Hi Chris
STM also describes a fabricated coil using a rare earth ring magnet from mc=
master carr, thick washer and cap screw. It was definitely easier for me to=
 make than taking apart and machining and putting back together a woofer sp=
eaker.=20
=A0
Hi Brett
Using STM equations I=A0have been wondering for quite a while=A0how to lowe=
r the gain to prevent clipping. It seemed the whole idea was to have a larg=
e gain in the equations for the desired frequency range. I put a voltage di=
vider at the output but that won't prevent clipping of the amp before the d=
ivider. Any ideas? I would like two output levels to expand my sensors rang=
e. It seems recording two levels would be easier to achieve than a higher r=
esolution A/D , however it would double the number of recording channels.=
=20
=A0
Regards
Barry


--- On Mon, 9/15/08, ChrisAtUpw@.......  wrote:

From: ChrisAtUpw@....... 
Subject: Re: Vertical Seismometer with Feedback, Transducer Question
To: psn-l@..............
Date: Monday, September 15, 2008, 3:15 PM


In a message dated 2008/09/15, brett3nt@............. writes:


Hi Brett,=20

=A0=A0=A0=A0=A0=A0 It is perfectly practicable to use rectangular coils and=
 quad NdFeB magnet blocks for a force feedback actuator. Modern speakers se=
em to use Ferrite magnets, which are both relatively weak and very temperat=
ure sensitive. The pole gaps have very little clearance. This may be a limi=
ting factor when the feedback coil is moving in an arc of a circle. The arm=
 needs to be very rigid and accurately positioned. Sean used to machine a l=
arger gap and use many more turns on his coil. This was relatively easy to =
do when using a cylindrical Alnico magnet. It my be very difficult to do us=
ing a ferrite system glued solid with epoxy.

=A0=A0=A0=A0=A0=A0 Strangely enough, I can just manage with a 12 bit ADC. T=
he problem with the 'bare' 16 bit ADC chips is that there is usually >3 bit=
s of digital noise on them, so their dynamic range is seriously restricted.=
 This problem can easily be avoided with the fast ADCs by taking say 32 rea=
dings in rapid sequence and adding them together to give one low noise read=
ing. You can actually get 18 bit resolution from a 16 bit ADC this way at l=
ow sample rates.
=A0=A0=A0=A0=A0=A0 Larry is now marketing ADCs with reduced digital noise. =
Dynamic range should not be a problem in practice with a +/- 32768 count ra=
nge and 1 bit noise.=20
=A0=A0=A0=A0=A0=A0 The factors that are likely to limit you are environment=
al, ocean and circuit noise, in that order.


It's only when you go to a 24 bit digitizer that you can have high sensitiv=
ity to weak signals and still be able to display large mid-distance quakes =
without clipping.=A0=20

=A0=A0=A0=A0=A0=A0 The resolution that you can actually get with a "24 bit"=
 ADC depends quite strongly on the sample rate. It may be as low as 18 bits=
.. They tend to be quite expensive and you need a 50% increase in your digit=
al storage capacity over a 16 bit system. Some have a very low input voltag=
e range. This generates a requirement for very low noise electronics =3D ex=
pensive.

The instrument itself will probably not be the limit when using 16 bits. Th=
e highest=20

coil currents will be needed at the highest frequencies, so if you don't pu=
sh for too high an upper corner frequency, you should be also a little bett=
er off.

=A0=A0=A0=A0=A0=A0 This is why you need to restrict the resistance of the f=
eedback coil to <100 Ohms maximum and also keep down the coil inductance. R=
estricting the maximum frequency to 10 Hz is likely to be quite adequate fo=
r an amateur seismologist in most locations and eases both the electronics =
and the digital storage requirements.


To understand the clipping issues for your location, determine what your=20
maximum acceleration is likely to be, often expressed in % of g, then use F=
=20
=3D m A to determine the peak force and calculate what that corresponds to =
in=20
terms of coil current.=A0 I suspect that's what you have already=20
done.=A0 Balancing sensitivity vs clipping level seems to be a fundamental=
=20
problem, which often results in the use of more than one type of instrument=
=20
in seismically active areas.

