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Seismometer

:: EPE Chat Zone ­:: ­Radio Bygones Message Board :: » EPE Forum Archives 2010 - » Archive through 15 February, 2010 » Seismometer « Previous Next »

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alec_t
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Username: alec_t

Post Number: 144
Registered: 03-2009

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Posted on Wednesday, 20 January, 2010 - 12:26 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hi,
It's probably just pie-in-the-sky, but I may one day get around to making a seismometer of the Lehman type. This involves an inertial mass mounted as a horizontal pendulum. The problem then is to sense linearly, in a non-contact way, movement of the mount relative to the mass. As a sensor there are several options which come to mind:-
1) optical, e.g. with a Moire fringe type grating or a variable mask, and a photo-sensor,
2) capacitive, where the varying capacitance between the mass and an adjacent pad varies the frequency of an oscillator or unbalances a bridge,
3) inductive, where the mass or an attached ferrite rod moves in a coil to vary the frequency of an oscillator or unbalance a bridge.

Any tips about using the above sensors, or suggestions for other sensors?

Regards, Alec
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741
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Username: 741

Post Number: 120
Registered: 08-2005

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Posted on Wednesday, 20 January, 2010 - 01:01 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

One issue is that movement is in 3 dimensions
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gajjer
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Username: gajjer

Post Number: 283
Registered: 05-2007

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Posted on Wednesday, 20 January, 2010 - 05:32 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hi alec
there was one in EPE a while back. I think it used a mask and photo sensor. Probably the simplest.
I have seen ones that use LVDTs but that is a lot more difficult to set up.
I think a bit of experimentation is required. I wouldn't think it needs to be absolutely linear because really you are looking at relative readings and frequency.

cheers
gaj
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alec_t
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Username: alec_t

Post Number: 145
Registered: 03-2009

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Posted on Wednesday, 20 January, 2010 - 06:04 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

@741
I take your point, but so far as I know even commercial seismometers are single-axis ones, which need orienting according to which dimension is of interest. To cover 3 dimensions you need 3 seismometers.
If you check out the following link, for example, you can get live seismic data, from the San Andreas Fault and other areas in California, and can select the dimension to create your own seismogram using a commercial seismometer:-

http://www.ncedc.org/bdsn/make_seismogram.html

@gajjer
I daresay EPE would have looked into the options, so the mask/photo-sensor looks favourite.

Thanks guys.
Cheers, Alec
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kevinbrunt
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Username: kevinbrunt

Post Number: 87
Registered: 02-2007

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Posted on Thursday, 21 January, 2010 - 12:58 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

If you're intending to build your own seismoc sensor, there's a lot of useful stuff here.

http://psn.quake.net/

(In particular, in "Information, Equipment and Software")

Also, there's an interesting (and cheap) sensor described at

http://jclahr.com/science/psn/chapman/piezo/index.html

which uses a piezo sounder disc as the sensing element.
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ant
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Post Number: 563
Registered: 05-2006

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Posted on Thursday, 21 January, 2010 - 01:03 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hello all,

For a digital output, the ruled scale used on digital calipers etc might be of use?

Regards Ant
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alec_t
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Post Number: 150
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Posted on Thursday, 21 January, 2010 - 11:10 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Thanks for the info, guys. Gives me a good starting point.

Cheers, Alec
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boris
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Username: boris

Post Number: 474
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Posted on Friday, 22 January, 2010 - 03:15 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

A few years ago, I proposed a solution to the earthquake problem. It doesn’t prevent them, but it ‘manages’ them to minimise casualties and damage.

You evacuate the affected area, then pump the fault full of water, then set off several large explosions inside the fault. This triggers the earthquake releasing the built up stress.

The amount of stress increases over time, and so does the magnitude of the resultant quake. More frequent ‘forced’ quakes, makes for smaller magnitude. I figure this only needs to be done once every 20 to 100 years, depending on the location.

Seismologists already have GPS units accurate to the mm installed around various fault lines. This, plus other clues, gives them a good estimate of the stress and potential quake magnitude.

The local building code must state that new buildings must be able to withstand a magnitude ‘n’ quake. Then when the predicted magnitude reaches this ‘n’ point, the forced quake is provided. This system would save most of the lives and property previously lost to ‘surprise’ quakes.
Thank you for not breeding.
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boris
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Post Number: 477
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Posted on Friday, 22 January, 2010 - 03:32 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Back on topic.

I built a fairly sensitive seismometer using a cheap piezo-disk, recovered from a birthday card that played a tune. The disk was oriented parallel/flat with the ground, suspended at 2 opposite edges with two pillars. A D-cell battery (Used only for its mass), was glues to the middle of the disk on its side, so the line of contact was perpendicular to a line between the pillars. This caused the disk to bend a little in the middle, along the line of contact with the battery, pre-stressing it and increasing its sensitivity (I think).

An op-amp, a CRO, and bingo. I could clearly see the effects of people walking around in the house next door, a match dropped onto the carpet 2 or 3 meters away, a car ‘kangarooing’ on the street, trucks on the main road 100 meters away, trains passing a couple of miles away, etc.

Dunno what kind of seismometer you’d call it, but I guess it worked mostly in the vertical axis.
Thank you for not breeding.
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phantastron
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Username: phantastron

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Posted on Tuesday, 26 January, 2010 - 08:08 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hi Alec, I have read your enquirery with interest. With regard to EPE they certainly do have a circuit for a seismograph. It temploys a magnet and a Hall effect device transistor. You can find more details in the EPE for Oct 2007. Hope this is of some use.
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alec_t
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Username: alec_t

Post Number: 166
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Posted on Tuesday, 26 January, 2010 - 11:36 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Thanks for the pointer, phantastron.

Regards, Alec
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steerpike
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Username: steerpike

Post Number: 467
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Posted on Wednesday, 27 January, 2010 - 10:44 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)


quote:

I built a fairly sensitive seismometer using a cheap piezo-disk




There was also one a few years ago in one of the mags (a footstep detector actually) based on a face-down loudspeaker, with a large cork glued to its cone, and the weight of the speaker resting on the cork/cone. The chassis & magnet of the speaker made up the inertial mass.

Different idea:
If you used a leaf spring to support the mass, (e.g., a hacksaw blade) you could glue a strain gauge onto the flat surface of the blade, and as the blade flexed, read the strain gauge output.
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boris
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Post Number: 491
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Posted on Thursday, 28 January, 2010 - 07:23 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Or a large magnet on the end of the spring, and a hall sensor.
Thank you for not breeding.

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