PSN-L Email List Message

Subject: Re: Homade Geophone
From: John or Jan Lahr JohnJan@........
Date: Sat, 06 Aug 2005 17:16:08 -0700

Hi Chris,

Thanks for the feedback.

At 03:46 PM 8/6/2005, you wrote:
>In a message dated 06/08/2005, JohnJan@........ writes:
>Hi John,
>     You may need to bend a bit of wire straight at both ends of the 
> spring and clamp it. One of the problems with spring suspensions is 
> noise from the loop ends. You could bore a small hole through the 
> 'mass' bolt just under the head and clamp the wire between two 
> washers? Put a similar clamp bolt on the frame? Noise can also be 
> minimised by bending the wire to a V instead of a loop end and 
> using a slightly sharper V of hard metal with a slightly rounded 
> edge, like they do in weighing scales.

Good suggestions.

>     You may get better results from the suspension if, instead of 
> points, you use a couple of stainless steel ball bearings on the 
> base column and either a square polished lathe tool or a glass 
> microscope slide on the end of the arm. 1/4" bearings should do 
> fine. If you just want to 'try it out' you should be able to buy 
> several sizes of ordinary ball bearings from a cycle shop. You do 
> need to place a drop of thin oil at the contact point to inhibit corrosion.

I tired ball bearings but the boom is so heavy that they tended to 
slip off of the support  I guess the way to compensate would be to 
tilt the glass slide so that it would tend to support the weight of 
the boom.  I think that the friction is too high with the pointed 
drill bits, so I'll next try the pivot pictured here:

I think the ball bearings would be easier for a teacher to set up, so 
that might be the final solution.

>     I would be tempted to use a U magnet for the mass, like in the 
> AS-1 and either a relay or a miniature transformer coil. Larry 
> sells relay coils. To get good signals, you really need a high 
> gradient magnetic field and lots of turns, preferably on a square section coil.
>     Mouser sell a range of small transformers. You remove the I 
> strips from both ends of the yoke, free up a central E strip on 
> both sides with a knife blade and then push it out using blocks and 
> a vice. You can then peel off and extract the remaining E strips to 
> leave you with a nice square sensor coil on a plastic former. I use 
> them for distance sensors with all the E strips replaced on one 
> side, in the same way that Sean Morrissey did.
>     If you use a single vertical magnet, you will likely pick up a 
> lot of magnetic noise from the power wiring in your house, from 
> fridges, central heating, electric cookers, from bikes, cars, 
> lorries, trains and from the earth's field. You will still pick up 
> noise with a U magnet, but it will be less. Both N & S poles face 
> downward and you are not sensitive to side to side forces. A 
> totally soft iron shielded Neo quad array would probably be too 
> heavy. The best way is still to put the coil on the end of the arm 
> with a brass weight and sit the magnet array out of harm's way on the floor!

I know I'm asking for problems with the current setup, but it does 
have the advantage of being quick to assemble!  As long as some 
surface waves show up occassionally, I'll be satisfied.  I guess the 
best approach would be to have a series of possible designs, from the 
most crude to the most advanced.  Then the student, educator or 
amateur could select which one to start with and if some success is 
obtained, would be encouraged to build another of higher difficulty 
and/or greater expense.

>     I notice that you mentioned damping magnets. Could you make an 
> air damper with two different sizes of drinks cans, or one drinks 
> can and an open topped tin set on the ground? The airflow 
> resistance would come from the small circular but long air gap in 
> between the cans? Maybe this is not too practical. It could be 
> quite sensitive to air pressure variations. Just an off the cuff idea.

In my current model I'm not damping at all.  I think the wide boom is 
acting like an air damper, or the friction at the "point hinge" is 
damping quite a bit.  I like the drink-can damper idea -- much better 
than messy oil and less expensive than magnets.  Just as long as it 
isn't too hard to get things aligned so there is no scraping between 
the cans.  The outer container could be a glass so that one could see 
if there is contact at any point.Maybe the gap could be slightly 
larger but the damping still effective with water.  One would have to 
top off the water from time to time!

>     You can get quite significant damping by drilling a round 
> copper bar and sliding one pole of a cylindrical Neo magnet just inside it.
>     I have tried every combination that I can think of for 'flat' 
> magnetic damping systems. BY FAR THE MOST EFFECTIVE SYSTEM is to 
> use rectangular Neo magnets mounted on two mild steel plates held 
> apart by mild steel bolts. I use 3.5" x 2" x 1/4" bright mild steel 
> plates and 1/4" mild steel bolts. On one inner face you place a N 
> and a S Neo bar magnet pair, with the long sides together. Facing 
> this on the other face is a S and a N Neo bar magnet pair. The 
> poles are on the flat faces of the magnets. You suspend an Al or Cu 
> damping blade in the central gap so that it moves at right angles 
> to the long magnet join and overlaps the magnets by maybe 3/8". 
> This avoids edge force effects between the blade and the magnets. 
> The fields from the rear of the magnet pairs are closely linked by 
> the soft iron, so that most of the mmf drives the fields in the 
> central gap. This system gives a very sharp rate of change of field 
> at the central join, allowing strong damping on a cental moving Cu / Al plate.

The damping from the magnets taped to the side of the boom worked OK 
and might be OK (as pictured here: ) for the most 
crude setup, if some kind of air damping doesn't do it.  A more 
advanced version with moving coil might then use the system you describe above.

>     Have fun!
>     Regards,
>     Chris Chapman

I'll be visiting my kids in California for the next 10 days, so 
progress will be delayed!  I am having fun.

Actually the zero level (or minus one?) sensor is shown in this and 
the next few pictures from the day camp I helped out with this summer:
The kids wound their own coils and arranged the suspension for a 
magnet.  In the process they learned that magnet wire has a coating 
that must be removed at the attachment points, that a moving magnet 
creates a small voltage, that a seismogram displays the time history 
of the movement of the magnet within the coil, and that fast motion 
generated a larger signal than slow motion (velocity sensor).



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