Guralp Systems CMG-3TB - Manual (Page 69)

Operator's guide
CENTRE
This command re-centres the masses. If the masses are clamped, or if
the sensor mass positions do not exceed ± 1.2 V, the
CENTRE
command does nothing. Otherwise, it attempts to zero the output of
the vertical, N/S and E/W sensors in sequence by exerting small extra
forces on the boom. For the vertical sensor, a motor-driven adjuster
presses a small spring lever against the boom until the mass position
sensor indicates an offset close to zero. In the case of the horizontal
sensors, the sensor frame is tilted on its base plate. Again, the
controller monitors the mass position sensor and stops the centring
process once it reaches its lowest offset.
This graph shows a typical centring process:
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 more spaced out 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. The mass position
output does not change for a while, as the true mass position is
outside the range of the output. In this case, the pulses cause the
mass to overshoot the central position, and 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.
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