Operator's guide
Afer successful centring, the mass position output should be in the
range 0.1 – 0.8 V. If the centring process leaves the mass position output
above ±1.1 V, you should start another centring cycle by activating the
CENTRE
command again. You will probably need to initiate the centring
process several times before the masses are adequately centred.
5.3 The feedback system
The output from a modern broadband seismometer does not depend on
the natural characteristics of the instrument. Instead, the period 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. Afer a mplifcation, t his vo ltage g e nerates a c u rrent in t h e
force transducer coil which tends to force the mass back toward
equilibrium. The feedback loop has a sufciently 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 shif, 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 feld of constant strength. The magnetic
circuit and magnet/pole assembly in the 3V are designed so that the
feld s trength f rom t he f eedback t ransducer i s c onstant o ver l arge
defections and current levels. Tests have shown that the mechanical
suspension system and electronics of a 3V instrument are linear to
beier t han 1 07 d B ( source: m easurements m ade a t A SL d uring
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
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