PSN-L Message - Subject: RE: New subscriber

Hi John, Hi Chris and others, You can find what many consider to be the original PSN amp design at http://pw2.netcom.com/~shammon1/equip.htm#Plans in the right column. In my document files, I also have an older design done by Jan Froom which dates back before 1989 which includes a 12-bit A/D. Pete Row designed the amp and filter found on the website and freely gave it to the members of the PSN group in 1990 at one of the first PSN meetings we held. That was at a time when the group was less than 20 members. Pete does analog design as a profession and spent an hour describing the circuit operation in detail. I'm not a designer so I wont even attempt to go there... Below that, on the same web page, you will find another design which came from Sean Morrissey. Sean has passed as may know and was also a world class designer. I have been using Pete's design for 15 years and it is easy to build and diagnose when an op-amp fails. I currently have five amp/filter sets connected to Larry's serial A/D board. You can find example seismic data on Larry's site under the ATx prefix from Aptos, CA. The ATZ, ATN and ATE are HS-10 geophones and the data from AT1 and AT2 are from Lehman's. Regards, Steve Hammond PSN San Jose Aptos, CA www.publicseismicnetwork.com -----Original Message----- From: psn-l-request@.............. [mailto:psn-l-request@................. Behalf Of ChrisAtUpw@....... Sent: Sunday, February 27, 2005 7:53 PM To: psn-l@.............. Subject: Re: New subscriber In a message dated 27/02/2005, jpopelish@........ writes: I am an electrical engineer residing in the Shenandoah valley in central Virginia. Analog circuit design is my forte. I have been interested in geology, volcanism and earthquakes for a long time. After discussing seismometer amplifiers with someone, recently, in sci.electronics.design, I started searching the web for home built seismometer and sensor designs. One thing that struck me about many of the sensor designs is their lack of optimisation and sophistication. Either this means that the sensor is not the limiting part of most designs or else it means that considerable improvement is possible. Hi John, Welcome! I can't comment on your discussions - I can't find the website you quote. Seismometer amplifiers do need quite specialised design with low noise, low drift, high gain and good filters with a low pulse overshoot. I don't know where you are finding the 'many' amplifier designs? There is one on Larry's website which uses LT1007s and is optimised. You need to optimise both current and voltage noise sources. LT1007s, OP07s and OP27s can all give satisfactory performance. If you wish to use very long periods where 1/f noise is a limitation, MAX432 and chopper amp circuits are available. Don't confuse apparently simple with unsophisticated! You are trying to get the amplifier noise a factor of 10 lower than the seismic noise. The ready availability of inexpensive but powerful NdFeB magnets has allowed the use of smaller sensor coils and magnets with increased sensitivity. After puzzling a bit over how I might design an inductive sensor that would improve upon the simple solenoidal coil and horse shoe magnet approach, I think I have come up with a more sensitive design that also has noise cancelling capability that will help it reject line generated fields (AC hum), variations in the Earth's magnetic field caused by the solar wind and lightning magnetic fields. This is based on making two similar coils that produce equal and opposite signals when exposed to large, common, external fields, but produce equal and aiding signals when exposed to the relative movement between the coils and magnet structure. The magnet structure also has no net external field to interact with the geo field that might interact with the seismometer boom. I have purchased a batch of NeFeB magnets on EBAY and am awaiting a quote for construction of the 6 iron pole pieces to make one of these fist sized sensors. I will make the coil forms and wind the coils, myself. I will also make the signal amplifier and filter. If that all comes together, I will take a shot at building a Lehman type horizontal, long period pendulum. I suggest that you consider 1" square NdFeB magnets in a quad formation, NS opposing SN, in between two 1/4" thick rectangular mild steel plates, say 3.5" long by 2" wide. You wind a flat rectangular coil to half cover the magnet poles, say ~1" square. The coil is completely screened by the soft iron backing plates, which should be earthed to the same point as the seismometer frame and the amplifier inputs. You can use the same layout, but with thicker rectangular 1" x 1/2" magnets for an Al or Cu inductive damping plate. This design gives very low stray external magnetic fields. Try it - you will like it! AC hum is fairly low and is strongly filtered by your 3 to 10 Hz amplifier filters. You should use woven screen connecting cable. The main problem in domestic situations is in limiting interference coming in through the utility supply and from various domestic sources. Fridges, cookers and electrical heating systems can produce large surges. You may benefit from a line filter. It is preferable to make the seismometer arm and weight using non magnetic materials. Stainless steel water pipe is quite useful for the arm and you can buy brass screw clamp fittings quite easily, to fit. Don't use a knife blade or a point suspension. Ball on a flat, crossed cylinder, crossed wire and crossed foil suspensions are all OK. Single wire and single foil (Cardan hinge) may also be OK. See http://pages.prodigy.net/fxc/ http://pages.prodigy.net/fxc/JC.html http://www.jclahr.com/science/psn/gldn_psn.html & http://physics.mercer.edu/petepag/MKXVII.pdf I also have a DATAQ DI-194-RS to hook it up to a computer but no software other than what came with that unit. You won't be very happy with a 194 for very long, but it is a start. You do really need 16 bits resolution for this type of work. Remember that you also need good triggering, recording, display and data analysis software. And you do need 0.1 sec timing accuracy. Calculate how much storage space you would need at 20 sps for a single day? Eventually I want to add an optical beam sensor that will make the unit act as a tilt meter (true DC operation, similar to the differential capacitive bridge type pickup, but with much simpler support circuits) and allow experiments with feedback using the original inductive sensor as a linear motor. This should keep me busy for a year or more. You might find some information to interest you at http://jclahr.com/science/psn/index.html You can make OK optical sensors using large area photodiode pairs (7sq mm) and a tungsten filament lamp with either a resistance or a voltage stabilisation circuit. Infra red LEDS change their output by about a factor of 5 at constant current between 0 and 100 C, so you would need to use additional photodiode stabilisation if you used one of them. I can get down to about +/- 15 nano metres of noise, or less if I reduce the bandwidth below 10 Hz. You can also use NdFeB magnet quads and an A3515 Hall Effect sensor. See http://www.geocities.com/meredithlamb/page003.html Two pairs of rectangular magnets, one SN and the other NS, are mounted on parallel soft iron backing plates connected by mild steel bolts. The sensor is suspended in the central field join. There are also differential capacitor designs available - if you need sub nanometre resolution. These are a subject in themselves. Anyway, here is some "food for thought". Can I suggest that you visit http://psn.quake.net/maillist.html#archives and download the last few years' letters? There is a great deal of good information and experience detailed therein. Regards, Chris Chapman

