Subject: Re: Seismograph for school From: ChrisAtUpw@....... Date: Sat, 30 Sep 2006 22:15:14 EDT In a message dated 2006/09/30, tchannel@.............. writes: > Hi Folks, I have a question about the coil and magnet portion of a sensor. > I have seen very small coils and magnets used and very large ones. I guess > the common one is about a 25# pull magnet and a coil of about 500 ohms? I am > going to rebuild my assembly for two reasons, first I have been told my cow > magnet and small coil is just too small. > The second reason is I just found a great cheap powerful magnet. It is the > size of a deck of cards, about 50# pull and about $8.00 at Harbor Freight. > > Using the same (everything) but three different magnet/coils, undersized, > just right, and oversized, what effects would they have, on the gain, final > image, etc? > > I would create a "pancake" coil about the same size as the magnet, square > in shape and thin. > > Your thought or suggestions please. If this is a good magnet to use in a > coil assembly or using two in a damper assembly, I can give you the part > number etc. They are welding assembly magnets, two in a package. Hi Ted, You don't need this size or bulk. You need quite a few thousand turns on a coil with the ID clear of the OD of the magnet, if you use a cow magnet. You do need a coil of 350 to 1000 Ohms to match your opamp input impedance. The greater the number of turns and the greater the change of flux (= area x field) through the coil, the greater the output. If you put an unshielded magnet on the arm of a seismometer, you are likely to detect every pulse on the house utility power wiring, fridges switching on and off, passing cars and trucks and changes in the Earth's field.... The signal is likely to be very noisy. The sensitivity of a seismic detection system needs to be very high. The usual coil / magnet system involves mounting the coil on the arm, sitting your magnet on the baseplate and using a non magnetic mass - brass is easy to use, but you can also use lead or copper. You need a linear high sensitivity response. You have to allow for tilt drift over the week of about 3/8" with a Lehman and the sensitivity needs to stay constant and linear within this range. This can be provided with a rectangular coil of maybe 2000 turns and a quad NdFeB magnet system on mild steel backing plates. I have described my detector system using four 1" square x 1/8" thick NdFeB magnets on 1/4" thick mild steel backing plates in the second half of http://jclahr.com/science/psn/chapman/lehman/index.html You may be able to extract the primary winding from a small low power mains transformer and use that? This is the sort which has side by side, not overlapping, P & S windings and a square / rectangular former. I don't know what is available to you? Larry sells round relay coils, but I prefer a larger rectangular form. You may also be able to buy 'spare' mains coils for timers and water flow valves on washing machines. Some have a suitable pancake form. The coil needs to have an internal rectangular measurement in the direction of motion of maybe 1/2" to 3/4" so that its output will be constant, allowing for the tilt drift on the seismometer. I also use an identical mild steel adjustable frame for the damping, but it uses a 1/16" copper plate and 1" x 1/2" x 1/4" thick NdFeB bar magnets. The damping tongue is mounted horizontally and the damping magnet block is simply slid further over it until you get ~0.7 critical damping. If you deflect the arm 10 mm and release it, it should swing 0.5 mm beyond the balance point before returning to balance. The damping force that you need decreases as the period set is increased, and increases with the seismic mass, so it needs to be easily adjustable. This construction is easy to set up and use and it is NOT temperature sensitive. This magnet + tongue damping arrangement is very effective. You can buy suitable NdFeB magnets from www.kjmagnetics.com. Regards, Chris Chapman In a me= ssage dated 2006/09/30, tchannel@.............. writes:

Hi Folks,  I have a questi= on about the coil and magnet portion of a sensor. I have seen very small coi= ls and magnets used and very large ones. I guess the common one is about a 2= 5# pull magnet and a coil of about 500 ohms? I am going to rebuild my assemb= ly for two reasons, first I have been told my cow magnet and small coil is j= ust too small. 
The second reason is I just found a great cheap powerful magnet.  It i= s the size of a deck of cards, about 50# pull and about $8.00 at Harbor Frei= ght.

Using the same (everything) but three different magnet/coils, undersized,=20= just right, and oversized, what effects would they have, on the gain, final=20= image, etc?

 I would create a "pancake" coil about the same size as the magnet, square=20= in shape and thin. 

Your thought or suggestions please.  If this is a good magnet to use=20= in a coil assembly or using two in a damper assembly, I can give you the par= t number etc.  They are welding assembly magnets, two in a package.


Hi Ted,

       You don't need this size or bulk.

       You need quite a few thousand turns on=20= a coil with the ID clear of the OD of the magnet, if you use a cow magnet. <= BR>
       You do need a coil of 350 to 1000 Ohms=20= to match your opamp input impedance.

       The greater the number of turns and the= greater the change of flux (=3D area x field) through the coil, the greater= the output.

       If you put an unshielded magnet on the=20= arm of a seismometer, you are likely to detect every pulse on the house util= ity power wiring, fridges switching on and off, passing cars and trucks and=20= changes in the Earth's field.... The signal is likely to be very noisy. The=20= sensitivity of a seismic detection system needs to be very high.

       The usual coil / magnet system involves= mounting the coil on the arm, sitting your magnet on the baseplate and usin= g a non magnetic mass - brass is easy to use, but you can also use lead or c= opper. You need a linear high sensitivity response. You have to allow for ti= lt drift over the week of about 3/8" with a Lehman and the sensitivity needs= to stay constant and linear within this range. This can be provided with a=20= rectangular coil of maybe 2000 turns and a quad NdFeB magnet system on mild=20= steel backing plates.

       I have described my detector system usi= ng four 1" square x 1/8" thick NdFeB magnets on 1/4" thick mild steel backin= g plates in the second half of  http://jclahr.com/science/psn/chapman/l= ehman/index.html

       You may be able to extract the primary=20= winding from a small low power mains transformer and use that? This is the s= ort which has side by side, not overlapping, P & S windings and a square= / rectangular former. I don't know what is available to you? Larry sells ro= und relay coils, but I prefer a larger rectangular form. You may also be abl= e to buy 'spare' mains coils for timers and water flow valves on washing mac= hines. Some have a suitable pancake form.

       The coil needs to have an internal rect= angular measurement in the direction of motion of maybe 1/2" to 3/4" so that= its output will be constant, allowing for the tilt drift on the seismometer= ..

       I also use an identical mild steel adju= stable frame for the damping, but it uses a 1/16" copper plate and 1" x 1/2"= x 1/4" thick NdFeB bar magnets. The damping tongue is mounted horizontally=20= and the damping magnet block is simply slid further over it until you get ~0= ..7 critical damping. If you deflect the arm 10 mm and release it, it should=20= swing 0.5 mm beyond the balance point before returning to balance. The dampi= ng force that you need decreases as the period set is increased, and increas= es with the seismic mass, so it needs to be easily adjustable. This construc= tion is easy to set up and use and it is NOT temperature sensitive. This mag= net + tongue damping arrangement is very effective.

       You can buy suitable NdFeB magnets from= www.kjmagnetics.com.

       Regards,

       Chris Chapman Subject: Re: Seismograph for school From: "Geoffrey" gmvoeth@........... Date: Sat, 30 Sep 2006 19:27:59 -0700 Hello Chris; Can you tell us how to get a uniform magnetic field to cut the coil at right angles (normal) to the windings ? can you use two opposing horse shoe magnets with a coil in the middle ? i should imagine a sturdy custom built non magnetic bracket is needed as well as great care not to let the magnets clap together or the impact might weaken the magnets. i think those horshoe magnets with a hole in the middle of the u is the way to go. regards; geoff __________________________________________________________ Public Seismic Network Mailing List (PSN-L) Subject: Re: Seismograph for school From: ChrisAtUpw@....... Date: Sat, 30 Sep 2006 23:58:52 EDT In a message dated 2006/10/01, gmvoeth@........... writes: > Hello Chris; > Can you tell us how to get a uniform magnetic field > to cut the coil at right angles (normal) to the windings ? Hi Geoff, I use 4 off 1" sq x 1/8" rectangular NdFeB magnets on two 3.5" x 2" x 1/4" thick mild steel backing plates and mount the coil in between. You have N+S magnets on one face opposite to S+N magnets on the other face. This gives a high central field and a linear movement response. I use a rectangular pancake coil of about 2000 turns. It is quite effective! See http://jclahr.com/science/psn/chapman/lehman/index.html > can you use two opposing horse shoe magnets > with a coil in the middle ? This is likely to give a lower sensitivity and is more difficult to make / mount. Large Alnico U magnets are quite expensive. You benefit from the relatively high flux of the NdFeB magnets. Regards, Chris Chapman In a me= ssage dated 2006/10/01, gmvoeth@........... writes:

Hello Chris;
Can you tell us how to get a uniform magnetic field
to cut the coil at right angles (normal) to the windings ?


Hi Geoff,

       I use 4 off 1" sq x 1/8" rectangular Nd= FeB magnets on two 3.5" x 2" x 1/4" thick mild steel backing plates and moun= t the coil in between. You have N+S magnets on one face opposite to S+N magn= ets on the other face. This gives a high central field and a linear movement= response. I use a rectangular pancake coil of about 2000 turns. It is quite= effective!

       See  http://jclahr.com/science/psn= /chapman/lehman/index.html

can you use two opposing horse=20= shoe magnets
with a coil in the middle ?


