An active system based on the sky-hook concept requires the absolute velocity of the isolated mass M to drive the actuator. Accelerometers with an inertial mass m can be used for this task because of their capability to sense the absolute acceleration.
The figure above depicts a sensor-actuator-unit. The signal of the accelerometer is generated by a piezo electric disc and then feed in an electronic circuit. This circuit is designed to perform a time integration and to amplify the input signal. The output of the electronics is a current which is proportional to the absolute velocity. The feedback loop is closed by the electro dynamic voice coil actuator, which generates the compensation force in order to counteract the motion of the isolated device.