From: Brett Nordgren 

To: psnlist 
Sent: Mon, 24 Sep 2012 3:21
Subject: Geophone analog model


Hi All,

In connection with creating a Spice model of Dave's negative-resistance geo=
phone=20
damper/amplifier circuit I found that I needed to make an electrical equiva=
lent=20
model of the geophone he was using.  In the model, the input is a voltage s=
ignal=20
which represents ground motion, with 1V =3D 1 cm/second.  Three equivalent =
electrical=20
components are computed, Reqv, Ceqv and Leqv, which are analogs for the geo=
phone=20
damping, mass and support-spring stiffness.  In addition Rc and Lc, the act=
ual coil=20
resistance and inductance are included in the model.

http://bnordgren.org/seismo/GeophoneModels.pdf

This model should also work with any other physical, resonant, velocity sen=
sor such=20
as many Lehmans and spring-mass verticals, if you wanted to experiment with=
 modeling=20
them as electrical circuits to simulate their performance.

Hi Brett,=20

    What are the values of RL and Rd in your load circuit, please?=20

    This seems to be a voltage generator circuit representation. The whole=
=20
point of the Lippmann Patent is that you use the geophone as a CURRENT=20
generator and you follow it up with a current to voltage converter and a=20
top cut 'f' filter. The damping is then NOT 0.6, but it is massive and this=
=20
effectively holds the armature ~still in ~the same position. You connect=20
the geophone between earth and the -ve input of the opamp. The spring=20
constant takes no part in the response. =20
    One practical problem is that the resistance of the copper geophone coi=
l=20
changes with temperature and you need to keep the compensated resistance=20
value just +ve, to keep the overall circuit stable. You could guarantee thi=
s=20
by winding a copper compensation coil on the outside of the geophone, or by=
=20
using a Pt foil thermometer element with a metal film resistor in series.=
=20

    Regards,=20

    Chris

=20
From: Brett Nordgren <br=
ett3nt@.............>



To: psnlist <psnlist@..............>


Sent: Mon, 24 Sep 2012 3:21


Subject: Geophone analog model















Hi All,

In connection with creating a Spice model of Dave's negative-resistance geo=
phone 

damper/amplifier circuit I found that I needed to make an electrical equiva=
lent 

model of the geophone he was using.  In the model, the input is a voltage s=
ignal 

which represents ground motion, with 1V =3D 1 cm/second.  Three equivalent =
electrical 

components are computed, Reqv, Ceqv and Leqv, which are analogs for the geo=
phone 

damping, mass and support-spring stiffness.  In addition Rc and Lc, the act=
ual coil 

resistance and inductance are included in the model.

http://bnordgren.org/seismo/GeophoneModels.pdf

This model should also work with any other physical, resonant, velocity sen=
sor such 

as many Lehmans and spring-mass verticals, if you wanted to experiment with=
 modeling 

them as electrical circuits to simulate their performance.


Hi Brett, 



    What are the values of RL and Rd in your load circuit, please? 



    This seems to be a voltage generator circuit representation. The whole =


point of the Lippmann Patent is that you use the geophone as a CURRENT 

generator and you follow it up with a current to voltage converter and a 
top cut 'f' filter. The damping is then NOT 0.6, but it is massive and this=
 

effectively holds the armature ~still in ~the same position. You connect 
the geophone between earth and the -ve input of the opamp. The spring 

constant takes no part in the response.  

    One practical problem is that the resistance of the copper geophone coi=
l 

changes with temperature and you need to keep the compensated resistance 
value just +ve, to keep the overall circuit stable. You could guarantee thi=
s 

by winding a copper compensation coil on the outside of the geophone, or by=
 

using a Pt foil thermometer element with a metal film resistor in series. <=
br>


    Regards, 



    Chris