Guralp Systems CMG-3TB - Manual (Page 70)

CMG-3TB
After the sensor unlocks the masses, the first round of centring has to
move the N/S and E/W components all the way from their end stops,
whilst the
Z component is often closer to the proper position. Because
of this, the first Z centring operation takes much less time than the
others, and you may not notice it.
After successful centring, the mass position outputs should be in the
range 0.1 – 0.8 V. If the centring process leaves the mass position
outputs 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. 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 3TB 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 3TB instrument are
linear to better than 107 dB (source: measurements made at ASL
during evaluation for the USGS National Network.)
70 Issue C