Servo Type Sensor
The servo type sensor is commonly used in earthquake monitoring and measurements of micro tremors on civil engineering structures due to its high sensitivity, stability, and accuracy in the lower frequency range compared to other vibration transducers.
A motion is sensed by a displacement detector, a current is fed to the coil to get back the pendulum mass to the original position. This current will be proportional to the acceleration, that is converted to an output voltage. The servo type sensor is for the earthquake monitoring or measurements of micro tremor on the civil engineering structures because of its higher sensitivity and stability or more accurate phase responses in the lower frequency range than those of other vibration transducers.
Detectable down from DC
Advanced linearity and high resolution
High sensitivity and large output voltage
Excellent phase response in low frequency range
Stable operation because of armature flexure suspension free from friction or fatigue
Static sensitivity calibration is possible using the gravity acceleration
Long cable can be used because of the pretty low output impedance
Though the upper frequency limit of the servo sensor should be defined by the natural frequency of the pendurum that is affected by displacement negative feedback, it is actually limited by the installed low pass filter. Lower frequency components can be detected down to DC acceleration. However, lower limit will depend on the integral characteristic of the amplifier in case of convension to velocity or displacement.
Phase response of the servo sensor is favorable down from low frequency area (almost DC)