Capacitive sensing and electrostatic positioning of the miniSTEP test masses Original Research Article

The capacitive position sensing and electrostatic positioning is a multi-purpose subsystem of the miniSTEP instrument. Implemented inside each of the four differential cryogenic accelerometers, it will be designed first to accurately set up the position of the two test masses. In fact, all degrees of freedom of each nearly cylindrical masses, except the rotation about the revolution axis, shall be finely controlled. The system will allow an easy way to calibrate the SQUIDs and the superconductive magnetic suspension and to centre the two masses to satisfy the rejection of the gravity gradient and angular motion effects. During the Equivalence Principle measurement phase, it will provide electronic damping for the differential mode and the dither signal for the test mass charge control system. The position measurements will be also useful for the drag compensation system of the satellite. Taking advantage of highly accurate and low back-action sensors and of the easiness and in quite real time adjustment of the configuration parameters through digital control loops it could even constitute a back-up for the SQUID sensors and the magnetic bearings.