       This is likely to give a lower sensitiv= ity and is more difficult to make / mount. Large Alnico U magnets are quite=20= expensive. You benefit from the relatively high flux of the NdFeB magnets.
       Regards,

       Chris Chapman Subject: Re: Seismograph for school From: "Geoffrey" gmvoeth@........... Date: Sun, 1 Oct 2006 00:40:52 -0700 Hello Chris; http://jclahr.com/science/psn/chapman/lehman/index.html Yes, I like that design very much. Do you make a vertical version of that with a spring where the wire is ? I would like very much to either buy a kit or the plans from you to build my own vertical. I lack talent in your area I need help to make a decent sensor. Vertical is my only interest unless I can get a planer type sensor in the horizontal. Meaning a single sensor for both EW/NS. Please email me the details. gmvoeth@........... gmvoeth@......... gmvoeth@......... any one of those three addresses. I have seen a novel sensor at ASU about 1985 where two masses are N/S and E/W but vertically at a 45 degree angle or so making it possible to reproduce three dimensional motion with only two sensors. Or so it seemed to me. I have never seen that design on the Internet. regards; geoff __________________________________________________________ Public Seismic Network Mailing List (PSN-L) Subject: Re: Seismograph for school From: ChrisAtUpw@....... Date: Sun, 1 Oct 2006 15:39:26 EDT In a message dated 2006/10/01, gmvoeth@........... writes: > Hello Chris; > http://jclahr.com/science/psn/chapman/lehman/index.html > Yes, I like that design very much. > > Do you make a vertical version of that > with a spring where the wire is ? Hi Geoff, Not at the moment. Long period verticals are much more difficult to make than long period horizontals. The essential properties to give a 'zero length extension spring' were worked out by LaCoste in the mid 1930's. See http://psn.quake.net/bibliography.html To balance the vertical force of gravity on the mass requires a very constant force. You need a Ni-SpanC spring and a lot of careful design to enable you to get stable periods over about 10 secs. The elastic properties of steel springs are much too temperature sensitive. One alternative is to use a steel leaf spring, but to provide electronic force feedback to both compensate for incoming signals and to remove the drift. The feedback essentially tries to hold the mass stationary. See great example at http://www.bryantlabs.net/seismo.html Another alternative is to use a steel spring to produce a vertical with a period of 1 to 3 seconds, which is not too difficult to do and then extend the response electronically by x10 to x20 - as you can for a geophone (Roberts' circuit). You could probably extend the period of your 1 Hz geophone to about ~20 sec this way, but you would likely see some increase in sensor noise. This period extension method is attempted on the AS-1. See http://jclahr.com/science/psn/as1/index.html The basic period is about 1.5 sec and a response from 3Hz to 20 sec is claimed, but the manufactured item costs about US $600. Regards, Chris Chapman In a me= ssage dated 2006/10/01, gmvoeth@........... writes:

Hello Chris;
http://jclahr.com/science/psn/chapman/lehman/index.html
Yes, I like that design very much.

Do you make a vertical version of that
with a spring where the wire is ?


Hi Geoff,

       Not at the moment.

       Long period verticals are much more dif= ficult to make than long period horizontals. The essential properties to giv= e a 'zero length extension spring' were worked out by LaCoste in the mid 193= 0's. See http://psn.quake.net/bibliography.html To balance the vertical forc= e of gravity on the mass requires a very constant force. You need a Ni-SpanC= spring and a lot of careful design to enable you to get stable periods over= about 10 secs. The elastic properties of steel springs are much too tempera= ture sensitive.

       One alternative is to use a steel leaf=20= spring, but to provide electronic force feedback to both compensate for inco= ming signals and to remove the drift. The feedback essentially tries to hold= the mass stationary. See great example at http://www.bryantlabs.net/seismo.= html

       Another alternative is to use a steel s= pring to produce a vertical with a period of 1 to 3 seconds, which is not to= o difficult to do and then extend the response electronically by x10 to x20=20= - as you can for a geophone (Roberts' circuit). You could probably extend th= e period of your 1 Hz geophone to about ~20 sec this way, but you would like= ly see some increase in sensor noise.

       This period extension method is attempt= ed on the AS-1. See http://jclahr.com/science/psn/as1/index.html The basic p= eriod is about 1.5 sec and a response from 3Hz to 20 sec is claimed, but the= manufactured item costs about US $600.

       Regards,

       Chris Chapman Subject: Re: Seismograph for school From: "meredith lamb" paleoartifact@......... Date: Sun, 1 Oct 2006 14:04:12 -0600 Hi all, There is a choice of magnet distributors one can entertain for purchasing magnets of course. "Magnets4less" Lists magnets roughly for alot less (~1/3 to 1/4) the price of KJMagnetics. The problem with Magnet4less is they have a minimum order of ~$30.00.....and....their shipping/handling charge of ~$8 or so, is afew dollars more, and its the ultra slow UPS, and their personnel at that place. KJMagnetics ships fast, and usually a flat rate via USPS. KJMagnetics also has a minimum quantity order (~usually 10 of whatever size), as does Magnets4less which can get up to 20 minimum ....for the sizes entertained here. Its worth comparison shopping I suppose .....for example: KJMagnetics: 1x1x1/8"= qty of 10 @ 29.15 ($2.91 each) Magnets4less: 1x1x1/8"= qty of 20 @..... ($0.98 each) Well made horizontal Chris! Still trying to quess what the boom pivot actually is.....perhaps a single wire vertical between the blocks and the boom end center pivots on such?? (can't see on the photo) Take care, Meredith Lamb On 9/30/06, ChrisAtUpw@....... wrote: > > In a message dated 2006/10/01, gmvoeth@........... writes: > > Hello Chris; > Can you tell us how to get a uniform magnetic field > to cut the coil at right angles (normal) to the windings ? > > > > Hi Geoff, > > I use 4 off 1" sq x 1/8" rectangular NdFeB magnets on two 3.5" x 2" > x 1/4" thick mild steel backing plates and mount the coil in between. You > have N+S magnets on one face opposite to S+N magnets on the other face. This > gives a high central field and a linear movement response. I use a > rectangular pancake coil of about 2000 turns. It is quite effective! > > See http://jclahr.com/science/psn/chapman/lehman/index.html > > can you use two opposing horse shoe magnets > with a coil in the middle ? > > > > This is likely to give a lower sensitivity and is more difficult to > make / mount. Large Alnico U magnets are quite expensive. You benefit from > the relatively high flux of the NdFeB magnets. > > Regards, > > Chris Chapman Hi all,

There is a choice of magnet distributors one can entertain for purchasing
magnets of course.  "Magnets4less" Lists magnets roughly for alot less
(~1/3 to 1/4) the price of KJMagnetics.  The problem with Magnet4less
is they have a minimum order of ~$30.00.....and....their shipping/handling
charge of ~$8 or so, is afew dollars more, and its the ultra slow UPS, and
their personnel at that place.  KJMagnetics ships fast, and usually a flat rate
via USPS.  KJMagnetics also has a minimum quantity order (~usually 10
of whatever size), as does Magnets4less which can get up to 20 minimum
....for the sizes entertained here.  Its worth comparison shopping I suppose
.....for example:

KJMagnetics:    1x1x1/8"= qty of 10 @ 29.15 ($2.91 each)
Magnets4less:   1x1x1/8"= qty of 20 @..... ($0.98 each)

Well made horizontal Chris!  Still trying to quess what the boom
pivot actually is.....perhaps a single wire vertical between the blocks and
the boom end center pivots on such?? (can't see on the photo)

Take care, Meredith Lamb

 
On 9/30/06, ChrisAtUpw@....... <ChrisAtUpw@.......> wrote:
In a message dated 2006/10/01, gmvoeth@........... writes:

Hello Chris;
Can you tell us how to get a uniform magnetic field
to cut the coil at right angles (normal) to the windings ?


Hi Geoff,

       I use 4 off 1" sq x 1/8" rectangular NdFeB magnets on two 3.5" x 2" x 1/4" thick mild steel backing plates and mount the coil in between. You have N+S magnets on one face opposite to S+N magnets on the other face. This gives a high central field and a linear movement response. I use a rectangular pancake coil of about 2000 turns. It is quite effective!

       See  http://jclahr.com/science/psn/chapman/lehman/index.html

can you use two opposing horse shoe magnets
with a coil in the middle ?


        This is likely to give a lower sensitivity and is more difficult to make / mount. Large Alnico U magnets are quite expensive. You benefit from the relatively high flux of the NdFeB magnets.

       Regards,

       Chris Chapman

Subject: Re: Seismograph for school From: Bobhelenmcclure@....... Date: Sun, 1 Oct 2006 21:21:16 EDT Hi all, I agree with Chris Chapman on the design of magnets and coils for horizontal sensors. Those who have an interest in building their own sensors should take a look at mine on John Lahr's web site, especially page _http://www.jclahr.com/science/psn/mcclure/horiz2.html_ (http://www.jclahr.com/science/psn/mcclure/horiz2.html) I operate three sensors for Z, N, and E. They are adjusted to fairly short natural periods, ranging from 5 to 14 seconds. I extend their effective period to 24 seconds by a digital filter of my own design, with excellent results. The magnets I recommend are available for $5.50 each, 50mm x 18mm x 6mm block magnet, from _http://www.gaussboys.com_ (http://www.gaussboys.com) You will need four of them and two pieces of 1/4 inch flat steel, two inches by three inches, and four steel bolts to make a magnet assembly. You do not necessarily need separate damping magnets. Just shunt the pickup coil with the appropriate resistance. Bob McClure Locust Valley, NY
Hi all,
 
  I agree with Chris Chapman on the design of magnets and coils fo= r=20 horizontal sensors. Those who have an interest in building their own sensors= =20 should take a look at mine on John Lahr's web site, especially page http://www.jc= lahr.com/science/psn/mcclure/horiz2.html
 
  I operate three sensors for Z, N, and E. They are adjusted to fa= irly=20 short natural periods, ranging from 5 to 14 seconds. I extend their effectiv= e=20 period to 24 seconds by a digital filter of my own design, with excellent=20 results.
 
  The magnets I recommend are available for $5.50 each, 50mm x 18m= m x=20 6mm block magnet, from http://www.gaussboys.com  You wil= l need=20 four of them and two pieces of 1/4 inch flat steel, two inches by three inch= es,=20 and four steel bolts to make a magnet assembly. You do not necessarily need=20 separate damping magnets. Just shunt the pickup coil with the appropriate=20 resistance.
 
Bob McClure
Locust Valley, NY
 Subject: Re: Seismograph for school From: ChrisAtUpw@....... Date: Sun, 1 Oct 2006 22:58:49 EDT In a message dated 2006/10/02, Bobhelenmcclure@....... writes: > I agree with Chris Chapman on the design of magnets and coils for > horizontal sensors. Hi Bob, Thanks! Chris In a me= ssage dated 2006/10/02, Bobhelenmcclure@....... writes:

I agree with Chris Chapman on=20= the design of magnets and coils for horizontal sensors.


Hi Bob,

       Thanks!

