Geoff
The secret miracle device that I use is a series coupling capacitor in the amplifier chain. The value I use is 4 uF but I only have a 3 second pendulum. AC couple the stages and the DC component becomes irrelevant. Make sure that the coupling caps are NON-POLAR. Don't use an electrolytic here. You can make up large values by paralleling smaller values.
I hope this helps.
Pete

--- On Mon, 7/19/10, Geoffrey  wrote:

> From: Geoffrey 
> Subject: Re: Velocity Sensor Question ?
> To: psn-l@..............
> Date: Monday, July 19, 2010, 7:48 AM
> I understand the stability of the
> baseline is related to the
> DC nature of the balance between the + and - legs
> of the operational amplifier. Copper has
> a temperature coefficient which means it changes at a
> certian rate its resistance with temperature.
> If i balance both sides of the equation with identical
> physical properties, then I should expect
> they will change identically with temperature
> thus keeping the DC baseline steady.
> If you use only one copper coil on one leg this
> balance becomes impossible to achieve.
> This device will be located outside where
> temp changes are greatest.
> 
> I do not know where you guys are coming from
> with your stable DC baselines unless you use
> some kind of miracle device which I have never found.
> 
> Best Regards,
> geoff
> 
> 
> 
> ----- Original Message ----- From: "Barry Lotz" 
> To: 
> Sent: Sunday, July 18, 2010 5:21 PM
> Subject: Re: Velocity Sensor Question ?
> 
> 
> Hi Geoff
> If you are measuring velocity not displacement there
> shouldn't be any temperature affects except maybe the
> electronics. There was a post awile back with a link to Ebay
> for a coil winding gismo for <$20. I'll see if I can find
> the link. Regards
> Barry
> http://www.seismicvault.com 
> 
> 
> 
> ________________________________
> From: Geoffrey 
> To: psn-l@..............
> Sent: Sun, July 18, 2010 4:16:55 PM
> Subject: Re: Velocity Sensor Question ?
> 
> Here is the preamp.
> I was going to place it right at the geophone.
> The two exact coils will change in step
> with temperature to keep a stable DC
> baseline. Minimal drift.
> 
> http://gmvoeth.home.mchsi.com/AMP001.jpg
> 
> Any reason this should not be right ?
> 
> Will Larry sell only the coils ?
> I might get two to try my ideas.
> 
> I looked into getting ten wound but the company
> was outragious in its (retooling fees)
> It seems they cant simply do it even tho they
> wind the things for a living.
> 
> Thanks for your response.
> 
> Best Regards,
> geoff
> 
> PS: if PSN had a binary news server.
> we could post any kind of file
> for people to have/look at ?
> Could someone create a [alt.binaries.seismic.psn]
> news group, I understand there is a complex process
> but the creation is free and almost all ISP have
> one as part of their service. I have tried to understand
> the process but like Linux OS I cant understand it.
> I have tried Linux several times but cant make it
> functional.
> Fedora 10 installs OK but I just cant get it to be
> functional.
> 
> ----- Original Message ----- From: "Brett Nordgren" 
> To: 
> Sent: Sunday, July 18, 2010 10:57 AM
> Subject: Re: Velocity Sensor Question ?
> 
> 
> > Hi Geoff,
> > 
> > At 07:30 AM 7/18/2010, you wrote:
> >> Hello Mr. Nordgren;
> >> 
> >> The question I have is thus:
> >> 
> >> Is that coil right for the magnets or,
> >> Should it be rectangular in shape with the
> >> two vertical sides of the rectangle outside the
> magnetic flux ?
> > 
> > Ideally you want your sensor to be linear. If you move
> the coil at a constant speed over its mechanical range you
> would like to see a relatively constant voltage out. You can
> achieve that in two ways. Have the magnet pole faces large
> enough that all the coil wires stay well within the magnetic
> region, or alternatively, make them small enough that all
> the magnetic lines stay within the region filled with coil
> wires. When you have the edges of the magnetic field moving
> across the outside or inside edges of the coil, the
> linearity suffers, though perhaps not enough to worry about
> too much.
> > 
> >> I figure I may need like 1206 feet of 36
> AWG(B&S) copper enameled wire
> >> for a 2100 turns 500 ohm coil ?
> >> 
> >> The 2100 turns are of enamled wire without a heavy
> coat of enamel.
> >> 
> >> Is 2100 turns enough ?
> > 
> > I guess the question would have to be, enough for
> what? It all depends on how sensitive you plan to make your
> signal detection circuit--that is, how much amplifier gain
> do you plan to have and if you are connecting to an A/D
> device, what is its sensitivity? In general, I would try to
> start with the smallest wire and the largest number of turns
> I could easily manage. Coil resistance of several K Ohms
> wouldn't be unreasonable. The coil Larry sells I believe has
> 10,000 turns and is 9,000 Ohms.
> > 
> >> Would it be better if the coil had a copper or
> iron core ?
> > 
> > Definitely no. Iron would "suck in" the magnetic
> lines, away from the wires where you want them to be.
> Copper, unless it made a complete loop, wouldn't do much
> since it wouldn't be in the magnetic field, certainly
> nothing particularly helpful.
> > 
> >> This would mean a custom coil rectangular instead
> of circular.
> > 
> > Possibly you'd get slightly better linearity with a
> rectangular coil and rectangular magnet pole pieces, but
> either shape should work reasonably well for what you are
> trying to do.
> > 
> >> I think it may be possible to have two identical
> >> coils center tapped in the middle with a single
> >> rare earth magnet in the middle between the two
> >> coils then you have the right setup for
> >> a proper op amp differential circuit.
> > 
> > I may be wrong, but I am suspecting that you are
> wanting to connect one output to each of the two inputs of
> an op amp. If that is what you are thinking about, the
> problem will be too much gain. Typical op amps have voltage
> gains of 100's of thousands, or more often, millions.
> Generally for approximate analyses designers assume that
> their gain is infinite. That means that extremely tiny input
> signals (noise) would have the op amp output bouncing
> between its voltage limits--not very useful.
> > 
> > Usually op amps are connected up as single ended
> amplifiers using a two-resistor feedback circuit, which
> makes a very nice voltage amplifier.
> > 
> > See: http://www.analog.com/static/imported-files/tutorials/MT-032.pdf
> Figure 3.
> > 
> > If you do that, one coil will work fine, and by
> changing the resistor values you can choose the amplifier
> gain to complement your coil sensitivity. The only advantage
> I can think of for some kind of differential coil setup is
> that it might not be as sensitive to 60 Hz hum. Though it
> probably wouldn't be that hard to connect a differential
> coil to a single-ended amplifier, sort of like a guitar
> "hum-bucking" pickup.
> > 
> > An instrumentation amplifier *would* allow for
> independent connections to a pair of coils, but they tend to
> be a lot more expensive and their gain usually can't be
> adjusted as precisely.
> > 
> > See: http://www.analog.com/static/imported-files/tutorials/MT-032.pdf
> Figure 2.
> > 
> >> +COIL- NmagnetS +COIL-
> >> 
> >> THE left coil - is connected to right coil +
> >> which is then the ground.
> >> 
> >> Left Coil + goes to op amp +
> >> and
> >> Right coil - goes to op amp -
> >> 
> >> The relative motion is magnet fixed to ground
> >> and coil fixed to device.
> >> 
> >> Coil is stable and magnet moves right and left
> >> between the coils.
> >> 
> >> Possibly +/- 2mm of range of motion.
> >> 
> >> Each coil having 1050 to 2100 turns
> >> custom wound to be exactly the same.
> > 
> > Hope that's a start,
> > Brett
> > 
> > 
> > 
> >
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