Epe pic magnetometer

[olsteffe]

ApBerg
I have tried to join the Yahoo group, but the group owner does not respond so my membership in the magnetometer group is still pending .
As fore the Armstrong oscillator, how does it output square waves? it is my understanding that the output ould have to go true a relaxation oscillator (schmitt trigger) in addition.
Sincerely
olsteffe

[ApBerg]

Hello olsteffe,

Have send you a PM.

Regards.

Ap

[stephane.mg]

I've built one, but cant get it to work properly.
the two sensors are pretty unstable, they are fgm-3 and not 3h.
I read values like 93089 --- 93179 --- 93091 with for each sensor a move of about 180
that makes a difference of 100-200 units, whatever the position I put them to.
I've tried to align them with connectors, and on the axis, so Ive got one with 96xxx and another with 92xxx
on an empty place, without anything metallic.
and when I null the device, I still have a final difference of values very unstable.
when I grab the proble it's getting worse, 300 - 400 for a difference.
I'm a missing something ? why are the sensors so unstable ? I've spent 2 hours trying to align them, but their own unstability makes this impossible to work.
thanks if you can help me.

steph

[olsteffe]

Hello Steph
Do you have a picture of your setup?
It would be easier to tell how to remedy your problems then. 100 -200 Hz difference is a span of about 100 to 200 nT so it is a bit of, You should be able to reach a difference of about 10 Hz.

olsteffe

[stephane.mg]

thanks for helping me, here are the pics. the two sensors are 0.5m spaced, inside a aluminium corner. I've aligned the connectors, and tried to align horizontally and vertically, but Ive got up to -150 <-> +150 variation. And when I move the probe of 15-20 degrees it goes worse.
you also have a picture of the output of one of my fgm3 sensors, it's not perfectly square as you can see. the cable between the two sensors is shielded (audio helmet cable), so it's not interferences.

I have to add a precision, measuring between impulses from the fgm3, Ive got about 70hz, when I move a little the fgm3 that Im measuring, the impulses gets a little more space, from 1hz to 1hz, very stable and precise, but on the values displayed, Ive got 93000 and 93300 varying and unstable. It should display a 93000 and when I get one hz, display 93001 and so on ?

regards.

steph

[Alexismex]

Hello Stephane,
I also make experiment with Fm3 ...the secret it is with the rigidity of the axe better a square aluminum axe and also one of the Fg is mounted inside a tube and this tube inside another tube with 3 brass screw to adjust very fine the alignment because is not possible manually like i see in your photo...make a small research in the web and you will find this mounting procedure
And also it very important to make ajustment outside far of electrical disturbance, in the middle of a field!!!!
Alexis

[stephane.mg]

yes, the mounting procedure with the screws is explained, I was planning to do it but my major problem is here :
if I get the measurements of one fgm3, not moving at all, isolated from any magnetic disturbance, its constantly moving with a +-100 value !
So I have for example 93000, 93088 1 second later, 92790 another second later.
Even if I manage to perfectly align the two fgm3, there are still moving, and so the difference between the two values read are +-200 about, making the device completely unusable (the max difference displayed is 999)

I mistaked in the previous post, when I get a signal out of the fgm3, its kind of stable, talking about a resolution of 1khz, but surely not about 1hz.

Do you have such a signal when measuring with the oscilloscope ? are the value as unstable as mines when you look only at one fgm3 reading, without moving anything ?

regards

[olsteffe]

Hello
The sensors are very temperature sensitive so when testing I suggest you place them inside some styrofoam for insulation when testing.
The sensors produce some heat so you should let them run for like 20 min in the insulated compartment before taking measurements.
Daily variations in the earth magnetic field can be several tens of (nT) Hz.
Some "cheap" source of carbon fiber tubing in my country is ice hockey cues (15 euros). They are really stiff and temperature stable.
Touching your aluminum angle can cause deflection due to the sensors weight, minute but still there.
You must also think of thermal expansion when you are touching the aluminum angle (use gloves)
Make certain the angle is unconstrained, hang it from a single point in one end only and unly support it loosely with some foam in the other end.
Justsome thoughts, hope they can help.
olsteffe

[stephane.mg]

thanks for your reply but this is not actually the problem I face.
looking at the oscilloscope, the pulse from fgm3 seems stable, also after the 4013 flip flop.
But the problem is here : I read about a 41khz frequency on direct output of fgm3, same frequency on input of pic16f877, but it displays a 82000 for this sensor, it just doubles the values so it doubles the difference between the two sensors. This could maybe be my instability problem.
Do you have the same on your epe gradiometer ?

