Speaker
Description
LIGO, the Laser Interferometer Gravitational-wave Observatory, relies on real-time control. Feedback is fundamental to maintaining length and alignment control of the interferometer optics, to providing high performance seismic isolation, and for controlling many other key parts of the detector. LIGO uses a custom, linux-based system for fast control integrated with EPICs for slow control and state definition. We use thousands of control loops and monitor hundreds of thousands of operational parameters. The past decade of operation has taught us many valuable lessons as we work to achieve stable operation at low noise. I will focus on the seismic isolation systems, hybrid active/ passive platforms which help keep the LIGO optics quiet. These systems provide 10x isolation at the microseism (~1/6 Hz) and more than 100x isolation from ground motion above 1 Hz. We confront many of the classic issues for active control: feedback vs. feedforward, how to best combine multiple sensors for optimal performance and noise rejection, and how to operate multiple degrees-of-freedom in a rational way. These systems also provide a framework to discuss many of the practical problems encountered when running a high-performance facility. How do you turn on the system gracefully and recover your alignment? What can you do when the environmental conditions change (wind, earthquakes, and storms)? How do you monitor for system failures? I will also highlight how the isolation system integrates into the full facility and allows us to keep the mirrors at the ends of the 4 km arm cavities stabilized to each other to ~1e-13 meters and ~1e-9 radians rms. I hope this overview will lead to interesting discussions with the accelerator control community.