6 the coil constant – Guralp Systems CMG-6TD User Manual
Page 79
Calibrating the 6TD
6.6
The coil constant
The feedback coil constant K is measured at the time of manufacture, and
printed on the calibration sheet. Using this value will give good results at the
time of installation. However, it may change over time.
The coil constant can be determined by tilting the instrument and measuring
its response to gravity. To do this, you will need apparatus for measuring tilt
angles accurately.
1. Measure or look up the acceleration due to gravity, g, at your location.
2. Tilt the instrument slightly, and measure its attitude and the value of
the mass position output for the component you wish to calibrate.
To make it easier to measure mass positions, you should use the RESP
command to place the instrument into one-second response mode. To
do this, open a Terminal window in Scream! (see section 7 on page 80)
and issue the commands:
OK-1
1 RESP
MASSES?
0 RESP
3. Repeat this measurement for several tilt angles.
4. For the horizontal sensor, the input acceleration is given by a = g sin ,
φ
whilst for the vertical sensor, it is a = g ( 1 – cos φ ). The “1” in the
equation for the vertical acceleration accounts for the effect of the
internal gravity-compensation spring.
Calculate the input acceleration for each of the tilt angles used, and
plot a graph of mass position output against input acceleration.
5. The gradient of the line obtained gives the sensitivity of the coil (in
V/ms
-2
, if g was measured in m/s
-2
and the mass position in V).
6. The coil constant K is equal to this sensitivity divided by the value of
the displacement feedback resistor (which is given on the calibration
sheet).
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Issue F - February 2014