Development of a Fine Steering Tip/Tilt Mechanism for Space Applications

Positioning mechanisms are frequently required in advanced optical systems, as Earth Observation telescopes, to adjust the position of one or several optical elements, to compensate for thermal effects, settling due to launch or environmental loads, movements due to carbon fiber components contraction, or to compensate for spacecraft micro-vibrations or jitters. In this context, the present research activity aims at the development of a mechanism for steering the beam of a space telescope to correct for image motion. Defined as Fine Steering Tip/Tilt Mechanism (FSTTM), it has to move an optical payload (like a mirror) fulfilling three degrees of freedom: a translation along its optical axis (performing a so called “piston” motion), and rotations along two orthogonal axes lying on its optical surface (performing so called “tip” and “tilt” rotations). In the first year, after a definition of the system requirements and a literature review, the layout of the FSTTM has been defined: four piezoelectric actuators will move the optical payload, and four capacitive sensors will measure its displacements. After that, due to the limited stroke capability of piezoelectric actuators, a Compliant Mechanical Amplifier (CoMA) has been developed to preload and amplify the piezoelectric output displacements. Through several FE analyses, and a trade-off study among different configurations, the Mechanical Amplified Piezoelectric Actuator (MAiA) has been defined. In the second year, a breadboard of MAiA actuator has been realized, and several tests have been completed on it before the finalization of the FSTTM design. Using a Coordinate Measuring Machine (CMM), firstly piezoelectric actuator and CoMA performances have been measured. Then, the composite MAiA actuator has been tested, in presence and in absence of an external structure having a certain stiffness. Moreover, a MAiA FE model has been validated by measuring its eigenfrequencies though a resonance search test. This breadboard has been operated also in vacuum: installed in a Thermal-Vacuum Chamber (TVAC), the designed preload has been verified at the extremes of non-operative temperature range, and actuator performances have been tested in the operative temperature range. In conclusion of breadboard testing activity, the FSTTM design has been finalized, verifying the system under required quasi-static, sinusoidal, random, shock and thermo-elastic loads with FE analyses. In the third year, after manufacturing drawings, custom and commercial components have been procured and the FSTTM has been integrated. Assembled MAiA actuators, and the entire mechanism have been verified with the CMM, to also validate the feedback system composed by capacitive sensors. After that, the FSTTM has been verified in vacuum at several temperatures, also validating the capacitive sensors through optical measurements. Before the environmental tests, the FSTTM FE model has been correlated using eigenfrequencies and damping ratios derived from a resonance search test. Therefore, the FSTTM has been successfully tested under sinuosoidal and random vibrations, and in vacuum under eight thermal cycles where it has also been operated.