LRS Micro-g LaCoste FG5, Manual, Page: 22 / 149

Design: Components and Function
2
2.3. The Superspring
The superspring (Figure 2-9) is a long-period, active vertical isolator used to
compensate for small vertical motions of the first beam splitter. The superspring
has a short (20-cm) mainspring with a natural period of about 1 second. The
mainspring is contained in a support housing that is actively servo-controlled to
track the superspring mass at the end of the mainspring. The resulting system is
a long-period (30-60 second) spring-mass system which is suspended from the
interferometer base. The superspring isolated ground motions occurring at a
higher frequency than its own enhanced natural frequency.
2.3.1.SUPERSPRING MASS
The superspring mass contains a corner cube retroreflector and an optical glass
sphere .
2.3.2.SPHERE DETECTOR SYSTEM
The superspring sphere detector system (Figure 2-10) senses motions of the
superspring mass relative to the support housing. An infrared light emitting
diode (LED) located on the support housing directs light through an optical glass
sphere attached to the superspring mass. The sphere focuses the light onto a
split photodiode detector, also mounted on the support housing. The support
housing is itself servo-driven to cancel these motions using an electromagnetic
coil-type linear actuator (coil) is mounted between the support housing and the
superspring base. As vertical ground motion occurs the linear actuator moves
the support housing up or down as needed. The apparatus is constrained to
move only vertically by a linear way system constructed of five flexures (delta
rods) arranged in an upper V-shaped array, and a lower triangular array.
2-12