Inside the 3ESP
6.3
The feedback system
The output from a modern broadband seismometer does not depend on the natural
characteristics of the instrument. Instead, the period and damping of the sensor is
completely determined by a feedback loop which applies a force to the sensor mass
opposing any motion. The force required to
restrain
the movement of the mass can
then be used to measure the inertial force which it exerts as a result of ground
motion.
All CMG-3 series units are based on these general principles. The capacitative
position sensor for each mass produces a voltage proportional to the displacement of
the mass from its equilibrium position. After amplification, this voltage generates a
current in the force transducer coil which tends to force the mass back toward
equilibrium. The feedback loop has a sufficiently high gain to cancel the motion of
the mass. Since the mass is not moving, the forces acting on it must be balanced;
the feedback voltage then directly measures the force, and hence the acceleration,
which is being applied to the mass. The feedback loop introduces a phase shift,
which must be carefully controlled if the instrument is to remain stable over its
entire frequency range. This is achieved using compensation components in the
forward and feedback paths.
Force feedback seismometers of this type rely on the assumption that the force
transducer produces a field of constant strength. The magnetic circuit and
magnet/pole assembly in the 3ESP are designed so that the field strength from the
feedback transducer is constant over large deflections and current levels.
In a feedback seismometer with a displacement transducer, it is essential to monitor
the acceleration output. This provides the position of the displacement transducer
and therefore also the mass position, as the displacement transducer is attached to
the sensor inertial mass. The sensor should always be operated with the
displacement transducer centred or nulled, so that the response to input
acceleration is linear.
There are two types of feedback system which can be used in a 3ESP instrument,
known as
hybrid
and
conventional-response
feedback.
6.3.1 Hybrid feedback
The
hybrid
feedback circuit contains a single capacitor in parallel with a resistor,
resulting in a single dominant pole at 0.033 Hz (30 s). Below this frequency, the
response of the seismometer is flat to ground acceleration; above it, the response is
flat to velocity. (Other values for the acceleration-velocity corner can be provided
upon request.) Hybrid-feedback systems provide a stable response, particularly for
portable systems, with a high saturation level at high frequencies and a high
dynamic range at long periods.
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Issue G - February 2016
