how such narrow pulses are effective using a coil of 1.5mH.

I have been off the forum for quite a long time and would be interested to know if anyone else has experience with this detector. I shall take it to the local beach next week and see how it performs there.

Eric

[Ferric Toes]

Hello Eric; I an a relic hunter in the USA. I recently received one of those British surplus VMH3CS's from E-b. It is in good shape, tactile switch covers are good, has some scuff marks, arm rest is OK. The coil/lower shaft look new, not a scratch on them. Not good weather to get outside, so gave it an air test, on normal with sensitivity maxed. Got a little EMI here inside, but would detect a US nickel at 13 1/2 inches. I don't have any nuggets to test and used my wife's very thin and small baby ring. It is 10K and about the size of a pencil, won't read on my digital scales which start at one gram. It was detected at eight inches.

Hi TH'r,
Good to hear that you have acquired a VMH3CS and it is in good shape. The detection ranges indicate that all is working fine and that you should do well with it as a relic hunting detector. What instructions did you get with it? It is usually just the field card which is quite basic. In addition to the LED display and the internal piezo speaker, there is a vibration device that you can feel through the handle. This can be disabled as it is not necessary for TH work and increases current consumption. Press C and + and hold down while switching the control knob to ON (Normal). Release C and + after 1 second.

Did a piezo headphone come with it? I was out searching farmland a week ago and found that headphones were necessary as the internal speaker has limited volume. I used Gray Ghost piezo phones as low impedance phones do not work. The audio volume can be changed by setting the control switch to the speaker symbol and pressing + or - , to change the volume level. Another useful point I learned that day was that I got better results by searching in the 'mineralised' mode in that I was detecting significantly less small ferrous items; the search being for mainly for coins.

Let us know how you get on when you take it into the field.

Eric.

[TH'r]

Hello Eric. Yes, I got the head phone, also a seldom used backpack, the test piece, and a 6" x 8" laminated (very basic) field instruction card. Thank you for the headphone advice. The speaker volume is low and I can see a problem when walking through stubble or leavers. I have a set of the White's yellow surf phones, and think I understand the plug pin-out and your comments on it. After my VHM3CS goes through its self check out, LED #13 will continue to blink followed by a quick chirp in normal mode, in mineral mode it blinks and chirps twice. I think this is an "operator confidence signal" alerting the detectorist that the machine is on and in which mode. I would like to turn it off if possible. If you have access to such information, would you tell me which combinations of button pushing does what? I will be glad to PM you my E-mail address if you like. Thanks again.

Hi TH'r,
Good to hear that you have acquired a VMH3CS and it is in good shape. The detection ranges indicate that all is working fine and that you should do well with it as a relic hunting detector. What instructions did you get with it? It is usually just the field card which is quite basic. In addition to the LED display and the internal piezo speaker, there is a vibration device that you can feel through the handle. This can be disabled as it is not necessary for TH work and increases current consumption. Press C and + and hold down while switching the control knob to ON (Normal). Release C and + after 1 second.

Did a piezo headphone come with it? I was out searching farmland a week ago and found that headphones were necessary as the internal speaker has limited volume. I used Gray Ghost piezo phones as low impedance phones do not work. The audio volume can be changed by setting the control switch to the speaker symbol and pressing + or - , to change the volume level. Another useful point I learned that day was that I got better results by searching in the 'mineralised' mode in that I was detecting significantly less small ferrous items; the search being for mainly for coins.

Let us know how you get on when you take it into the field.

Eric.

[Ferric Toes]

Hello Eric. Yes, I got the head phone, also a seldom used backpack, the test piece, and a 6" x 8" laminated (very basic) field instruction card. Thank you for the headphone advice. The speaker volume is low and I can see a problem when walking through stubble or leavers. I have a set of the White's yellow surf phones, and think I understand the plug pin-out and your comments on it. After my VHM3CS goes through its self check out, LED #13 will continue to blink followed by a quick chirp in normal mode, in mineral mode it blinks and chirps twice. I think this is an "operator confidence signal" alerting the detectorist that the machine is on and in which mode. I would like to turn it off if possible. If you have access to such information, would you tell me which combinations of button pushing does what? I will be glad to PM you my E-mail address if you like. Thanks again.

Hi TH'r,
Yes, The chirp is a confidence signal and I know of no way that it can be turned off. This is essential for mine clearance as there is no background threshold tone to keep you informed that the detector is working.

I have a set of yellow surf phones, but they were too loud in the default audio setting so I made an inline attenuator which incorporated a capacitor to act as a high pass filter. This removed a rather rough 'edge' to the sound and a bit of processor sound that you do not hear normally. There is a plastic bodied six pin plug available in the market that matches the connector on the Vallon and is relatively cheap. You certainly don't want to be buying a MIL spec one. I did not need an attenuator with the Gray Ghost surf phones. You can of course turn the sound down on the detector, but the new value is not stored when you switch off. Please PM me.

Eric.

