Inside the 3ESP
6
Inside the 3ESP
6.1
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 0.5 s, whilst
the horizontal sensor has an unstressed flat spring with a natural period around 1 s.
CMG-3ESP 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 3ESP 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.
The signal voltages output by the 3ESP 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 3ESP also receives control signals, which are used to clamp and
unclamp 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.
6.2
Locking and unlocking
6.2.1 Manual control
The mass clamps in the 3ESP push each mass into precisely-machined cavities, so
that they are unable to move in any direction. The force required to disengage the
masses is more than 15 times the weight of the mass, allowing you to transport the
instrument safely.
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Issue G - February 2016