Hi John, Hi Chris and = others,

You can find what many consider to = be the=20 original PSN amp design at http://pw2.netco= m.com/~shammon1/equip.htm#Plans in=20 the right column. In my document files, I also have an older = design done=20 by Jan Froom which dates back before 1989 which includes a 12-bit = A/D. Pete=20 Row designed the amp and filter found on the website and freely gave it = to the=20 members of the PSN group in 1990 at one of the first PSN meetings we = held. That=20 was at a time when the group was less than 20 members. Pete does analog=20 design as a profession and spent an hour describing the circuit = operation=20 in detail. I'm not a designer so I wont even attempt to go there... = Below that,=20 on the same web page, you will find another design which came from = Sean=20 Morrissey. Sean has passed as may know and was also a world = class=20 designer. I have been using Pete's design for 15 years and it is easy to = build=20 and diagnose when an op-amp fails. I currently have five amp/filter sets = connected to Larry's serial A/D board. You can find example seismic data = on=20 Larry's site under the ATx prefix from Aptos, CA. The ATZ, ATN and ATE = are HS-10=20 geophones and the data from AT1 and AT2 are from Lehman's.

Regards, Steve Hammond PSN San=20 Jose

Aptos, CA=20

www.publicseismicnetwork.com=

-----Original Message-----
From:=20 psn-l-request@.............. = [mailto:psn-l-request@...............On Behalf=20 Of ChrisAtUpw@.......
Sent: Sunday, February 27, 2005 = 7:53=20 PM
To: psn-l@..............
Subject: Re: New=20 subscriber

In a message dated 27/02/2005, jpopelish@........ writes:
I am an = electrical engineer=20 residing in the Shenandoah valley in central Virginia. Analog = circuit=20 design is my forte. I have been interested in geology, = volcanism and=20 earthquakes for a long time. After discussing seismometer = amplifiers=20 with someone, recently, in sci.electronics.design, I started = searching the=20 web for home built seismometer and sensor designs.

One = thing=20 that struck me about many of the sensor designs is their lack of=20 optimisation and sophistication. Either this means that the = sensor is=20 not the limiting part of most designs or else it means that = considerable=20 improvement is possible.

Hi John,

Welcome! I can't comment on your = discussions - I=20 can't find the website you quote. Seismometer amplifiers do need quite = specialised design with low noise, low drift, high gain and good = filters with=20 a low pulse overshoot.

I don't know where you are finding the = 'many'=20 amplifier designs? There is one on Larry's website which uses LT1007s = and is=20 optimised. You need to optimise both current and voltage noise = sources.=20 LT1007s, OP07s and OP27s can all give satisfactory = performance. If you=20 wish to use very long periods where 1/f noise is a limitation, MAX432 = and=20 chopper amp circuits are available. Don't confuse apparently simple = with=20 unsophisticated! You are trying to get the amplifier noise a factor of = 10=20 lower than the seismic noise. The ready availability of inexpensive = but=20 powerful NdFeB magnets has allowed the use of smaller sensor = coils and=20 magnets with increased sensitivity.