       Chris Subject: Re: Seismograph for school From: "tchannel" tchannel@.............. Date: Sun, 1 Oct 2006 22:12:06 -0600 Hi Bob, Thanks for the great picture. I really liked your ideas and = also the great one that Chris made. Chris posted a drawing showing the magnet layout. Two magnets on top, = end to end N to S, I think, and on the bottom two magnets end to end S = to N. I find they make these magnets with the N and S poles in various = arrangement. I don't remember the terms but are your poles on the ends or on the Axially (through 6mm = thickness)? In other words, laying flat, face up, North and flip it = over and that side is South? I made my first sensor, using a cow magnet and a coil in the shape of a = spool, the cow magnet's, North pole, only fits into the spool hole, to = the center point. If the pendulum move right, the North pole moves from = center to the end of the spool, if it move left the North pole moves = from the center of the spool to the entrance of the spool. It does = work, but I see these are much better arrangement. After reading these postings, I have a much better ideas about the coil = size and proper wire size. On the other hand I see picture of huge huge = magnets and coil. Is there a measurement in mv's or other test to tell = if one has made a coil/magnet arrangement which is proper in output? Thanks, for the ideas, Ted ----- Original Message -----=20 From: Bobhelenmcclure@.......... To: psn-l@................. Sent: Sunday, October 01, 2006 7:21 PM Subject: Re: Seismograph for school Hi all, I agree with Chris Chapman on the design of magnets and coils for = horizontal sensors. Those who have an interest in building their own = sensors should take a look at mine on John Lahr's web site, especially = page http://www.jclahr.com/science/psn/mcclure/horiz2.html I operate three sensors for Z, N, and E. They are adjusted to fairly = short natural periods, ranging from 5 to 14 seconds. I extend their = effective period to 24 seconds by a digital filter of my own design, = with excellent results. The magnets I recommend are available for $5.50 each, 50mm x 18mm x = 6mm block magnet, from http://www.gaussboys.com You will need four of = them and two pieces of 1/4 inch flat steel, two inches by three inches, = and four steel bolts to make a magnet assembly. You do not necessarily = need separate damping magnets. Just shunt the pickup coil with the = appropriate resistance. Bob McClure Locust Valley, NY
Hi Bob,  Thanks for the great picture.  I really liked = your ideas=20 and also the great one that Chris made.
Chris posted a drawing showing the magnet layout.  Two magnets = on top,=20 end to end N to S, I think, and on the bottom two magnets end to end S = to=20 N.   I find they make these magnets with the N and S poles in = various=20 arrangement.  I don't remember the terms
but are your poles on the ends or on the Axially (through 6mm=20 thickness)?   In other words, laying flat, face up, North and = flip it=20 over and that side is South?
 
I made my first sensor, using a cow magnet and a coil in the shape = of a=20 spool,  the cow magnet's, North pole, only fits into the spool = hole, to the=20 center point.  If the pendulum move right, the North pole = moves from=20 center to the end of the spool, if it move left the North pole moves = from the=20 center of the spool to the entrance of the spool.   It does = work, but=20 I see these are much better arrangement.
 
After reading these postings, I have a much better ideas about the = coil=20 size and proper wire size.  On the other hand I see picture of huge = huge=20 magnets and coil.   Is there a measurement in mv's or other = test to=20 tell if one has made a coil/magnet arrangement which is proper in = output?
 
Thanks, for the ideas,  Ted
----- Original Message -----
From:=20 Bobhelenmcclure@....... =
Sent: Sunday, October 01, 2006 = 7:21=20 PM
Subject: Re: Seismograph for = school

Hi all,
 
  I agree with Chris Chapman on the design of magnets and = coils for=20 horizontal sensors. Those who have an interest in building their own = sensors=20 should take a look at mine on John Lahr's web site, especially page http://www= ..jclahr.com/science/psn/mcclure/horiz2.html
 
  I operate three sensors for Z, N, and E. They are adjusted = to=20 fairly short natural periods, ranging from 5 to 14 seconds. I extend = their=20 effective period to 24 seconds by a digital filter of my own design, = with=20 excellent results.
 
  The magnets I recommend are available for $5.50 each, 50mm = x 18mm=20 x 6mm block magnet, from http://www.gaussboys.com  = You will=20 need four of them and two pieces of 1/4 inch flat steel, two inches by = three=20 inches, and four steel bolts to make a magnet assembly. You do not = necessarily=20 need separate damping magnets. Just shunt the pickup coil with the = appropriate=20 resistance.
 
Bob McClure
Locust Valley, NY
Subject: Magnests and coil for PVC seismometer & more From: jonfr500@......... Date: Mon, 2 Oct 2006 05:45:38 +0000 Hi all My Gentoo Linux hard drive is dieing on me (it's not dead, it's giving erro= rs), so I have to use webmail until I can replace it and get my system back= up. I am going to start to build my PVC seismometer this month, what I am hopef= ully going to buy this month are three things. Pair of magnets (no larger t= hen 4 cm in diamater), a coil and a resistance (like they use in geophones)= .. I need a suggestions on the magets, the coil and the resistance. The natr= ual frequancy of the PVC seismometer is ment to be 1Hz, that is a good plac= e for me to start. Regards. J=F3n Fr=EDmann. __________________________________________________________ Public Seismic Network Mailing List (PSN-L) Subject: Sensor calibration From: Bobhelenmcclure@....... Date: Mon, 2 Oct 2006 22:00:43 EDT I have been asked about sensor calibration. For my thoughts, please go to _http://www.seismicnet.com/psnlist/index04.html_ (http://www.seismicnet.com/psnlist/index04.html) to see my letter on the subject, submitted on Tue. 13 Jul 2004 19:33:44 GMT. I have used both the methods described there for my own sensors. Cheers, Bob McClure
  I have been asked about sensor calibration. For my thoughts, ple= ase=20 go to
http://www.seismicne= t.com/psnlist/index04.html to=20 see my letter on the subject, submitted on Tue. 13 Jul 2004 19:33:44 GMT. I=20= have=20 used both the methods described there for my own sensors.
 
Cheers,
 
Bob McClure
 Subject: Re: Seismograph for school From: "tchannel" tchannel@.............. Date: Tue, 3 Oct 2006 10:28:57 -0600 Hi Bob, I have questions about your great Horz. sensor and the picture. 1. You used solder for the mass, is the mass volume fixed or is there a = wide range of weights, one could use. I guess the mass is to overcome = the friction of the hinge and stay stationary. The reason I ask is I = see a wide range of masses, but most around 5#. What is yours? and what = is your beam length? 2. Again the solder, is non magnetic is this important as I see many = things use as mass including iron? 3. I see you used a resistor as a damp, and not a external damper = device. I have read about doing this, but could not understand the = values for the resistor. You used 94k (in parallal?) acrossed the coil? = How did you arrive at that value? 4. You indicate to use a 100k in place of the 10k when using Larry's = amp, which I will be using. Why is that? 5. The placement of the sensor: Could it be on other than a concrete = floor, a normal wood floor? Could it be on carpet? I know that is not = the best choice, but many schools have such floors. Or should one not = bother with any other surface but a concrete floor? Thanks for your advise, and "well done!" Ted ----- Original Message -----=20 From: Bobhelenmcclure@.......... To: psn-l@................. Sent: Sunday, October 01, 2006 7:21 PM Subject: Re: Seismograph for school Hi all, I agree with Chris Chapman on the design of magnets and coils for = horizontal sensors. Those who have an interest in building their own = sensors should take a look at mine on John Lahr's web site, especially = page http://www.jclahr.com/science/psn/mcclure/horiz2.html I operate three sensors for Z, N, and E. They are adjusted to fairly = short natural periods, ranging from 5 to 14 seconds. I extend their = effective period to 24 seconds by a digital filter of my own design, = with excellent results. The magnets I recommend are available for $5.50 each, 50mm x 18mm x = 6mm block magnet, from http://www.gaussboys.com You will need four of = them and two pieces of 1/4 inch flat steel, two inches by three inches, = and four steel bolts to make a magnet assembly. You do not necessarily = need separate damping magnets. Just shunt the pickup coil with the = appropriate resistance. Bob McClure Locust Valley, NY
Hi Bob,  I have questions about your great Horz. sensor and = the=20 picture.
 
1. You used solder for the mass,  is the mass volume fixed or = is there=20 a wide range of weights, one could use.  I guess the mass is to = overcome=20 the friction of the hinge and stay stationary.  The reason I ask is = I see a=20 wide range of masses, but most around 5#.  What is yours? and what = is your=20 beam length?
 
2. Again the solder, is non magnetic is this important as I see = many things=20 use as mass including iron?
 
3.  I see you used a resistor as a damp, and not a external = damper=20 device.  I have read about doing this, but could not understand the = values=20 for the resistor.  You used 94k (in parallal?) acrossed the = coil?  How=20 did you arrive at that value?
 
4. You indicate to use a 100k in place of the 10k when using = Larry's=20 amp,  which I will be using.  Why is that?
 
5.  The placement of the sensor:  Could it be on other = than a=20 concrete floor, a normal wood floor?  Could it be on carpet?  = I know=20 that is not the best choice, but many schools have such floors.  Or = should=20 one not bother with any other surface but a concrete floor?
 
Thanks for your advise, and "well done!"   Ted
----- Original Message -----
From:=20 Bobhelenmcclure@....... =
Sent: Sunday, October 01, 2006 = 7:21=20 PM
Subject: Re: Seismograph for = school

Hi all,
 
  I agree with Chris Chapman on the design of magnets and = coils for=20 horizontal sensors. Those who have an interest in building their own = sensors=20 should take a look at mine on John Lahr's web site, especially page http://www= ..jclahr.com/science/psn/mcclure/horiz2.html
 
  I operate three sensors for Z, N, and E. They are adjusted = to=20 fairly short natural periods, ranging from 5 to 14 seconds. I extend = their=20 effective period to 24 seconds by a digital filter of my own design, = with=20 excellent results.
 
  The magnets I recommend are available for $5.50 each, 50mm = x 18mm=20 x 6mm block magnet, from http://www.gaussboys.com  = You will=20 need four of them and two pieces of 1/4 inch flat steel, two inches by = three=20 inches, and four steel bolts to make a magnet assembly. You do not = necessarily=20 need separate damping magnets. Just shunt the pickup coil with the = appropriate=20 resistance.
 
