Guralp Systems CMG-5T Compact - Manual (Page 21)

Operator's Guide
sensitive detector. The accelerometer feedback loop is completed with a
phase compensator and a feedback force transducer power amp. A
switchable input line allows the feedback loop to be broken, providing
open-loop response when required.
The differential output amplifier scales the output sensor sensitivity and
a second stage amplifier can be configured (at the factory) either as a
further cascaded gain stage or as a high-pass filter with unity gain.
4.1 The force transducer
The CMG-5T Compact is a force-feedback strong-motion accelerometer
which uses a coil and magnet system to generate the restoring feedback
force. Such accelerometers inherently depend on the production of a
constant-strength field in the magnet gap. Although the high quality
magnets used in the 5T Compact accelerometer are exceedingly stable
under normal conditions, if the sensor is sited in an area where the
background seismic noise is much higher than that of a vault
constructed in a seismically stable location, the flux density may be
affected by the external magnetic field generated by the feedback
transducer coil.
In order to minimise non-linearities in the feedback force transducer, the
5T Compact uses a symmetrical system of two magnets and two force
coils. Any increase in flux in one coil is cancelled by a corresponding
decrease in flux in the other, thus eliminating any non-linearity due to
lack of symmetry.
4.1.1 The sensor transfer function
Most users of seismometers find it convenient to consider the sensor as a
“black box” which produces an output signal V from a measured input x.
So long as the relationship between V and x is known, the details of the
internal mechanics and electronics can be disregarded. This
relationship, given in terms of the Laplace variable s, takes the form
( V / x ) (s) = G × A × H(s)
In this equation
•
G is the acceleration output sensitivity (gain constant) of the
instrument. This relates the actual output to the measured input
over the flat portion of the frequency response.
•
A is a constant which is evaluated so that A × H(s) is
dimensionless and has a value of 1 over the flat portion of the
frequency response. In practice, it is possible to design a system
transfer function with a very wide-range flat frequency response.
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