Guralp Systems CMG-3T - Manual (Page 43)

Operator's guide
and damping of the sensor is completely determined by a feedback
loop which applies a force to the sensor mass opposing any motion.
The force required to restrain the movement of the mass can then be
used to measure the inertial force which it exerts as a result of ground
motion.
All CMG-3 series units are based on these general principles. The
capacitative position sensor for each mass produces a voltage
proportional to the displacement of the mass from its equilibrium
position. After amplification, this voltage generates a current in the
force transducer coil which tends to force the mass back toward
equilibrium. The feedback loop has a sufficiently high gain to cancel
the motion of the mass. Since the mass is not moving, the forces acting
on it must be balanced; the feedback voltage then directly measures
the force, and hence the acceleration, which is being applied to the
mass. The feedback loop introduces a phase shift, which must be
carefully controlled if the instrument is to remain stable over its entire
frequency range. This is achieved using compensation components in
the forward and feedback paths.
Force feedback seismometers of this type rely on the assumption that
the force transducer produces a field of constant strength. The
magnetic circuit and magnet/pole assembly in the 3T are designed so
that the field strength from the feedback transducer is constant over
large deflections and current levels. Tests have shown that the
mechanical suspension system and electronics of a 3T instrument are
linear to better than 107 dB (source: measurements made at ASL
during evaluation for the USGS National Network.)
In a feedback seismometer with a displacement transducer, it is
essential to monitor the acceleration output. This provides the position
of the displacement transducer and therefore also the mass position, as
the displacement transducer is attached to the sensor inertial mass.
The sensor should always be operated with the displacement
transducer centred or nulled, so that the response to input acceleration
is linear.
There are two types of feedback system which can be used in a 3T
instrument, known as hybrid and conventional-response feedback.
These are both described below.
Hybrid feedback
The hybrid feedback method of operation is illustrated by the
following schematic diagram:
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