In a message dated 2006/10/08, Bobhelenmcclure writes:

> >   A wire size of #38 or less will allow this number of turns to fit 
> > comfortably within the gap field cross-section. The coil will have a 
> resistance low 
> > enough to permit resistive shunt damping of a pendulum weighing up to a 
> > kilogram, in my opinion. My sensors have a pendulum mass of about 0.1Kg, 
> and 
> > critical damping is achieved at about 30 kOhms. Since the coil resistance 
> is only 
> > 340 Ohms, the shunt damping imposes negligible loss on output sensitivity. 
> 
> > Even a kilogram mass would require only about 10% loss of output using 
> shunt 
> > damping.
> 
>        The damping force required also depends on the set period. What 
> period are you using?
>        What effect does this damping current have on the input noise in 
> practice? Can it be significant?
> 
> Hi Chris,
>     I bought cheap low temperature enameled magnet wire from Alltronics. I 
> have both #38 (3.97 thou OD) and #40 gauge (3.14 thou OD). I never tried to 
> use the #40 wire, as it is difficult for me to see, let alone handle. I strip 
> by burning off the enamel. My friend Victor frowns on that, and recommends 
> fine emery paper.

Hi Bob,

    Magnet wire is available from Alltronics in 1/4 lb reels, but it is Kynar 
insulated
       http://www.alltronics.com/cgi-bin/category.cgi?&category=MW&start=0

       If you buy the polyurethane insulated wire like Beldsol, you don't 
have to strip it. If you put a hot iron and solder on it, the insulation just 
melts - no problems - but I have only found this wire down to 36 AWG - 5 thou OD.

       With the Kynar insulated wire, you have to strip it first before you 
can solder it. I usually use the edge of a wax candle flame (or a match) to 
first burn off this insulation. (You have to be very careful with a butane 
lighter not to melt the wire) Then I pull it fairly gently several times through two 
small pads of the very fine wire wool. This both cleans it effectively and 
hardens it a bit. 

       I once only wound two coils with 40,000 turns of 44 AWG copper wire 
with 1 thou paper interleaving to measure paramagnetic susceptability - it took 
me two whole days.... 

>   My horizontal sensors are easily shunt damped. On one of them set to 12 
> second period, I measured a Q of 1.1 with a shunt resistance (including the 
> amplifier) of 66 kOhms. My formula for Q is Q = K * R / P. The value for K is 
> therefore 0.2. The mass of the coil and solder weight is about 100 grams. The 
> coil resistance is 340 Ohms, the number of turns is 1100, the field strength 
> is ~0.8T, the field length per turn is 0.1m, and the sensor output is 85 
> v-s/m. If you know your own sensor's output, pendulum mass, and period, you can 
> work out your own value for K from the above information, and determine what 
> shunt damping resistance you would need. However, remember that the coil 
> resistance, in series with the shunt resistance (in parallel with the amplifier 
> input resistance) is the damping resistance.

       What is P please?

> Volts= B*L*(dx/dt)
> Force=B*L*I
> I= Volts/R = B*L*(dx/dt)/R
> Force= B*L*B*L*(dx/dt)/R
> Force / (dx/dt)= (B*L)^2 / R 
> 
>   I have not checked yet to see if the above equation is consistent with my 
> observed damping versus resistance.

    In the force equation, isn't the force proportional to the number of 
turns, whereas the inductance is proportional to the square of the number of turns 
and depends on the magnetic return path?

       How does the length of the pendulum, the set period and the mass 
factor into these equations, please?

>    Shunt damping makes it easy for me to check my sensors. I measure their 
> natural period by disconnecting the shunt, and discharging a small capacitor 
> across the sensor-amplifier terminals. The decay of oscillation amplitude 
> lets me make sure that the partially undamped oscillation has the expected Q 
> value, and the time between zero crossings gives me the natural period. I would 
> abandon the sport of seismometry if I could not control damping this way.

       I prefer to have my damping and sensors on separate fittings and their 
setup independant, but I can see the attraction of variable resistive damping 
if you choose very powerful sensor magnets and a low to moderate seismic 
mass. 

       I tend to use rather smaller / thinner magnets for sensing and they do 
not have a serious diamagnetic repulsion problem, although I have observed 
this type of effect. 
       I use wide Cu plate for the damping, so that the arm hits the side 
stops before the outer edge of the damping plate gets close to the outside edge 
of a magnet. I try to 'design out' problems when possible.

       If you are using feedback sensors with electronically extended 
periods, Cu plate damping is a lot quieter than velocity feedback from a 
differentiated position signal.
   
      Have you any comparisons of the input noise due to shunt damping vs 
plate damping - or the noise when undamped and when damped? Won't the induced 
current generate additional noise directly?  

