  Low-frequency seismic noise poses a significant challenge in
gravitational-wave detectors, prompting the use of active seismic isolation
systems. These systems employ advanced control configurations, including
feedback control, sensor correction, and sensor fusion, to mitigate seismic
disturbances. Optimal performance of these systems is achieved through the
application of the H-infinity method, which optimizes control filters. The
H-infinity method was employed to optimize a sensor correction filter and
complementary filters for sensor fusion for a KAGRA Type-B suspension,
resulting in the development of an instrumentation-optimal ``super sensor''.
The super sensor exhibits comparable overall displacement noise RMS performance
to a relative sensor, yet offers significantly better band-limited RMS
performance around the secondary microseism frequency, making it highly robust
to seismic noise. This highlights the efficacy of the H-infinity method in
generating seismic isolation filters tailored to instrument limitations,
thereby transforming the system into a fully functional active seismic
isolation system.