Bob McClure
Locust Valley, NY
Subject: Locating earthqukes with winquake From: jonfr500@......... Date: Tue, 3 Oct 2006 18:20:26 +0000 Hi Is it possible for me to locate earthquakes with Winquake ? Becose I am goi= ng to setup a remote station soon, problay after 2 - 3 months (depending on= few things). The remote station won't be internet connected, but I will co= llect data from it once a month. Sometimes next week I will also get one sensor reading regarding a earthqua= ke that I did detect this summer, I am going to attemt to locate that earth= quake to some exstent just by using two stations. Regards. J=F3n Fr=EDmann. __________________________________________________________ Public Seismic Network Mailing List (PSN-L) Subject: Re: Locating earthqukes with winquake From: ChrisAtUpw@....... Date: Tue, 3 Oct 2006 18:53:16 EDT In a message dated 2006/10/03, jonfr500@......... writes: > Is it possible for me to locate earthquakes with Winquake ? Because I am > going to setup a remote station soon, probably after 2 - 3 months (depending on > few things). The remote station won't be internet connected, but I will > collect data from it once a month. > > Sometimes next week I will also get one sensor reading regarding an > earthquake that I did detect this summer, I am going to attempt to locate that > earthquake to some existent just by using two stations. Hi Jon, Before you spend any money.... The P wave may travel at ~8 km /sec. You are going to need fairly widely separated stations to get a reasonably accurate angle estimate, depending on the distance. You can determine the approximate distance from the P-S interval. Can you get on-line signals from one of the Icelandic seismic stations as your second source? At least to try out? If you are going to use this method, you will need precision timing at all times, about 0.1 sec or better. You are lucky if the software clock on a computer is accurate to 30 sec / day! They are hopeless for seismic work. This is likely to need a GPS receiver at both stations. Could you use an internet link? What sort of sensors were you planning to use? Regional and teleseismic quakes need a response down to 0.5 Hz. Regards, Chris Chapman In a me= ssage dated 2006/10/03, jonfr500@......... writes:

Is it possible for me to locate= earthquakes with Winquake ? Because I am going to setup a remote station so= on, probably after 2 - 3 months (depending on few things). The remote statio= n won't be internet connected, but I will collect data from it once a month.=

Sometimes next week I will also get one sensor reading regarding an earthqua= ke that I did detect this summer, I am going to attempt to locate that earth= quake to some existent just by using two stations.


Hi Jon,

       Before you spend any money....

       The P wave may travel at ~8 km /sec. Yo= u are going to need fairly widely separated stations to get a reasonably acc= urate angle estimate, depending on the distance. You can determine the appro= ximate distance from the P-S interval. Can you get on-line signals from one=20= of the Icelandic seismic stations as your second source? At least to try out= ?

       If you are going to use this method, yo= u will need precision timing at all times, about 0.1 sec or better. You are=20= lucky if the software clock on a computer is accurate to 30 sec / day! They=20= are hopeless for seismic work. This is likely to need a GPS receiver at both= stations.

       Could you use an internet link?

       What sort of sensors were you planning=20= to use? Regional and teleseismic quakes need a response down to 0.5 Hz.

       Regards,

       Chris Chapman Subject: Re: Locating earthqukes with winquake From: jonfr500@......... Date: Tue, 3 Oct 2006 23:23:37 +0000 Hi The distanst between the stations is going to be ~11 km. I am going to use = gps clock on both stations, it is a standard that I have implimented at cur= rent date. All IMO stations are digital stations and I cannot lisen to them. They also= use there own specalised decoding software, that I don't have accsess to. = But I can request a data from IMO and then I get a sac binary format files,= that I convert to psn format. The second station is going to use 4.5Hz geophone, like the one I have. The= second station is going to sit directly on a bedrock, no sand, nothing of = such short. I cannot have a internet connection, becose this is out in the = country side and the location doesn't have a fone line. Even then the only = connection option is a ISDN at 9.6kbps as speed, if I don't want to pay pr = min, charge. Regards. J=F3n Fr=EDmann. __________________________________________________________ Public Seismic Network Mailing List (PSN-L) Subject: Re: Seismograph for school From: ChrisAtUpw@....... Date: Tue, 3 Oct 2006 21:00:29 EDT In a message dated 2006/10/03, tchannel@.............. writes: > 1. You used solder for the mass, is the mass volume fixed or is there a > wide range of weights, one could use. I guess the mass is to overcome the > friction of the hinge and stay stationary. The reason I ask is I see a wide range > of masses, but most around 5#. What is yours? and what is your beam length? Hi Ted, The period of a simple pendulum T = 2 x Pi x sqrt(L / g) where L is t he length in m and g = 9.81 m / sec^2. With a garden gate arrangement you can extend the period by x20 or more, but this gets increasingly difficult since the suspension angles get tiny and you may get dimensional / temperature associated lack of stability. Make the cross bar width ~1/3 the base length. There are pendulum design sheets on John Lahr's website. If A is the angle between the vertical and the rotation axis of the gate, you multiply g in the equation above by sine A. You will be using tilts of less than 1 degree. A 22" arm (1.5 ec) set to 20 sec requires an angle of 0.322 deg; set to 30 sec it requires 0.143 deg. The period is independent of the mass of the pendulum, but depends on the 'radius of gyration' of the whole moving system. A heavy connecting beam can significantly reduce this. A reasonable mass is 1 to 2 lbs with about a 2 ft arm - 1.5 sec. If you use more weight than this, you will need a strong top suspension. I suggest that you work out this force and look up the strength of your wire? The idea behind the suspension is that there should be as little friction as possible. Pre electronic seismometers used mechanical lever gain linkages to move the pen and hence needed a large mass to overcome the friction. > 2. Again the solder, is non magnetic is this important as I see many things > use as mass including iron? I don't use any magnetic materials on the boom 'in principle'. You won't know for sure what is a magnetic interaction and what is seismic. If I am taking the time and effort to build a seismometer, avoiding obvious problems just makes good sense. Like put the pickup coil on the arm and the magnets on the baseplate! > 3. I see you used a resistor as a damp, and not a external damper device. > I have read about doing this, but could not understand the values for the > resistor. You used 94k (in parallel?) acrossed the coil? How did you arrive > at that value? Experiment. Exactly the same principle - any relative movement induces a voltage in the coil proportionate to the magnetic field, the number of turns on the coil, the velocity and the circuit resistance provides the damping loss. The lower this resistance, the greater the loss, but this also reduces the output voltage. Not significantly in this design, since the magnetic field and coupling are very high and the mass is low. > 4. You indicate to use a 100k in place of the 10k when using Larry's amp, > which I will be using. Why is that? Because an input resistor of 10 K is less than the required damping resistor for this particular sensor design. > 5. The placement of the sensor: Could it be on other than a concrete > floor, a normal wood floor? Could it be on carpet? I know that is not the best > choice, but many schools have such floors. Or should one not bother with any > other surface but a concrete floor? You are likely to find wood floors both noisy and lacking tilt stability - can be critical for a long period Lehman. I can't say no, but try it with little expectation of success? You can put wide melamine shelving on a carpet and add weight to hold it firm, but the thicker the carpet the lower the stability. Expect drift with temperature, humidity and time. Bricks? On a concrete floor, I stick on 2" squares of 1/8" SS plate with pool adhesive to provide a good base for the adjusting screws. Regards, Chris Chapman In a me= ssage dated 2006/10/03, tchannel@.............. writes:

1. You used solder for the mass= ,  is the mass volume fixed or is there a wide range of weights, one co= uld use. I guess the mass is to overcome the friction of the hinge and stay=20= stationary. The reason I ask is I see a wide range of masses, but most aroun= d 5#.  What is yours? and what is your beam length?


Hi Ted,

       The period of a simple pendulum T =3D 2= x Pi x sqrt(L / g) where L is the length in m and g =3D 9.81 m / sec^2. Wit= h a garden gate arrangement you can extend the period by x20 or more, but th= is gets increasingly difficult since the suspension angles get tiny and you=20= may get dimensional / temperature associated lack of stability. Make the cro= ss bar width ~1/3 the base length. There are pendulum design sheets on John=20= Lahr's website. If A is the angle between the vertical and the rotation axis= of the gate, you multiply g in the equation above by sine A.  You will= be using tilts of less than 1 degree. A 22" arm (1.5 ec) set to 20 sec requ= ires an angle of 0.322 deg; set to 30 sec it requires 0.143 deg.
       The period is independent of the mass o= f the pendulum, but depends on the 'radius of gyration' of the whole moving=20= system. A heavy connecting beam can significantly reduce this. A reasonable=20= mass is 1 to 2 lbs with about a 2 ft arm - 1.5 sec. If you use more weight t= han this, you will need a strong top suspension. I suggest that you work out= this force and look up the strength of your wire?
       The idea behind the suspension is that=20= there should be as little friction as possible. Pre electronic seismometers=20= used mechanical lever gain linkages to move the pen and hence needed a large= mass to overcome the friction.

2. Again the solder, is non mag= netic is this important as I see many things use as mass including iron?

       I don't use any magnetic materials on=20= the boom 'in principle'. You won't know for sure what is a magnetic interact= ion and what is seismic. If I am taking the time and effort to build a seism= ometer, avoiding obvious problems just makes good sense. Like put the pickup= coil on the arm and the magnets on the baseplate!

3.  I see you used a resis= tor as a damp, and not a external damper device.  I have read about doi= ng this, but could not understand the values for the resistor.  You use= d 94k (in parallel?) acrossed the coil?  How did you arrive at that val= ue?


       Experiment. Exactly the same principle= - any relative movement induces a voltage in the coil proportionate to the=20= magnetic field, the number of turns on the coil, the velocity and the circui= t resistance provides the damping loss. The lower this resistance, the great= er the loss, but this also reduces the output voltage. Not significantly in=20= this design, since the magnetic field and coupling are very high and the mas= s is low.

4. You indicate to use a 100k i= n place of the 10k when using Larry's amp, which I will be using. Why is tha= t?


       Because an input resistor of 10 K is l= ess than the required damping resistor for this particular sensor design.
5.  The placement of the=20= sensor: Could it be on other than a concrete floor, a normal wood floor? Cou= ld it be on carpet? I know that is not the best choice, but many schools hav= e such floors. Or should one not bother with any other surface but a concret= e floor?


       You are likely to find wood floors both= noisy and lacking tilt stability - can be critical for a long period Lehman= ..  I can't say no, but try it with little expectation of success? You c= an put wide melamine shelving on a carpet and add weight to hold it firm, bu= t the thicker the carpet the lower the stability. Expect drift with temperat= ure, humidity and time. Bricks? On a concrete floor, I stick on 2" squares o= f 1/8" SS plate with pool adhesive to provide a good base for the adjusting=20= screws.

