A Simple Force Balance Accelerometer/seismometer Based on a Tuning Fork Displacement Sensor

Seismometers and MEMS accelerometers use the force-balance principle to obtain measurements. In these instruments the displacement of a mass object by an unknown force is sensed using a very highresolution displacement sensor. The position of the object is then stabilised by applying an equal and opposite force to it. The magnitude of the stabilising force is easily measured, and is assumed to be equivalent to the unknown force. These systems are critically dependent on the displacement sensor. In this paper we use a resonant quartz tuning fork as the sensor. The tuning fork is operated so that its oscillation is lightly damped by the proximity of the movable mass object. Changes in the position of the mass object cause changes in the phase of the fork’s resonance; this is used as the feedback variable in controlling the mass position. We have developed a novel acceleration sensor using this principle. The mass object is a piezoelectric bimorph diaphragm which is anchored around its perimeter, allowing direct electronic control of the displacement of its centre. The tuning fork is brought very close to the diaphragm centre, and is connected into a self-oscillating feedback circuit which has phase and amplitude as outputs. The diaphragm position is adjusted by a feedback loop, using phase as the feedback variable, to keep it in a constant position with respect to the tuning fork. The measured noise for this sensor is approximately 10.0 mg in a bandwidth of 100 Hz, which is substantially better than equivalent commercial systems.