Design and development of the point-ahead angle mechanism for the lase interferometer space antenna (LISA)

The Point Ahead Angle Mechanism (PAAM) for ESA’s Laser Interferometer Space Antenna (LISA) mission will compensate the out-of-plane point-ahead angle between three satellites flying 5 million kilometres apart. The PAAM consists of a mirror supported by flexures allowing the mirror to rotate with a maximum stroke of ±1 mrad. The mirror is actuated in 0.14 μrad steps by two redundant linear Piezo LEGS® actuators driving a sine-bar. Since the actuators are self-locking, a special lever performing the role of a linear mechanical differential is used to provide redundancy. The design uses high-precision flexures to minimise mirror parasitic piston displacements in the picometres range. The angle is driven in closed loop using two capacitive sensors. This paper presents the mechanism at the design stage of an elegant breadboard (EBB) ready for tests. The performance requirements are summarized, then the overall concept of the mechanism is described, at last the key aspects of the detailed design are discussed: flexures design, kinematic structure, kinematic mount, redundant piezo-actuation, sensors and control. Optimisation of the design has shown that the selected high-precision design can meet the stringent requirements of fine positioning and severe environments.