I also found this, one of my sensors, put to n-s position display a low frequency, when it goes on oscilloscope about 40khz, the flip flop makes a 20khz signal, with pulse twice the width of the normal ones, so the value displayed is 40000 and pretty stable. I dont understand this behavior, maybe the 4013 is not good, or maybe the sensor...

[olsteffe]

Just wondering are you using the 20 Mhz crystal? And are you using the QA (1) AND QB (13) output of the IC2?
olsteffe

[stephane.mg]

Yes off course, but I think I've understood the problem.

Look at the picture I've provided for the output of one fgm-3

You'll see that the pulse goes up, a little down, up again and below 0v to stabilize.

I think that this part of the signal at the end triggers again the flip flop and makes it switch again, so I've got a square pulse at exactly the same frequency than the fgm3, and normally the flip flop should divide by 2 the frequency. The microcontroler multiplies by two the frequency, considering that the flip flop has done its job well, so I've got a frequency twice as much as I should have (fgm3 40khx, flip flop Qa at 40khz, and final dispay at 80000)

The fgm-3, brand new seems to be not correct as the signal is not as on this site :
http://www.reeve.com/FGMSensors.htm

but like on the picture I've provided at the thread's start.

I've mailed speake about this problem, I'll also try to put a diode on the fgm3's output to stop the signal to go under 0v at the end...

[olsteffe]

Should have catched that one

When you said you measured the same frequency at the FGM output and then again at the input of the PIC the answer was actually out in the day (flip flop frequency divider on me). There is a damping or freewheeling diode inside the sensor could be it has failed.

Keep us informed and hope you sort this out with Speake.

olsteffe

[stephane.mg]

Hi,
I've tried to change the 4013, and put a diode to avoir the signal to go under the 0v, no success.

I've mailed today Bill Speake about this problem, and I'm waiting for his answer about the strange signal those fgm-3 have.

I'll asked someone at a serious electronical shop to sell me a easy solution to try to make the flip flop work efficiently, but I'm really disappointed by the fgm-3 results.

[ivconic]

Look... to understand better FGM-3 behavior; i suggest you to wire up only power +5V (with cap an resistor) and output connect to oscilloscope.
Put it on table (with no metal arround) oriented N-S. Than observe waveform. Than turn it slowly in W-E orientation and see changes. Than use some iron piece to experiment. Close up iron to side of sensor and observe. Than close iron to front and observe. Play a bit with various orientations and various metals. All the time observe signal behavior on scope (phase, frequency and amplitude).
I think that will clear all doubts about FGM-3 sensor and its behavior.
Regards!

[stephane.mg]

Hi ivconic, first thanks for your help.
I've already put my oscilloscope directly on the fgm-3 output, while deconnecting it from the flip flop.
I've played with n-s e-w orientation, saw the frequency changes of the signal, and saw that the signal is normally between 40khz and 50khz at my location.
But the main problem I have, is that the flip flop also have a 40khz-50khz output, on Q, fed to the pic16f877 on pins 2, and 3 for the bottom and top sensors, so I included this picture to show you the problem.
you'll see a first 0-4v, then a decrease, then a 1v-4v, then a decrease up to -1v, then stabilisation at 0v.
The second decrease seems to be enough to trigger again the flip flop, so the frequency is not divided by two as it should be.
So my problems comes from the two sensors itself, they muse have some defective components inside. I've mailed Speake two times since yesterday, still no answer.

Regards,

steph