PSN-L Message - Subject: Re: Sensor and Damping magnets

In a message dated 26/03/2006, gmvoeth@........... writes:

<= FONT=20 style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size= =3D2>it is my=20 understanding you want the highest uniform magnetic field possible and you= =20 want the wires to move within that field perpendicular. So to compliment t= he=20 field you will put two horseshoe type magnets in opposition with a gap the= n=20 have
the coil in the middle with one half cutting the N/S poles and the= =20 other half cutting the S/N poles.
It seems best if the coil is smaller= =20 then the magnet
faces and is rectangular rather than=20 circular.

Hi Geoff, Ian,

If you=20 could get several of those old military magnetron magnets like they use=20= in=20 the old days of electronics that are used for Physics demonstrations tod= ay=20 at the universities you most probably would have the best magnet you can= =20 get. But they look like they weigh 100 lbs each.

They can be used but they are very bulky and=20 don't usually have the pole shape that you want.

You are much better off with rectangular coils=20 and rectangular magnets.

You need two adjacent areas of opposing high=20 intensity field. NdFeB bar magnets are not very expensive, but they are very= =20 powerful.

The 'I' is soft iron plate.

The 'B' are the support bolts with 3 off nuts=20 locking the plates in place.

The 'N' and 'S' are 1" square x 1/8" Neo magnet= s=20 with poles on the square face.

The 'CC-------CC' is a rectangular frame coil w= ith=20 an open centre and maybe 3,000 to 5,000 turns, about 1" wide by 1,1/4" long.= The=20 coil overlaps the ends of the magnets coming out of the paper. I used=20 4 paxolin rods and thin sheet held together with epoxy glue to make the= =20 coil frame. Make a reinforced hole in the centre so that you can mount the f= rame=20 on a bolt in a hand drill to wind on the wire.

Sensor. I have exaggerated the coil to magnet separation.

Magnets 1"squarex1/8" thick with poles on the square face

B &nbs= p; &n= bsp; = &nbs= p; &n= bsp; B

IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII= IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII

B &nbs= p; NNNNNNNNNNSSSSSSSSSS = B

B &nbs= p; &n= bsp; = &nbs= p; &n= bsp; B

B &nbs= p; =20 <- CC------------------CC =20 -> = ; B

B &nbs= p; CC------= ------------CC &n= bsp; B

B &nbs= p; &n= bsp; = &nbs= p; &n= bsp; B

B &nbs= p; =20 SSSSSSSSSSNNNNNNNNNN &n= bsp; =20 B

IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII= IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII

B &nbs= p; &n= bsp; = &nbs= p; &n= bsp; B

Damper. Cu is the wide copper damping plate, 1/16" to 1/8"=20 thick

Magnets 1"x1/2"widex1/4" thick with poles on the 1"x1/2" face

View shows the 1/2"x1/4" magnet ends

B &nbs= p; &n= bsp; 3.5" long &nbs= p; &n= bsp; B = ;=20 Mild Steel Plate

IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII= IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII=20 1/4" thick x 2" wide

B &nbs= p; &n= bsp;NNNNNSSSSS &n= bsp; = B

B <-=20 CuCuCuCuCuCuCuCuCuCuCuCu -> B 1/4" mild ste= el=20 bolts

B &nbs= p; =20 SSSSSNNNNN = &nbs= p; =20 B full thread + 3 nuts

IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII= IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII

B &nbs= p; &n= bsp; = &nbs= p; &n= bsp; =20 B

For a Lehman, I use two 1/4" thick bright mild steel=20 plates, 3.5" long by 2" wide. The corners are drilled to take 1/4" mild stee= l=20 set screws 2.5" long with mild steel washers and nuts. The two plates a= re=20 held maybe 3/4" to 1" apart for a sensor, using three nuts on every bolt.=20 A bolt is firmly secured to one plate using a nut. The second plate is=20 mounted in between pairs of nuts on the free thread, to allow the separation= of=20 the plates to be adjusted and locked. Four NdFeB magnets 1/8" thick by 1" sq= uare=20 are mounted on the inside faces of the plates. A N+S pair on one face i= s=20 opposed by a S+N pair on the other. The sensor coil is mounted at the centre= of=20 the high intensity magnetic field.

This construction gives quite an effective magn= etic=20 and electrostatic screen around the coil. The external stray field is low. T= he=20 sensitivity is high due to the high field and the response is reasonably lin= ear.=20 If you use rectangular instead of round coils, it can be made very highly li= near=20 over +/-1/2" movement.

