Hi all,

  There has been a lot of discussion about damping lately, and so I am going 
to put my two cents worth in.

  I operate an old vertical Strengnether, fairly light in construction.  It 
is presently set for a natural period of about 3.4 seconds, and about 0.8 
damping.  I believe that somewhat under-damped is best.  For checking the 
instrument, I rigged up a circuit consisting of two double pole switches, a 
high value resistor, and a battery.  The circuit is placed between the 
seismometer and the amplifier.  One of the switches is used to reverse the 
battery polarity.  The other switch, in one position, simply connects the 
sensor to the amplifier.  In the other position, the amplifier is left open 
circuited, and the sensor is connected to the battery and the series 
resistor.  The small current through the sensor coil drives the pendulum away 
from balance.  While recording at faster than normal sampling rate, I throw 
the switch to this position for a few seconds to displace the pendulum, then 
switch back to normal operation to observe the velocity waveform as the 
pendulum returns to equilibrium.

  I have written a numerical simulation program which can duplicate this kind 
of test for any natural period and damping I choose.  Using Paint, I can 
transparently paste its waveforms over the seismometer waveforms to get a 
match.  One does have to recognize that the amplifier's impulse response is 
another factor to complicate things.  I should really add that to the 
simulation program.

Bob McClure
Hi all,



  There has been a lot of discussion about damping lately, and so I am going to put my two cents worth in.



  I operate an old vertical Strengnether, fairly light in construction.  It is presently set for a natural period of about 3.4 seconds, and about 0.8 damping.  I believe that somewhat under-damped is best.  For checking the instrument, I rigged up a circuit consisting of two double pole switches, a high value resistor, and a battery.  The circuit is placed between the seismometer and the amplifier.  One of the switches is used to reverse the battery polarity.  The other switch, in one position, simply connects the sensor to the amplifier.  In the other position, the amplifier is left open circuited, and the sensor is connected to the battery and the series resistor.  The small current through the sensor coil drives the pendulum away from balance.  While recording at faster than normal sampling rate, I throw the switch to this position for a few seconds to displace the pendulum, then switch back to normal operation to observe the velocity waveform as the pendu
lum returns to equilibrium.



  I have written a numerical simulation program which can duplicate this kind of test for any natural period and damping I choose.  Using Paint, I can transparently paste its waveforms over the seismometer waveforms to get a match.  One does have to recognize that the amplifier's impulse response is another factor to complicate things.  I should really add that to the simulation program.



Bob McClure