       Regards,

       Chris Chapman Subject: Re: Locating earthqukes with winquake From: "Thomas Dick" dickthomas01@............. Date: Tue, 3 Oct 2006 20:20:07 -0500 what about using radio to connect to you computer like they are doing in Washington State, USA? __________________________________________________________ Public Seismic Network Mailing List (PSN-L) Subject: Re: Locating earthqukes with winquake From: jonfr500@......... Date: Wed, 4 Oct 2006 01:51:45 +0000 Hi There is a big mountin in between. I can't brodcast over it. I also think t= hat I might need a premission for such type of radio transmission, it probl= ay costs money. Regards. J=F3n Fr=EDmann. __________________________________________________________ Public Seismic Network Mailing List (PSN-L) Subject: Re: Seismograph for school From: Ben Bradley benbradley@............... Date: Tue, 03 Oct 2006 21:59:10 -0400 ChrisAtUpw@....... wrote: > In a message dated 2006/10/03, tchannel@.............. writes: >>2. Again the solder, is non magnetic is this important as I see many things >>use as mass including iron? > > > I don't use any magnetic materials on the boom 'in principle'. You > won't know for sure what is a magnetic interaction and what is seismic. If I am > taking the time and effort to build a seismometer, avoiding obvious problems > just makes good sense. Like put the pickup coil on the arm and the magnets on > the baseplate! This makes good sense for eliminating magnetic effects on the boom, but you then have electric connections to something on the boom. How do you do connect to the coil and minimize friction and "spring"-type interference to the boom from the connecting wires? I have my own ideas in this area that I think are good, but I'd like to hear what others do for this. > Regards, > > Chris Chapman > __________________________________________________________ Public Seismic Network Mailing List (PSN-L) Subject: Building a lehman seismometer From: jonfr500@......... Date: Wed, 4 Oct 2006 02:18:59 +0000 Hi In my planning of building a PVC seimsmometer, I also plan to build a Lehma= n seismometer, if I can keep it small enugh. But I need a suggestion for ma= terial and some mesurements and proper drawings before I can start. I also = want to keep the frequancy of the Lehman seismometer around 1Hz. Regards. J=F3n Fr=EDmann. __________________________________________________________ Public Seismic Network Mailing List (PSN-L) Subject: Re: Seismograph for school From: ChrisAtUpw@....... Date: Tue, 3 Oct 2006 22:54:19 EDT In a message dated 2006/10/04, benbradley@............... writes: > How do you do connect to the coil and minimize friction and "spring"-type > interference to the boom from the connecting wires? Hi Ben, The usual method is to use two small coils, or 'hairpins' maybe 2" long, of the finest insulated copper wire that you can get, which is likely to be 36 gauge, 5 thou, across the lower bearing. If you mount them vertically, they have the least torque. A small torque which is constant will not matter and it is balanced out during levelling. The electrical resistance will be very small compared to that of the coil. If you use wire like Beldsol, it has polyurethane insulation. You wrap it around the terminal post, apply a hot soldering iron and resin cored solder and the insulation just melts. You do not need to 'strip' the wire. Regards, Chris Chapman In a me= ssage dated 2006/10/04, benbradley@............... writes:

How do you do connect to the co= il and minimize friction and "spring"-type
interference to the boom from the connecting wires?


Hi Ben,

       The usual method is to use two small co= ils, or 'hairpins' maybe 2" long, of the finest insulated copper wire that y= ou can get, which is likely to be 36 gauge, 5 thou, across the lower bearing= .. If you mount them vertically, they have the least torque. A small torqu= e which is constant will not matter and it is balanced out during levell= ing. The electrical resistance will be very small compared to that of the co= il. If you use wire like Beldsol, it has polyurethane insulation. You wrap i= t around the terminal post, apply a hot soldering iron and resin cored solde= r and the insulation just melts. You do not need to 'strip' the wire.

       Regards,

       Chris Chapman Subject: Re: Building a lehman seismometer From: ChrisAtUpw@....... Date: Tue, 3 Oct 2006 23:23:14 EDT In a message dated 2006/10/04, jonfr500@......... writes: > I also plan to build a Lehman seismometer, if I can keep it small enough. > But I need a suggestion for material and some mesurements and proper drawings > before I can start. I also want to keep the frequency of the Lehman > seismometer around 1Hz. Hi Jon, The whole point about a Lehman type seismometer is that you take a pedulum which would have a period of about 1.3 to 1.5 sec if mounted vertically and extend the period to maybe 20 sec. The sensor will respond from 20 sec to the 10 Hz of your amplifier filter. I suspect that an arm of between 45 and 60 cm would be fine for you. Don't try to make it smaller. This should pick up your seismic signals very well. I make a T / L frame with U channel Al with 5 mm Al plate for the corner supports. I don't know what materials are available to you. You can use 50 x 25 mm, but I prefer 75 x 25 mm section. Regards, Chris Chapman In a me= ssage dated 2006/10/04, jonfr500@......... writes:

I also plan to build a Lehman s= eismometer, if I can keep it small enough. But I need a suggestion for mater= ial and some mesurements and proper drawings before I can start. I also want= to keep the frequency of the Lehman seismometer around 1Hz.

Hi Jon,

       The  whole point about a Lehman ty= pe seismometer is that you take a pedulum which would have a period of about= 1.3 to 1.5 sec if mounted vertically and extend the period to maybe 20 sec.= The sensor will respond from 20 sec to the 10 Hz of your amplifier filter.=20= I suspect that an arm of between 45 and 60 cm would be fine for you. Don't t= ry to make it smaller. This should pick up your seismic signals very well.
       I make a T / L frame with U channel Al=20= with 5 mm Al plate for the corner supports. I don't know what materials are=20= available to you. You can use 50 x 25 mm, but I prefer 75 x 25 mm section.
       Regards,

       Chris Chapman Subject: ebay auction From: Bob Barns royb1@........... Date: Wed, 04 Oct 2006 09:50:06 -0400 INTENSITRON Earthquake Intensity Computer (seismometer) Item number: 160037359005 ends Oct 8 Bob __________________________________________________________ Public Seismic Network Mailing List (PSN-L) Subject: Re: Building a lehman seismometer From: "tchannel" tchannel@.............. Date: Wed, 4 Oct 2006 11:00:34 -0600 Hi Everyone, I did find some 38 gauge magnet wire, but thought you = might like to see this on ebay. 37 gauge Item number: 290035960569 one = pound. Thanks, Ted ----- Original Message ----- From: ChrisAtUpw@.......... To: psn-l@................. Sent: Tuesday, October 03, 2006 9:23 PM Subject: Re: Building a lehman seismometer In a message dated 2006/10/04, jonfr500@......... writes: I also plan to build a Lehman seismometer, if I can keep it small = enough. But I need a suggestion for material and some mesurements and = proper drawings before I can start. I also want to keep the frequency of = the Lehman seismometer around 1Hz. Hi Jon,=20 The whole point about a Lehman type seismometer is that you = take a pedulum which would have a period of about 1.3 to 1.5 sec if = mounted vertically and extend the period to maybe 20 sec. The sensor = will respond from 20 sec to the 10 Hz of your amplifier filter. I = suspect that an arm of between 45 and 60 cm would be fine for you. Don't = try to make it smaller. This should pick up your seismic signals very = well. I make a T / L frame with U channel Al with 5 mm Al plate for = the corner supports. I don't know what materials are available to you. = You can use 50 x 25 mm, but I prefer 75 x 25 mm section. Regards, Chris Chapman
Hi Everyone,  I did find some 38 = gauge magnet=20 wire, but thought you might like to see this on ebay.  37 gauge = Item=20 number: 290035960569 one pound.
 
Thanks, Ted
 
----- Original Message ----- = From: ChrisAtUpw@.......
Sent: Tuesday, October 03, 2006 = 9:23=20 PM
Subject: Re: Building a lehman=20 seismometer

In a=20 message dated 2006/10/04, jonfr500@......... = writes:

I also plan to build a Lehman seismometer, if I can keep = it=20 small enough. But I need a suggestion for material and some = mesurements and=20 proper drawings before I can start. I also want to keep the = frequency of the=20 Lehman seismometer around 1Hz.


Hi Jon,=20

       The  whole point = about a=20 Lehman type seismometer is that you take a pedulum which would have a = period=20 of about 1.3 to 1.5 sec if mounted vertically and extend the period to = maybe=20 20 sec. The sensor will respond from 20 sec to the 10 Hz of your = amplifier=20 filter. I suspect that an arm of between 45 and 60 cm would be fine = for you.=20 Don't try to make it smaller. This should pick up your seismic signals = very=20 well.

       I make a T / L frame = with U=20 channel Al with 5 mm Al plate for the corner supports. I don't know = what=20 materials are available to you. You can use 50 x 25 mm, but I prefer = 75 x 25=20 mm section.

      =20 Regards,

       Chris Chapman=20
Subject: Maine US Quakes From: "Thomas Dick" dickthomas01@............. Date: Thu, 5 Oct 2006 09:36:31 -0500 Thsi in our local paper this morning ... I quote Bar Harbor, Maine title of article "Water table drops after quake" Moinday's earthquake lowered the water level by 3 1/2 feet at one lo0cation in a national park but wasn't expected to hurt the water supply, officials said. By Wednesday, the water level was still falling but beginning to level off, said Gregory Stewart, a hydrologist water level was observed at one of the ageny's monitoring wells in Acadia National Park. The earthquake registered a magnitude of 3.9 and followed quakes with magnitudes of 3.5 and 2.5 on Sept. 22 and 26. and Maine was one place they felt safe putting atomic generating plants! __________________________________________________________ Public Seismic Network Mailing List (PSN-L) Subject: Re: Building a lehman seismometer From: "tchannel" tchannel@.............. Date: Thu, 5 Oct 2006 09:46:28 -0600 Hi Chris, A few questions: 1 Does your coil and damper assemblies just set on the aluminum base, = free to move for adjustment, or are they fixed to the base? 2 You cleaned the plates, which hold the magnets, and then painted the = exterior, but not the surface the magnets touch. Would the paint = interfere with something on that surface? 3 This is a hard question. Assuming I have finished my sensor, and = leveled it, what is the procedure for adjusting the period? I know the = target angle. I would guess one would have to start at that point, with the base = level, the pendulum level, and everything plumb. You mentioned the formula for determining the angle of the hinge, I have = that information. Not knowing the correct procedure, and not being able = to measure that small of an angle, I would have converted the angle into = a decimal of inches, and measured the difference from the top hinge to = the bottom hinge. Maybe you could walk me through this procedure? 4 I get confused with many of these terms, so I hope you understand this = question. If the target period is say 10 seconds, would the arm move, = when pulled 10mm, verrrrrrrrrry slowly, taking 10 seconds to complete = one cycle? 5 After it is leveled and the period is set, Is it then that you can = check the damp, by moving the arm 10mm and releasing it? I know what an underdamped situation would look like. But is there way = to tell if it is over damped? Thanks, Ted ----- Original Message -----=20 From: ChrisAtUpw@.......... To: psn-l@................. Sent: Tuesday, October 03, 2006 9:23 PM Subject: Re: Building a lehman seismometer In a message dated 2006/10/04, jonfr500@......... writes: I also plan to build a Lehman seismometer, if I can keep it small = enough. But I need a suggestion for material and some mesurements and = proper drawings before I can start. I also want to keep the frequency of = the Lehman seismometer around 1Hz. Hi Jon,=20 The whole point about a Lehman type seismometer is that you = take a pedulum which would have a period of about 1.3 to 1.5 sec if = mounted vertically and extend the period to maybe 20 sec. The sensor = will respond from 20 sec to the 10 Hz of your amplifier filter. I = suspect that an arm of between 45 and 60 cm would be fine for you. Don't = try to make it smaller. This should pick up your seismic signals very = well. I make a T / L frame with U channel Al with 5 mm Al plate for = the corner supports. I don't know what materials are available to you. = You can use 50 x 25 mm, but I prefer 75 x 25 mm section. Regards, Chris Chapman
Hi Chris,   A few = questions:
 
1 Does your coil and damper = assemblies just=20 set on the aluminum base, free to move for adjustment, or are they fixed = to the=20 base?
 