For induced current damping, I=20 use four NdFeB bar magnets 1"x1/2"x1/4" thick in two opposing squa= res,=20 with the same 1/4" steel plate mounting, but with a much smaller magnet=20 separation. I use a copper damping plate, 1/16" to 1/8" thick as=20 appropriate, a bit over 2" wide and 2.5" free length. This allows=20= the=20 arm to swing +/-1/2" without the edge of the copper plate overlapping the ed= ge=20 of the 1" magnet square. The N+S join of the magnet pairs is set perpendicul= ar=20 to the direction of motion. The damping is adjusted by varying by the length= of=20 copper tongue overlapping the magnet square and also by varying the separati= on=20 of the 1/4" mild steel plates and hence the magnet separations. Most of the=20 damping occurs close to the central N/S magnet join. This is a very effectiv= e=20 and easily adjusted design of damper.

I have also used soft Al damping plates, but Al= is=20 usually more strongly paramagnetic than the copper is diamagnetic and it has= a=20 higher resistivity. I use the damping plate to physically limit the sideways= =20 travel of the seismometer arm. If the plate is made narrower and the arm dri= fts,=20 you may get 'odd' force effects as the edge of the plate gets close to=20= the=20 outside edge of the magnets. Al is usually mildly paramagnetic, but copper i= s=20 usually diamagnetic.

<= FONT=20 style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size= =3D2>I can't=20 imagine how you could safely create a one inch gap with those monstrous=20 magnets. They would take off a finger or two if you were not careful in=20 keeping them apart.

With some difficulty and great care= !=20 Neo magnets start to get dangerous above about 1/4" thick, 1/2"=20 square. I have a pair 2" square by 1/2" thick. I can't put eno= ugh=20 finger force on them to even slide them apart. I use wood blocks=20 and an A frame work bench. Two of these magnets will 'jump=20 together' if less than about a foot apart. Only ever=20 handle ONE free magnet at a time!

<= FONT=20 style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size= =3D2>No one=20 talks about safety when playing with springs and stuff so I guess no amate= urs=20 have had any serious accidents trying to build these=20 seismometers.

Oh yes they do! Sean's EMails carried several=20 warnings about dry wall scraper spring systems.

Construction.

Cut and file the 2"x1/4" bright mild steel to=20 length. Mark out the 3 or 4 bolt holes to choice on one plate, the= n=20 clamp the two plates together and drill all the clearance holes through= =20 both plates at the same time. Mark the plate orientation with a centre=20 punch. This ensures perfect bolt alignment. Degrease, then coat the ste= el=20 with acid anti rust liquid / paste, leave for ~10 min and wipe off with= a=20 dry tissue. Let the plates dry. Mark the positions of the magnets,=20 nuts and washers and paint all other surfaces with Hammerit= e or=20 a similar anti rust paint.

Lightly grease the bolts, the clearance holes,=20 exposed iron and the nuts. Fully assemble one plate with bolts and fing= er=20 tight nuts. Check that the other plate fits on cleanly and fully tighten the= =20 nuts. Fit a second nut on each bolt leaving about 3/4" thread between the nu= ts.=20

Lightly grease and wipe the magnet mounting spa= ces.=20 Separate a magnet, clean any dust or debris off it using gaffer sticky tape,= =20 apply it to the edge of the steel plate and slide it into position. Check th= at a=20 second magnet has opposite top surface polarity and slide it into position.=20 Check the orientation of the free plate and assemble opposite polarity magne= ts=20 on it - a N/S magnet pair on one plate opposes S/N pair on the other.

Slide the free plate onto the mounting bolts,=20 recheck the magnet polarity, adjust the nuts to give the desired magnet gap=20 separation and fit the lock nuts. Wipe off any excess grease.

http://www.amazingmagnets.com/&n= bsp;=20 and http://www.kjmagnetics.com/ in= the=20 US stock a wide range of NdFeB magnets amongst many other sources.

Geoff, you raised the question of how great a S= /N=20 ratio you can get. My 16 bit ADC gives me a 90 dB range. When I was=20 developing my LVDT sensor, I managed to get about 7 nano metre noise ov= er a=20 6 mm linear movement with 10 Hz bandwidth. This is not far short of 120= dB.=20 However, my local ambient ground noise is well over 10x this and it is stron= gly=20 dependant on the weather, the local traffic and the time of day. T= his=20 is my practical concern and it is the major limitation for a lot of 'amateur= '=20 systems. Many professional systems use deep boreholes remote from human=20 activity. I agree with Dave on the desirability of burying geophones to= =20 reduce the pickup of surface noise.

With a low noise geophone / coil type amplifier= ,=20 you should be able to get quite a bit below 1 micro V input noise for 10 Hz=20 bandwidth without using 'special' circuits. If you were prepared to build a=20 chopper amplifier, you would likely get below 100 nano V.

Roger Sparks commented on the presence of ocean= =20 microseisms masking small quakes. It is possible to use a twin Tee variable=20 width bandstop filter to remove ~50 dB of this signal. The signals are=20 usually quite narrow band, although they do vary in period around the world=20= and=20 with time.

Regards,

Chris Chapman

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