Guralp Systems CMG-1T - Manual (Page 17)

Operator’s Guide CMG-1T Ocean Bottom
Seismometer
September, 1999, Issue C Section 3,
Page 1
3. THE FEEDBACK SYSTEM DESCRIPTION
In a practical broadband seismometer the natural characteristics of the
seismometer are never used. The period and damping of the sensor is
completely determined by a feedback loop which supplies a counter-force to the
inertial mass sufficient to oppose any overall motion. The force required to
restrain the movement of the mass is then a measure of the inertial force exerted
by the mass due to the ground motion.
The feedback control operates as described below:
The capacitative position sensor provides a voltage proportional to the
displacement of the mass from its equilibrium position. This voltage, after
amplification, generates a current in the force transducer coil which tends to
force the mass back to its equilibrium position. With a high loop gain the
motion of the mass is effectively cancelled thus providing the force balance
condition. The feedback voltage is then a measure of the force and thus of the
acceleration applied to the mass.
In order to obtain stable operation over the whole frequency range the feedback
loop phase shift has to be carefully controlled. This is achieved by
compensation components in the forward and feedback paths as shown in the
following block diagrams.
The CMG-1T sensor is supplied with one of two different types of feedback
system. The main difference between the two systems is in the feedback
electronics used to implement the required responses. In the HYBRID circuit
the feedback components between the output signal and the seismometer mass
consist only of a single capacitor in parallel with a resistor. This results in only
a single pole at the specified frequency. The output of the seismometer at
frequencies below this frequency is proportional to ground acceleration. This
contrasts with the CONVENTIONAL RESPONSE feedback system which has
an additional parallel feedback circuit consisting of a non-inverting integrator in
series with a resistor. The response of the arrangement gives a double pole at
the specific frequencies. In all cases, signal in frequency bands longer than the
specified velocity corner frequencies (eg. 10 sec, 30 sec, 100 sec and 360 sec.)
can be recorded using the mass position outputs. However, high pass circuitry
is recommended to remove the sensor output offsets.