Guralp Systems CMG-5T Compact - Manual (Page 17)

Operator's Guide
calibration by tilting the sensor by 90° and varying the
calibration signal until it precisely compensates for the signal
generated due to gravity.
12. Calibrate any other sensors connected to the digitizer in the
same way. You must wait for the previous calibration to finish
before doing this: clicking Inject now has no effect whilst the
Calibration enable relay is open.
If you prefer, you can inject your own signals into the system at any
point (together with a Calibration enable signal, if required) to provide
independent measurements and to check that the voltages around the
calibration loop are consistent. For reference, a DM24-series digitizer
will generate a calibration signal of around 16,000 counts (4V) when set
to 100% (sine-wave or step) and around 10,000 counts (2.5V) when set to
50%.
3.3.1 Open-loop calibration
The CMG-5T Compact exposes a logic level control line on the signal
connector which switches the instrument into open-loop mode whilst it
is activated. In this mode, force feedback to the masses is disabled,
leaving each one free to oscillate at the fundamental resonance
frequency of its spring. Voltages representing the acceleration of the
masses can be measured on the normal channels.
On the Hand-held Control Unit, the logic line is connected to the
1 SEC VEL/BB VEL switch. Move this switch to “1 SEC VEL” to enable
open-loop mode. You can carry out calibration experiments whilst in
this mode. Move the switch back to “BB VEL” to restore normal
operation.
Opening the feedback loop merely enables the masses to move freely. If
the masses are already near their equilibrium positions, switching to
open-loop mode will not have a large effect.
3.4 The calibration pack
All Güralp sensors are fully calibrated before they leave the factory.
Both absolute and relative calibration calculations are carried out. The
results are given in the calibration pack supplied with each instrument.
3.4.1 The calibration sheet
The calibration sheet provides the measured acceleration output over the
flat portion of the sensor frequency response in units of volts per metre
per second squared (V/ms -2 ). Because the sensor produces outputs in
differential form (also known as push-pull or balanced output), the
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