=0A
=0A=
From: Geoffrey <gmvoeth@...........>
To: psn-l@..............
Sent: Sun, July 18, 2010 4:16:55 PM
Subject: Re: Velocity Sensor Que=
stion ?
Here is the preamp.
I was going to place it right =
at the geophone.
The two exact coils will change in step
with tempera=
ture to keep a stable DC
baseline. Minimal drift.
http://gmvoeth.=
home.mchsi.com/AMP001.jpg
Any reason this should not be right ?
<=
BR>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 it=
s (retooling fees)
It seems they cant simply do it even tho they
wind=
the things for a living.
Thanks for your response.
Best Rega=
rds,
geoff
PS: if PSN had a
binary news server.
we could post any kind of file
&nb=
sp; for people to have/look at ?
Could someone crea=
te a [alt.binaries.seismic.psn]
news group, I understand t=
here is a complex process
but the creation is free and alm=
ost all ISP have
one as part of their service. I have trie=
d to understand
the process but like Linux OS I cant under=
stand it.
I have tried Linux several times but cant make i=
t functional.
Fedora 10 installs OK but I just cant get it=
to be functional.
----- Original Message ----- From: "Brett Nordgre=
n" <
brett3nt@.............>
To: <
psn-l@webtro=
nics.com>
Sent: Sunday, July 18, 2010 10:57 AM
Subject: Re:
Velocity Sensor Question ?
> Hi Geoff,
>
> At 0=
7:30 AM 7/18/2010, you wrote:
>> Hello Mr. Nordgren;
>> <=
BR>>> The question I have is thus:
>>
>> Is that c=
oil right for the magnets or,
>> Should it be rectangular in shape=
with the
>> two vertical sides of the rectangle outside the magne=
tic flux ?
>
> Ideally you want your sensor to be linear. =
; If you move the coil at a constant speed over its mechanical range you wo=
uld like to see a relatively constant voltage out. You can achieve th=
at in two ways. Have the magnet pole faces large enough that all the =
coil wires stay well within the magnetic region, or alternatively, make the=
m small enough that all the magnetic lines stay within the region filled wi=
th coil wires. When you have the edges of the magnetic field moving a=
cross the outside or inside edges of the coil, the linearity
suffers, though perhaps not enough to worry about too much.
>
&g=
t;> I figure I may need like 1206 feet of 36 AWG(B&S) copper enamele=
d 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 quest=
ion would have to be, enough for what? It all depends on how sensitiv=
e you plan to make your signal detection circuit--that is, how much amplifi=
er gain do you plan to have and if you are connecting to an A/D device, wha=
t is its sensitivity? In general, I would try to start with the small=
est 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.
>
>> W=
ould it be better if the coil had a copper or iron core ?
>
> Definitely no. Iron would "suck in" the magnetic lines, awa=
y 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 me=
an a custom coil rectangular instead of circular.
>
> Possibly=
you'd get slightly better linearity with a rectangular coil and rectangula=
r magnet pole pieces, but either shape should work reasonably well for what=
you are trying to do.
>
>> I think it may be possible to h=
ave two identical
>> coils center tapped in the middle with a sing=
le
>> rare earth magnet in the middle between the two
>> =
coils then you have the right setup for
>> a proper op amp differe=
ntial circuit.
>
> I may be wrong, but I am suspecting that yo=
u 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 thousan=
ds, or more often, millions. Generally for approximate analyses desig=
ners assume that their gain is infinite. That means that extremely ti=
ny input signals (noise) would have the op amp output bouncing between its =
voltage limits--not very useful.
>
> Usually op amps are conne=
cted up as single ended amplifiers using a two-resistor feedback circuit, w=
hich makes a very nice voltage amplifier.
>
> See: http://www.=
analog.com/static/imported-files/tutorials/MT-032.pdf Figure 3.
&g=
t;
> If you do that, one coil will work fine, and by changing the re=
sistor values you can choose the amplifier gain to complement your coil sen=
sitivity. The only advantage I can think of for some kind of differen=
tial 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 different=
ial coil to a single-ended amplifier, sort of like a guitar "hum-bucking" p=
ickup.
>
> An instrumentation amplifier *would* allow for inde=
pendent connections to a pair of coils, but they tend to be a lot more expe=
nsive and their gain usually can't be adjusted as precisely.
>
&g=
t; See:
http://www.analog.com/static/imported-files/tutor=
ials/MT-032.pdf Figure 2.
>
>> +COIL- Nmagn=
etS +COIL-
>>
>> THE left coil - is connected to right c=
oil +
>> which is then the ground.
>>
>> Left C=
oil + goes to op amp +
>> and
>> Right coil - goes to op =
amp -
>>
>> The relative motion is magnet fixed to groun=
d
>> and coil fixed to device.
>>
>> Coil is
stable and magnet moves right and left
>> between the coils.
&=
gt;>
>> Possibly +/- 2mm of range of motion.
>>
&=
gt;> Each coil having 1050 to 2100 turns
>> custom wound to be =
exactly the same.
>
> Hope that's a start,
> Brett
&g=
t;
>
>
> _____________________________________________=
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