High Accuracy Position and Scanning Mechanisms
The need for accurate pointing of spacecraft antennas and / or optical terminals / payloads is virtually required on the large majority of space missions. Radio frequency links and to a larger extent inter-satellite optical links are strongly depending upon the pointing system and associated performances. Pointing mechanisms constitutes the core technology of pointing systems, and determine the overall system architecture and final performances. High pointing accuracy, low mass / power consumption, long life, high reliability are typical driving requirements for space pointing mechanisms.
Accuracy requirements are typically ranging from small fractions of a degree (for high gain antennas), down to micro-radians for inter-satellite optical links, and even nano-radians for instrument optical delay lines applications. Drive / control electronics for open or closed loop pointing control strategies is developed in parallel and proceeds toward an higher level of integration, aiming at a reduced mass, higher performances, low cost (via a batch production approach). Scanning mechanisms share most of the technological components and challenges with pointing mechanisms. Their typical application field is instrument technologies for Earth observation purposes, and small optical terminals for inter-satellite links, which are promising candidate for micro-machining technology applications.
The figure shows a 3 degrees-of-freedom long-stroke / high-resolution tip/tilt mechanism developed by Micromega Dynamics (B). It is based on the magnetic bearing technology and designed for the future space interferometric missions. The achieved positioning and steering resolutions are respectively 1nm over 5mm stroke and 10nrad over 5mrad for the two angular ranges. Thanks to the innovative magnetic bearing design, the power dissipation in the mechanism (under gravity compensated conditions) is less than 1mW, which makes it a good candidate also for infrared interferometric applications where cryogenic conditions have to be implemented and dissipated power minimized. In addition, this elegant breadboard model has successfully passed an environmental (thermal & vibration) and functional test campaign. This mechainism has been tested in low gravity conditions during a parabolic flight test. Performances were as expected even if the low frequency regime could not be verified due to the limited duration of the reduced gravity conditions.
A Soft Stewart platform was developed at University of Brussels (ULB) in collaboration with Micromega Dynamics. It consists of a 6 degrees-of-freedom hexapod whose soft legs are provided with sensors and magnetic actuators. This allows the implementation of an active isolation control scheme, which consists in the active damping of the 6 suspension modes of the platform. The figure on lower right shows the benefit of the active isolation control. Contrary to the passive isolation, the active system allows damping the suspension modes while preserving the nice –40dB/dec isolation curve. This Stewart platform can be used to isolate either a sensitive payload from the bus or the bus from a perturbation (e.g. RWA reaction wheels or cryocoolers). In addition, the proposed platform can be provided with high-resolution positioning and steering capabilities. The actual platform has successfully passed parabolic flight tests.
LISA Point Ahead Angle Mechanism
In the LISA mission, the distance variation between free-floating masses in spacecrafts is measured through laser interferometry. Due to seasonal orbit evolution and the time delay of the traveling light, for each telescope of the LISA arm, the incoming and outgoing laser beams do not lay on the same plane. In addition, their relative angle changes with time. A Point Ahead Mechanism is positioned on the LISA Optical Bench to orient the outgoing beam to the right direction The Point-Ahead Angle Mechanism steers the outgoing laser beam within the following range, essentially steering a mirror mounted on the mechanism. High stability for the mirror is requested because translation noise of the reflecting mirror is seen by the LISA system as an optical pathlength (OPD) change Angular jitter of the reflecting mirror results in pathlength noise if pivot point is offset wrt the incoming beam
OPD and angular jitter requirements are defined through noise shape function between 0.1 mHz and 1 Hz as follows:
Two parallel technology development activities have been carried out. PAAM parformances after environmental testing have been performed on a dedicated metrology set-up developed at Albert Einstein Institute at Hannover.
Last update: 7 May 2014