=A0=A0=A0=A0=A0=A0 Which is a problem generated by the assumed requirement =
to be capable of measuring seismic signals below the environmental backgrou=
nd levels anywhere on Earth - irrespective of the actual noise at your loca=
tion. Part of the "specmanship" which goes quite a way to explaining the co=
st of commercial seismometers.

=A0=A0=A0=A0=A0=A0 Regards,

Hi Chris
STM also describes a fabricated coil using a rare earth ring magnet from mcmaster carr, thick washer and cap screw. It was definitely easier for me to make than taking apart and machining and putting back together a woofer speaker. 
 
Hi Brett
Using STM equations I have been wondering for quite a while how to lower the gain to prevent clipping. It seemed the whole idea was to have a large gain in the equations for the desired frequency range. I put a voltage divider at the output but that won't prevent clipping of the amp before the divider. Any ideas? I would like two output levels to expand my sensors range. It seems recording two levels would be easier to achieve than a higher resolution A/D , however it would double the number of recording channels. 
 
Regards
Barry


--- On Mon, 9/15/08, ChrisAtUpw@....... <ChrisAtUpw@.......> wrote:

From: ChrisAtUpw@....... <ChrisAtUpw@.......>
Subject: Re: Vertical Seismometer with Feedback, Transducer Question
To: psn-l@..............
Date: Monday, September 15, 2008, 3:15 PM


In a message dated 2008/09/15, brett3nt@............. writes:


Hi Brett, 

       It is perfectly practicable to use rectangular coils and quad NdFeB magnet blocks for a force feedback actuator. Modern speakers seem to use Ferrite magnets, which are both relatively weak and very temperature sensitive. The pole gaps have very little clearance. This may be a limiting factor when the feedback coil is moving in an arc of a circle. The arm needs to be very rigid and accurately positioned. Sean used to machine a larger gap and use many more turns on his coil. This was relatively easy to do when using a cylindrical Alnico magnet. It my be very difficult to do using a ferrite system glued solid with epoxy.

       Strangely enough, I can just manage with a 12 bit ADC. The problem with the 'bare' 16 bit ADC chips is that
 there is usually >3 bits of digital noise on them, so their dynamic range is seriously restricted. This problem can easily be avoided with the fast ADCs by taking say 32 readings in rapid sequence and adding them together to give one low noise reading. You can actually get 18 bit resolution from a 16 bit ADC this way at low sample rates.
       Larry is now marketing ADCs with reduced digital noise. Dynamic range should not be a problem in practice with a +/- 32768 count range and 1 bit noise. 
       The factors that are likely to limit you are environmental, ocean and circuit noise, in that order.


It's only when you go to a 24 bit digitizer that you can have high sensitivity to weak signals and still be able to display large mid-distance quakes without clipping.  


       The resolution that you can actually get with a "24 bit" ADC depends quite strongly on the sample rate. It may be as low as 18 bits. They tend to be quite expensive and you need a 50% increase in your digital storage capacity over a 16 bit system. Some have a very low input voltage range. This generates a requirement for very low noise electronics = expensive.

The instrument itself will probably not be the
 limit when using 16 bits. The highest 

coil currents will be needed at the highest frequencies, so if you don't push for too high an upper corner frequency, you should be also a little better off.


       This is why you need to restrict the resistance of the feedback coil to <100 Ohms maximum and also keep down the coil inductance. Restricting the maximum frequency to 10 Hz is likely to be quite adequate for an amateur seismologist in most locations and eases both the electronics and the digital storage requirements.


To understand the clipping issues for your location, determine what your 
maximum acceleration is likely to be, often expressed in % of g, then use F 
= m A to determine the peak force and calculate what that corresponds to in 
terms of coil current.  I suspect that's what you have already 
done.  Balancing sensitivity vs clipping level seems to be a fundamental 
problem, which often results in the use of more than one type of instrument 
in seismically active areas.


       Which is a problem generated by the assumed requirement to be capable of measuring seismic signals below the environmental background levels anywhere on Earth - irrespective of the actual noise at your location. Part of the "specmanship" which goes quite a way to explaining the cost of

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