Guralp Systems CMG-3T - Manual (Page 35)

Operator's guide
5 Inside the 3T
5.1 The sensors
The horizontal and vertical sensors are similar in design. The inertial
mass in both cases consists of a transducer coil and a leaf-spring
suspended boom which swings on a frictionless hinge. A triangular
spring supports the weight of the mass; in the vertical sensor this
spring is pre-stressed with a natural period of around 0.5 seconds,
whilst the horizontal sensor has an unstressed flat spring with a
natural period of around 1 second. CMG-3T sensors have no spurious
resonances below 140 Hz, and weigh around 180 g. The small boom
size and stiff springs allow three independent instruments to be
mounted within the casing, together with all the associated feedback
electronics.
The 3T functions by monitoring the position of each mass with a
capacitative position sensor. The three sensors are identical. Signals
from the sensors are fed into an electronic processing unit, which is
mounted in a screened compartment above the mechanical
components (see below for details on the feedback circuitry.)
When the instrument is being transported, the masses are locked
securely in their frames so as to relieve strain on the support hinges.
This locking is performed by a small motor-driven clamp in response
to a signal from the surface controller unit.
Before using the instrument, the boom of the vertical sensor must be
levelled and the bases of the horizontal sensors tilted, so that the
masses are centred in their equilibrium positions. These adjustments
are made by small DC motors controlled remotely.
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