Recently, servo-seismometers (negative feedback seismometers) are used for various fields in seismology and earthquake engineering. First, this report explains five fundamental negative feedback seismometers with a displacement or a velocity transducer. Next, composite negative feedback seismometers, such as STS, VSE and CMG seismometers, are explained. Also, new negative feedback seismometers that have two feedback paths, are discussed. Finally, seismometer noise, the representations of instrument noise and seismometer characteristics, and, some problems in borehole observation by using negative feedback seismometers are discussed.
