5 test installation – Guralp Systems CMG-6T User Manual
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Operator's Guide
7. Adjust the relevant potentiometer until the signal meter reads
zero.
2.4.1 Remote centring option
For instruments fitted with the remote centring option, the mass
position offsets are adjusted using a micro-controller and three digital
potentiometers that replace the standard electromechanical
potentiometers.
When installing the instrument, ensure that it is levelled accurately by
checking that the bubble in the level lies within the central circle.
On power-up, the micro-controller will automatically zero the mass
positions of all three axes simultaneously. Zeroing can be further
triggered via the “centre” control line on pin U. (On instruments
without the digital nulling option, pin U is the 'Acc/Vel' line).
Automatic zeroing takes approximately forty-five seconds to complete,
after which the sensor reverts to long period operation and the nulling
module enters a low power “sleep” mode. During nulling, the sensor's
outputs will fluctuate as the pots are adjusted in a binary search before
settling with a mass position of ±0.5V. If the sensor is poorly levelled,
the micro-controller will make three attempts to zero the mass before
giving up and using the closest match.
A test mode is available to check the operation of the digital centring
pots. This mode is entered by holding the centre line low during
power up. The unit will then set the pots to maximum for thirty
seconds; then minimum for thirty seconds; then to the centre position
for six minutes. The centre line must be held low continuously,
otherwise the unit will abort the test mode and null the sensors as
normal.
2.5 Test installation
This section gives an overview of how to set up a CMG-6T and begin
recording data. We recommend that you set up a test instrument in
your office or laboratory as a “dry run” to gain a basic understanding of
the system and to check that it is functioning as expected.
This test installation will use the instrument's default settings. Data
will be digitised using a CMG-DM24 and received using Güralp
Systems' Scream! software, available from our website
July 2011
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