[tifred]

Hello Eric
Can you give us the reference or the address for the helmet connector for the Vallon?
Thank you

[Qiaozhi]

Summary of Vallon VMH3CS Characteristics

Having re-read all posts in this thread, I have noted the important details below:

1. Earliest sampling time appears to be 25us. Eric deduced this by connecting the Vallon coil to a standard PI preamp.
2. Drive voltage = 6.5V.
3. Pulse width = 50us.
4. Pulse current = 1A (rises to full value in 10us, and flat for 40us).
5. Current drain is 340mA average.
6. Operating time = 30 hours.
7. Power source = 3x D-type batteries.
8. Coil = 1.5mH, 3 ohm DC resistance, f0 = 156kHz, C = 694pF (including connecting cable).
9. Rd = 735 ohms (optimum value for critical damping from calculation).
10. Coil wire is 32 AWG enamelled copper.
11. Pulse period = 1020us, 510us between +ve and -ve pulses.

The detector must be using high voltage with current limiting to get the coil current to 1A in 10us, and then remain flat-topped for a further 40us.
Combining a high-resolution ADC with a DSP may conceivably allow sampling further up the decay curve, but who knows?

Consider this ->

A 0.3gm gold nugget with a TC of 3.3us. Therefore 5*TC = 16.5us.
The target signal will have decayed to 0.67% after 5 TCs, and 0.05% at 25us. This is a miniscule amount of target signal to extract from the noise, even for some clever DSP algorithms. The puzzling thing about all this is why anyone would choose to use a 1.5mH coil for minimum-metal mine detection?

To try and discover the answer I ran two simulations side-by-side. One with a 300uH coil and the other with a 1.5mH coil, and compared the results. With reference to the summary of the Vallon characteristics, we can calculate that the 1.5mH will contain 750uJ of energy when charged to 1A. To put the same amount of energy into a 300uH coil would require 2.24A.

Rather than complicate matters by using a high-voltage with current limiting, I simply inserted an appropriate series resistor in each current path and made the pulse width long enough that the coil current would flat-top at the required value. Then it was possible to make a valid comparison and determine which solution yields the best result. The analysis was also performed using a mono-polar configuration, as the design is symmetrical and the results will be the same for both polarities.

From the simulation plot results (for target with 3.3us TC) it is abundantly clear that the 300uH coil wins hands down when detecting a 3.3us target. The simulation includes a parametric sweep of the mutual inductance between the coils and their targets in order to compare the target response with the no-target signal at the preamp output. V(out1) is the 300uH coil response, and there is a large change in the decay curve. However, for V(out2), which is the 1.5mH coil response, the change is very small in comparison.

Perhaps Vallon's decision to use a 1.5mH coil is related to the elimination of ground response by sampling later, and using the power of the DSP algorithms to compensate for having a much weaker target signal; plus using a high-voltage source with current limiting to overcome the high inductance at switch-on, and thus allowing a higher pulse rate.

In conclusion, I really cannot see any advantage to using a high inductance coil for this application. The simulation results demonstrate beyond doubt that a 300uH coil would be a much better choice. Previously I considered whether the damping resistor was being switched in after the flyback had reached maximum, but examining the scope images show no evidence of this happening. It's almost as if Vallon went out of their way to make things difficult for themselves by deciding to use an inappropriately high inductance coil wound with enamelled copper wire, and then having to circumvent these difficulties with an overly complex design.

One final point ... nowhere in the Vallon brochure does it even mention minimum-metal mines. So this detector may not even be intended for that purpose anyway, and that's why it performs well as a coin detector (see second simulation with target TC of 10us).

Target with 3.3us TC:


Target with 10us TC:

[Qiaozhi]

Perhaps I should add a few comments about the simulation results ->

For the 300uH coil:
V(tx1) = TX voltage
I(L1) = coil current
I(L2) = eddy current response in target
V(out1) = preamp output

tx1, L3, L4 and out2 are for the 1.5mH coil.

TX-on delays are set such that TX-off for both coils occur at the same point in time.

Maybe the designers at Vallon were simply too DSP-obsessed when working on this detector.
Either that we're missing the bleedin' obvious.

[Ferric Toes]

Perhaps I should add a few comments about the simulation results ->

For the 300uH coil:
V(tx1) = TX voltage
I(L1) = coil current
I(L2) = eddy current response in target
V(out1) = preamp output

tx1, L3, L4 and out2 are for the 1.5mH coil.

TX-on delays are set such that TX-off for both coils occur at the same point in time.

Maybe the designers at Vallon were simply too DSP-obsessed when working on this detector.
Either that we're missing the bleedin' obvious.

Great work George.
We may know more when I look at the RX amplified waveform sometime soon. I'm am endeavouring to connect up one set of boards on a baseplate so that I can poke around with a scope. I think I have identified the preamp IC and, with difficulty, see what is happening on pin 6. Next thing is to find the drive pulse for the correct Mosfet and trigger the scope from that TX-off edge.
One obvious thing that I missed is the apparent absence of a reservoir capacitor for the TX stages. There is one 1000uF 16V cap on the distribution board, but that is not where you would expect it to be. It seems to be across the battery input.

Eric.

[Ferric Toes]

Hello Eric
Can you give us the reference or the address for the helmet connector for the Vallon?
Thank you

The connector is made by Souriau and described as UTS Hi Seal. The part number is UTS6JC10E6. It is a good connector, fully waterproof, easy to assemble, solder cup terminals, and made in your home country . As it is a plastic shell and the bulkhead connector on the Vallon is metal, I put a light smear of silicon grease on the latter, mainly the bayonet pins, so that it mates up smoothly without any stress on the plastic.

Eric.