Guralp Systems CMG-40T - Manual (Page 31)

Operator's guide
may need to increase R by adding further resistors to the circuit.
4.5 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 the acceleration due to gravity, g, at your location.
2. Tilt the instrument slightly, and measure its attitude and the gain of the
mass position output for the component you wish to calibrate.
3. Repeat this measurement for several tilt angles.
4. For the vertical sensor, the input acceleration is given by a = g sin φ,
whilst for the horizontal sensor, it is a = g ( 1 – cos φ ).
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/m/s 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, given on the calibration sheet.
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