FGM-3 Alignment and Calibration

[damiafix]

Hi guys,
I've bought and built the Carl's Gradiometer, now I've some problem with the alignment and calibration of the twoo FGM-3.
the distance between each sensor, is 1.30 m.
When I push on the power, I turn the tube in the N/S position, and then i rotate it of 360° (slowly):



then this first calibration procedure (of 10 seconds) I place the tube in an E/W orientation, for the FGM-3 alignment; in this position, with the threshold pot to low (I've used a 4.7k pot, here it's very hard to find 5k pots) I hear a low frequency pulse sound (the frequency increases with the rotation of the potentiometer) if I turn of 180° the tube (as the calibration rotation -down-up) I hear a change of the frequency, so when I hear a hight frequency sound, I stop the rotation and adjust the sensor to reduce the frequency, but it's very hard! With any adjustment, I hear ever a low frequency pulse sound (never stop). The Gradiometer work, but now the sensibility is very low.
Someone can help me for a good calibration, and aligment procedure?
Photographs or videos are welcome!

All the best, and many thanks for your help, D.

[damiafix]

Hi guys,
this is the calibration procedure that I use for the alignment of the two FGM-03, look the picture, please:



With a friend I've built a wood support, with an inclination of 57° (I'm in Italy) for the 360° slow rotation of the mag probe around itself ; then I've placed the tube in a E/W orientation, and level (in the picture all the sensors are pointed to N, with the pin contacts oriented to the S). I've rotated and rolled the tube for to hear a highest frequency sound, and in this position, I've tryed to adjust the sensor, for to reduce the frequency. At the end of the procedure, unfortunately, when I rotate the tube, I hear changes of frequency, and when I place the probe vertically to the ground, the VCO play continuously highest frequency, in every case! What is wrong? What do you think about?

All the best, D.

[olsteffe]

Hello
You should turn it in the plane of inclination. (head over heel)

Regards
olsteffe

[damiafix]

Hi Olsteffe,
many thanks for your reply! I'm sorry but maybe I don't have understood; I should turn the tube, with the wood support placed at the end of the tube (Picture-1)? Or, turn it as in the Picture-2?

Picture 1:


Picture 2:


In the project of the FMX1 (Carl's Gradiometer) Carl write this:

At this point you could run off and start using the mag, but for optimal performance we need to make one easy mechanical
adjustment. Recall that a single sensor is sensitive to its orientation within the magnetic field. For the two-sensor arrangement, the
mixed-frequency output is sensitive to their misalignment to each other. If they are perfectly aligned, then each sensor will detect
the exact same magnetic field (in the absence of a target) and the gradient will be zero even if the whole tube is rotated. If they are
not aligned, then one sensor could detect a slightly different field, and this will change with the movement and orientation of the
tube. All calibration and alignment procedures should be done well away from undesirable magnetic fields, including iron targets
and electrical (AC) lines.
Before we can align the sensors, we first need to allow the SCL007 to perform a sensor calibration. According the literature,
the calibration is done during the first 10 seconds or so after power-up during which time it expects the sensors to see a minimum
and maximum earth field. The described method is to hold the tube in a north-south orientation pointing the north end upwards at
the angle of the field’s inclination, about 67° to the horizontal in the UK, a little less in the US, and 0° at the equator. Switch on the
power and slowly rotate the tube, head over heels, through 360° during the first 10 seconds. That’s all there is to it; this procedure
should be done every time the mag is turned on for use.
To align the sensors, turn on the unit and go through the sensor calibration described above. Place the tube in an east-west
orientation (and level) in a way that you can rotate or roll the tube. I built a simple wooden “U” stand with “V” notches cut in the
uprights. With the tube in a starting position listen to the output frequency. If there is no output sound or the frequency is less than
10Hz you may need to go ahead and adjust the VCO to get an appropriate frequency. Now slowly rotate the tube by about 180° and
listen for a frequency change. If there is one, it indicates that the sensors are misaligned. I found an effective method is to rotate the
tube to find the highest output frequency; with the tube held at this position adjust the sensor to reduce the frequency. Continue this
until you can rotate the tube with little or no frequency change.

Maybe for my bad english I have not understood the procedure, I'm sorry!


Best Regards, D.

[olsteffe]

Like you show in picture 2, the vertical rotation.

olsteffe

[damiafix]

Hi Olsteffe,
many thanks for your reply! I will try this procedure, and then I'll let you know the result.

Best Regards, D.

[olsteffe]

Hello again D
You could try and use a digital voltmeter directly at the pin 14 and 15 and to ground of the DAC. Then 0 V = perfect alignment.
Then adjust the treshold of the VCO so you get a comfortable put put frequency when there are no anomalies.

Regard olsteffe

[olsteffe]

Hello
I have just made a rude sketch of a suggestion for a sensor mount. It moves in one plane only using a M3 skrew inside a M5 skrew giving differential precision adjustment possibilities. One turn of the blue skrew = 0.4mm movement 1/4 turn equals .1mm and so on. both sensors would have to be mounted in similar mounts but orthogonal to eachother (90 degrees)

[damiafix]

Dear Olsteffe, many thanks for your help, I'll try again with your precious suggestion!

All the best, D.

[olsteffe]

Hello
Just to clarify the direction of the inclination angle. It is pointing as shown in the sketch. A normal misunderstanding is that it is pointet into the earth from the north.

Best regards
olsteffe

[damiafix]

Hi Olsteffe,
many thanks again, for your reply and courtesy! I've some doubts about the probe measures. In my case, the distance between each sensor is 1.3 m (circa) and not 50cm, as you can see in this schematic (Probe from Epe Project):

What do you think about?

Best Regards, D.

[olsteffe]

Sensormount II

Hello
I realised that the ball and socket is to complicated. New design suggestion is given.

Best regards
olsteffe

[olsteffe]

Damiafix
I think your biggest problem would be mechanical stability, if you are not using carbonfiber tubing, the flexing of the tubing would give false readings.
Secondly, what size does the objects have and at what depths are they?

olsteffe

[damiafix]

Hi Olsteffe,
I'm using PVC tubes, please look the follow picture, for the detailed measures:

the structure seem to be stable, never flexions. I've used a electrical tape, between the first and the second external tube (34mm to 32mm).
For the deep and the size of the objects, I would be happy to see, ojects as this:


at least 2 mt.

Do you think that's possible?

All the best, D.

[olsteffe]

I think you are pushing the limits if that is a german steel helmet 6 feet under. Based on experience with a fôerster gradiometer. if there is a bunch of them in the same spot you might see it.

By the way, is there anyone who has comments or question on the sensor mount I have suggested. Please comment. I will try and find the time to draw it up in 3D and post it here.

Sincerely
olsteffe