2 You cleaned the plates, which = hold the=20 magnets, and then painted the exterior, but not the surface the magnets=20 touch.  Would the paint interfere with something on that=20 surface?
 
3 This is a hard question.  = Assuming I=20 have finished my sensor, and leveled it, what is the procedure for = adjusting the=20 period?  I know the target angle.
I would guess one would have to start = at that=20 point, with the base level, the pendulum level, and everything=20 plumb.
You mentioned the formula for = determining the angle=20 of the hinge, I have that information.  Not knowing the correct = procedure,=20 and not being able to measure that small of an angle, I would have = converted the=20 angle into a decimal of inches, and measured the difference from the top = hinge=20 to the bottom hinge.  Maybe you could walk me through this=20 procedure?
 
4 I = get confused=20 with many of these terms, so I hope you understand this question.  = If the=20 target period is say 10 seconds, would the arm move, when pulled 10mm,=20 verrrrrrrrrry slowly, taking 10 seconds to complete one = cycle?
 
5  After it is leveled and the = period is set,=20 Is it then that you can check the damp, by moving the arm 10mm and = releasing=20 it?
I know what an underdamped situation = would look=20 like.  But is there way to tell if it is over damped?
 
Thanks, Ted
 
 
 
 
----- Original Message -----
From:=20 ChrisAtUpw@.......
Sent: Tuesday, October 03, 2006 = 9:23=20 PM
Subject: Re: Building a lehman=20 seismometer

In a=20 message dated 2006/10/04, jonfr500@......... = writes:

I also plan to build a Lehman seismometer, if I can keep = it=20 small enough. But I need a suggestion for material and some = mesurements and=20 proper drawings before I can start. I also want to keep the = frequency of the=20 Lehman seismometer around 1Hz.

Hi Jon,=20

       The  whole point = about a=20 Lehman type seismometer is that you take a pedulum which would have a = period=20 of about 1.3 to 1.5 sec if mounted vertically and extend the period to = maybe=20 20 sec. The sensor will respond from 20 sec to the 10 Hz of your = amplifier=20 filter. I suspect that an arm of between 45 and 60 cm would be fine = for you.=20 Don't try to make it smaller. This should pick up your seismic signals = very=20 well.

       I make a T / L frame = with U=20 channel Al with 5 mm Al plate for the corner supports. I don't know = what=20 materials are available to you. You can use 50 x 25 mm, but I prefer = 75 x 25=20 mm section.

      =20 Regards,

       Chris Chapman=20 Subject: Re: Building a lehman seismometer From: ChrisAtUpw@....... Date: Thu, 5 Oct 2006 15:26:42 EDT In a message dated 2006/10/05, tchannel@.............. writes: > 1 Does your coil and damper assemblies just set on the aluminum base, free > to move for adjustment, or are they fixed to the base? Hi Ted, My arm has a support wire fixture and then a mass right at the end of the arm. The support fixture carries a damping tongue in the direction of the hinge and a coil on the mass side. Both the sensor and damper magnets slide on the base strip, for mounting and adjustment of their position. They have small guide strips underneath which hold them parallel. > 2 You cleaned the plates, which hold the magnets, and then painted the > exterior, but not the surface the magnets touch. Would the paint interfere with > something on that surface? I suggest that you paint everything with Hammerite or similar, but try to get it smooth and level. I etch coated the magnet position with phosphoric acid, allowed it to react and dry overnight and then put CL anticorrosion car grease on it. I was not certain that I could get enough damping initially, but I found that I could overdamp the system quite easily, so I increased the damper magnet spacing. > 3 This is a hard question. Assuming I have finished my sensor, and levelled > it, what is the procedure for adjusting the period? Maybe you could walk me > through this procedure? You first lower the base plate / strip at the mass end. Then you mount the arm and make any necessary solder connections for the coil wiring. Then you put the magnet units on the base and adjust the top wire so that both the coil and the damping plate are central within the magnet spacings. The first time, you may need to make adjustments of the magnet block heights, so that the arm is parallel to the base. Then you slide the damper free and adjust the cross levels so that the arm is stable in the central position. If you can't get a stable central position, you need to adjust the suspension so that the top hinge is nearer the mass end - or the bottom fitting is further away. You may mark the end of the arm with an inkline and also put a central mark on the base. You deflect the arm and time a swing. It may be about 5 sec initially. Then you slowly raise the base at the mass end of the arm with the adjusting screw, testing to determine the period, until you get the period you want. As the period is lengthened, you will probably need to trim the cross level slightly. Then you slide the damping magnet over edge of the tongue, deflect the arm 10 mm and release it. It may help to stick a bit of graph paper to the base to be able the measure the position of the damping block. 0.7 damping is obtained when the arm swings 0.5 mm past the zero position and then falls back again. The amount of damping decreases as the set period increases, so it needs to be easily adjustable. If the critical position is obtained with the tongue less that half covering the magnets, increase the damper magnet spacing. > 4 I get confused with many of these terms, so I hope you understand this > question. If the target period is say 10 seconds, would the arm move, when > pulled 10mm, very slowly, taking 10 seconds to complete one cycle? That is correct. Assuming that the arm is not damped, you deflect it and then release it. It takes 2.5 sec to get to zero, overshoots and comes back to zero in 5 sec, overshoots again and comes back to zero from the initial direction in another 5 sec. Total 5 + 5 = 10 sec. The period is the time for one complete oscillation cycle. You want at least twice this. > 5 After it is leveled and the period is set, Is it then that you can check > the damp, by moving the arm 10mm and releasing it? Correct, you set up the period first and then you slide the damper further over the tongue in small steps until it is just a bit under critical. > I know what an underdamped situation would look like. But is there way > to tell if it is over damped? As you increase the damping from zero, the time that the arm takes to fall back through / to the balance position increases slightly. If you overdamp the arm, it never swings through the zero position and the more you damp it, the longer it takes to get there. Look for the arm swinging just past the zero line, but without another oscillation. Regards, Chris Chapman In a me= ssage dated 2006/10/05, tchannel@.............. writes:

1 Does your coil and damper ass= emblies just set on the aluminum base, free to move for adjustment, or are t= hey fixed to the base?


Hi Ted,

       My arm has a support wire fixture and t= hen a mass right at the end of the arm.
       The support fixture carries a damping t= ongue in the direction of the hinge and a coil on the mass side.
       Both the sensor and damper magnets slid= e on the base strip, for mounting and adjustment of their position. They hav= e small guide strips underneath which hold them parallel.  

2 You cleaned the plates, whic= h hold the magnets, and then painted the exterior, but not the surface the m= agnets touch.  Would the paint interfere with something on that surface= ?


       I suggest that you paint everything wi= th Hammerite or similar, but try to get it smooth and level. I etch coated t= he magnet position with phosphoric acid, allowed it to react and dry overnig= ht and then put CL anticorrosion car grease on it. I was not certain that I=20= could get enough damping initially, but I found that I could overdamp the sy= stem quite easily, so I increased the damper magnet spacing.

3 This is a hard question. Ass= uming I have finished my sensor, and levelled it, what is the procedure for=20= adjusting the period? Maybe you could walk me through this procedure?=

       You first lower the base plate / strip= at the mass end. Then you mount the arm and make any necessary solder conne= ctions for the coil wiring. Then you put the magnet units on the base and ad= just the top wire so that both the coil and the damping plate are central wi= thin the magnet spacings. The first time, you may need to make adjustments o= f the magnet block heights, so that the arm is parallel to the base.
       Then you slide the damper free and adju= st the cross levels so that the arm is stable in the central position. If yo= u can't get a stable central position, you need to adjust the suspension so=20= that the top hinge is nearer the mass end - or the bottom fitting is further= away. You may mark the end of the arm with an inkline and also put a centra= l mark on the base. You deflect the arm and time a swing. It may be about 5=20= sec initially. Then you slowly raise the base at the mass end of the arm wit= h the adjusting screw, testing to determine the period, until you get the pe= riod you want. As the period is lengthened, you will probably need to trim t= he cross level slightly.
       Then you slide the damping magnet o= ver edge of the tongue, deflect the arm 10 mm and release it. It may help to= stick a bit of graph paper to the base to be able the measure the position=20= of the damping block. 0.7 damping is obtained when the arm swings 0.5 mm pas= t the zero position and then falls back again.
       The amount of damping decreases as the=20= set period increases, so it needs to be easily adjustable. If the critical p= osition is obtained with the tongue less that half covering the magnets, inc= rease the damper magnet spacing.

4 I get confused with many of=20= these terms, so I hope you understand this question.  If the target per= iod is say 10 seconds, would the arm move, when pulled 10mm, very slowly, ta= king 10 seconds to complete one cycle?


       That is correct. Assuming that the arm= is not damped, you deflect it and then release it. It takes 2.5 sec to get=20= to zero, overshoots and comes back to zero in 5 sec, overshoots again and co= mes back to zero from the initial direction in another 5 sec. Total 5 + 5=20= =3D 10 sec. The period is the time for one complete oscillation cycle. You w= ant at least twice this.

5  After it is leveled an= d the period is set, Is it then that you can check the damp, by moving the a= rm 10mm and releasing it?


