The LIGO Interferometer Sensing and Control System

The LIGO Interferometer Sensing and Control System (ISC) is a large and highly distributed Multiple Input Multiple Output (MIMO) control system that is used to control the length and alignment degrees of freedom of the interferometers. The 4 kilometer Fabry-Perot cavity lengths are controlled to better than 10-13 meters (rms) and the angular degrees of freedom are controlled to better than 10-8 radians. This paper will describe the real-time digital servo control systems that have been designed, developed and implemented for the LIGO Length Sensing and Control (LSC) [1] and Alignment Sensing and Control (ASC) [2] systems. In addition, the paper will describe how these controls, along with the suspended optic controls [3], have been integrated into the overall LIGO control and data acquisition system [4].