Control of the double pendulum optics suspension system of a 30 m triangular Fabry–Pérot cavity

We present a technique for the control of a double pendulum suspension system used to carry the optical elements of a 30 m long Fabry–Pérot cavity, a prototype for the mode cleaner of the VIRGO experiment. The movement of the payload is sensed with a charge coupled device camera, and the feedback forces are applied to the intermediate stage of the suspension via coil–magnet systems. The control strategy is explained and the application of the system for the longitudinal locking of the cavity is demonstrated.