OASIS: a new development for future high-accuracy optical pointing

High accuracy pointing performances and also large autonomy from the host satellite are required for the future optical payloads designed to perform astronomy, laser communication, planetary observation, or other missions. In order to prepare these future applications, SFIM Industries/Ets d'Asnieres (SFIM/EA) has undertaken under CNES (French Space Agency) contract, the development of a new generic system able to fit a large variety of satellites, instruments and missions. To meet the stringent pointing stability constraints, a few microradians, the design has mainly to minimize friction and to control flexible modes. The mechanism is a two axis single stage turret designed around two identical joints, linked together by an isostatic truss structure. The structure is optimized using an integrated design approach according to the flexible modes control problem. The high angular stiffness to friction ratio of the articulation is obtained through an optimized design of the ball bearings. These are protected during launch by a new locking mechanism. The residual friction wires torques are linearized by a mechanical device which provides in addition a closed loop friction compensation. The control torques are produced by a brushless torque motor optimized for low ripple and detent torques. The control laws design is based upon H(infinity) methods which have been adapted to robust control of flexible systems. The control loop uses a combination of optical encoder and accelerometer outputs or in a best way a low frequency payload ecartometry signal combined with accelerometers outputs. A feasibility study was carried out in 1993 and has demonstrated the validity of the concept. A demonstrator is to be tested in the end of 1995.