After puzzling = a bit over=20 how I might design an inductive sensor that would improve upon the = simple=20 solenoidal coil and horse shoe magnet approach, I think I have come = up with=20 a more sensitive design that also has noise cancelling capability = that will=20 help it reject line generated fields (AC hum), variations in the = Earth's=20 magnetic field caused by the solar wind and lightning magnetic = fields. This=20 is based on making two similar coils that produce equal and opposite = signals=20 when exposed to large, common, external fields, but produce equal = and aiding=20 signals when exposed to the relative movement between the coils and = magnet=20 structure. The magnet structure also has no net external field = to=20 interact with the geo field that might interact with the seismometer = boom.

I have purchased a batch of NeFeB magnets on EBAY and = am=20 awaiting a quote for construction of the 6 iron pole pieces to make = one of=20 these fist sized sensors. I will make the coil forms and wind = the=20 coils, myself. I will also make the signal amplifier and = filter. =20 If that all comes together, I will take a shot at building a Lehman=20 type
horizontal, long period pendulum.

I suggest that you consider 1" = square NdFeB=20 magnets in a quad formation, NS opposing SN, in between two 1/4" = thick=20 rectangular mild steel plates, say 3.5" long by 2" wide. You wind a = flat=20 rectangular coil to half cover the magnet poles, say ~1" square. The = coil is=20 completely screened by the soft iron backing plates, which should be = earthed=20 to the same point as the seismometer frame and the amplifier inputs. = You can=20 use the same layout, but with thicker rectangular 1" x 1/2" = magnets for=20 an Al or Cu inductive damping plate. This design gives very low stray = external=20 magnetic fields. Try it - you will like it!

AC hum is fairly low and is strongly = filtered by=20 your 3 to 10 Hz amplifier filters. You should use woven = screen=20 connecting cable. The main problem in domestic situations is in = limiting=20 interference coming in through the utility supply and from various = domestic=20 sources. Fridges, cookers and electrical heating systems can = produce=20 large surges. You may benefit from a line filter. It is preferable to = make the=20 seismometer arm and weight using non magnetic materials. Stainless = steel water=20 pipe is quite useful for the arm and you can buy brass screw clamp = fittings=20 quite easily, to fit.

Don't use a knife blade or a point=20 suspension. Ball on a flat, crossed cylinder, crossed wire and crossed = foil=20 suspensions are all OK. Single wire and single foil (Cardan hinge) may = also be=20 OK. See http://pages.prodigy.net/fxc/&= nbsp; http://pages.prodigy.net/fx= c/JC.html =20 http://www.jclah= r.com/science/psn/gldn_psn.html &=20 http://physics.merc= er.edu/petepag/MKXVII.pdf

I also have a=20 DATAQ DI-194-RS to hook it up to a computer but no software other = than what=20 came with that unit.

You won't be very happy with a 194 for = very long,=20 but it is a start. You do really need 16 bits resolution for this type = of=20 work. Remember that you also need good triggering, recording, display = and data=20 analysis software. And you do need 0.1 sec timing accuracy. Calculate = how much=20 storage space you would need at 20 sps for a single day?

Eventually I=20 want to add an optical beam sensor that will make the unit act as a = tilt=20 meter (true DC operation, similar to the differential capacitive = bridge type=20 pickup, but with much simpler support circuits) and allow = experiments with=20 feedback using the
original inductive sensor as a linear = motor. =20 This should keep me busy for a year or more.

You might find some information to = interest you=20 at http://jclahr.com/scien= ce/psn/index.html You=20 can make OK optical sensors using large area photodiode pairs (7sq mm) = and a=20 tungsten filament lamp with either a resistance or a voltage=20 stabilisation circuit. Infra red LEDS change their output by about a = factor of=20 5 at constant current between 0 and 100 C, so you would need to = use=20 additional photodiode stabilisation if you used one of them. I can get = down to=20 about +/- 15 nano metres of noise, or less if I reduce the bandwidth = below 10=20 Hz.

You can also use NdFeB magnet quads and = an A3515=20 Hall Effect sensor. See http://www.ge= ocities.com/meredithlamb/page003.html Two=20 pairs of rectangular magnets, one SN and the other NS, are = mounted on=20 parallel soft iron backing plates connected by mild steel bolts. The = sensor is=20 suspended in the central field join.

There are also differential capacitor = designs=20 available - if you need sub nanometre resolution. These are a subject = in=20 themselves.

Anyway, here is some "food for thought". =

Can I suggest that you visit http://psn.quake.net= /maillist.html#archives and=20 download the last few years' letters? There is a great deal of good=20 information and experience detailed therein.

Regards,

Chris=20 Chapman

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