## PSN-L Email List Message

Subject: Re: Vertical Seismometer with Feedback, Transducer Question
From: Barry Lotz barry_lotz@.............
Date: Wed, 17 Sep 2008 19:33:58 -0700 (PDT)

```Hi Brett
After my email I played around with the various components. I have=A0STM's =
Mcad program, but I was able to present the data in Excel as you did. I cur=
rently have a max output of around 2000-2500 v/m/s for my sensors. I was ab=
le to reduce this down to around 400 by reducing the mass to around 0.2 kg =
and reducing r from ~240000 to around 100000. I then adjusted the other fee=
dback components to get a good response at the ends of the pass band and an=
appropriate damping value. I then could use this as one output, increase o=
utput back up by ~6x =A0or more and make this my second output. I'm not sur=
e if a gain reduction of 6 is enough to make much of a difference. Right no=
w I'm "clipping" with a magnitude of 7.0 @ about 4000 km or 5.0 @ ~200 km. =
Looking at my log-log graph of distance vs counts a gain reduction of 6 wou=
ld correspond to an increase in non- clipped magnitude of=A0~ 0.7 at a give=
n distance. Hardly seems worth the bother.=A0
Barry
=A0

--- On Wed, 9/17/08, Brett Nordgren  wrote:

From: Brett Nordgren
Subject: Re: Vertical Seismometer with Feedback, Transducer Question
To: psn-l@..............
Date: Wednesday, September 17, 2008, 8:27 AM

At 07:32 PM 9/15/2008 -0700, you wrote:
>Hi Chris
>STM also describes a fabricated coil using a rare earth ring magnet from=
=20
>mcmaster carr, thick washer and cap screw. It was definitely easier for me=
=20
>to make than taking apart and machining and putting back together a woofer=
=20
>speaker.
>
>Hi Brett
>Using STM equations I have been wondering for quite a while how to lower=
=20
>the gain to prevent clipping. It seemed the whole idea was to have a large=
=20
>gain in the equations for the desired frequency range. I put a voltage=20
>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=20
>sensors range. It seems recording two levels would be easier to achieve=20
>than a higher resolution A/D , however it would double the number of=20
>recording channels.
>

Hi Barry,

As you suggest, the circuit itself will have clipping limits, which will=20
come into play when you increase the A/D clipping level.  In order to=20
determine what those limits are, I'd need to know quite a bit more about=20
exact circuit.  The most likely limit will be in both the voltage and=20
current capabilities of the amplifier which drives the derivative branch=20
capacitor, though several other factors will also relate to that.  Some of=
=20
those are the mass value, the forcing coil constant, N/A, the coil=20
resistance and just possibly its inductance, and the derivative capacitor=
=20
value to name the most important factors.  Given those (except possibly for=
=20
the inductance) combined with the derivative branch driver amplifier peak=
=20
voltage and current capabilities, one should be able to plot clipping=20
velocity vs frequency, which I think is what you are looking for.

http://bnordgren.org/seismo/loop7.zip  which has a lot of tools for=20
relating component values and instrument performance.  Although it doesn't=
=20
directly relate to clipping levels, it does provide some useful tools=20
related to them.  Sorry for the lack of documentation, but there are three=
=20
worksheets at the beginning which define what the variables mean and show a=
=20
little of how to use the workbook.  You will want to let Excel run macros=
=20
if you are planning to do more than just look at the example data, which is=
=20
currently set to be an approximate model of Allan Coleman's MkXX=20
design.  Please feel free to ask about anything that needs=20
explaining.  Someday I'd like to write some more about how to use it to do=
=20
a feedback design and any questions or comments will help me to do that bet=
ter.

Regards,
Brett

Hi Brett
After my email I played around with the various components. I have STM's Mcad program, but I was able to present the data in Excel as you did. I currently have a max output of around 2000-2500 v/m/s for my sensors. I was able to reduce this down to around 400 by reducing the mass to around 0.2 kg and reducing r from ~240000 to around 100000. I then adjusted the other feedback components to get a good response at the ends of the pass band and an appropriate damping value. I then could use this as one output, increase output back up by ~6x  or more and make this my second output. I'm not sure if a gain reduction of 6 is enough to make much of a difference. Right now I'm "clipping" with a magnitude of 7.0 @ about 4000 km or 5.0 @ ~200 km. Looking at my log-log graph of distance vs counts a gain reduction of 6 would correspond to an increase in non- clipped magnitude of ~ 0.7 at a given distance. Hardly seems worth the bother.
Barry

--- On Wed, 9/17/08, Brett Nordgren <brett3nt@.............> wrote:
From: Brett Nordgren <brett3nt@.............>Subject: Re: Vertical Seismometer with Feedback, Transducer QuestionTo: psn-l@..............Date: Wednesday, September 17, 2008, 8:27 AMAt 07:32 PM 9/15/2008 -0700, you wrote:
>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.
>

Hi Barry,

As you suggest, the circuit itself will have clipping limits, which will
come into play when you increase the A/D clipping level.  In order to
determine what those limits are, I'd need to know quite a bit more about
exact circuit.  The most likely limit will be in both the voltage and
current capabilities of the amplifier which drives the derivative branch
capacitor, though several other factors will also relate to that.  Some of
those are the mass value, the forcing coil constant, N/A, the coil
resistance and just possibly its inductance, and the derivative capacitor
value to name the most important factors.  Given those (except possibly for
the inductance) combined with the derivative branch driver amplifier peak
voltage and current capabilities, one should be able to plot clipping
velocity vs frequency, which I think is what you are looking for.

http://bnordgren.org/seismo/loop7.zip  which has a lot of tools for
relating component values and instrument performance.  Although it doesn't
directly relate to clipping levels, it does provide some useful tools
related to them.  Sorry for the lack of documentation, but there are three
worksheets at the beginning which define what the variables mean and show a
little of how to use the workbook.  You will want to let Excel run macros
if you are planning to do more than just look at the example data, which is
currently set to be an approximate model of Allan Coleman's MkXX
design.  Please feel free to ask about anything that needs
explaining.  Someday I'd like to write some more about how to use it to do
a feedback design and any questions or comments will help me to do that better.

Regards,
Brett

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