PSN-L Email List Message
Subject: Feedback and nonlinearities
From: Brett Nordgren brett3nt@.............
Date: Sat, 13 Aug 2011 13:49:19 -0400
To all,
I think it might be helpful to briefly outline why feedback designs
have been so successful in making the best seismometers, particularly
with respect to system nonlinearities such as exist in the springs of
vertical instruments.
The feedback electronics are constructed to force the instrument to
have the desired dynamic characteristics (frequency response). To do
this successfully, at all frequencies of interest there must be high
loop gain. At the lower frequencies, loop gain can be thought of as
the ratio of the force being exerted on the mass by the feedback
system to the 'dF=KdX' force exerted by the spring supporting the
proof mass. In our designs we maintain a minimum loop gain of 100 at
all frequencies below the spring-mass resonance, which rises to be
over 100,000 at the lowest sub-milliHertz frequencies. The mass is
affected by a force from the feedback loop which is, at a minimum,
100 times greater than the first-order spring force variation. I
have assumed that all nonlinear spring forces must be somewhat
smaller than this first-order force, so for them, their domination by
the feedback must be even greater as it acts to ensure linear
ground-motion response.
In summary, it is well known that feedback acts in a way to mask or
eliminate system nonlinearities, whether originating in the spring,
the position sensor or the pivot flexures.
If we needed to observe subtle instrument nonlinearities, we would
have to turn the feedback off, though if we did, the instrument would
unfortunately quit working, as it would no longer be able to maintain
the required precise centering.
Regards,
Brett
__________________________________________________________
Public Seismic Network Mailing List (PSNLIST)
To leave this list email PSNLIST-REQUEST@.............. with
the body of the message (first line only): unsubscribe
See http://www.seismicnet.com/maillist.html for more information.
[ Top ]
[ Back ]
[ Home Page ]