Servo accelerometers :
These use the closed loop servo systems of force-balance, torque-balance or null-balance to provide close accuracy. Acceleration causes a seismic mass to move.
The motion is detected by one of the motion-detection devices, which generate a signal that acts as an error signal in the servo-loop.
The demodulated and amplified signal is then passed through a passive damping network and then applied to the torquing coil located at the axis of rotation of the mass.
The torque is proportional to the coil current, which is in turn proportional to the acceleration.