Embedded model control: sub-microradian horizontality of the nanobalance thrust-stand

The measuring principle of the nanobalance thrust-stand is the high sensitivity to length variations of a Fabry-Perot cavity embraced by two balanced tilting plates. The nanobalance aims to thrust accuracy less than 1 muN. The plate flexible joints are fixed to an athermic spacer so as to minimize thermo-elastic deformations. Unavoidable plate unbalance makes the cavity length sensible to spacer lateral acceleration and tilt. At very low frequencies, less than 10 mHz, only tilt is significant as it may drift well above target accuracy. Tilt must be reduced to fractions of microradian in the instrument measurement bandwidth below 1 Hz. To cope with milliradian tilt, large stroke (>1 mm) piezoelectric actuators are coordinated with small-stroke. Coarse and fine clinometers allow large range and high accuracy. Embedded model control has been employed.