       Correct, you set up the period first a= nd then you slide the damper further over the tongue in small steps until it= is just a bit under critical.

    I know what= an underdamped situation would look like.  But is there way to tell if= it is over damped?


       As you increase the damping from zero,=20= the time that the arm takes to fall back through / to the balance position i= ncreases slightly. If you overdamp the arm, it never swings through the zero= position and the more you damp it, the longer it takes to get there. Look f= or the arm swinging just past the zero line, but without another oscillation= ..

       Regards,

       Chris Chapman Subject: Sensor magnet and coil design From: Bobhelenmcclure@....... Date: Thu, 5 Oct 2006 16:11:35 EDT I submitting this note in order to give others the benefits of my own sometimes trial-and-error efforts to build a seismic sensor. I am sure that Chris Chapman will continue to contribute, as well. Both Chris Chapman and I favor the use of Neodymium Iron Boron magnets arranged in a four-pole structure for generating the magnetic field needed for velocity sensing and damping in seismic sensors. The magnets are powerful, relatively cheap, and easily obtained. The magnets I favor are block magnets, 6 mm thick, 18 mm wide, and 50 mm long. They are magnetized in the thickness direction and are of the composition known as N38. They have a coercive force of about 12,000 Oersteds (CGS units), corresponding to 1.2 Teslas in the SI system. Construction of a four-pole magnet assembly is very simple, but also very hazardous. These magnets are attracted to steel and to each other with a force of about 57 pounds per square inch, and any skin caught between them can be badly nipped. You must keep loose magnets very far away from steel or each other and wear heavy leather work gloves when handling them. The magnets are brittle, and will break if they crash together or with steel. The pole plates are mild steel, thick enough to carry the flux of the magnetic circuit. More on that later. One of the plates must have clearance holes for 3/16" bolts, one hole in each corner. The plates, for the above magnet dimensions, should measure two inches by at least three inches. Slide two magnets onto each plate, with opposite poling on each plate. When you have one magnet in position on a plate, carefully hold the second magnet above the first. You will feel either an attraction or a repulsion of the second magnet toward the one on the plate. Make sure the force is repulsive, then slide the second magnet into place beside the first. When the magnets are positioned on each plate, make a wooden or plastic shim somewhat thinner than the final magnet gap you intend to use. I would use a shim about six inches long and the same width as the paired magnets, which would be about 1.5 inches for the example. With the shim covering the magnets of one plate, hold the second plate over it, getting no closer than enough to determine if the force between them is attractive or repulsive. The correct alignment is indicated by an attractive force. You now place the upper plate (magnets down) on the shim at one end of the lower plate and slide the upper plate and magnets up over the lower magnets. You now have a sandwich of upper and lower assemblies tightly squeezing the shim in between. Place four bolts in the plate with the holes. Note than these bolts are used to hold the plates apart, not together. Use the bolts to jack the plates apart to get the desired gap spacing, and then you can withdraw the shim. If each of your plates have clearance holes drilled in them, you will need two nuts for each screw. Now it is time to discuss what field strength you get, and what thickness of steel is required. Suppose for my example using 6 mm thick magnets, I use a gap of 6 mm. The resulting distance between plates is 18 mm, 12 mm filled with magnet, and 6 mm with air. I call the ratio of total magnet thickness to total plate separation the filling factor. To a first approximation, the gap field the coercive force of the magnetic material multiplied by the filling factor. In this example, the coercive force is 12 KOe and the filling factor is 2/3, so the gap field is 8 KOe. The accuracy of this estimate depends on the magnet width compared to the gap size. If the gap becomes appreciable compared to the width, you will get more fringing field and less-than-expected gap field. I have written a program that solves LaPlace's equation by iteration which lets me estimate more exactly what the penalty is for any geometry. For my example magnet structure, I use 1/4 inch thick steel plates. I find that there is a very slight saturation of the plates in the region between magnet pairs. When I use 24 mm wide magnets, there is a lot of saturation and a lot of external stray field around the whole assembly. I have to put steel side plates on the assembly to eliminate the saturation and the stray field. Steel can carry a flux density of about 20 to 24 KOe without saturation. Without saturation, the magnetic circuit of a 4-pole structure consists of a closed rectangular flux path, with lines traveling through magnets, gaps, and steel. The steel must carry the same total number of lines of flux as the gap and magnets. The number of lines is proportional to the gap flux density multiplied by the magnet width. In the example of 8 KOe and 18 mm width, this corresponds to 8,000 times 1.8, or 14,000 lines per lineal centimeter of magnet structure. The quarter-inch-thick steel (0.625 cm) must carry a flux density of 22,000 KOe. It does so, barely. The next topic is coil design. By following the above magnet assembly design principles, you know the approximate gap field, so by also knowing the magnet length you will be able to estimate the number of coil turns you will need to achieve a given output sensitivity in volts per meter per second. If the end loops of the coil extend beyond the ends of the magnet, each turn is immersed in a total field length of twice the magnet length. In the example design, the magnet length is 50 mm, so L per turn is 0.1 m. The output voltage generated by the moving coil is Volts = B * N * L * Vel, where B is field strength in Teslas, N is number of turns, L is length per turn in the field, and Vel is velocity in meters per second. One Tesla corresponds to 10,000 Oersteds. Suppose n = 1100, B = 0.8 Tesla, and L = 0.1 meter. Then Volts/Vel = 0.8 * 1100 * 0.1 = 88 v-s/m, which is a good number to strive for. A wire size of #38 or less will allow this number of turns to fit comfortably within the gap field cross-section. The coil will have a resistance low enough to permit resistive shunt damping of a pendulum weighing up to a kilogram, in my opinion. My sensors have a pendulum mass of about 0.1Kg, and critical damping is achieved at about 30 kOhms. Since the coil resistance is only 340 Ohms, the shunt damping imposes negligible loss on output sensitivity. Even a kilogram mass would require only about 10% loss of output using shunt damping. There is one significant complication to this type of magnet and coil design, having to do with the fact that pure copper is diamagnetic, and coil and its pendulum are subject to forces other than the desired restoring force of a garden gate pendulum. In the example design this diamagnetic force is a decentering force, making it very difficult to adjust the sensor to a state of stable long period equilibrium. I reduce this effect by essentially potting the coil in acrylic plastic, which is also diamagnetic, and having the boundaries of the plastic extend well outside the edges of the magnetic field. Using a heavier pendulum would be of great benefit, as well. Bob McClure
  I submitting this note in order to give others the benefits of m= y=20 own sometimes trial-and-error efforts to build a seismic sensor. I am sure t= hat=20 Chris Chapman will continue to contribute, as well.
 
  Both Chris Chapman and I favor the use of Neodymium Iron Boron=20 magnets arranged in a four-pole structure for generating the magnetic field=20 needed for velocity sensing and damping in seismic sensors. The magnets are=20 powerful, relatively cheap, and easily obtained. The magnets I favor ar= e=20 block magnets, 6 mm thick, 18 mm wide, and 50 mm long. They are magnetized i= n=20 the thickness direction and are of the composition known as N38. They have a= =20 coercive force of about 12,000 Oersteds (CGS units), corresponding to 1.2 Te= slas=20 in the SI system.
 
  Construction of a four-pole magnet assembly is very simple, but=20= also=20 very hazardous. These magnets are attracted to steel and to each other with=20= a=20 force of about 57 pounds per square inch, and any skin caught between them c= an=20 be badly nipped. You must keep loose magnets very far away from steel or eac= h=20 other and wear heavy leather work gloves when handling them. The magnets are= =20 brittle, and will break if they crash together or with steel. The pole plate= s=20 are mild steel, thick enough to carry the flux of the magnetic circuit. More= on=20 that later. One of the plates must have clearance holes for 3/16" bolts, one= =20 hole in each corner. The plates, for the above magnet dimensions, should mea= sure=20 two inches by at least three inches.
 
  Slide two magnets onto each plate, with opposite poling on each=20 plate. When you have one magnet in position on a plate, carefully hold the=20 second magnet above the first. You will feel either an attraction or a repul= sion=20 of the second magnet toward the one on the plate. Make sure the force is=20 repulsive, then slide the second magnet into place beside the first. When th= e=20 magnets are positioned on each plate, make a wooden or plastic shim somewhat= =20 thinner than the final magnet gap you intend to use. I would use a shim abou= t=20 six inches long and the same width as the paired magnets, which would be abo= ut=20 1.5 inches for the example. With the shim covering the magnets of one plate,= =20 hold the second plate over it, getting no closer than enough to determine if= the=20 force between them is attractive or repulsive. The correct alignment is=20 indicated by an attractive force. You now place the upper plate (magnets dow= n)=20 on the shim at one end of the lower plate and slide the upper plate and magn= ets=20 up over the lower magnets. You now have a sandwich of upper and lower assemb= lies=20 tightly squeezing the shim in between.
 
  Place four bolts in the plate with the holes. Note than these bo= lts=20 are used to hold the plates apart, not together. Use the bolts to jack the=20 plates apart to get the desired gap spacing, and then you can withdraw the s= him.=20 If each of your plates have clearance holes drilled in them, you will need t= wo=20 nuts for each screw.
 
  Now it is time to discuss what field strength you get, and what=20 thickness of steel is required. Suppose for my example using 6 mm thick magn= ets,=20 I use a gap of 6 mm. The resulting distance between plates is 18 mm, 12 mm=20 filled with magnet, and 6 mm with air. I call the ratio of total magnet=20 thickness to total plate separation the filling factor. To a first=20 approximation, the gap field the coercive force of the magnetic material=20 multiplied by the filling factor. In this example, the coercive force is 12=20= KOe=20 and the filling factor is 2/3, so the gap field is 8 KOe. The accuracy of th= is=20 estimate depends on the magnet width compared to the gap size. If the gap=20 becomes appreciable compared to the width, you will get more fringing field=20= and=20 less-than-expected gap field. I have written a program that solves LaPlace's= =20 equation by iteration which lets me estimate more exactly what the penalty i= s=20 for any geometry.
 
