Guralp Systems CMG-3ESPCD - Manual (Page 77)

CMG-3ESPCD Digital Broadband Seismometer Inside the 3ESPCD
7 Inside the 3ESPCD
7.1 Inside the 3ESPCD
7.1.1 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 around half a second, whilst the horizontal sensor has an unstressed
flat spring with a natural period around 1 second. CMG-3ESPCD 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 3ESPCD 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.
See section 3 on page 8 for detailed instructions on how to set up your
3ESPCD installation.
The signal voltages output by the 3ESPCD are proportional to ground velocity,
and are transmitted from the instrument on balanced differential lines. In
addition, mass position signals are sent as single-ended circuits referred to
analogue ground on the output plug. The 3ESPCD also receives control
signals, which are used to clamp and un-clamp the masses, and to run the
motors which level and centre the instrument once in position. Finally, a line
is provided for you to apply a calibrating voltage to the force transducers,
thereby measuring the deflection sensitivity.
7.1.2 The control system
The internal operations of the 3ESPCD are supervised by a control
microprocessor, which drives the mass clamping and centring adjustment
MAN-C3E-0002 77 Issue B - November 2013