       Regards,

       Chris Chapman
In a me=
ssage dated 2006/10/08, Bobhelenmcclure writes:



>   A wire size of=
 #38 or less will allow this number of turns to fit 

> comfortably within the gap field cross-section. The coil will have a re=
sistance low 

> enough to permit resistive shunt damping of a pendulum weighing up to a=
 

> kilogram, in my opinion. My sensors have a pendulum mass of about 0.1Kg=
, and 

> critical damping is achieved at about 30 kOhms. Since the coil resistan=
ce is only 

> 340 Ohms, the shunt damping imposes negligible loss on output sensitivi=
ty. 

> Even a kilogram mass would require only about 10% loss of output using=20=
shunt 

> damping.



       The damping force required also depend=
s on the set period. What period are you using?

       What effect does this damping current h=
ave on the input noise in practice? Can it be significant?



Hi Chris,

    I bought cheap low temperature enameled magnet wire from=20=
Alltronics. I have both #38 (3.97 thou OD) and #40 gauge (3.14 thou OD). I n=
ever tried to use the #40 wire, as it is difficult for me to see, let alone=20=
handle. I strip by burning off the enamel. My friend Victor frowns on that,=20=
and recommends fine emery paper.




Hi Bob,



    Magnet wire is available from Alltronics in 1/4 lb reels,=
 but it is Kynar insulated

       http://www.alltronics.com/cgi-bin/categ=
ory.cgi?&category=3DMW&start=3D0



       If you buy the polyurethane insulated w=
ire like Beldsol, you don't have to strip it. If you put a hot iron and sold=
er on it, the insulation just melts - no problems - but I have only found th=
is wire down to 36 AWG - 5 thou OD.



       With the Kynar insulated wire, you have=
 to strip it first before you can solder it. I usually use the edge of a wax=
 candle flame (or a match) to first burn off this insulation. (You have to b=
e very careful with a butane lighter not to melt the wire) Then I pull it fa=
irly gently several times through two small pads of the very fine wire wool.=
 This both cleans it effectively and hardens it a bit. 



       I once only wound two coils with 40,000=
 turns of 44 AWG copper wire with 1 thou paper interleaving to measure param=
agnetic susceptability - it took me two whole days.... 



  My horizontal sensors ar=
e easily shunt damped. On one of them set to 12 second period, I measured a=20=
Q of 1.1 with a shunt resistance (including the amplifier) of 66 kOhms. My f=
ormula for Q is Q =3D K * R / P. The value for K is therefore 0.2. The mass=20=
of the coil and solder weight is about 100 grams. The coil resistance is 340=
 Ohms, the number of turns is 1100, the field strength is ~0.8T, the field l=
ength per turn is 0.1m, and the sensor output is 85 v-s/m. If you know your=20=
own sensor's output, pendulum mass, and period, you can work out your own va=
lue for K from the above information, and determine what shunt damping resis=
tance you would need. However, remember that the coil resistance, in series=20=
with the shunt resistance (in parallel with the amplifier input resistance)=20=
is the damping resistance.




       What is P please?



Volts=3D B*L*(dx/dt)

Force=3DB*L*I

I=3D Volts/R =3D B*L*(dx/dt)/R

Force=3D B*L*B*L*(dx/dt)/R

Force / (dx/dt)=3D (B*L)^2 / R 



  I have not checked yet to see if the above equation is consist=
ent with my observed damping versus resistance.




    In the force equation, isn't the force proportional to th=
e number of turns, whereas the inductance is proportional to the square of t=
he number of turns and depends on the magnetic return path?



       How does the length of the pendulum, th=
e set period and the mass factor into these equations, please?



   Shunt damping make=
s it easy for me to check my sensors. I measure their natural period by disc=
onnecting the shunt, and discharging a small capacitor across the sensor-amp=
lifier terminals. The decay of oscillation amplitude lets me make sure that=20=
the partially undamped oscillation has the expected Q value, and the time be=
tween zero crossings gives me the natural period. I would abandon the sport=20=
of seismometry if I could not control damping this way.




       I prefer to have my damping and sensors=
 on separate fittings and their setup independant, but I can see the attract=
ion of variable resistive damping if you choose very powerful sensor magnets=
 and a low to moderate seismic mass. 



       I tend to use rather smaller / thinner=20=
magnets for sensing and they do not have a serious diamagnetic repulsion pro=
blem, although I have observed this type of effect. 

       I use wide Cu plate for the damping, so=
 that the arm hits the side stops before the outer edge of the damping plate=
 gets close to the outside edge of a magnet. I try to 'design out' problems=20=
when possible.



       If you are using feedback sensors with=20=
electronically extended periods, Cu plate damping is a lot quieter than velo=
city feedback from a differentiated position signal.

   

      Have you any comparisons of the input noise d=
ue to shunt damping vs plate damping - or the noise when undamped and when d=
amped? Won't the induced current generate additional noise directly?  <=
BR>


       Regards,



       Chris Chapman