Hello Bob,

Thanks for taking the time to provide feedback.  Please see my 
responses/questions below.

Jason

At 06:47 PM 06/30/2004, you wrote:
>Hi Jason,
>
>   I have some suggestions. Others please feel free to disagree.
>
>1. Greatly reduce the mass of the pendulum. A mass of 5 pounds is probably 
>too much for your superstructure. As the pendulum moves, it exerts a 
>transverse moment on the support structure and baseplate, making it hard 
>to achieve a stable long period. High mass also makes shunt resistance 
>damping practically impossible.

How much weight is preferable--is there a rule of thumb (or better yet, 
mathematical formulas) based on the boom size and/or other parameters?

>2. Stiffen the superstructure laterally with cross bracing.

It seems pretty sturdy but I can easily add another cross-brace.

>3. Having a magnet on the pendulum makes the sensor responsive to stray 
>magnetic fields. Put the coil on the pendulum and the magnet on the base. 
>While you are at it, place a second magnet on the opposite side of the 
>coil, with poles reversed. You will get twice as much sensitivity, much 
>better linearity of response, and half as much shunt conductance required 
>for a given damping level. With the coil mounted on the pendulum, you 
>might not need any extra mass, and that would make shunt damping easier as 
>well.

Easy enough, but where should I run the wires?  Along the boom or up the 
suspension?

>4. Be sure to use a cover which shields the sensor from ambient 
>temperature change and drafts. You may need a heater inside at the top to 
>prevent convection of air upward from the baseplate.

Will make an enclosure of Celotex.  How about a small appliance lamp instead?

>5. Put a one microfarad capacitor across the input terminals of your 
>amplifier. I have amplifiers of differing designs, and they all oscillate 
>internally if there is no shunt capacitance across the input terminals 
>when the source is inductive (i.e., a sensor coil). You cannot see this 
>oscillation at the output terminals; it manifests itself as excess output 
>noise, and in the case of my DC amplifiers, a large and fluctuating bias.

In parallel with the damping resistor?

>   If you want to see my sensors, visit John Lahr's web site at the pages 
> he kindly provides for my amateur efforts:
>
>      http://www.jclahr.com/science/psn/mcclure/
>
>Regards,
>Bob McClure
>Locust Valley, NY


Hello Bob,


Thanks for taking the time to provide feedback.  Please see my
responses/questions below.


Jason


At 06:47 PM 06/30/2004, you wrote:

Hi Jason,


  I have some suggestions. Others please feel free to
disagree.


1. Greatly reduce the mass of the pendulum. A mass of 5 pounds is
probably too much for your superstructure. As the pendulum moves, it
exerts a transverse moment on the support structure and baseplate, making
it hard to achieve a stable long period. High mass also makes shunt
resistance damping practically impossible. 



How much weight is preferable--is there a rule of thumb (or better yet,
mathematical formulas) based on the boom size and/or other
parameters?  


2. Stiffen the
superstructure laterally with cross bracing. 



It seems pretty sturdy but I can easily add another 
cross-brace.


3. Having a magnet
on the pendulum makes the sensor responsive to stray magnetic fields. Put
the coil on the pendulum and the magnet on the base. While you are at it,
place a second magnet on the opposite side of the coil, with poles
reversed. You will get twice as much sensitivity, much better linearity
of response, and half as much shunt conductance required for a given
damping level. With the coil mounted on the pendulum, you might not need
any extra mass, and that would make shunt damping easier as well.



Easy enough, but where should I run the wires?  Along the boom or up
the suspension?


4. Be sure to use a
cover which shields the sensor from ambient temperature change and
drafts. You may need a heater inside at the top to prevent convection of
air upward from the baseplate.



Will make an enclosure of Celotex.  How about a small appliance lamp
instead?


5. Put a one
microfarad capacitor across the input terminals of your amplifier. I have
amplifiers of differing designs, and they all oscillate internally if
there is no shunt capacitance across the input terminals when the source
is inductive (i.e., a sensor coil). You cannot see this oscillation at
the output terminals; it manifests itself as excess output noise, and in
the case of my DC amplifiers, a large and fluctuating bias. 



In parallel with the damping resistor?


  If you want
to see my sensors, visit John Lahr's web site at the pages he kindly
provides for my amateur efforts:


    
http://www.jclahr.com/science/psn/mcclure/



Regards, 

Bob McClure 

Locust Valley, NY