flat spring in a geophone

[thschrader]

Hi,
I am running a simulation (flat spring in a geophone) with 2D-elements
and 3D-output option. Is there a possibiblity to adjust the thickness
of the result mesh? I had a look at the calculix manual, but cannot find
a parameter. Thanks.

output3D.JPG (17.92 KiB) Viewed 1832 times

spring_geophone.JPG (128.58 KiB) Viewed 1832 times

[bernd]

Hi,
I am running a simulation (flat spring in a geophone) with 2D-elements
and 3D-output option. Is there a possibiblity to adjust the thickness
of the result mesh? I had a look at the calculix manual, but cannot find
a parameter. Thanks.
output3D.JPG
spring_geophone.JPG

thickness of result mesh = thickness of geometry thus thickness of shellthickness object in object tree

[thschrader]

thickness of result mesh = thickness of geometry thus thickness of shellthickness object in object tree

ok, got it. Thanks Bernd.
diameter spring 17 mm, thickness 0,05 mm, Material CuBe-alloy with E=135 GPa.
Using 2D-mesh with 3D-visual is a very nice option!
Second Eigenmode at 95 Hz (=spurious frequency?)

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[HarryvL]

Using 2D-mesh with 3D-visual is a very nice option!
Second Eigenmode at 95 Hz (=spurious frequency?)
result_mesh.JPG

Nice analysis Thomas. Why do you think the second eigenmode is spurious? It looks creadible to me. I guess the first eigenmode is a simple vertical motion?

[thschrader]

I guess the first eigenmode is a simple vertical motion?

Yes.
Last week I renovated my home. What did I found? My old diploma-thesis from 1994!
Theme: shape-optimization of a geophone-spring with FEM.
I used MSC-Nastran/Patran for this. As a student I worked at the institute of statics and dynamics
(university of Hannover, germany) to get some additional money for beer .
The institute had an assignement with oyo-geospace to analize a analog seismic geophone. So they grabbed me to do that…
The goal is to design the springs at top/bottom of the geophone (the cylinder is the moving coil,
the fixed magnet is inside the cylinder) for perfect linearity and vertical movement alone.
But the coil can tilt and the spring-arms can flutter. This induces spurious frequencies in the measurement.
Lets rebuilt this with FC.
Thomas
https://www.isd.uni-hannover.de/
https://www.geospace.com/sensors/
http://csegjournal.com/assets/pdfs/arch ... r_freq.pdf

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[bernd]

cool stuff thomas, I made a separate topic out of this ...

[thschrader]

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According to the data-sheet, the first eigenfrequency should be at 10 Hz.
But I cant find the technical drawings anymore, so I estimated the geometry from the pictures in my thesis.
However, the frequency-distance between mode 1+2 is ten times higher than the first eigenfrequency.
Everything is fine...

geospace_seismometer.JPG (62.35 KiB) Viewed 1716 times

[HarryvL]

I guess the first eigenmode is a simple vertical motion?

Yes.
Last week I renovated my home. What did I found? My old diploma-thesis from 1994!
Theme: shape-optimization of a geophone-spring with FEM.
I used MSC-Nastran/Patran for this. As a student I worked at the institute of statics and dynamics
(university of Hannover, germany) to get some additional money for beer .
The institute had an assignement with oyo-geospace to analize a analog seismic geophone. So they grabbed me to do that…
The goal is to design the springs at top/bottom of the geophone (the cylinder is the moving coil,
the fixed magnet is inside the cylinder) for perfect linearity and vertical movement alone.
But the coil can tilt and the spring-arms can flutter. This induces spurious frequencies in the measurement.
Lets rebuilt this with FC.
Thomas
https://www.isd.uni-hannover.de/
https://www.geospace.com/sensors/
http://csegjournal.com/assets/pdfs/arch ... r_freq.pdf
geophone_analysis.JPG

Ach so! Spurious as in undesirable. Not spurious as in non-physical.

By the way I read that the “spurious” modes are above 100Hz. So your 95Hz for the second mode is credible?!

What frequency did you find for your first (vertical mode)? This is the one that should sit somewhere between 10 and 100 Hz if I understand the paper well.

[HarryvL]

Moment mal! It of course depends heavily on the mass that is vibrating. I guess in a geophone the spring connects a magnet to a coil. If the outer housing is the magnet and the inner object the coil then the coil vibrates inside the magnet ?! I think you just modeled vibration of the spring without the coil? If you would attach a mass to the spring (coil) then the natural frequency goes down. The frequency is inversely proportional the the square root of the vibrating mass, so the coil would have to be 100 times heavier than the spring to drop the frequency by a factor of 10.

[thschrader]

I think you just modeled vibration of the spring without the coil? If you would attach a mass to the spring (coil) then the natural frequency goes down.

The moving part is the coil (the outer red cylinder). The magnet (blue) is fixed and clamped between the springs.
Coil mass is approx 10-11 gramms (the mass of the magnet is approx 60 gramms).
As you can see from the response-curve, the highest sensitivity is at 10 Hz (first eigenfrequency=resonance-frequency).
The geophone should behave like a single mass-spring system (only one eigenfrequency).
But in reality this wont work. The design-goal of the springs is to "shift" the 2. eigenfrequency (and all others) far enough away
(here at 360 Hz) to get a good measure without "gost-signals". Most seismic analysis need a frequency-range
between 1-100 Hz, so this geophone seems perfect for that.
The 95Hz 2. mode from my first calculation is for one spring alone. for the whole system this movement
(the tilting of the inner plate) is not possible, because of the clamping at the inner magnet.
Thomas

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