Guralp Systems CMG-5T - Manual (Page 18)

CMG-5T
The mass and the capacitor plates are energised by a two-phase
transformer driver, forming a differential capacitor. This acts as a
capacitive transducer, whose signal is then demodulated with a phase-
sensitive detector. The accelerometer feedback loop is completed with
a feedback loop compensator and a feedback force transducer power
amp.
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 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 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 vaults built in
seismically stable locations, 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 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.
4.2 Frequency response
The frequency response of each component is provided as amplitude
and phase plots.
When testing the instrument to confirm that it meets its design
specification, the range of frequencies used are concentrated over
about 3 decades (i.e. 1000 : 1) of excitation frequencies. Consequently,
the frequency plots of each component are provided in normalised
form. Each plot marks the frequency cut-off value (often quoted as “–
3dB” or “half-power” point).
Güralp Systems performs frequency response tests on every sensor at
the time of manufacture. All records are archived for future reference.
18 Issue E