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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