Seismology --- One of my other interests.....

[davenn]

(thinks...)
So the beam needs lots of mass to counter disturbances from air movement, but a heavy beam tends to bend the pole it depends from so you are forced to adjudicate a trade-off.

yes it is a tradeoff, most amateur systems use less that 1kg, many 0.5 kg or less and then house the unit is a relatively airtight enclosure... styrofoam box, inverted glass or perspex fish tank ( ya can still see the beast then )

I have revised my one down to 1kg max. The other big problem is ....
yes the heavier mass is more stable BUT its much much harder to dampen the oscillation than with a very light mass.

And that is where I am currently having a crap of a problem trying to sort out. by the end of saturday nite I was about ready to scrap the whole thing !!!
its a hell of a job getting the dampening to work. currently have a couple of rare earth magnets with the aluminium "blade" passing betyween them and its just not doing anything. The boom just continues to merrily oscillate. I have to get the dampening strong enough to stop the oscillation within 1 to 1.5 cycles.
The guys are saying that the strength of the magnetic field needs to be such that I can feel strong resistance to passing the aluminium through it by hand. its proving really difficult to achieve.

I find it hard to get past my jewelled bearing. Amethyst crystals terminate in sharp pyramids which could provide vertical support too, against a dimple in a suitable surface.
It is a bit surprising that something as flexible as aluminium is used. It would have been less surprising to me if you used rock for both the supporting and the oscillating elements of the seismometer.

some guys use angle iron or even galvanised piping. But a reasonably sturdy aluminium frame isnt any real problem.


Dave

 

[poor mystic]

What does the magnetic circuit look like , Dave?

 

[daddles]

And that is where I am currently having a crap of a problem trying to sort out. by the end of saturday nite I was about ready to scrap the whole thing !!!
its a hell of a job getting the dampening to work. currently have a couple of rare earth magnets with the aluminium "blade" passing betyween them and its just not doing anything. The boom just continues to merrily oscillate. I have to get the dampening strong enough to stop the oscillation within 1 to 1.5 cycles.

I think I understand your problem -- or at least empathize with it... I haven't looked specifically, but I'd imagine you're using the strongest magnets you can get your hands on, so there's no easy way to make the eddy currents larger except by making things mechanically closer -- and that only works to a point.

Here's a suggestion to think about: could you put a coil in where you currently have the sheet of metal for the eddy current brake (or change the design to do so)? You'd then create a current in this coil to counteract the movement (i.e., create the damping you want). The big advantage, of course, is that you have great control over this current.

A concern about doing this, of course, is that this created field could interfere with your measurement. Off the top of my glistening bald head, I see two possible solutions. First, try using something like some mu metal for shielding. This might attenuate the created field enough. Second, switch the field on and off at a frequency that can be ignored by the detection system. Sort of inverse synchronous detection if you will. Of course, I don't think you'd need to get that sophisticated; a low pass filter could protect your seismic signal.

Has anyone tried this yet?

 

[poor mystic]

or maybe use thinner aluminium brake discs...

 

[(*steve*)]

The problem with magnetic braking is that the braking is proportional to velocity ( or possibly velocity squared). Either way, low speed, low braking.

Do you have a complete magnetic circuit between the poles? Lacking this you will get a poorer braking effect.

The other thing you could consider is magnetic repulsion. have a magnet mounted on the pendulum, with another magnet (pair of magnets probably) mounted so that you have N-N and S-S together. This should provide a strong force which centres the pendulum. It may also make deflection rather non-linear. This may be useful as it would reduce sensitivity for large events thus extending the measuring range of your instrument.

something like this:

Fixed magnet S==N N==pendulum==S S==N Fixed magnet
pendulum motion <------>

 

[davenn]

poor mystic What does the magnetic circuit look like , Dave?

here is one design that I will be closely following except I will be using 4 x rectangular
magnets instead of the 4 disc ones this guy is using

4 magnets 2 above the aluminium and 2 below.

now it has been impressed upon me that I MUST use a steel frame (as in the 2 red painted steel plates) and steel bolts this improves the magnetic field density through the aluminium and helps to confine the magnetic field within the frame.

just bought the 4 bar magnets from Jaycar yesterday $20 each !! ouch

I was also told that the bar magnets work much better than the disc ones.
I am using disc ones for the sensor coil they seem to be ok in that application.

cheers
Dave

 

[poor mystic]

Yes, the thing is to reduce magnetic resistance as much as possible. The biggest contributor of magnetic resistance to a magnetic circuit is any air gap.
The aluminium braking conductor must be treated as an air gap in the calculations of magnetic resistance. Therefore it makes sense to use as thin a braking conductor as possible, and to align the magnets so that the air gap is minimal.

 

[davenn]

ok have just drawn this up.....

Also note I am following the design that I posted that big pic link to some days ago
In that the aluminium "blade" is connected to the pendulum boom and not the magnet frame as in the pic I posted in previous post

ok so you are looking end on of the 4 bar magnets, so you can see how the N's and the S's are arranged. A 1 to 3mm gap between either side of the aluminium and the magnets

Dave

 

[poor mystic]

I've made a new diagram, attached, which I think is what you need. The most important thing to note is that the airgap must be minimal; thousandths of inches count!

 

[poor mystic]

Here is a much better diagram

 

[davenn]

Here is a much better diagram

So you are looking at a horseshoe style magnet there ?
I have seen horseshoe magnets used but usually 2 of them one of either side of the Al blade. (maybe I should use the word - vane - instead of blade )

I built a new magnet arrangement yesterday using the layout I drew above with the 4 bar magnets. WOW it really works.
The magnets have a 5 mm gap and with a 1mm thick Al vane leaves 2mm clearance either side.
leaving the bolts long out the bottom of the magnet frame as above means that it sits nicely over the 50mm wide main seismo frame and slides happily back and forward to adjust the dampening.

am now
now for some respectable shakes to sort out the amount of dampening etc.

cheers
Dave

 

[poor mystic]

Yes, I thought a horseshoe would be most conservative of magnetic flux, which likes to keeps to low reluctance paths and really hates air-gaps!

PS Congratulations on successfully engineering your magnetic brake!