Calibration and testing of the ST7 capacitive sensor using an optical interferometer with fiber optic input and output

We describe the calibration and operation of the ST7 GRS capacitive sensor system. A novel optical interferometer, featuring both fiber optic delivery and readout, is used for calibration of the capacitive sensor system and as a witness sensor during testing. The capacitive sensor showed 1 nm/√Hz performance from 1 mHz to 10 Hz. Above 10 mHz the optical system showed significantly better performance than the capacitive sensor, with a noise floor at 2 × 10−11 m/√Hz and a longer measurement standoff distance. The optical system was limited to 5 nm/√Hz at 1 mHz due to thermal disturbances of the laboratory environment, and could be reduced with a revised design and/or improved thermal stability.