Advancements of the ELT M1 figure control loop: edge sensor fault detection and management of actuator non-linearities in a large MIMO system

The primary mirror of ESO's Extremely Large Telescope contains 798 hexagonal segments, which are equipped with position actuators (PACT) for segment piston-tip-tilt actuation and edge sensors (ES) to measure the relative segment displacements. PACT and ES are used for M1 figure control, i.e. for maintaining the reference shape of the primary mirror. Due to the sheer number of used sensors (4524), ES failures cannot be excluded. In order to minimize the influence of such failures on observations, an automated approach for ES failure handling has been developed. Besides identification of one or more malfunctioning sensors, replacement values for the erroneous sensor signals are calculated and fed back into the figure loop, enabling uninterrupted observations. The figure loop position actuators are hybrid actuators, combining brushless DC motors for a large range of motion with Piezo actuators for high accuracy and fast settling. Motions larger than the Piezo range (∼10μm) are restricted to a maximum velocity of 100μm/s. Since the figure loop is controlled in modal space such a rate limit introduces non-linearities in this large Multi-Input Multi-Output (MIMO) system resulting in cross-coupling of modes and undesirable transient behavior. A custom management scheme for MIMO systems with actuator non-linearities has been developed and successfully tested. It minimizes overshoots and cross-coupling between modes during transitions considering the PACT velocity limitation. In addition, this scheme simplifies closure of the figure loop from any initial condition. This paper covers advancements in both areas. After a brief description of the relevant subsystems, the algorithm to detect faulty ES is introduced. The closely related calculation of ideal replacement values is shown and simulation results illustrate the effectiveness of the Fault Detection, Isolation & Recovery (FDIR) measures for the ELT M1. Then the theory of considering actuator non-linearities in large MIMO systems is introduced. A custom scheme is derived and applied to the ELT M1 Figure Loop. Simulation results and measurements taken with ESO's seven segment M1 Test Facility are presented.