Figure 2.13 shows a typical control

Figure 2.13 shows a typical control topology of an active isolation system. The isolation platform is supported by suspension springs so that the motion can be controlled with actuators. Vibration of the platform is measured by inertial sensors (seismometers), which sense the platform accelerations relative to an inertial frame, and the signals are sent to the feedback controller. Since inertial sensors have poor sensitivities at low frequencies, the platform is positioned at DC with respect to the support structure by using relative position sensors. In the feedback controller, signals from inertial sensors and position sensors are blended to connect the two control loops. Additional isolation performance is obtained by using feedforward inertial sensors measuring the ground vibration. Detailed discussion on the active isolation system can be found in reference [49].