  For my example magnet structure, I use 1/4 inch thick steel plat= es.=20 I find that there is a very slight saturation of the plates in the region=20 between magnet pairs. When I use 24 mm wide magnets, there is a lot of=20 saturation and a lot of external stray field around the whole assembly. I ha= ve=20 to put steel side plates on the assembly to eliminate the saturation and the= =20 stray field. Steel can carry a flux density of about 20 to 24 KOe without=20 saturation. Without saturation, the magnetic circuit of a 4-pole structure=20 consists of a closed rectangular flux path, with lines traveling through=20 magnets, gaps, and steel. The steel must carry the same total number of line= s of=20 flux as the gap and magnets. The number of lines is proportional to the gap=20= flux=20 density multiplied by the magnet width. In the example of 8 KOe and 18 mm wi= dth,=20 this corresponds to 8,000 times 1.8, or 14,000 lines per lineal centime= ter=20 of magnet structure. The quarter-inch-thick steel (0.625 cm) must carry a fl= ux=20 density of 22,000 KOe. It does so, barely.
 
  The next topic is coil design. By following the above magnet=20 assembly design principles, you know the approximate gap field, so by also=20 knowing the magnet length you will be able to estimate the number of coil tu= rns=20 you will need to achieve a given output sensitivity in volts per meter per=20 second. If the end loops of the coil extend beyond the ends of the magnet, e= ach=20 turn is immersed in a total field length of twice the magnet length. In the=20 example design, the magnet length is 50 mm, so L per turn is 0.1 m.
 
  The output voltage generated by the moving coil is Volts =3D B *= N * L=20 * Vel, where B is field strength in Teslas, N is number of turns, L is lengt= h=20 per turn in the field, and Vel is velocity in meters per second. One Tesla=20 corresponds to 10,000 Oersteds. Suppose n =3D 1100, B =3D 0.8 Tesla, and L=20= =3D 0.1=20 meter. Then Volts/Vel =3D 0.8 * 1100 * 0.1 =3D 88 v-s/m, which is a good num= ber to=20 strive for.
 
  A wire size of #38 or less will allow this number of turns to fi= t=20 comfortably within the gap field cross-section. The coil will have a resista= nce=20 low enough to permit resistive shunt damping of a pendulum weighing up to a=20 kilogram, in my opinion. My sensors have a pendulum mass of about 0.1Kg, and= =20 critical damping is achieved at about 30 kOhms. Since the coil resistance is= =20 only 340 Ohms, the shunt damping imposes negligible loss on output sensitivi= ty.=20 Even a kilogram mass would require only about 10% loss of output using shunt= =20 damping.
 
  There is one significant complication to this type of magnet and= =20 coil design, having to do with the fact that pure copper is diamagnetic, and= =20 coil and its pendulum are subject to forces other than the desired restoring= =20 force of a garden gate pendulum. In the example design this diamagnetic forc= e is=20 a decentering force, making it very difficult to adjust the sensor to a stat= e of=20 stable long period equilibrium. I reduce this effect by essentially potting=20= the=20 coil in acrylic plastic, which is also diamagnetic, and having the boundarie= s of=20 the plastic extend well outside the edges of the magnetic field. Using a hea= vier=20 pendulum would be of great benefit, as well.
 
Bob McClure
 Subject: Re: Building a lehman seismometer From: "tchannel" tchannel@.............. Date: Thu, 5 Oct 2006 15:50:06 -0600 Thanks, Chris for your reply. Most helpful I am in the process of building a sensor bases on your prototype, and = great drawings. I have most of the parts located, again with your help = and others on the mailing list. I will send pictures as I go along, if = you like, and if you tell me how to send them. I regards to the angle between the bottom and top hinge, a fraction of a = degree, how do you measure for that? Or do you adjust and measure the = period, instead? Chris, I see on your drawing you did not need to brace the vertical arm = of your device. Looks very strong. How did you attach that vertical to = the cross member? Many Thanks, Ted ----- Original Message -----=20 From: ChrisAtUpw@.......... To: psn-l@................. Sent: Thursday, October 05, 2006 1:26 PM Subject: Re: Building a lehman seismometer In a message dated 2006/10/05, tchannel@.............. writes: 1 Does your coil and damper assemblies just set on the aluminum = base, free to move for adjustment, or are they fixed to the base? Hi Ted, My arm has a support wire fixture and then a mass right at the = end of the arm. The support fixture carries a damping tongue in the direction = of the hinge and a coil on the mass side. Both the sensor and damper magnets slide on the base strip, for = mounting and adjustment of their position. They have small guide strips = underneath which hold them parallel. =20 2 You cleaned the plates, which hold the magnets, and then painted = the exterior, but not the surface the magnets touch. Would the paint = interfere with something on that surface? I suggest that you paint everything with Hammerite or similar, = but try to get it smooth and level. I etch coated the magnet position = with phosphoric acid, allowed it to react and dry overnight and then put = CL anticorrosion car grease on it. I was not certain that I could get = enough damping initially, but I found that I could overdamp the system = quite easily, so I increased the damper magnet spacing. 3 This is a hard question. Assuming I have finished my sensor, and = levelled it, what is the procedure for adjusting the period? Maybe you = could walk me through this procedure? You first lower the base plate / strip at the mass end. Then = you mount the arm and make any necessary solder connections for the coil = wiring. Then you put the magnet units on the base and adjust the top = wire so that both the coil and the damping plate are central within the = magnet spacings. The first time, you may need to make adjustments of the = magnet block heights, so that the arm is parallel to the base. Then you slide the damper free and adjust the cross levels so = that the arm is stable in the central position. If you can't get a = stable central position, you need to adjust the suspension so that the = top hinge is nearer the mass end - or the bottom fitting is further = away. You may mark the end of the arm with an inkline and also put a = central mark on the base. You deflect the arm and time a swing. It may = be about 5 sec initially. Then you slowly raise the base at the mass end = of the arm with the adjusting screw, testing to determine the period, = until you get the period you want. As the period is lengthened, you will = probably need to trim the cross level slightly. Then you slide the damping magnet over edge of the tongue, = deflect the arm 10 mm and release it. It may help to stick a bit of = graph paper to the base to be able the measure the position of the = damping block. 0.7 damping is obtained when the arm swings 0.5 mm past = the zero position and then falls back again. The amount of damping decreases as the set period increases, so = it needs to be easily adjustable. If the critical position is obtained = with the tongue less that half covering the magnets, increase the damper = magnet spacing.=20 4 I get confused with many of these terms, so I hope you understand = this question. If the target period is say 10 seconds, would the arm = move, when pulled 10mm, very slowly, taking 10 seconds to complete one = cycle? That is correct. Assuming that the arm is not damped, you = deflect it and then release it. It takes 2.5 sec to get to zero, = overshoots and comes back to zero in 5 sec, overshoots again and comes = back to zero from the initial direction in another 5 sec. Total 5 + 5 = =3D 10 sec. The period is the time for one complete oscillation cycle. = You want at least twice this. 5 After it is leveled and the period is set, Is it then that you = can check the damp, by moving the arm 10mm and releasing it? Correct, you set up the period first and then you slide the = damper further over the tongue in small steps until it is just a bit = under critical. I know what an underdamped situation would look like. But is = there way to tell if it is over damped? As you increase the damping from zero, the time that the arm = takes to fall back through / to the balance position increases slightly. = If you overdamp the arm, it never swings through the zero position and = the more you damp it, the longer it takes to get there. Look for the arm = swinging just past the zero line, but without another oscillation. Regards, Chris Chapman
Thanks, Chris for your reply. Most=20 helpful
 
   I am in the process of = building a=20 sensor bases on your prototype, and great drawings.   I have = most of=20 the parts located, again with your help and others on the mailing = list.  I=20 will send pictures as I go along, if you like, and if you tell me how to = send=20 them.
 
I regards to the angle between the = bottom and top=20 hinge, a fraction of a degree, how do you measure for that? Or do you = adjust and=20 measure the period, instead?
 
Chris, I see on your drawing you did = not need to=20 brace the vertical arm of your device. Looks very strong.  How did = you=20 attach that vertical to the cross member?
 
Many Thanks, Ted
 
 
----- Original Message -----
From:=20 ChrisAtUpw@.......
Sent: Thursday, October 05, = 2006 1:26=20 PM
Subject: Re: Building a lehman=20 seismometer

In a=20 message dated 2006/10/05, tchannel@..............=20 writes:

1 Does your coil and damper assemblies just set on the = aluminum=20 base, free to move for adjustment, or are they fixed to the=20 base?


Hi=20 Ted,

       My arm has a support = wire=20 fixture and then a mass right at the end of the=20 arm.
       The support fixture = carries a=20 damping tongue in the direction of the hinge and a coil on the mass=20 side.
       Both the sensor and = damper=20 magnets slide on the base strip, for mounting and adjustment of their=20 position. They have small guide strips underneath which hold them=20 parallel.  

2 You cleaned the plates, which hold the magnets, and = then=20 painted the exterior, but not the surface the magnets touch.  = Would the=20 paint interfere with something on that surface?

       I=20 suggest that you paint everything with Hammerite or similar, but try = to get it=20 smooth and level. I etch coated the magnet position with phosphoric = acid,=20 allowed it to react and dry overnight and then put CL anticorrosion = car grease=20 on it. I was not certain that I could get enough damping initially, = but I=20 found that I could overdamp the system quite easily, so I increased = the damper=20 magnet spacing.

 3 This is a hard question. Assuming I have finished my = sensor,=20 and levelled it, what is the procedure for adjusting the period? = Maybe you=20 could walk me through this procedure?

       You=20 first lower the base plate / strip at the mass end. Then you mount the = arm and=20 make any necessary solder connections for the coil wiring. Then you = put the=20 magnet units on the base and adjust the top wire so that both the coil = and the=20 damping plate are central within the magnet spacings. The first time, = you may=20 need to make adjustments of the magnet block heights, so that the arm = is=20 parallel to the base.
       Then you = slide=20 the damper free and adjust the cross levels so that the arm is stable = in the=20 central position. If you can't get a stable central position, you need = to=20 adjust the suspension so that the top hinge is nearer the mass end - = or the=20 bottom fitting is further away. You may mark the end of the arm with = an=20 inkline and also put a central mark on the base. You deflect the arm = and time=20 a swing. It may be about 5 sec initially. Then you slowly raise the = base at=20 the mass end of the arm with the adjusting screw, testing to determine = the=20 period, until you get the period you want. As the period is = lengthened, you=20 will probably need to trim the cross level slightly.
      =20 Then you = slide the=20 damping magnet over edge of the tongue, deflect the arm 10 mm and = release it.=20 It may help to stick a bit of graph paper to the base to be able the = measure=20 the position of the damping block. 0.7 damping is obtained when the = arm swings=20 0.5 mm past the zero position and then falls back=20 again.
       The amount of damping = decreases=20 as the set period increases, so it needs to be easily adjustable. If = the=20 cri