Guralp Systems CMG-3ESPCD - Manual (Page 81)

CMG-3ESPCD Digital Broadband Seismometer Inside the 3ESPCD
1. The BUSY LED pulses to indicate that it is centring the Z component.
Each pulse corresponds to a small force on the mass. The pulses
become briefer as this goes on, until a pulse is missed, signifying that
no corrective impulse is needed.
2. The N/S component follows in the same way. If the true mass position
is outside the range of the output, you may not see the output change
for some time. If the pulses cause the mass to overshoot the central
position, a second group of pulses in the opposite direction is applied
to bring it into line.
3. The E/W component follows in the same way. All three masses are
now centred and the process completes.
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.
7.1.3 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.
MAN-C3E-0002 81 Issue B - November 2013