Precision Calibration of Seismic Sensors

Seismic feedback sensors offer a unique opportunity to perform precision calibration on such devices.

At their core is an inertial mass attached to a suspension system with one degree of freedom. The position and the motion of the mass are detected by a primary transducer, usually a capacitor or a linear variable displacement transformer (LVDT). In addition, the inertial mass has a secondary transducer, typically a coil/magnet arrangement which converts the electronic information generated by the feedback loop into a mechanical restoring force acting on the mass.

At Gaiacode we use the coil constant of the feedback coil as the main parameter for calibrating all our broad-band sensors, which have a high loop-gain. The first calibration step is to tilt the sensors (both vertical and horizontal) on a precision tilt table. Using an 8.5 digit multimeter and precision angle measurement we can calculate the coil constant in units of A/m/s2 with an accuracy of better than 1 part in 10000.

Knowing the coil constant we then inject a wide range of frequencies into the coil using a high precision signal generator and measure the reaction of the mass to this input using the primary transducer.

This procedure gives us a precise measurement of the sensor's output sensitivity value or its frequency response in amplitude and phase, depending on the type of calibration signal injected. In addition, these measurements also evaluate the system linearity and the total harmonic distortion to ground signals.

In this paper we describe the details of this calibration technique and give examples of measurements for systems with responses both in velocity and acceleration.

Author: Cansun Guralp (Gaiacode)